The St. Croix River rises in the northwestern corner of Wisconsin, out of Upper St. Croix Lake in
Douglas County, near
Solon Springs, approximately south of
Lake Superior. It flows south to
Gordon, then southwest. It is joined by the
Namekagon River in northern
Burnett County, where it becomes significantly wider. A few miles downstream the St. Croix meets the boundary between Minnesota and Wisconsin, which it demarcates for another until its
confluence with the
Mississippi River. , where the clearer waters of the St. Croix meet the muddier Mississippi River. Other major
tributaries include the
Kettle River,
Snake River, and
Sunrise River joining from the west, and the
Apple River,
Willow River, and
Kinnickinnic River joining from the east. Just below
Stillwater, Minnesota the river widens into Lake St. Croix, and eventually joins the
Mississippi River at
Prescott, Wisconsin, approximately southeast of
St. Paul, Minnesota.
Geomorphology The presence of older
glacial deposits within the St. Croix River basin proves that the
Laurentide Ice Sheet has repeatedly glaciated this region during the
Pleistocene Epoch. Within this region these older deposits consist of gray
calcareous tills associated with glaciofluvial yellowish-brown
sand and
gravel and
glacial lacustrine silts and
clays of the Pierce Formation and overlying reddish-brown tills and associated sand and gravel of the River Falls Formation. Any glacial landforms associated with these earlier glacial deposits have been destroyed by erosion. These earlier deposits occur as erosional remnants either capping hills or buried by the latest advance of the
Laurentide Ice Sheet over this area during the
Last Glacial Maximum. The
drainage basin of the St. Croix River last
deglaciated between about 19,000 and 14,000
BP calibrated (16,000 and 12,000 14C uncalibrated). During this time, the Superior Lobe retreated from the maximum extent of the Superior Lobe at the Emerald
moraine northeastward to Thompson and Nicherson moraine complex at the edge of the Lake Superior basin. During the retreat of the Superior Lobe, a blanket of reddish-brown tills containing beds of fine sand and silt was left behind by the melting of the retreating ice sheet. During glacial retreat, meltwater drainage from the intermediate St.Croix moraine established the precursor to the St. Croix River. The retreat of the Superior lobe into the
Lake Superior basin created small,
proglacial lakes. Later, these lakes coalesced to form a large proglacial lake called proglacial
Lake Duluth, within the western Superior basin. The multiple lake levels of proglacial Lake Duluth included the
Duluth level and an older and smaller
epi-Duluth level. At first, glacial
meltwater drained from the epi-Duluth level and a smaller precursor proglacial lake, named
Lake Nemadji through the Moose Lake (Portage) outlet into the Kettle River and into the St. Croix River. The flow of glacial meltwater from glacial Lake Nemadji and Lake epi-Duluth during the retreat of the Superior Lobe caused the rapid entrenchment of the St. Croix River and formation of a
strath terrace, known as the
Chengwatana surface. As the Superior Lobe retreated, the Duluth level was established when another outlet, the Brule outlet, opened and the Moose Lake outlet was abandoned. The opening of the Brule outlet allowed a massive and sudden outflow of glacial meltwater through the Brule outlet and down St. Croix River and excavation of a deep inner channel, which includes
the Dalles, into the Chengwatana surface. This massive flow of meltwater also created the
giant potholes of
Interstate Park between 10,800 and 10,600 BP calibrated. The Brule outlet was abandoned when Lake Superior Lobe retreated from the
Keweenaw Peninsula and opened lower eastward draining outlets. This caused the Duluth level to drop abruptly to post-Duluth levels and water to cease flowing into Brule outlet and down the St. Croix River. The cessation of water outflow through the Brule outlet disconnected the St. Croix River from the Lake Superior basin and created the northward flowing
Bois Brule River. ==Conservation efforts==