Traverse Gap

The Traverse Gap is an ancient river channel occupied by Lake Traverse, Big Stone Lake and the valley connecting them at Browns Valley, Minnesota. It is located on the border of the U.S. states of Minnesota and South Dakota. Traverse Gap has an unusual distinction for a valley: it is crossed by a continental divide, and in some floods water has flowed across that divide from one drainage basin to the other. Before the Anglo-American Convention of 1818 it marked the border between British territory in the north and U.S. – or earlier, French – territory in the south.

The Traverse Gap was cut at the end of the last ice age. The Laurentide Ice Sheet decayed and receded as the Wisconsonian glaciation drew to a close, and Glacial Lake Agassiz formed from its meltwaters. Outlets to the north were blocked by the glacier, and the outlet to the south was dammed by the Big Stone Moraine which had been left by the retreat of the ice sheet. Lake Agassiz filled until it overtopped the moraine about 11,700 years ago. The resulting enormous outflow of the lake carved a natural spillway through the moraine, through which cascaded Glacial River Warren. This great river not only created the gap, it also cut the valleys of the present-day Minnesota and Upper Mississippi Rivers. River Warren drained Agassiz twice more over the next 2300 years, separated by intervals when the ice sheet receded sufficiently to uncover other outlets for Lake Agassiz. About 9400 years ago, Agassiz found a permanent outlet to the north. With its former source now draining elsewhere, River Warren ceased to flow, and the spillway gorge became the Traverse Gap.

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