thumb|285x285px|Glacial Lake Wisconsin 20,000 years ago with modern counties for geographical context.
Glacial Lake Wisconsin was a prehistoric proglacial lake that existed from approximately 18,000 to 14,000 years ago, at the end of the last ice age, in the central part of present-day Wisconsin in the United States.
Formation and demise
Before the last glacier, a somewhat different Wisconsin River drained the north-central part of the state, running around the east end of the Baraboo Hills. Around 18,000 years ago, the Green Bay lobe of the Laurentide ice sheet crept in from the east, abutting against the Baraboo Hills. With that outlet closed, the water backed up, filling the basin to the north and west, forming Glacial Lake Wisconsin.
The water rose to as deep as 160 feet, with a surface area eight times the size of modern Lake Winnebago, a large, cold lake that stretched north to the site of Wisconsin Rapids. Eventually it found a new outlet, flowing west to the Mississippi via the east fork of the Black River near City Point. and Roche-a-Cri, for example. The lake existed for thousands of years, with storms and ice scouring sand off those bluffs. Streams from the glacier to the north and east also carried in sand and silt which settled at the bottom of the lake, roughing in the flat sandy Central Plain that we see today when we follow I-90/94.
About 14,000 years ago, as the climate warmed, the glacier began to retreat. The lake water reopened the path around the Baraboo Hills. Once the trickle began, it quickly melted a larger channel through the ice and became a torrent. In a catastrophic flood, most of the huge lake probably drained out the south end in no more than a few weeks - possibly a few days. Upstream, the current cut new channels through the lake-bottom sand. After removing the lake-bottom sand, it cut canyons through the weak Cambrian sandstone beneath, which had existed long before the lake, forming the Dells of the Wisconsin River, that are now largely beneath the high water created by damming the river. Boat tours today show the portions that remain above water.