Coming down Minot’s north hill recently, I was again struck by the immensity of the Souris River Valley compared to the relatively small flowage of the present-day Souris River. How could that large valley have formed? The Souris River is an underfit river, sometimes called a misfit river. The river valley indicates a much larger river formed the valley in the past. Other rivers, including the Pembina, James, and Sheyenne rivers formed in a similar fashion.
Toward the end of the last ice age, as the glaciers were melting, a massive lake, glacial Lake Regina was formed. The southern end of the lake was near present Day Weyburn, Saskatchewan, and the lake stretched perhaps thousands of square miles to the northwest. At some point, the water broke out at the southern end resulting in a cataclysmic release of water that flowed southeastward cutting out what is today the Souris River and Des Lacs river valleys.
The segments upstream from the Velva area, are occasionally referred to as the Souris and Des Lacs Spillways. However, glacial Lake Souris at that time occupied much of the present Souris River Valley and adjacent land further downstream. As a result, the morphology of the Souris River valley in these two stretches is noticeably different.
The Pembina River Valley was formed in a similar fashion, only from meltwater flowing out of glacial Lake Souris and into glacial Lake Agassiz. The river starts northeast of Turtle Mountain and flows southeastward across southern Manitoba before dipping down into North Dakota northwest of Walhalla then eastward into the Red River.
Glacial Lake Souris also released a torrent of water that led to the formation of the Sheyenne River Valley as well as the James River Valley. As some of you know, the Sheyenne deposited large amounts of sand at the river’s delta into glacial Lake Agassiz. Today we know that area as the sandhills of Richland and Ransom Counties. John Bluemle in the 3rd edition of his “The Face of North Dakota” helps put that flow into perspective, and I quote “The result was the nearly instantaneous carving of the entire Sheyenne River Valley, which flowed, rim-to-rim, full of water – a river probably larger than the Mississippi River at New Orleans.”