Draining of Glacial Lake Agassiz

About 13,000 years ago the melting glaciers that covered Minnesota and Canada created a vast lake, bigger than all the Great Lakes of today combined. Geologists later named this Lake Agassiz (AH-ga-see), for the Swiss geologist Louis Agassiz. The lake drained twice: first to the south, to form the channel of the Minnesota River and the Upper Mississippi in the Twin Cities, and then, 1,600 years later, to form the course of the Red River of the North.

Between 17,000 and 21,000 years ago the last North American glacier reached its maximum depth and extent, covering all but the southeast corner of Minnesota. Its slow melting and retreat then began. Some 13,000 years ago its meltwater formed Glacial Lake Agassiz, which covered most of what is now Saskatchewan and Alberta, plus a sliver of northwest Minnesota and eastern North Dakota. Its waters were contained by the ice sheet to the north and a natural dam to the south, since named the Big Stone Moraine. (A moraine is a ridge of debris formed by a glacier.)

About 11,700 years ago the lake breached the moraine near present-day Browns Valley, Minnesota, at a place now called Traverse Gap. The flow of water and debris surged south, then southeast, with unimaginable force, creating what geologists later named the Glacial River Warren. This river carved a wide, deep valley—now the Minnesota River valley—which turned north at Mankato, because it hit bedrock, joined the much smaller Mississippi River at Fort Snelling, and joined the St. Croix at Hastings. It thus formed the river network around which all human habitation in southern Minnesota has since been organized.

When River Warren’s surge reached what is now downtown St. Paul, about 10,000 years ago, it flowed atop the shelf of limestone upon which the city now stands. When it reached the edge of that shelf it met much softer rock—shale, with sandstone beneath. It then proceeded to cut deeply into that material, forming the basin still visible south of downtown. In time the flow dug a deep chasm, and the water cascading over the limestone edge formed a waterfall 175 feet tall and 2,700 feet across (later named River Warren Falls) spanning the entire basin from downtown to Cherokee Heights.

The force of falling water ate away at the sandstone under the limestone shelf, and little by little the edges of the shelf broke off, again and again and again. Thus the falls marched slowly upstream to the west, progressively diminishing in height and force, until they reached the Mississippi at the site of Fort Snelling. It then followed the Mississippi north, split off at Minnehaha Creek to form Minnehaha Falls (where the shelf and softer rock below can still be seen), and continued north to what became Minneapolis. Dakota people called the water feature Owamniyomni: Turbulent Waters. After 1680, when Father Louis Hennepin saw the site and named it for the patron saint of Padua, Italy, it became known to Euro-Americans as St. Anthony Falls.

While Glacial River Warren worked its way east, then north, the glacier, far to the north in Canada, continued to melt. Lake Agassiz re-formed, though smaller. About 7,400 years ago it broke through the weakening ice to the north and east, and flowed toward Hudson’s Bay, creating the shallow valley of the Red River of the North, which extends from Lake Traverse to the south to Winnipeg, and beyond, in a shallow, gently sloping valley. The breakthrough eventually drained all the accumulated melt water, and this marked the end of Lake Agassiz. In time the valley of the Red River proved to be fertile farmland, thus attracting European immigration and forming the population centers of western Minnesota and eastern North and South Dakota. The two successive drainings of Lake Agassiz created two of modern Minnesota’s three great river corridors: the Minnesota and the Red, along with the Mississippi.