Glacial Lake Duluth

Lake Duluth was a proglacial lake that formed in the Lake Superior drainage basin as the Laurentide ice sheet retreated. The oldest existing shorelines were formed after retreat from the Greatlakean advance (previously called the Valders), sometime around 11,000 years B.P. Lake Duluth formed at the western end of the Lake Superior basin. Lake Duluth overflowed south through outlets in Minnesota and Wisconsin at an elevation of around 331 m above sea level.

As the Superior lobe of the Laurentian Ice Sheet melted, water became trapped along the ice margin and formed several independent lakes. The modern water basin in which these lakes stood have been used for their names. Lake Nemadji occupied the headwaters of the Nemadji River, Lake Brule occupied part of the Brule River basin, and Lake Ontonagon was in the Ontonagon River basin. These lakes are all west of the Keweenaw Peninsula. To the east, the ice melt drained into Green Bay or Lake Michigan through channels crossing the western part of the upper peninsula of Michigan. Lake Duluth extended a few miles farther east than the Keweenaw Peninsula, to the border of the Huron Mountains, east of Keweenaw Bay. In the early part of the recession of the ice front Lake Duluth, with an outlet from the Brule River Valley through the St. Croix River Valley, was present in the western part of the Lake Superior Basin. A little later the ice border retreated sufficiently to allow the small independent lakes to become a part of Lake Duluth or to be drained by the lowering of the water level, for in general the water level was lowered as these lakes merged with Lake Duluth

One of the bordering lakes was present when the Superior ice lobe was at its full extent. It stood in the part of the St. Louis River drainage basin northwest of the border of the Superior lobe

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