Lake Souris

The Glacial Lake Souris occupied the basin of the Souris River from the most southern portion of this river's loop in North Dakota to its elbow in Manitoba, where it turned sharply northward and passed through the Tiger Hills. The length of Lake Souris was about 170 miles, from latitude 48° to latitude 50°35', and its maximum width, north of Turtle Mountain, was nearly 70 miles. It was situated near the far southeast corner of the large glacial Lake Agassiz, separated from it by another small glacial body, Glacial Lake Hind.

North of the Souris basin, an arm of this lake extended along the Assiniboine River from Griswold and Oak Lake to some distance above the mouth of the Qu'Appelle River in Saskatchewan; and the main body of the lake was deeply indented on the east by the high oval area of Turtle Mountain, and forms, with overlying drift deposits, the massive terrace of the Coteau du Missouri on the west.

The mouth of Lake Souris, where it first outflowed to Lake Agassiz by the Big Coulée River and the Sheyenne River, was approximately 1,600 to 1,500 feet above the present sea-level, being gradually cut down about 100 feet by the stream. Because of subsequent changes which are known to have taken place in the relative elevation of the land and water surfaces in this district, the shore-line of the northern part of the lake at the end of its time of outflow to the Sheyenne would now have an elevation of about 1,600 feet at Langs Valley. Therefore, when its channel of discharge was transferred to the new course by Pelican Lake and along the Pembina River, Lake Souris was suddenly lowered about 125 feet to the level of the top of the bluffs of Langs Valley, and a further lowering of 110 feet was afterward effected by the gradual erosion of this valley. The lake was wholly drained by this outlet.

...
Wikipedia