The posted
coords will take you to the center of Lake St. Croix, a riverine
lake which formed as the result of the natural sediment deposition
and damming process which takes place as a river matures.
The Mississippi, Minnesota and St. Croix Rivers were carved over
11,000 years ago as proglacial lakes Agassiz and Duluth drained at
the end of the last Ice Age. These glacial lake outburst floods
were cataclysmic in magnitude. Torrents of water raged across this
region for over a year before these giant lakes were drained. Deep
gorges and valleys were carved. Upper substrate was eroded. The
force of the these rivers cut deeply into the faultines down to the
most resistant layers of bedrock.
These active glacial rivers are classified as a
“young” or “youthful river”. A youthful
river is a river with a steep gradient that has very few
tributaries and flows quickly. Its channels erode deeper rather
than wider. The gradient is controlled primarily by tectonics. The
discharge is controlled by climate. The sediment load is controlled
by various factors including climate, geology in the headwaters and
the stream gradient. Fast moving water is capable of carrying, or
suspending, a much greater volume of sediment than slow moving
water, thus enhancing it ability to cut a path through varying
stratas.
As time passed and the glacial lakes drained, the volume of
water flowing through these channels has greatly reduced. These
rivers are now classified as a “mature river.” The
gradient has become less steep than those of youthful rivers and
flows are slower. A mature river is fed by many tributaries and has
more discharge than a youthful river. Its channels erode wider
rather than deeper. As the speed of the water slows, the ability to
suspend particulate matter also declines. The original deeply
incised channels of the river valleys begin to fill with sediment.
Sediment begins to accumulate and form deltas where tributaries
meet the main river flow. This ongoing cycle continues to alter the
flow and course of the maturing rivers. These tributary fans dammed
the main rivers, forming riverine lakes.
The gradient and flow rate of each river is unique and subject
to weather and watershed. The flow rate of the Mississippi was so
greatly reduced that the ongoing deposition of sediment nearly
dammed the St. Croix River at the confluence at Prescott, WI. The
accumulation of river debris and sediment is now known as Point
Douglas Park. The hydraulic damming action of the reduced flow, and
subsequent backfilling, caused the formation of Lake St. Croix
which extends from Prescott, WI northward through Stillwater,
MN.
The following tasks
are required to log this EarthCache:
Please forward in an e-mail the
following:
1. An estimation of the width of the
active channel of St. Croix River from the end of the causeway to
the Minnesota shoreline, and reference your unit of
measurement;
2. The elevation reading at GZ on
the causeway;
3. The distance you walked from the
arch to the posted coords; and.......
4. Posting a photograph of yourself or GPS at this site is now an
optional requirement. Posting interesting photographs of this
glacial footprint to educate other visitors to this EarthCache and
to document your experience here are appreciated.