Burrill Lake is one of about 135 estuaries of various sizes in NSW that drain to the sea. These include rivers, bays, creeks, lakes, lagoons and inlets. Of these about 90 are coastal lakes and lagoons bigger than one hectare. Burrill Lake is one of the 70 or so coastal lakes and lagoons which alternate between being open or closed to the ocean. These are known as Intermittently Closed and Open Lakes and Lagoons (ICOLLs).
ICOLLs are located all along the NSW coast. Most occur south of Sydney where catchments are generally smaller in size and experience lower average rainfall. Average wave activity is also generally higher and can push sand into estuary mouths.
ICOLLs are separated from the ocean by a sand beach barrier or berm. This entrance barrier forms and breaks down depending on the movement and redistribution of sand and sediments by waves, tides, flood flows and winds.
ICOLLs open and close to the ocean naturally in a constant but irregular cycle. When there is sufficient water flowing into the lake or lagoon from the catchment area (usually following heavy rainfall), water levels in the ICOLL will rise. The water levels in ICOLLs can often rise rapidly (within hours or days depending on the catchment size) in response to higher rain events (e.g. 50mm+). Eventually the water in the ICOLL will spill over the entrance sand berm and drain to the ocean. The force of the backed up water then quickly scours an entrance channel through the beach and reopens the ICOLL to the ocean. When ICOLLs are open they become tidal with seawater moving into and out of the estuary with the daily tidal cycle.
ICOLLs close when the ocean waves and tides push sand from offshore into the entrance, which gradually closes the entrance channel. Without further large freshwater flows into the estuary from the catchment, the ICOLL will remain closed to the sea. When ICOLLs are closed they do not exchange water with the ocean and water levels within them fluctuate depending on rainfall, catchment inflows and evaporation.
During wetter times many ICOLLs remain constantly open to the ocean. In times of drought and reduced rainfall ICOLL entrances close more frequently and stay closed for longer periods of time. It is common for many ICOLLs to remain closed for several years at a time. As well as rainfall, specific local conditions such as catchment size and headlands near the entrance determine if an ICOLL is mostly open or mostly closed. However historical records show that about 70% of the ICOLLs in NSW are closed for the majority of the time.
Burrill Lake is inland from the ocean and is joined to it by a short river known as the Inlet. In addition to sand being washed ashore by wave activity, sediment is eroded from the river banks when water flows are faster, carried down the river and deposited where flows are slower.
To log this cache you will need to visit the lake entrance and answer the following three questions.
1. Was the Lake Entrance open or closed at the time of your visit?
2. The land on the far side of the entrance is significantly different from the land on the near side of the entrance. Please describe this difference.
3. In what way does this difference contribute to the closure of the Lake?
Please send me the answers by e-mail. Do not include them in your log. You do not need to wait for my reply before logging. If there is a problem with your answer I will contact you for clarification. More than one cacher may log the cache with just one e-mail, but please include all names in the e-mail.
Reference:NSW Dept. of Primary Industries