Presihajoča jezera na Pivškem EarthCache

(English)  (Fotografije / Photos)  (Konec teksta / End of Text)

Kazalo

1. Uvod
2. Geomorfološke značilnosti Pivške kotline
3. Hidrogeološke značilnosti Pivške kotline
4. Podzemne jame
5. Tektonske značilnosti Pivške kotline
6. Presihajoča jezera
7. Dostop
8. Zemeljski zaklad
9. Viri
10. Fotografije

Uvod

V sušnem obdobju se reka Pivka pojavlja na stiku apnencev in flišnih kamnin med Prestrankom in Rakitnikom, samo nekaj kilometrov pred svojim ponorom v Postojnsko jamo, medtem ko je pri Zagorju podzemna kraška voda deset metrov pod dolinskim dnom presihajoče reke Pivke. V času visokih vod se podzemeljske vode iz plitkega kraškega vodonosnika zgornjega dela doline (Zgornja Pivka) razlijejo na površje in zapolnijo strugo Pivke, ki takrat neprekinjeno teče od izvira pri Zagorju do ponora. Istočasno je na Zgornji Pivki poplavljenih tudi več manjših kraških globeli, ki se tako spremene v presihajoča kraška jezera. Zaradi stalnosti vsakoletnih poplav je med domačini za večino omenjenih kotanj v uporabi ledinsko ime jezero. Ker niso vse kotanje poplavljene hkrati, pač pa je to odvisno od količine padavin, se razlikuje tudi število teh jezer, ki ga lahko najdemo v različni literaturi. Zdi se, da je najbolj splošno sprejeto, da je presihajočih Pivških jezer 17, zato smo se tudi mi v opisu tega zemeljskega zaklada odločili za to število.

Geomorfološke značilnosti Pivške kotline

Na podlagi najdb kosti in zobovja poginulih živali strokovnjaki ugotavljajo, da so v vodah Pivške kotline že v daljni preteklosti, pred približno 400.000 leti (mindelsko-riška medledena doba), živeli pritlikavi povodni konji Hippopotamus pentlandi. Tekom geološke zgodovine so se pritoki reke Pivke, zlasti z Javornikov, zaradi zakrasevanja postopno umaknili v podzemlje, pred tem pa so ključno sodelovali pri izoblikovanju teras Pivške kotline.

Pivška kotlina je velika depresija med visokimi kraškimi planotami Nanosom (1313 m), Hrušico (1264 m), Javorniki (1268 m), Slavenskim ravnikom (600 – 700 m), Snežnikom (1796 m) in flišnim povodjem Notranjske Reke. Severni del te velike kotline je iz neprepustnih flišnih kamnin s površinskim  odtokom. Južni del, Zgornja Pivka, pa je nastal na apnencih in ima nekatere lastnosti kraškega polja.

Kotlina Zgornje Pivke je dolga okrog 16 km in 4 – 5 km široka. Njeno površje se blago spušča od 620 m nadmorske višine pri Koritnicah ob vznožju Snežniškega masiva na JV, do 520 m na SZ delu pri Prestranku oziroma Rakitniku, kjer reka Pivka prestopi na flišne kamnine. Pivka je največja reka v kotlini. Vodo dobiva s fliša in kraških izvirov v Zgornji Pivki, ponika pa v Postojnski jami v SV delu kotline. Površje Zgornje Pivke je obenem nagnjeno tudi od vznožja Javornikov proti JZ, tako da je pas najnižjega uravnanega sveta ob vznožju niza vzpetin, ki Zgornjo Pivko ločijo od flišnih pokrajin v porečju notranjske Reke. Ta prečna nagnjenost dna je bolj izrazita kot vzdolžna.

Površje kotline Zgornje Pivke lahko razdelimo na dve večji morfološki enoti: uravnano dno kotline in višjo skalno teraso, ki se vleče vzdolž Javornikov. Vanjo so poglobljene kotanje Pivških jezer. Dno kotline je najnižji del doline vzdolž reke Pivke. Med Rakitnikom oziroma Prestrankom in Pivko je široka od 1 – 2 km, od Pivke proti Zagorju pa se nekajkrat zoži in nato spet odpre v širšo ravnico. V taki razširitvi južno od Zagorja izvira tudi reka Pivka. Značilnost dna kotline so uravnana tla ob Pivki. Dno je popolnoma ravno le v bližini Pivke, drugod je nekoliko valovito in dvignjeno nekaj metrov nad strugo reke Pivke, ponekod so v njem tudi nizki kopasti vrhovi. Na zahodni strani ravno dno počasi prehaja v pobočja, na vzhodni strani pa povečini ostro v strma pobočja višje terase, ki se dviguje 10 – 20 m nad njim. Živoskalno dno pokriva mestoma tanka plast ilovnatih ali peščenih sedimentov. V dnu ni vrtač, so pa številni izviri in tudi ponori, vendar morfološko niso izraziti. K uravnanemu dnu Zgornje Pivke spada tudi večja kotanja pri Knežaku. Dolga je do 3 km ter do 1,5 km široka. Tam kotanjo loči od sklenjenega dna kotline višja terasa. Dno pri Knežaku je uravnano, k temu prispeva še obsežen vršaj grušča, ki so ga prinesle vode s Snežnika nad Koritnicami. Na dnu stoji nekaj izoliranih kopastih vrhov, na primer Obroba (644 m). Dno kotanje ima oster prehod v strmo ježo terase na vzhodni strani.

Višja skalna terasa leži med dnom kotline in Javorniki. V severnem delu je široka do 3 km, pri Petelinjskem jezeru okrog 1,5 km, južneje pod Zagorjem pa je široka 4 km in sega od Javornikov do vznožja Gradišča ter tako loči dno pri Knežaku od dna kotline ob Pivki. Terasa je nagnjena proti JZ. Ob Javornikih je v višini 650 – 670 m, tam v opaznem pregibu preide v pobočja Javornikov. Ob zahodnem robu pa je v nadmorski višini med 550 m in 570 m oziroma 10 – 15 m nad ravnico ob Pivki. Terasa je ponekod že močno razčlenjena. Taka je zlasti v severnem delu, najbolje pa je ohranjena med Trnjem, Bačem in Palčjem. Površje terase razčlenjujejo posamezni kopasti vrhovi, predvsem pa številne vrtače. Vzdolž nižjega zahodnega roba so vrtače večje, tu je tudi več velikih vrtač in kotanj z ravnim dnom, v katerih se pojavljajo občasna Pivška jezera.

Večina kotanj Pivških jezer so vrtačam podobne koncentrične depresije z velikim uravnanim dnom in ostrim pregibom med dnom in pobočji. Večji jezeri sta Palško in Petelinjsko, ki imata daljšo os dna jezera večjo od 1500 m. Ostale kotanje imajo premere med 100 in 300 m. Kotanje so torej veliko večje kot vrtače, ki jih poplave ne dosegajo. Imajo živoskalno dno, ki pa je prekrito s plastjo sedimenta in prsti, ki običajno ne presega debeline 0,5 m. Prehod ravnega dna v pobočje je v večini primerov izrazit in oster. Podobno so oblikovana pobočja dna kotline proti višji terasi. Dno jezerskih kotanj leži v višini, ki ga še doseže nihanje gladine kraške podtalnice. Ko se ta skozi številne kraške kanale dvigne, se v njih pojavi voda, istočasno pa se pojavi tudi v dnu doline, kjer izvira in teče Pivka. Pojav vode na površju omogoča močnejše raztapljanje apnenca, to pa je usmerjeno predvsem bočno, kar je razširilo dno teh kotanj. Proces je še danes aktiven, o čemer priča oster pregib med dnom in pobočji.

Kotanje Pivških jezer so torej nastale s preobrazbo vrtač ali večjih reliefnih uleknin, ki so segale v območje nihanja kraške podtalnice. Visoka gladina kraške podtalnice omogoča pojavljanje vode na površju in nastanek jezer, vpliva pa tudi na to, da se prst in sedimenti tu ne spirajo navzdol v kras. Dna jezer so zato pokrita z debelejšo prstjo, kot jo najdemo na okoliškem kraškem površju.

Hidrogeološke značilnosti Pivške kotline

Masiv Javornikov ima funkcijo kraškega vodonosnika, v katerem se vode pretakajo pretežno podzemno. Na zahodnem obrobju v dolini Pivke je vodonosnik v stiku z aluvialnimi in flišnimi kamninami, ki so slabše prepustne in predstavljajo lokalno hidrogeološko pregrado, ob kateri se predvsem ob visokem vodostaju pojavljajo kraški izviri. Debelina teh kamnin pa je relativno majhna in tudi pod njimi je razvit kraški vodonosnik, ki se pod Pivško kotlino razteza proti Vipavski dolini. Na jugu se Javorniki nadaljujejo v kraški masiv Snežnika. V tem delu poteka kraška razvodnica s prispevnim območjem rek Reke, Riječine in Kolpe, njen položaj pa je zaradi posebnih značilnosti krasa (podzemno pretakanje, bifurkacija) praktično nemogoče natančno določiti.

Hidrogeološka karta širšega območja

Legenda: 1. vidna in pokrita narivnica, 2. Viden in pokrit prelom, 3. površinski tok, 4. Presihajoče jezero, 5. večji izvir, 6. stalni izvir, 7. občasni izvir, 8. ponor, 9. točka injiciranja sledila, 10. s sledenjem dokazana glavna in stranska podzemna vodna zveza, 11. kraški vodonosnik, 12. razpoklinski  vodonosnik, 13. medzrnski vodonosnik, 14. zelo slabo prepustne plasti.

Na severni in vzhodni strani kraški vodonosnik omejuje območje med Planinskim in Cerkniškim poljem, ki ga gradijo predvsem zgornjetriasni dolomiti. Ti so slabše prepustni od apnencev, seka pa jih tudi prelomna cona Idrijskega preloma, ki ima zaradi zdrobljenega, milonitiziranega dolomita vlogo hidrogeološke pregrade, ob kateri podzemne vode v kraških izvirih iztekajo na površje.

V kraškem vodonosniku se podzemne vode pretakajo večinoma po razširjenih razpokah in kraških kanalih v različnih smereh proti izvirom na obrobju. Na osnovi hidrogeoloških raziskav, še posebej sledilnih poskusov, so bile dokazane smeri odtekanja proti izvirom na Planinskem polju (Malenščica, Unica, Škratovka) in v Rakovem Škocjanu (Kotliči, Prunkovec), proti izvirom Vipave v Vipavski dolini ter proti kraškim izvirom ob reki Pivki. Območje južno od izvirov Pivke se vsaj deloma odteka proti izviru Podstenjšek in drugim kraškim izvirom v povodju reke Reke, vendar pa bolj  podrobne raziskave o položaju razvodnice zaenkrat še niso bile opravljene.

Hidrogeološka zgradba

Hidrogeološka karta območja presihajočih Pivških jezer

Legenda: 1. kraški vodonosnik, 2. Medzrnski vodonosnik, 3. zelo slabo prepustne plasti, 4. stalni površinski tok, 5. Občasni površinski tok, 6. presihajoče jezero, 7. kraški izvir, 8. kraška jama, 9. ponor

Ožje območje presihajočih Pivških jezer gradijo predvsem dobro zakraseli kredni apnenci, ki imajo značilnosti dobro prepustnih kamnin s kraško-razpoklinsko poroznostjo. V njih so razvite številne površinske in podzemne kraške oblike. Prsti na apnencih je relativno malo in še ta je pogosto nezvezna, zato je ponekod površje precej kamnito. Padavine s površja neposredno prenikajo v bolj ali manj vertikalni smeri skozi razpokane, zakrasele apnence vadozne cone globlje v kras, v stalno zaliti coni vodonosnika pa se nato raztekajo v različnih smereh proti izvirom na obrobju kraškega masiva.

V dolini reke Pivke so na apnencih odložene kvartarne aluvialne naplavine, ki so relativno slabše prepustne in predstavljajo lokalno hidrogeološko pregrado. Ob njih se ob visokem vodostaju, ko se gladina podzemne vode v krasu dvigne nad nivo kontakta, pojavljajo občasni kraški izviri, iz katerih voda napaja površinski tok reke Pivke. V daljših obdobjih brez padavin pa se nivo podzemne vode spusti pod ta prelivni rob in izviri presušijo. Voda iz površinske Pivke se izgublja v podzemlje skozi številne ponore v strugi.

Na eocenskem flišu, ki je zelo slabo prepusten, se na območju med Prestrankom in Postojno vode pretakajo pretežno površinsko. Debelina tega fliša je relativno majhna in tudi v večjih globinah pod njim je razvit kraški vodonosnik. Eocenski fliš v dolini reke Reke je del Komenske narivne grude, na katero je narinjen masiv Javornikov (Snežniška narivna gruda). Predstavlja neprepustno odlago kraškega vodonosnika in se na površje prikaže tudi v tektonskem oknu pri Knežaku. Na stik apnenca in fliša so vezani kraški izviri, ki napajajo pritoke Reke.

Značilnosti podzemnega pretakanja

V zahodnem robu Javornikov je podzemna voda dosegljiva v jami Fužina pri Stari vasi, v Breznu v Kobiljih grižah in v Matijevi jami ob Palškem jezeru. V okviru raziskav za vodooskrbo so bile izvrtane tudi tri vrtine, in sicer pri Fužini, pri Kobiljih grižah in pri Žejskih izvirih, kjer so v osemdesetih letih občasno spremljali nihanje nivoja podzemne vode, kasneje pa so bile vrtine zamašene in merjenje ni več možno. Stalna je voda v vodnjakih v Pivki in Zagorju. Geološki zavod Ljubljana je na območju med izviri Pivke, Koritnicami in Šembijami izvrtal več vrtin in v njih krajše obdobje spremljal nihanje nivojev podzemne vode. Znani so še nivoji stalnih kraških izvirov na obrobju masiva Javornikov. Ob visokem vodostaju gladina kraške vode doseže površje v občasnih izvirih ob Pivki in v presihajočih Pivških jezerih. Predvsem s stališča določitve maksimalnih nivojev so zanimivi podatki o gladini jezer, zbrani ob poplavah v novembru 2000.

Na osnovi vseh teh podatkov je mogoče sklepati o generalnih smereh pretakanja podzemne vode in okvirno tudi o razponu med najvišjimi in najnižjimi nivoji vode v različnih delih kraškega vodonosnika. Za območje reke Pivke je bilo tako ugotovljeno, da je ob nizkem vodostaju v njenem zgornjem toku podzemna voda okrog 10 m, v spodnjem toku pa tudi več kot 20 m pod suho strugo. Ob visokem vodostaju je struga zalita, aktivni pa so tudi izviri ob strugi. Glede na višino vodostaja je zalitih tudi več ali manj jezer. Ker pa so vse omenjene raziskave potekale le v krajših časovnih obdobjih in so bile meritve le občasne, je za podrobnejšo analizo značilnosti nihanja nivoja podzemne vode ob različnih hidroloških pogojih podatkov premalo.

Več o smereh in značilnostih podzemnega pretakanja smo izvedeli iz rezultatov do sedaj opravljenih sledilnih poskusov. V jamo Požiralnik pod Kremenco so 26. septembra 1955 injicirali 10 kg uranina in ugotovili povezavo z izviri Unice in Malenščice. Navidezno hitrost pretakanja so ocenili na okrog 0,5 – 0,6 cm/s. Ob visokem vodostaju 14. novembra 1967 so v ponor na Palškem jezeru injicirali 10 kg vijoličasto obarvanih trosov in ugotovili povezavo s Trnskimi izviri, navidezno hitrost pa ocenili na 9 cm/s. Naslednji dan so ponorne vode na Petelinjskem jezeru obarvali s 3,5 kg uranina, voda pa se je podzemno pretakala proti Žejskim izvirom z navidezno hitrostjo 1 cm/s. V Kneške ponikve so 26. februarja 1968 ob pretoku 5 l/s injicirali 1 kg uranina in ugotovili povezavo z izvirom Videmščica in navidezno hitrost pretakanja 1,2 cm/s. Potok Stržen, ki ga napajajo vode izvira Fužina pri Stari vasi, je bil obarvan dvakrat. Najprej so ob nizkem vodostaju (pretok Stržena 30 l/s) 23. februarja 1982 v požiralnik v strugi Stržena pri Rakitniku injicirali 10 kg uranina in opazovali vodni tok v Planinski jami ter izvire Malenščice, Škratovko in Kotliče. Čeprav je opazovanje trajalo dalj časa, sledila nikjer niso zaznali. Zato so sledenje Stržena izvedli še enkrat 9. avgusta 1988. Tudi v tem primeru je bil vodostaj nizek s pretokom Stržena 10 l/s. Injicirali so 20 kg rodamina in dodatno opazovali še izvire Vipave in Timave. V prej omenjenih izvirih na obrobju Planinskega polja in Rakovega  Škocjana se sledilo spet ni pojavilo, ugotovljena pa je bila podzemna vodna zveza z izviri Vipave (navidezna hitrost pretakanja okrog 0,4 cm/s). Na Timavi je bilo opazovanj premalo, da bi lahko rezultate uporabili kot zanesljive. Hkrati so 9. avgusta 1988 Pivko v Trnju obarvali z 20 kg uranina. Na tem mestu je v strugi več požiralnikov, v katere ob nizkem vodostaju Pivka v celoti ponikne, ob višjem pa je kapaciteta požiralnikov premajhna in vode delno odtekajo naprej po strugi. Sledenje je potekalo ob nizkem vodostaju, ko se je Pivka v celoti izgubljala v požiralnik. Z izjemo Vipave in Timave se je sledilo pojavilo v vseh ostalih opazovanih izvirih. Ugotovljene so bile naslednje navidezne hitrosti podzemnega pretakanja: proti vodnemu toku v Planinski jami in izvirom Malenščice okrog 0,5 cm/s ter proti Pivki pri Prestranku in Strženu pri Rakitniku okrog 0,2 cm/s.

Metodologija sledilnih poskusov je z leti še napredovala in zanesljivost rezultatov se veča. Zato so še posebej zanimivi rezultati sledenja, ki je bilo izvedeno poleti 1997. Na območju vojaškega vadišča Poček na Javornikih so ob nizkem vodostaju 10. junija v dnu skalne vrtače injicirali 4 kg uranina in nato na 11 točkah zajemali vzorce približno 1 leto. Rezultati so pokazali, da se pomemben delež vode odteka podzemno proti izvirom Malenščice (55 % povrnjenega sledila) in izvirom Vipave (26% povrnjenega sledila). Navidezna dominantna hitrost toka v obeh smereh je bila okrog 0,7 cm/s. Tako je bil ugotovljen glavni tok podzemne vode iz kraškega vodonosnika Javornikov proti izvirom na Planinskem polju, potrjeno pa je bilo tudi podzemno odtekanje pod flišem Pivške kotline proti Vipavski dolini. V manjših deležih se je sledilo pojavilo še v drugih izvirih na Planinskem polju (Unica, Škratovka) in v Rakovem Škocjanu (Rak pred Prunkovcem, Prunkovec, Kotliči). Najbolj počasno, a zanesljivo dokazano, pa je bilo pretakanje proti najbližjemu zajetju v Stari vasi (navidezna dominantna hitrost 0,29 cm/s). Pojav sledila v Žejskih izvirih ter v Pivki pri Prestranku ni bil dovolj prepričljiv, da bi lahko podzemno vodno zvezo zanesljivo potrdili.

Zanimiva je ugotovitev, da se je sledilo pojavljalo v izvirih Malenščice po vsakih močnejših padavinah še eno leto po injiciranju. Iz tega je moč zaključiti, da se tudi vsako onesnaženje na kraškem površju deloma že zelo hitro pojavi v izvirih na obrobju, deloma pa se zadrži v podzemlju in ga potem vsake močnejše padavine v daljšem časovnem obdobju spet spirajo na površje. Tudi hitrosti pretakanja se spreminjajo s hidrološkimi pogoji.

Hidrološke razmere

Hidrološke razmere obravnavanega območja se odražajo v hidroloških značilnostih reke Pivke. Napajajo jo kraške vode in po različno dolgih odsekih površinskega toka spet ponika v kraško podzemlje. Izvira pri Zagorju in njena celotna dolžina je 26 km, od tega je le v spodnjem delu 11 km stalnega površinskega toka. V zgornjem delu je v sušnih obdobjih nivo podzemne vode tudi 10 in več metrov pod strugo Pivke, ki je zato večinoma suha. Po močnejših padavinah pa se nivo dvigne in aktivirajo se občasni kraški izviri, ki jo napajajo. Ob visokih vodah Pivka tudi poplavlja, in sicer obsegajo poplavne površine 7,5 km², od tega pripada presihajočim kraškim jezerom 2,2 km². Največ poplav je jeseni, razmeroma pogoste so zimske poplave, podobno tudi spomladi.

V srednjem toku pri Prestranku Pivka ni stalna in se pojavlja le v obdobjih višjega vodostaja. Voda v strugi pri Prestranku pomeni, da kraški izviri delujejo in da so jezera vsaj deloma zalita. V spodnjem toku tik pred ponorom v Postojnsko jamo pa je Pivka suha le izjemoma. K značilno višjim pretokom v tem delu nekaj prinese pritok Nanoščice in drugih manjših potokov s fliša, pa tudi dodatno napajanje iz številnih kraških izvirov.

Podzemne jame

Jame na Pivškem (Kartografija Jurij Hajna)

Na širšem območju presihajočih Pivških jezer  je registriranih in raziskanih 87 jam in brezen s skupno dolžino 2.617 m in skupno globino 1.255 m. V času italijanske okupacije (1918 – 1943) je bilo na tem področju registriranih že 31 jam, ki imajo tudi italijansko katastrsko številko VG (Venezia – Giulia). Gostota jam (0,7 / km²) na tem področju je nekoliko nižja, kot jo je izračunal Gams za celotno Pivško kotlino (1,1 / km²). Globino 50 m presegata zgolj Tičkova jama in Brezno v Slavenskih senožetih. Na Pivškem prevladujejo krajše jame do 50 m (85 %), le 4 jame so daljše od 100 m: Drskovška golobina, Trnska jama - nekoč že pomembna turistična jama, Jama v kamnolomu pri Matenji vasi ter Mrzla jama.

Deset najdaljših jam v neposredni bližini Pivških jezer (stanje na dan 1. junij 2005; Arhiv ZRK 2005).

Najštevilčnejše so suhe jame (95 %). Najbolj znana jama na Pivških jezerih je estavela Matijeva jama, v kateri je zlasti zanimiv skalni relief, saj odseva izrazita nihanja vodne gladine. V času italijanske okupacije je vojska iz jame črpala vodo za svoje potrebe. Danes je vhod v jamo izrazito preoblikovan, saj jo je Jugoslovanska ljudska armada (JLA) v preteklosti uporabljala kot cilj topniškega obstreljevanja.

Načrt Matijeve jame (Palško jezero) z izmerjenimi vodostaji

Jame skrivajo številne informacije o preteklih dobah. Jamski sedimenti, ostanki živali in drugi predmeti v sedimentih na Pivškem ne služijo zgolj za razvozlavanje človekove zgodovine, ampak nam pomagajo tudi pri interpretaciji nekdanjih klimatskih razmer.

Tektonske značilnosti Pivške kotline

Čez Pivško kotlino poteka pomemben regionalni nariv, nariv Snežnika, ki loči Snežniško narivno grudo od paravtohtona Komenske narivne grude. Tektonsko okno pri Knežaku je dokaz starejših narivnih deformacij. Pokrajina je prerezana s številnimi mlajšimi prelomi, od katerih so najpomembnejši Raški, Predjamski in Selški prelom. Pokrajina Zgornje Pivke je tektonsko precej aktivna, kar kažejo potresi v zadnjih letih. Kot najbolj potencialni aktivni prelom se kaže Raški prelom oziroma kakšen od  spremljajočih prelomov severno od Raškega preloma, npr. Šembijski prelom in morda tudi Selški prelom. Kraška presihajoča jezera Zgornje Pivke so razvita v zgornjekrednih apnencih in so od nariva Snežnika oddaljena 0,5-4 km proti SV. Večina jezer je oblikovana ob Selškem prelomu.

Geološka karta Pivške kotline s presihajočimi jezeri (sestavila S. Šebela)

Legenda:  1-železnica, 2-reka s smerjo toka, 3-občasni tok vode, 4-hrib z nadmorsko višino, 5-mesto, 6-manjši kraj, 7-kraška jama, 8-presihajoče jezero, 9-aluvij (kvartar), 10-fliš (eocen), 11-apnenec (paleogen), 12-prevladujoči apnenec (kreda), 13- nariv: močnejši in šibkejši, 14-desni zmik ob horizontalnem prelomu, 15-prelom: ugotovljen in pokrit, 16-antiklinala, 17- sinklinala.

Pivška kotlina je izredno zanimiva ne samo zaradi kraških presihajočih jezer ter kraške hidrologije, ampak tudi zaradi zanimive geološke zgradbe. V tem delu poteka pomemben regionalni nariv, imenovan nariv Snežnika, ki med seboj loči dve pomembni tektonski enoti. Severnejša Snežniška narivna gruda je narinjena na paravtohton Komenske narivne grude. Omenjeni nariv lahko sledimo nekoliko južno od Gemone v Italiji, čez Vipavsko dolino, kot nariv Snežnika ter SV od Reke na Hrvaškem, to je na dolžini vsaj 160 km. Za Pivško kotlino in področje Snežnika je značilna zmerna tektonska aktivnost, ki se kaže v zgodovinskih in sodobnih potresih. Geoloških in kraških zanimivosti v Pivški kotlini torej ne manjka.

Večina ozemlja današnje Slovenije pripada Jadranski mikroplošči. Alpska orogenetska veriga je rezultat konvergence med Afriško in Evrazijsko ploščo. Jadranska mikroplošča je v veliki meri podrinjena pod Jadransko morje in prekrita z debelo plastjo mladih nesprijetih sedimentov, kot je primer v Padski nižini. Geološke, seizmične in seizmično tomografske raziskave dokazujejo veliko verjetnost, da se V, Z in S robovi Jadranske mikroplošče podrivajo in ustvarjajo predgorje okoliškim deformiranim conam. Jadranska mikroplošča se verjetno počasi premika glede na sosednjo Evrazijsko ploščo. Glavni aseizmični izdanek Jadranske mikroplošče je Istrski polotok Slovenije in Hrvaške. Na podlagi GPS meritev opazujemo v zahodni Sloveniji očitne in ostre (nekaj mm/ leto) desne (±transpresivne) gradiente vzdolž Savskega preloma in Periadriatske prelomne cone, kar nakazuje, da je bočno izrivanje v SV Alpah še vedno aktivno, saj ga povzroča rotacija Jadranske mikroplošče v nasprotni smeri urinega kazalca.

Poleg starejših narivnih prelomov (narivanje v miocenu) zasledimo tudi mlajše neotektonske prelome. Med njimi sta najpomembnejša Predjamski in Raški prelom, ki oba kažeta značilnosti desnega zmika. Za Pivško kotlino je pomemben tudi Selški prelom, ki se iz Prestranškega ravnika nadaljuje proti JV vse do Koritnic in domnevno še naprej proti JV v zahodno pobočje Snežnika. Večina presihajočih Pivških jezer se nahaja v območju (širine 1,5 km) Selškega preloma.

Na raziskanem terenu so najstarejše kamnine dobro plastnati apnenci ter dolomitne in apnene breče (K1,2 – spodnja do zgornja kreda), ki jih najdemo na območju Bača, Koritnic in V od Ilirske Bistrice. Sledijo zgornjekredni apnenci (prevladujejo sivi rudistni apnenci), ki gradijo Javornike. Med Snežniškim prelomom in Ilirsko Bistrico najdemo vremske plasti, ki jih uvrščajo v kredo in paleogen. Iz obdobja eocena imamo flišne laporje, peščenjake in numulitne breče. Dno Pivške kotline je deloma pokrito z rečnimi sedimenti. Aluvij prekriva tudi dna največjih kraških depresij-presihajočih jezer. Presihajoča jezera Pivške kotline so razvita v zgornje krednih apnencih.

Potresi

Širše območje Ilirske Bistrice je potresno eno najbolj aktivnih predelov Slovenije. Eden najmočnejših potresov v srednji Evropi (dne 25.1.1348) z žariščem na območju Dobrača in Beljaka je močno prizadel tudi ozemlje današnje Slovenije. Potres je med drugim uničil tudi utrjen grad Novi grad pri Podgradu blizu Ilirske Bistrice.

Od 159 izbranih potresov iz obdobja od 2.2.1834 do 3.12.1981 je bil najmočnejši (Mm=5.2) tako imenovani Cerkniški potres (januar 1926) z epicentrom v Javornikih. V časopisu Edinost (Trst, 8. januar 1926) zasledimo, da so v Postojni čutili nad 50 potresnih sunkov. V Postojnski jami naj bi se zrušil stalagmit, ki je meril skoraj 1 meter v premeru. Drugi najmočnejši potres predstavlja potres na območju Ilirske Bistrice in Snežnika dne 31.1.1956 z magnitudo Mm=5.1 in globino ipocentra 7 km. V obdobju od 2.2.1834 do 3.12.1981 je bil v Pivški kotlini najmočnejši potres lociran v dolini Pivke (Pivka-Postojna) dne 22.10.1900 z magnitudo (Mm=4.5). Potresi so v Pivški kotlini in na območju Ilirske Bistrice precej pogosti, s povprečno globino žarišč med 7 in 16 km.

Presihajoča jezera

Ob visokih kraških vodah se pojavijo številne poplavljene površine, ki jih domačini glede na velikost in trajanje poplav različno imenujejo, npr. luže, močila, loke, lokve, mlake, ribniki, jezera ipd. Glede na literaturne vire in pogovore z domačini danes lahko z opisnimi in lokalno-pomenskimi toponimi k presihajočim Pivškim jezerom prištevamo večje število občasno z vodo napolnjenih kraških depresij.

Pri poimenovanju nekaterih jezer obstaja več različic, mi bomo v nadaljevanju navajali najbolj razširjena imena. Če se ob visokih vodah odpravimo na Pivško, na poti od severa proti jugu naletimo na sledeča jezera. Po dnu kraškega dola Jeredovce je včasih potekala pot na Bile. Dno Krajnikovega dola je redkeje poplavljeno. V najnižje ležečem Petelinjskem jezeru (532 m nad morjem) se voda zadrži najdlje, povprečno 6 mesecev na leto. Po površini je največje Palško jezero. Najmočnejši izvir, ki ga polni, je estavela Matijeva jama. Kot povedo domačini, se v Klenskem dolu pri Kleniku občasno zadrži voda. Danes se za to poplavljeno površino uporablja tudi ime Klensko jezero. Zaradi nasutja dela depresije, kjer se pojavi Radohovsko jezero, ki edino leži na levi strani reke Pivke, se je njegova površina precej zmanjšala. V bližini vasi Parje leži Parsko jezero, kamor odtekajo vode iz Malega in Velikega Drskovškega jezera ter naprej proti kraškemu izviru Mišnik. Domačini vedo povedati, da se vode na dnu Velikega Zagorskega jezera, ki je po površini sicer manjše od Malega Zagorskega, zadržujejo le krajši čas. V Kljunovem ribniku kraška voda bruha z velikim pritiskom zlasti iz umetne vrtine in teče v strugo Pivke. Severovzhodni del te depresije se imenuje Mali dol. Za  gradom Kalc se v dnu Velikega dola občasno zadrži voda. Če potujemo po stari vojaški poti iz Zagorja proti Baču, lahko ugledamo še dve kraški depresiji, ki sta redkeje poplavljeni: večje Bačko jezero in nekoliko višje ležeče Laneno jezero. V dnu Kalskega jezera je izkopan kanal, ki visoke vode odvodnjava v strugo reke Pivke. Šembijsko jezero je najjužnejše presihajoče jezero, ki se pojavi pri Šembijah in je bilo v dosedanjih prispevkih o presihajočih jezerih s Pivškega pogosto prezrto.

Osnovne značilnosti Pivških presihajočih jezer

Površina Šembijskega jezera je bila v času izjemno visokih voda jeseni 2000 precej velika. Voda iz višje ležečih Narič se je površinsko pretakala v nižje ležečo depresijo, imenovano Jezero. V istem obdobju sta bili zaradi visokih voda vasi Knežak in Bač prometno odrezani. Visoke vode leta 2000 so bile res izjemne, vendar domačini pogosto omenjajo, da se je v zadnjih desetletjih povprečen čas visokih voda v nekaterih jezerih skrajšal ter da se na nekaterih območjih poplavljena površina krči.

Prisotnost nekaterih presihajočih jezer lahko dobro napovemo, npr. z opazovanjem toka reke Pivke. Tok Pivke skozi Prestranek in vsaj deloma razlita struga Pivke pri Rakitniku sta zanesljiva znaka, da so v kraškem zaledju visoke vode, da delujejo kraških izviri, ki vsaj deloma napolnijo kraška jezera na Zgornji Pivki. Tudi v Podtaboru pri Podstenjšku domačini s preprostim opazovanjem izvira ob cesti vedo, da če le-ta ob visokih spomladanskih in jesenskih vodah oživi, je Šembijsko jezero zagotovo polno.

Na Zgornji Pivki se globina podtalnice giblje med 560 m n.v. v južnem delu doline v okolici Knežaka in 516 m n.v. v Matijevi jami na Palškem jezeru. Matijeva jama je značilna estavela z izrazitim nihanjem vodostajev (40 m in več). Ob visokih vodah novembra 2000 se je voda v jami dvignila za dodatnih 8 m nad do sedaj najvišje izmerjenim vodostajem in je dosegla višino 566 m. Eden izmed vzrokov za pojavljanje kraških presihajočih jezer na Zgornji Pivki je tudi omejena prepustnost vodnih kanalov v smeri kraškega izvira Malni (Planinsko polje), kar v deževnih obdobjih povzroča poplave vzdolž struge Pivke ter obenem poplave v manjših kraških globelih, ki se takrat spremene v jezera. Nihanje vodostajev v jezerih, ki ležijo bližje reki Pivki, je manjše kot v od nje bolj oddaljenih jezerih. V osnovi je zapolnitev jezer posledica dejstva, da je napajanje v območju izrazitejše od odtoka. V splošnem je na Zgornji Pivki izoblikovan bolj ali manj enoten plitek kraški vodonosnik, vendar pa lahko na podlagi podatkov o nihanju vodostajev v različnih jezerih sklepamo, da je prepustnost v vodonosniku zelo različna.

Na Zgornji Pivki je gladina podtalnice v splošnem nagnjena proti severu, z izjemo hidrološkega zaledja Šembijskega jezera, kjer visi proti severozahodu. Voda iz Šembijskega jezera napaja kraški izvir Podstenjšek, ki je desni pritok reke Reke (jadransko povodje), medtem ko je reka Pivka z ostalimi šestnajstimi jezeri del hidrografskega zaledja Ljubljanice (črnomorsko povodje). Ta predel Zgornje Pivke tako predstavlja razvodje med jadranskim in črnomorskim povodjem. Kljub dejstvu, da je Šembijsko jezero v ozkem hidrološkem pogledu očitno del jadranskega povodja, ga glede na geologijo in druge naravne značilnosti obravnavamo skupaj z ostalimi presihajočimi Pivškimi jezeri.

Z  izjemo Palškega in Malega Drskovškega jezera, se vsa jezera napajajo in praznijo pretežno skozi aluvialne naplavine, ki prekrivajo njihova zakrasela dna. Prvi dve se dodatno napajata in praznita skozi manjše izvorne in ponorne jame. Plasti aluvialnih naplavin skoraj gotovo prispevajo k trajanju ojezeritve posameznih kotanj.

Vodne povezave

Kras v Sloveniji hrani velike količine vode. Kljub intenzivnemu proučevanju podzemeljskih vodnih povezav v zadnjem času je to le drobec pri poznavanju pretakanja kraških voda. Podobno je tudi na Pivškem, klasičnem krasu, kjer je eno izmed porečij Ljubljanice in kjer je hidrološko stanje izjemno zapleteno. Na Zgornji Pivki poteka razvodnica med črnomorskim in jadranskim povodjem. Površinska in podzemeljska bifurkacija v epifreatični in freatični coni vključuje precej veliko zaledje Pivke z zapletenimi vodnimi povezavami in različnimi odtoki v različne smeri. Danes z gotovostjo lahko trdimo, da med reko Pivko in nekaterimi jezeri obstajajo neposredne podzemeljske povezave. Sama beseda Pivka je sinonim za požiralnik, ki pije vodo, malo jezerce, ki počasi odteka, ali za senožeti pri Zagorju, kjer votlikava tla popijejo vodo in čez katere poteka rečna struga. Glavnemu izviru reke Pivke iz Pivšc se nizvodno pridružijo še številni manjši.

Porečje Pivke sestavlja okrog 140 km vodnih tokov, od tega jih je 34 % stalnih, 66% pa občasnih. Sezonskim poplavam na Pivki je podvrženih okrog 16 % sveta Pivške kotline. Ob izdatni namočenosti se namreč v strugo Pivke pa tudi na širšem območju ob njej iz številnih bruhalnikov prelivajo visoke vode, medtem ko nizke odtekajo po podzemeljskih kanalih nižje neposredno proti Planinskemu polju. Ob visokih vodah uravnavajo višino podzemeljskih voda izviri ob Pivki. To se kaže zlasti v razporeditvi vodnih gladin presihajočih jezer; v jezerih bližje Pivki (v Parskem, obeh Drskovških in Zagorskih ter v Kalskem) se gladina dvigne le za nekaj metrov nad površinsko Pivko, v bolj oddaljenih pa več. V suši se voda umakne 20 m pod strugo Pivke. Domačini omenjajo, da so se otroci v tolmunih Pivke v njenem zgornjem toku še pred nekaj desetletji v času poletnih počitnic kopali. V času kolin pa je bilo v strugi v Zgornji Pivki še vedno toliko vode, da so jo lahko uporabljali za čiščenje črevja za koline. V vmesnem času so strugo Pivke kosili. Po ocenah reka Pivka na koncu svoje poti po Pivški kotlini vnese v Postojnsko jamo okoli 4000 ton erodirane kamnine letno.

Visoke vode novembra 2000

V jeseni 2000, posebno v oktobru in novembru, je nad zahodno in srednjo Evropo prevladovalo globoko območje nizkega zračnega pritiska. Ves mesec november so si v zelo kratkih razmikih sledile hladne fronte, katerih posledica je bilo obilno  deževje predvsem na zahodu Slovenije. Posledica advekcije vlažnih in toplih zračnih mas so bile lokalno obilne in dolgotrajne padavine. Ponekod je oktobra in novembra padlo tudi do štirikrat več padavin, kot je povprečna količina za omenjena meseca.

Podatki o padavinah z referenčnih meteoroloških postaj Postojna, Ilirska Bistrica in Jurišče za september, oktober in  november 2000 kažejo, da je bila na Pivškem povprečna količina padavin za vse tri mesece skupaj (obdobje 1961-1990) skoraj dvakrat presežena. Obilne padavine so dosegle višek v novembru, ko je bilo na meteoroloških postajah Postojna in Jurišče zabeleženih samo sedem dni brez dežja in dan manj na meteorološki postaji Ilirska Bistrica. Prvo neprekinjeno obdobje s padavinami je bilo po podatkih z meteoroloških postaj Postojna, Ilirska Bistrica in Jurišče zabeleženo med 30. oktobrom in 11. novembrom (230.1, 275.8, 332.6 mm), drugo med 13. in 22. novembrom (163, 191.2, 217.5 mm) in tretje med 24. in 26. novembrom (63.5, 77, 53.7 mm). V novembru 2000 je količina padavin na Pivškem trikrat presegla povprečno količino padavin za obdobje 1961-1990 (Postojna 455.2 mm, Ilirska Bistrica 544 mm, Jurišče 601.6 mm), kar se je odrazilo v izrazitem dvigu nivoja podtalnice.

Vodnatost stalnih kraških izvirov se je hitro povečala, oživeli so tudi številni pobočni kraški izviri, kar je povzročilo obsežne poplave ob toku Pivke. Dvig podtalnice se je odrazil z zalitjem 17 presihajočih jezer, izjemoma pa so bile poplavljene tudi nekatere višje ležeče kraške kotanje. Gladina vode se je v dveh presihajočih jezerih dvignila za okoli 10 m nad srednji vodostaj (Palško jezero, Šembijsko jezero), približno tolikšen je moral biti tudi dvig gladine podtalnice v kraškem vodonosniku. Tako visoki vodostaji so se pojavili po približno sedemdesetih letih. Vendar pa obstajajo očitne razlike v dvigu vodostajev med posameznimi jezeri. Eden izmed poglavitnejših vzrokov je najverjetneje tudi stopnja razvitosti podzemnih vodnih kanalov. Nekatere izmed poplavljenih kraških kotanj imajo verjetno bolje razvite podzemeljske povezave kot ostale, kar povzroča razlike v nihanju nivoja podtalnice znotraj območja. To se je odrazilo v hitri zapolnitvi nekaterih jezer, ki so tako dosegla rekordne vodostaje, medtem ko so vodostaji posameznih jezer komaj dosegli srednje vrednosti.

Dolina Zgornje Pivke s kraškimi presihajočimi jezeri in poplavnimi površinami novembra 2000

Legenda: 1. jezero in poplavna površina 2. srednji vodostaj Palškega jezera 3. površinski tok med jezeri 4. reka Pivka s pritoki 5. Naselje 6. grad Kalc 7. površje pod 600 m n.v. 8. površje nad 600 m n.v. (Poplave vzdolž struge Pivke niso označene.)

Izrazitejše nihanje vodostajev je bilo zabeleženo v jezerih, ki so bolj oddaljena od reke Pivke. Na poplavnem območju v  okolici Knežaka in Bača ter v okolici Šembijskega jezera se je tako nivo podtalnice dvignil za približno 30 m in več nad srednji nivo. Rekordni vodostaji so bili zabeleženi tudi v nekaterih drugih jezerih, še posebno na Palškem jezeru, kjer se je v tistem obdobju stranski vhod v Matijevo jamo nahajal skoraj 20 m pod gladino jezera. Pritisk podtalnice, ki je silila na površje iz kraškega vodonosnika, je bil izjemno velik in voda je na območju jezer vrela na dan v številnih drobnih izvirkih. Na posameznih območjih, kjer je bila travna ruša pregosta, se je pod njo naredila svojevrstna vodna blazina. Ob pritisku na napeto površino se je »blazina« upognila, predrla pa se ni. Pojav je bil opazovan v okolici Šembijskega in Palškega jezera.

Profil Pivke in presihajočih jezer v novembru 2000

Legenda: 1. jezero 2. poplavljeno območje 3. srednji vodostaj jezera 4. struga Pivke 5. Površinski tok 6. podzemni tok 7. kraški izvir 8. ponor

Dostop

Do tega zemeljskega zaklada pridete iz Postojne, od koder greste v smeri Ilirske Bistrice oziroma Reke na Hrvaškem. Cesta vas po nekaj kilometrih pripelje do naselja Pivka, kjer v centru ne zavijete desno po prednostni cesti, ampak greste naravnost v smeri Knežaka. Druga možnost je pot iz Ilirske Bistrice v smeri Šembij in Knežaka.

Kot je razvidno iz opisa zaklada, je presihajočih jezer 17, nekatera od njih so že pred Pivko. Za to, da lahko odgovorite na zahtevana vprašanja, boste morali obiskati nekaj lokacij, ki so med seboj kar precej oddaljene. Za pot od ene do druge boste potrebovali avto ali vsaj kolo, za pešačenje so razdalje prevelike. Pri iskanju lokacij si pomagajte s točkami poti, ki so navedene v nadaljevanju. Sledijo si v vrstnem redu od severa proti jugu, oziroma od Postojne proti Ilirski Bistrici.

Ker je lokacij več in so med seboj precej oddaljene, si vzemite dovolj časa, vsaj pol dneva. Še zlasti to velja, če boste ta zemeljski zaklad kombinirali s serijo tradicionalnih zakladov, ki je tudi posvečena presihajočim jezerom na Pivškem. V tem primeru vam lahko iskanje vseh zakladov vzame tudi več kot en dan.

Za to, da boste lahko uspešno odgovorili na vprašanja, ki so pogoj za vpis tega zemeljskega zaklada, boste morali obiskati štiri od sedemnajstih presihajočih Pivških jezer, in sicer Palško, Petelinjsko, Veliko Drskovško in Malo Drskovško jezero. Preostala jezera  za uspešnost lova na ta zaklad niso ključna, kljub temu pa priporočamo ogled vseh naštetih lokacij. Jezera so z avtom ali kolesom dostopna skozi celo leto, razen takrat, ko je pokrajina zasnežena, saj poljskih poti in kolovozov, ki vodijo do lokacij, ne plužijo.

Da bi vam olajšali iskanje lokacij, na katerih boste poiskali odgovore na vprašanja, naj navedemo nekaj pomožnih orientacijskih točk, ki vam bodo pri tem v pomoč:

1.       Petelinjsko jezero

a.       Parkirate lahko v vasi pri točki PJ03P0, kjer vas kažipot usmeri proti jezeru, do katerega je še približno 1,5 km.

b.      Če niste preveč občutljivi glede avtomobila ali imate terenca, se lahko pripeljete čisto do jezera, do točke PJ03.

2.       Palško jezero

a.       Parkirate lahko v vasi, pri točki PJ04P0.

b.      Pot je dovolj dobra tudi za običajne avtomobile do točke PJ04P1.

c.       Tisti s terenci ali nekoliko manj občutljivi se lahko zapeljete vse do točke PJ04P2.

d.      Točka PJ04P3 je pri ostankih cerkve sv. Marjete. Tu se konča pot, ki je primerna za vozila, ter se nadaljuje s stezo, ki pripelje do Matijeve jame.

3.       Veliko Drskovško jezero

a.       Avto lahko pustite pri razcepu takoj za mostom, pri točki PJ08P1.

b.      Za manj občutljive voznike je možen dostop vse do jezera, do točke PJ09.

4.       Malo Drskovško jezero

a.       Avto lahko pustite pri razcepu takoj za mostom, pri točki PJ08P1. Če boste nadaljevali z avtom, tukaj zavijte levo.

b.      Če vam ni pretirano žal avta, se lahko zapeljete v neposredno bližino jezera, do točke PJ08P2.

Zemeljski zaklad

Za registracijo tega zemeljskega zaklada morate odgovoriti na naslednja vprašanja:

1.       Pojdite do točke PJ03 pri Petelinjskem jezeru. Poglejte v smeri (azimutu) 55° in povejte, kaj vam najbolj pade v oči?

2.       Pojdite do točke PJ04 pri Palškem jezeru. Stojite na enem od njegovih bregov. Ocenite razdaljo do nasprotnega brega. Gre po vašem mnenju za 500 m, 900 m ali 1,4 km?

3.       Na točki PJ04MJ ob Palškem jezeru je estavela Matijeva jama. Kaj je estavela? (Opomba: da odgovorite na to vprašanje, ni treba iti do jame, če pa se že boste za to odločili, bodite izredno previdni!)

4.       Pojdite do točke PJ09 pri Velikem Drskovškem jezeru. Raziskave so pokazale, da voda iz tega jezera nima neposredne povezave do reke Pivke. Kam torej odteka po vašem mnenju?

5.       Upoštevajoč prejšnji odgovor, kje je po vašem mnenju odtok iz jezera  glede na točko PJ09, na kateri stojite (smer neba, levo/desno)?

6.       Pojdite do točke PJ08 pri Malem Drskovškem jezeru. Na robu tega jezera sta dve manjši jami, ena na JV in druga na SZ. Zanimivo je, da se obe imenujeta Podmou (podmol), le da je ena izvir in druga ponor. Glede na to, kar veste o jezerih in kar vidite na terenu, kaj mislite, katera je katera? Utemeljite svoj odgovor.

7.       Katera je najpomembnejša značilnost, po kateri se Šembijsko jezero razlikuje od ostalih šestnajstih Pivških presihajočih jezer?

8.       Kot neobvezno nalogo pa naredite fotografijo vas ali vašega GPSja pri enem od jezer in jo pripnite vašemu vpisu.

Odgovore pošljite skozi GC profil. Potem lahko zabeležite vaš obisk, ne da bi čakali na posebno dovoljenje. V kolikor bo z vašimi odgovori kaj narobe, vas bomo kontaktirali.

Če ste fotografirali, priložite zanimive fotografije.

(Slovene)  (Photos)  (End of Text)

Table of Contents

1. Introduction
2. Geomorphology of the Pivka basin
3. Hydrogeology of the Pivka basin
4. Underground caves
5. Tectonic characteristics of the Pivka basin
6. Intermittent lakes
7. Access
8. EarthCache
9. Sources
10. Photos

Introduction

In dry seasons the Pivka river appears at the contact between limestones and flysch rocks near Prestranek, only a few kilometers ahead of its ponor into Postojnska jama. Near Zagorje the underground karst waters are about ten meters below the valley bottom of periodical Pivka river. During high waters, the underground water from the shallow karst aquifer of the upper part of the basin (Upper Pivka) pour over the surface and fill the Pivka riverbed, which then runs continuously from a spring near Zagorje to the ponor. During the same period also several small karst depressions are flooded in Upper Pivka and changed into intermittent karst lakes. The majority of the usually flooded karst depressions in Upper Pivka are because of their regular annual flooding traditionally termed lakes. Because not all basins are flooded simultaneously - this depends on rainfall - the number of these lakes, which can be found in various literature, varies. It seems, however, that the most generally accepted number of the Pivka intermittent lakes is 17, so in the description of this EarthCache we opted for this number, too.

Geomorphology of the Pivka basin

On the basis of found bones and teeth of dead animals experts deduced that in waters of the Pivka basin in the past, approximately 400.000 years ago (Mindel-Riss interglacial period), a dwarf hippo Hippopotamus pentlandi lived. Due to karstification, during geological history surface affluents of the Pivka river, especially from the Javorniki mountains, have gradually retreated to the underground. Previously these streams took an active part in the formation of terraces in the Pivka valley.

The Pivka basin is a large depression among high karst plateaus of Nanos (1313 m), Hrušica (1264 m), Javorniki (1268), low plateau Slavenski ravnik (600 – 700 m), Snežnik (1796 m), and flysch hills that form a catchment area of the Reka river. The northern part of the basin was formed in noncarbonate flysch rocks with superficial drainage, while the southern part of the Upper Pivka basin was formed in limestone and has some characteristics of a karst polje. The Upper Pivka basin is about 16 km long and 4 – 5 km wide part of the basin. Its surface gently tilts from the foot of the Snežnik mountain on SE from 620 m at Koritnice to 520 m on the NW at Rakitnik, where the Pivka river leaves limestone and enters flysch part of the basin. This river, which is the largest in the basin, collects water from flysch and karst springs in the Upper Pivka basin and finally sinks into Postojnska jama cave on the NE part of the Pivka basin. At the same time the surface of the Upper Pivka basin is inclined from the foot of Javorniki mountains towards SW. Zone of the lowest levelled area is at foothills which separate the Upper Pivka basin from the flysch landscape of the Reka river valley. This transverse inclination is more explicit as the longitudinal one.

The surface of the Upper Pivka basin can be divided into two distinctive morphological units: levelled bottom of the basin and higher rocky terrace along the Javorniki mountains. Depressions of the Pivka lakes are deepened into the terrace. Levelled bottom is the lowest part of the basin and is extended along the river Pivka. Between the villages of Rakitnik, Prestranek and Pivka it is 1 – 2 km wide;  between Pivka and Zagorje it narrows few times and widens again into a wider plain. In the bottom of such a widening south from Zagorje the karst spring of the river Pivka lies. The characteristic of the basin bottom is levelled ground along the Pivka river. It is completely levelled only in close proximity of the Pivka river; elsewhere it is slightly undulated and elevated several meters above the riverbed.

Somewhere on this surface there are also low, about 10 meters high isolated hills. There is a gradual transition of the surface into the slopes on the western side, while on the eastern side of the bottom there is a sharp transition into the slopes of the higher terrace, which raises 10 – 20 m above it. Rocky bottom of the surface is somewhere covered with a thin layer of clay and sandy sediments. There are no dolines in the bottom, but numerous springs and ponors, which are small and morphologically not very distinctive. A large closed depression at Knežak also belongs to the large levelled surface of the Upper Pivka. It is 3 km long and up to 1.5 km wide. This depression is separated from the surface along the Pivka riverbed with the higher terrace. Its bottom near Knežak is levelled and at Koritnice partly covered with fossil alluvial fans deposited by stream that flowed in the Pleistocene from the Snežnik mountain. There are also some isolated hills, hums like Obroba (644 m). The bottom of this depression has also a sharp edge and steep slope to the higher terrace on the eastern side.

The higher rocky terrace lies between the bottom of the basin and the Javorniki mountains. In its nortern part it is 3 km wide, at Petelinjsko jezero lake about 1.5 km, and further to the south at Zagorje 4 km. It extends from the Javorniki mountains to the foot of the Gradišče hill and separates the depression at Knežak from the levelled surface along the Pivka river. The terrace is inclined towards SW. Along the foot of the Javorniki mountains its altitude is between 650 and 670 m, where it passes to the slopes of the Javorniki. On the western side of the terrace it ends at the elevations between 550 and 570 m, with the sharp edge of 10 - 15 m above the bottom of the basin along the Pivka river. The terrace is somewhere, mostly in the northern part, expressively dissected. It is best preserved between the settlements Trnje, Bač and Palčje. The surface of the terrace is dissected by isolated peaks and mainly by many dolines. Along its western edge the dolines tend to be larger; there are several big dolines and depressions with levelled bottoms in which intermittent Pivka lakes appear.

Most of the depressions of the Pivka lakes are doline-like, closed with large levelled bottoms and sharp transition from the bottom to the slope above it. The largest lakes are Palško jezero and Petelinjsko jezero with longer axis of the lake bottom larger than 1500 m. The other depressions have bottom diameter between 100 and 300 m. The depressions of the lakes are much larger than dolines which bottoms are not reached by the floods. Their rocky bottoms are covered by a layer of sediment and soil which in most cases is not thicker than 0.5 m. Transition from a levelled bottom to the steep slope is morphologically strict and distinctive. Slopes of the bottom basin to the higher terrace are similarly developed. The bottoms of the lake depressions are in that level that is still reached by the oscillations of the karst water table. When water in numerous channels rises it appears on the surfaces of these depressions and also in the bed of the Pivka river. The appearance of water on the surface enables stronger dissolution of limestone; mainly laterally what causes widening of the depression bottoms. Steep slopes indicate that the corrosion is especially strong on the edges of the lake depressions and that the process is still active nowadays.

The depressions of the Pivka lakes were formed by the transformation of dolines or larger relief bends in the zone of the karst water oscillations. High karst water table enables appearance of water on the surface and formation of the lakes. It also influences that soil and sediments are not washed away deeper into the karst. For this reason the soil in the lake bottoms is thicker than on the surrounding karst surface.

Hydrogeology of the Pivka basin

The Javorniki range functions as a karst aquifer in which groundwater flow is dominant. On the western border in the Pivka basin it is in contact with less permeable alluvial and flysch rocks. They act as a hydrogeological barrier at which karst springs emerge specially during high waters. And as their thickness is relatively small, a karst aquifer is developed also below them in the direction towards the Vipava valley. On the southern side the Javorniki range traverses into the Snežnik karst massif. In this part the karst watershed with the catchments of rivers Reka, Riječina and Kolpa is located. Due to specific characteristics of karst (groundwater flow, bifurcation) it is practically impossible to define its position precisely.

Hydrogeological map of broader area

Legend: 1. Visible and covered thrust, 2. Visible and covered fault, 3. Surface flow, 4. Intermittent lake, 5. Big spring, 6. Permanent spring, 7. Intermittent spring, 8. Ponor, 9. Injection point, 10. Main and secondary direction of underground water flow, proved by tracing tests, 11. Karst aquifer, 12. Fissured aquifer, 13. Porous aquifer, 14. Very low permeable rocks

On the northern and eastern side the karst aquifer is bordered by less permeable Upper Triassic dolomite in the area between the Planina and Cerknica karst poljes. This is intersected by the fault zone of the Idrija fault in which dolomite is crushed and millonitized. At this hydrogeological barrier underground water emerges through karst springs to the surface.

In a karst aquifer underground water flows mainly through widened fissures and karst channels in different directions towards the springs at the border. Based on hydrogeological researches, especially tracer tests, groundwater flow was proved towards the springs at the Planina polje (Malenščica, Unica, Škratovka) and in Rakov Škocjan (Kotliči, Prunkovec), and also towards the Vipava spring in the Vipava valley and karst springs in the Pivka basin. The area south from the Pivka springs is at least partly drained towards the Podstenjšek spring and other karst springs in the Reka river basin, but more detailed researches of the watershed position have not yet been done.

Hydrogeological structure

Hydrogeological map of the area of Pivka intermittent lakes

Legend: 1. Karst aquifer, 2. Porous aquifer, 3. Very low permeable rocks, 4. Permanent surface flow, 5. Intermittent surface flow, 6. Intermittent karst lake, 7. Karst spring, 8. Karst cave, 9. Ponor

The area of the karst intermittent lakes of Pivka is mostly of well karstified Cretaceous limestones, which are characterised as well permeable rocks with karst-fissured porosity. In these, numerous surface and underground karst features are developed. The layer of soil on the limestone is relatively thin and frequently not consolidated, so the surface is very rocky. Precipitation infiltrates directly from the surface in more or less vertical direction through the vadose zone deeper into the karst, and then in the phreatic zone groundwater flows in different directions towards the springs at the border of the karst massif.

In the Pivka basin Quaternary alluvial sediments are deposited on limestones. They are less permeable and act as a local hydrogeological barrier. As during high waters the water table in karst rises above this contact, intermittent karst springs emerge and supply the surface stream of the Pivka river. In longer periods without precipitation the water table falls below the contacts and springs become dry. Water from the surface flow of Pivka sinks in the underground through several ponors in the riverbed. On very low permeable Eocene flysch in the area between Prestranek and Postojna surface drainage prevails. But the thickness of this flysch is relatively small and also in greater depths below it karst aquifer is developed. The Eocene flysch in the Reka valley belongs to the Komen thrust sheet on which the Javorniki massif (Snežnik thrust sheet) is thrust. It has a role as an impermeable base of the karst aquifer and is visible on the surface also in the Knežak tectonic window. At the contact between limestone and flysch, karst springs emerge which supply the tributaries of the Reka river.

Characteristics of groundwater flow

In the western part of the Javorniki range groundwater is reachable in the caves Fužina pri Stari vasi, Brezno v Kobiljih grižah and Matijeva jama. Three boreholes were drilled near Fužina pri Stari vasi, Brezno v Kobiljih grižah and Žeje spring in order to research their potential for water supply. In the 1980s the water table was occasionally measured in them, but later they were plugged and measurements are no longer possible. Water is permanent in the wells in settlements Pivka and Zagorje. Several boreholes were drilled in the area between the Pivka springs and the villages Koritnice and Šembije, and during a short period of time the water table was occasionally measured in them by the Geological Survey of Ljubljana. Also the altitudes of permanent karst springs at the border of the karst massif are known. During high waters karst water reaches the surface in intermittent karst springs and lakes in the Pivka basin. To define maximum water levels the data on the altitudes of lakes gathered during the floods in November 2000 are specially interesting.

Based on the above mentioned data it is possible to make some general conclusions about the directions of groundwater flow and about the range between maximum and minimum levels of groundwater in different parts of the karst aquifer. The water table in the upper part of the Pivka basin is during low waters around 10 m below a dry riverbed, and in the lower part more than 20 m below it. After heavy rains the riverbed is filled with water, and karst springs nearby are active. Dependent on the height of the water table also, more or fewer karst lakes emerge. But as all mentioned measurements were performed only occasionally and in short time periods, there is not enough data for more precise analysis of the characteristics of water table oscillations at different hydrological conditions.

More information about directions and characteristics of groundwater flow were obtained by tracer tests. In the cave Požiralnik pod Kremenco 10 kg of uranine were injected on 26 September 1955 and the underground connections with the Unica and Malenščica springs were proved. The apparent flow velocity was estimated to be around 0.5-0.6 cm/s. During high waters on 14 November 1967 around 10 kg of coloured spores were injected in the ponor at the Palško jezero and the connection with the Trnski izviri spring was defined. The apparent flow velocity was estimated at 9 cm/s. On the next day sinking waters at the Petelinjsko jezero were coloured by 3.5 kg of uranine. Underground water flow with an apparent velocity of 1 cm/s towards the Žejski izviri spring was proved. 1 kg of uranine was injected on 26 February 1968 in the sinking stream near Knežak (discharge 5 l/s). The underground connection with the Videmščica spring and an apparent flow velocity of 1.2 cm/s were established.

The Stržen brook, which is supplied by the Fužina spring, was traced twice, firstly during low waters (discharge 30 l/s) on 23 February 1982 when 10 kg of uranine were injected into the sinking stream near the village Rakitnik. Although water flow in the Planinska jama cave as well as Malenščica, Škratovka and Kotliči springs were observed in a longer period, no tracer was detected. Therefore another tracer test was carried out on 9 August 1988. In the Stržen stream (discharge 10 l/s) 10 kg of rhodamine were injected and additionally the Vipava and Timava springs were observed. In the before-mentioned springs at Planina polje and Rakov Škocjan the tracer was again not detected, but the underground water connection with the Vipava springs (apparent velocity around 0.4 cm/s) was proved. The number of samples of the Timava spring was not sufficient to use the results as reliable. At the same time on 9 August 1988 also the Pivka river near the village Trnje was coloured by 20 kg of uranine. There are several ponors in the riverbed in which the river disappears during low waters. At high waters the capacity of ponors is too low and the river at least partly continues its surface flow. The tracer test was performed during low waters and tracer was detected in all observed springs with the exception of Vipava and Timava springs. The following apparent velocities of groundwater flow were obtained: towards the stream in Planinska jama and the Malenščica spring around 0.5 cm/s, and towards the Pivka river near Prestranek and the Stržen stream near Rakitnik around 0.2 cm/s.

Due to the development of the methodology of tracer tests in recent years the quality of their results increases. Therefore the results of tracing in summer 1997 are especially interesting. On the military training site Poček on the Javorniki range 4 kg of uranine were injected at the rocky bottom of a doline on 10 June 1997 at low waters. Water samples were collected at 11 locations in the period of approximately 1 year. It was proved that an important share of water flows underground towards the Malenščica spring (55 % of recovered tracer) and the Vipava spring (26 % of recovered tracer). The apparent dominant velocity in both directions was around 0.7 cm/s. In this way the main groundwater flow from the Javorniki karst aquifer towards the springs on the Planina polje, but also the underground flow below the flysch of Pivka basin towards the Vipava valley were proved. In lower concentrations the tracer was also detected in other springs at Planina polje (Unica, Škratovka) and Rakov Škocjan (Rak, Prunkovec, Kotliči). The slowest, but reliably proved, was the connection with the nearest Fužina spring (apparent dominant flow velocity 0.29 cm/s). The detection of tracer in the Žejski izviri and in the Pivka river near Prestranek was not convincing enough to confirm the underground water connection.

Also interesting is the increase of concentration of tracer in the springs after each precipitation event even one year after injection. We can conclude that also each pollution at the surface can reach the springs at the border very fast, but is partly also retained in the underground and then washed out by each heavier precipitation event in a longer time period. Flow velocities are influenced by hydrological conditions, too.

Hydrological conditions

Hydrological conditions of the studied area are reflected in hydrological characteristics of the Pivka river. It is recharged by karst waters and at the ends of several sections of surface flow it sinks again in the karst underground. The Pivka river emerges near the village Zagorje and its total length is 26 km. Only 11 km in its lower part is permanent surface flow. In the upper part the water table is, during low waters, 10 m or more below the riverbed which is therefore mostly dry. After heavier precipitation the water table rises and also intermittent karst springs along the Pivka river are active. At very high waters some parts of the basin are flooded to an area of 7.5 km², about 2.2 km² of which belongs to intermittent karst lakes. Floods are regular in autumn, and relatively often in winter and also in spring.

In the middle part of Pivka river near Prestranek the surface flow is not permanent and is limited to the periods of high waters. Surface flow of the Pivka near Prestranek indicates that also karst springs are active and intermittent lakes are at least partly flooded. In the lower part of the Pivka near the sinking point into Postojnska jama the riverbed is dry only exceptionally. Significantly higher discharges in this part are due to the inflows from the Nanoščica stream and other smaller tributaries from the flysch area, but also due to an additional supply from numerous karst springs.

Underground caves

Caves in the Pivka basin (Cartography Jurij Hajna)

In the wider area of the Pivka intermittent lakes 87 caves and shafts are registered and explored with a total length of 2617 m and a total depth of 1255 m. In the same region during Italian occupation (1918 – 1943) 31 caves were registered, which have also Italian cadastre number VG (Venezia – Giulia). Cave density 0.7 caves per km² is slightly lower compared to Gams’ calculations for the whole Pivka valley (1.1 / km²). Only Tičkova jama and Brezno v Slavenskih senožetih exceed a depth of 50 meters. In the Pivka region short caves up to 50 m (85%) prevail; only 4 are longer than 100 m: Drskovška golobina, Trnska jama (in the past already an important tourist cave), Jama v kamnolomu pri Matenji vasi and Mrzla jama.

10 longest caves in the immediate vicinity of the Pivka lakes (situation 1st June 2005; Arhiv IZRK 2005).

The most famous cave in the area of Pivka intermittent lakes is estavelle Matijeva jama, due also to its cave rocky relief, which clearly reflects distinctive underground water oscillations. In the time of Italian occupation the army pumped water from the cave for its own needs. The cave entrance is today completely altered because the Yugoslav people’s army used it as a target of bombardments during manoeuvres.

Matijeva jama (Palško jezero) with measured water levels

Caves contain a lot of information about past periods. Cave sediments, animal remains and other objects in sediments in the Pivka valley do not serve only for an assessment of human history but can be also very useful for interpretations of the former climatic conditions.

Tectonic characteristics of the Pivka basin

The important regional thrust called Snežnik thrust, that divides the Snežnik thrust sheet from the parautochthon of the Komen thrust sheet, runs through the Pivka basin. A tectonic window near Knežak is proof of older thrusting deformations. The landscape is cut by numerous younger faults among which the most important are Raša, Predjama and Selce faults. The area of Upper Pivka is tectonically quite active, which is shown by earthquakes in recent years. It looks as if the most active is the Raša fault or one of its northern parallel faults, for example Šembije fault or maybe Selce fault. Karst intermittent lakes of Upper Pivka are developed in Upper Cretaceous limestones. The lakes are situated 0.5-4.0 km NE from the Snežnik thrust. Most of the lakes are situated along the Selce fault.

Geological map of Pivka basin with intermittent lakes (compiled by S. Šebela)

Legend: 1-railroad, 2-river with flow direction, 3-occasional stream, 4-hill with above sea altitude, 5-town, 6-smaller town, 7-karst cave, 8- intermittent lake, 9-alluvium (Quaternary), 10-flysch (Eocene), 11-limestone (Paleogene), 12- mostly limestone (Cretaceous), 13- thrust fault: stronger and weaker, 14-dextral movement along horizontal fault, 15-fault: established and covered, 16-anticline, 17-syncline

The Pivka basin is a very interesting region, not only because of the intermittent karst lakes and karst hydrology but also due to its interesting geological structure. In this area runs an important regional fault that divides two tectonic units and is called Snežnik thrust. The northern Snežnik thrust sheet is thrust over the parautochthon of the Komen thrust sheet. The thrust can be tracked south from Gemona in Italy, through the Vipava valley, as the Snežnik thrust and NE from Rijeka in Croatia, which represents a total length of 160 km. For Pivka basin and for Snežnik region the moderate tectonic activity is characteristic of what can be seen in historical and recent earthquakes. There are plenty of geological and karstological characteristics in Pivka basin.

The biggest part of Slovenia belongs to the Adriatic (Adria) microplate. The Alpine orogenetic belt is a result of convergence between African and Eurasian plates. The Adria microplate is mostly submerged beneath the Adriatic Sea and is covered by thick layers of young unconsolidated sediments in the river Po basin (Italy). Geologic, seismic and seismic-tomography researches show the probability that the E, W and N edges of the Adria microplate are underthrusting and forming the foreland to these surrounding deforming zones. The Adria microplate is probably moving slowly regarding the neighboring Eurasian plate. Adria’s major aseismic outcrop is the Istria peninsula of Slovenia and Croatia. On the bases of GPS measurements in western Slovenia we observe a significant and sharp (few mm/year) dextral (±transpressive) gradient along the Sava fault, Periadriatic zone, suggesting that lateral extrusion in the NE Alps is still active and being driven by ccw rotation of Adria.

Besides older thrusting deformations from the Miocene we can detect also younger neotectonic movements. The most important are the Predjama and Raša faults, which both show characteristics of dextral movement. For the Pivka basin the Selce fault is also very important. The fault runs from Prestranški ravnik towards SE to Koritnice and supposedly further on towards SE into the Snežnik Mountain. Most of the Pivka intermittent lakes are situated within the wider (1.5 km) area of Selce fault.

In the studied area the oldest rocks are well-stratified limestones and dolomite and limestone breccias (K1,2 – Lower to Upper Cretaceous), which can be found around Bač, Koritnice and E from Ilirska Bistrica. Following these are Upper Cretaceous limestones (predominantly gray rudist limestones) that form the Javorniki Mountains. Between Snežnik fault and Ilirska Bistrica there are the Vreme beds that belong to the period between Cretaceous and Paleogen. From the Eocene period we find flysch marls, sandstones and nummulitid breccias. The bottom of the Pivka basin is partly covered with river sediments. The alluvium covers also the bottoms of bigger karst depressions-intermittent lakes. The intermittent lakes of Pivka basin are developed in Upper Cretaceous limestone.

Earthquakes

The wider area of Ilirska Bistrica is seismically one of the most active parts of Slovenia. One of the strongest earthquakes in middle Europe (January 25, 1348), with hypocenter in the area of Dobrač and Beljak, strongly influenced the area of Slovenia. Among others the earthquake destroyed the Novi grad castle near Podgrad close to Ilirska Bistrica.

Of the selected 159 earthquakes for the period between February 2, 1834 and December 3, 1981 the strongest one  (Mm=5.2) was so-called Cerknica earthquake (January 1926) with epicenter in Javorniki Mountains. In the Edinost newspaper (Trieste, January 8, 1926) we read that in Postojna more than 50 earthquakes were felt. In Postojnska jama the stalagmite measuring almost 1 m in diameter collapsed. The second strong earthquake in the region of Ilirska Bistrica and Snežnik occurred on January 31, 1956 with the magnitude Mm=5.1 and at a depth of 7 km. In the period between February 2, 1834 and December 3, 1981 the strongest earthquake in the Pivka basin was located between Pivka and Postojna on October 22, 1900 with magnitude Mm=4.5. Earthquakes in Pivka basin and in the region of Ilirska Bistrica are quite frequent with the mean depth of 7-16 km.

Intermittent lakes

During high karst waters many surfaces become submerged. Based on the expanse and duration of floods these flooded depressions have in Slovene different expressions, i.e. luže, močila, loke, lokve, mlake, ribniki, jezera, etc. From the literature and conversations with local people about descriptive and locally used toponyms, several temporarily water-filled karst depressions could be considered as Pivka intermittent lakes and any one particular lake could have several different names.

In this description we will be using the most common, standardized names of the lakes. If we decide to cross the Pivka valley during high waters, we can encounter several lakes from the north to the south. In the bottom of the karst depression Jeredovce there once existed a path to the village of Bile. The bottom of Krajnikov dol is rarely submerged. In the lowest situated Petelinjsko jezero (532 m a.s.l.) water is retained longest, for an average of 6 months per year. Palško jezero has the biggest surface with estavelle Matijeva jama as the strongest spring. Natives say that water remains temporarily in Klenski dol near village of Klenik. Today some people also use the expression Klensko jezero for the same flooded surface. Due to the filling of a part of a depression with building material where Radohovsko jezero appears, its surface area has shrunk. It is the only lake which lies on the left bank of the Pivka river. Near the village of Parje is Parsko jezero where waters from Malo Drskovško jezero and Veliko Drskovško jezero inflow and further to the karst spring Mišnik. People say that waters remain for a shorter period in the bottom of the smaller Veliko Zagorsko jezero, compared to the neighbouring Malo Zagorsko jezero. In Kljunov ribnik karst water springs with high pressure, especially from the borehole, and flows directly to the bed of the Pivka river. The northeastern part of the same depression is named Mali dol. Behind the castle Kalc in the bottom of Veliki dol depression water is retained occasionally. If we travel further on the old military road from Zagorje to Bač, another two karst depression that are rarely flooded can be seen: the larger Bačko jezero and at higher altitude Laneno jezero. In the bottom of Kalsko jezero a dug channel serves to drain high waters into the Pivka river. Near the village Šembije the most southern lake, Šembijsko jezero, is formed. In previous studies of the Pivka intermittent lakes this one was often overlooked.

Basic characteristics of the Pivka intermittent lakes.

During high waters in the year 2000 its surface was quite huge. Waters from higher lying Nariče flowed on the surface to a lower depression named Jezero. In the same period the villages Bač and Koritnice were cut off from traffic communication. The high waters in 2000 were exceptional, but people often mention that in the last few decades average retention time of high water in some lakes has shortened and that the flooded surface of some areas has contracted.

Presence of some intermittent lakes could be well predicted, e.g. with monitoring of the Pivka river flow. Its continuous flow through the village Prestranek and at least its part overflow at the village Rakitnik are trustworthy signals that there are high waters in the karst hinterland and that many karst springs are activated which have filled up some karst lakes in the Upper Pivka. Natives from Podtabor at Podstenjšek can predict the presence of Šembijsko jezero with simple monitoring of one spring near the road. If it becomes active during high spring and autumn waters, Šembijsko jezero is filled.

The height of the water table in the Upper Pivka aquifer ranges from 560 m asl on the southern part of the valley (the area around Knežak) to 516 m asl in the Matijeva jama cave, situated at Palško jezero. Matijeva jama is a typical estavelle with considerable oscillations of water level (40 m and more). During the high waters in November 2000 water rose for additional 8 m above the maximum height recorded previously, reaching 566 m. Another reason for the appearance of the lakes in Upper Pivka is the limited permeability of the water channels towards the Malni springs (Planinsko polje), causing flooding around the Pivka riverbed in wet seasons and at the same time floods in small karst depressions which become lakes. The fluctuation of the water levels in the lakes situated closer to the Pivka riverbed is smaller than in the lakes situated farther from it. The overflowing in the lakes is basically the consequence of the fact that underground recharging is stronger then subsurface runoff from the region. In general the Upper Pivka forms a more or less uniform shallow aquifer, but according to the data about the oscillations of water levels in different lakes, the permeability in the aquifer varies significantly.

The water table of the Upper Pivka karst aquifer is generally inclined northwards, with the exception of the hydrological background of Šembijsko jezero, where it is inclined to the northwest. The water from Šembijsko jezero recharges the Podstenjšek karst spring which is a right tributary of the Reka river (Adriatic Sea basin) while the Pivka river with associated intermittent lakes is a part of the recharge area of the Ljubljanica river (Black Sea basin). Therefore the southern part of the Upper Pivka presents a watershed between the Adriatic and Black seas. Because of the geology and other natural characteristics, Šembijsko jezero is discussed together with the other intermittent lakes of Pivka, but from the narrower hydrological view it obviously belongs to the Adriatic Sea basin.

With the exception of Palško jezero and Malo Drskovško jezero, all the other lakes recharge and discharge mainly through the alluvial sediments which cover their karstified bottoms. The previous two are additionally recharged and discharged also from the small spring or ponor caves. Layers of alluvial deposits almost certainly contribute to the flood duration of the lakes.

Water connections

Karst in Slovenia preserves huge amounts of water. Despite recent detailed investigations of underground water connections there is still only a superficial knowledge. The situation in the Pivka valley is like that of a part of the Ljubljanica river basin hinterland of the Slovene Classical karst. The hydrological situation of the Pivka is very complex. In the Upper Pivka there is the watershed between the Black sea and Adriatic sea catchment areas. Superficial and underground bifurcation in epiphreatic and phreatic zones includes the vast hinterland of Pivka with complex water connections and different outflow in different directions. It can be said today with certainty that between the Pivka river and some lakes there exist direct underground connections. The word Pivka is a synonym for a ponor, where water sinks, a little lake which slowly drains away or even for a grassland near village Zagorje where water sinks into hollow ground and where a river channel passes over it. Several small springs downstream join the main Pivka rising at Pivšce.

Pivka’s river basin comprises of approximately 140 km of water flows; 34% of them are permanent and 66% temporary. It is estimated that 16% of the Pivka valley is subject to seasonal floods. After heavy precipitations, high waters from many springs pour into and over the Pivka’s riverbed. In the meantime low waters pass through underground channels directly to Planinsko polje. During high karst waters, springs near the Pivka river regulate the level of the underground water. This could be seen in the arrangement of the lakes’ levels. In the case of lakes closer to the Pivka river, e.g. Parsko jezero, Malo Drskovško jezero, Veliko Drskovško jezero, Malo Zagorsko jezero, Veliko Zagorsko jezero and Kalsko jezero the lake level is just a few meters above the Pivka river, whilst it is higher in more distant lakes. In dry periods the waters withdraw 20 m below the Pivka riverbed. Locals mentioned that a few decades ago children used to swim in the pools of the Upper Pivka river during summer holidays. In wintertime there was enough water in the Upper Pivka river for preparing pig intestines for sausage-making. In between these periods the riverbed was mowed. It is estimated that the Pivka river after its passage along the Pivka valley carries 4000 tons of eroded rocks into Postojnska jama annually.

High waters in November 2000

In the autumn of 2000, especially in October and November, a deep low air pressure area over Western and Middle Europe prevailed. During November cold fronts were following at short intervals, causing abundant precipitation especially in the western part of Slovenia. Due to the advection of moist and warm air, heavy local precipitation was of long duration. In some places the monthly amounts of precipitation in October and November were four times the average.

The amount of precipitation in September, October and November 2000 at the reference meteorological stations Postojna, Ilirska Bistrica and Jurišče show, that in the region of Pivka the precipitation for all three months together was nearly two times the average of the period 1961-1990. Heavy precipitation reached its maximum in November when only 7 days without rain were recorded at the meteorological stations Postojna and Jurišče and six at the meteorological station Ilirska Bistrica. The first continuous period of rainfall was, according to the data from the reference meteorological stations Postojna, Ilirska Bistirca and Jurišče, recorded from October 30 to November 11 (230.1, 275.8, 332.6 mm), the second from November 13 to 22 (163, 191.2, 217.5 mm) and the third from November 24 to 26 (63.5, 77, 53.7 mm). In November 2000, the amount of precipitation in the region of Pivka was three times the average monthly amount for the period 1961-1990 (Postojna 455.2 mm, Ilirska Bistrica 544 mm, Jurišče 601.6 mm), which resulted in the intense rising of the water table.

The discharges of permanent karst springs rapidly increased; in addition karst springs also emerged at higher elevations, causing extensive flooding around the Pivka riverbed. The rise of groundwater table resulted in the filling of the 17 intermittent lakes; exceptionally some other karst depressions at higher elevations were also flooded. The water levels in the two of the regularly flooded lakes (Palško jezero, Šembijsko jezero) rose by about 10 m above the mean values, showing that the water table in the karst aquifer rose by about the same value. Such a high water table has not been seen for about 70 years. However, obvious differences in water level rising occurred between the lakes. One major reason is apparently the stage of the development of the underground water channels. Some of the flooded karst depressions have probably stronger underground water connections than others, causing the differences in fluctuations of groundwater level inside the area. Thus some of the lakes were flooded more rapidly than others and reached record values, while on the other hand some lakes hardly reached the mean water levels.

The valley of Upper Pivka with intermittent karst lakes and flooded areas in November 2000

Legend: 1. Lake and flooded areas, 2. Mean surface of Palško jezero 3. Surface stream between the lakes 4. Pivka river with tributaries 5. Settlement 6. Kalc castle 7. Surface below 600 m asl 8. Surface above 600 m asl. (Floods along the Pivka river are not marked.)

It was observed that the changes of water levels were greater in the lakes situated farther from the Pivka river. In the flooding area around the settlements of Knežak and Bač and in the area of Šembijsko jezero the water table rose about 30 m and more above its mean values. Record heights of water levels were also recorded in other lakes, especially in Palško jezero, where the side entrance to the Matijeva jama cave was almost 20 m below the lake surface at that time. The pressure of the underground water emerging at the surface from the karst aquifer was very high and the water was flowing from numerous small springs. At some locations, where the turf was more compact, some kind of »natural waterbed« formed beneath it. When pressed, the turf moved, but did not break. This phenomenon was observed in the area of Šembijsko jezero and Palško jezero.

The profile of Pivka riverbed and the intermittent lakes in November 2000

Legend: 1.Lake 2. Flooded area 3. Mean water level of the lake 4. Pivka riverbed 5. Surface stream 6. Underground water flow 7. Karst spring 8. Ponor

Access

To get to this EarthCache, one should go from Postojna in the direction of Ilirska Bistrica and Rijeka in Croatia. After a few kilometers the road will bring you to Pivka, where in the center do not turn right on the priority road, but go straight in the direction of Knežak. Another option is to go from Ilirska Bistrica towards Šembije and Knežak.

As can be seen from the description of the EarthCache, there are 17 intermittent lakes, some of them already before Pivka. In order to answer the required questions, you will need to visit a few sites that are quite distant from one another. To get from one to the other you will need a car or bike at least, since for walking distances are too long. When searching for locations refer to the waypoints, which are listed below. They are listed from north to south, or from Postojna towards Ilirska Bistrica.

As there are several locations, some of them quite remote, reserve enough time, at least half a day. This is particularly true if you combine this EarthCache with a series of traditional caches, which is also dedicated to Pivka intermittent lakes. In this case, finding all the caches can take more than one day.

To be able to successfully answer the required questions you will need to visit four of the seventeen Pivka intermittent lakes, namely Palško, Petelinjsko, Veliko and Malo Drskovško lake. The remaining lakes are not key for successful logging of this EarthCache. However, we still recommend visiting all these sites. The lakes are accessible by car or bicycle throughout the year, except when the landscape is covered with snow, as farm roads and paths that lead to sites are not plowed.

To help you find the sites where you will find answers to questions, here are some additional waypoints:

1.       Petelinjsko jezero

a.       You can park in the village at waypoint PJ03P0, where a sign will direct you to the lake which is approx. 1.5 km away.

b.      If you are not too sensitive about your car or are driving a 4x4, you can go all the way to the lake, to waypoint PJ03.

2.       Palško jezero

a.       You can park in the village, at waypoint PJ04P0.

b.      Road is good enough for ordinary cars up to waypoint PJ04P1.

c.       If you are not too sensitive about your car or are driving a 4x4, you can go all the way to waypoint PJ04P2.

d.      Waypoint PJ04P3 is at the remains of the church of St. Margaret. Here ends the road suitable for cars and continues with a single-track leading to Matijeva jama cave.

3.       Veliko Drskovško jezero

a.       You can leave your car at the junction right after the bridge, at waypoint PJ08P1.

b.      If you are not too sensitive about your car or are driving a 4x4, you can go almost to the lake, to waypoint PJ09.

4.       Malo Drskovško jezero

a.       You can leave your car at the junction right after the bridge, at waypoint PJ08P1. If you are going to drive to the lake, turn left here.

b.      If you are not too sensitive about your car or are driving a 4x4, you can go almost to the lake, to waypoint PJ08P2.

EarthCache

To register this EarthCache you must answer the following questions:

1.       Go to waypoint PJ03 near Petelinjsko jezero.  What can you see at bearing (azimuth) 55°?

2.       Go to waypoint PJ04 near Palško jezero. You’re standing on one of its sides. Estimate the distance to the opposite lakeside. In your opinion, is it 500 m, 900 m or 1.4 km?

3.       At waypoint PJ04 MJ there is the estavelle Matijeva jama.  What is estavelle? (Note: to answer this question you do not need to go to the cave. But if you do go, be extremely careful!)

4.       Go to waypoint PJ09 near Veliko Drskovško jezero. Research has shown that water from this lake does not have a direct connection to River Pivka. What do you think where the water goes then?

5.       Based on the previous answer, where do you think the sink through which water flows out of the lake is located, relative to waypoint PJ09 where you’re standing (left/right, east/west/north/south)?

6.        Go to waypoint PJ08 near Malo Drskovško jezero. At the sides of the lake there are two small caves, one at SE and other at NW point of the lake. Interestingly, both caves are called Podmou but one is a source and the other a sink. Based on what you see around you and what you know about the lakes, which is which in your opinion? Elaborate your answer.

7.       What is the most important distinction that differentiates Šembijsko jezero from remaining sixteen Pivka intermittent lakes?

8.       Optionally, take a photo of yourself or your GPS at one of the lakes and attach it to your log.

Send your answers in Slovene or English through my GC profile. After that you can log your visit, don't wait for permission from me. If there’s something wrong with your answers I will contact you.

If you took photos, attach them to your log.

Logs without the answers sent to me or logs containing the answers will be deleted!

(English)

Viri / Sources:

*          Mulec J., Mihevc A., Pipan T., 2005: Presihajoča jezera na Pivškem / Intermittent lakes in the Pivka basin. – Acta carsologica, 34/3, 543-565, Ljubljana

*          Šebela S., 2005: Tektonske zanimivosti Pivške kotline / Tectonic sights of the Pivka basin. – Acta carsologica, 34/3, 566-581, Ljubljana

*          Petrič M., Kogovšek J., 2005: Hidrogeološke značilnosti območja presihajočih Pivških jezer / Hydrogeological characteristics of the area of intermittent karst lakes of Pivka. – Acta carsologica, 34/3, 599-618, Ljubljana

*          Kovačič G., Habič Š., 2005: Kraška presihajoča jezera Pivke (JZ Slovenija) ob visokih vodah novembra 2000 / Intermittent karst lakes of Pivka basin (SW Slovenia) during high waters in november 2000. – Acta carsologica, 34/3, 619-649, Ljubljana

*          Ravbar N., Šebela S., 2004: The karst periodical lakes of Upper Pivka, Slovenia / Kraška presihajoča jezera Zgornje Pivke, Slovenija. – Acta carsologica, 33/1, 159-173, Ljubljana

Slike / Photos  (Preskoči / Skip)

Petelinjsko jezero avgusta 2012 / in August 2012

Palško jezero avgusta 2012 / in August 2012

Palško jezero avgusta 2012 / in August 2012

Palško jezero avgusta 2012 / in August 2012

Palško jezero avgusta 2012 / in August 2012

Cerkev sv. Marjete pri Palškem jezeru / Church of St. Margaret at Palško jezero

Parsko jezero avgusta 2012 / in August 2012

Malo Drskovško jezero avgusta 2012 / in August 2012

Veliko Drskovško jezero avgusta 2012 / in August 2012

Veliko Drskovško jezero avgusta 2012 / in August 2012

Kljunov ribnik avgusta 2012 / in August 2012

Navpični vhod v Matijevo jamo / Vertical entrance to Matijeva jama

Struga, ki vodi v Matijevo jamo / Water channel to Matijeva jama

(Vrh / Top)