Glacial History of Minnesota - Lake Formation - Proglacial Lakes - Glacial Lake Agassiz

Glacial Lake Agassiz

The largest of all the proglacial lakes was Lake Agassiz, a small part of which occupied the present Red River Valley of Minnesota and North Dakota. Glaciers to the north blocked the natural northward drainage of the areas. As the ice melted, a proglacial lake developed southward of the ice. The water overflowed the continental divide at Browns Valley, Minnesota; drained through the Traverse Gap and cut the present Minnesota River valley. The amount of discharge was staggering. It helped the adjacent Mississippi River to form a very large valley in the southeastern Minnesota.

The river that drained from Lake Agassiz is called the Glacial River Warren. It flowed over the top of a recessional moraine at Browns Valley. As the water eroded away, the glacial deposits the level in the lake dropped. Eventually enough large boulders were left behind that a boulder pavement was produced, which inhibited further downward cutting. The lake level was thus stabilized for a while. During the few decades when the level was constant, waves on the lake produced noticeable beaches along the shoreline. Glacial outwash was also being deposited on the bottom of the lake. Eventually the boulders at the lake outlet were eroded downstream and the river then could erode downward through a mix of sediment sizes. Again, a boulder pavement formed and, as before, the lake level stabilized at a lower level, again forming another set of beaches.

After further retreat of the ice into Canada, lower outlets were uncovered to Hudson Bay, and the Minnesota Valley outlet was abandoned. The continental divide at Browns Valley become the headwaters for the north flowing Red River of the North and southeast flowing tributary of the Mississippi River, the Minnesota River.

During its existence, Lake Agassiz may have been the largest freshwater lake to ever have existed. The lakebed composed of lake muds and silts is one of the flattest regions of Earth and is extremely fertile. No bedrock erosion lakes exist there because the ice was too thin to erode. No kettle lakes are found on the lakebed because lakebed deposits would have filled their depressions.