Near the end of the Pleistocene, 10-15,000 years ago, large meltwater lakes formed along the retreating edge of the Continental Ice Sheet. One of these, Lake Souris, extended from north-central North Dakota into Manitoba, intermittently connecting with Glacial Lake Agassiz, to its east.
All of these lakes expanded and contracted depending upon their interconnections, the regional climate and the rate of meltwater production. Dammed by tongues of ice or moraines of glacial debris, some would occasionally break through their retaining wall, sending a torrent of water across the flat landscape of the Northern Plains. One such event involved Glacial Lake Regina of southern Saskatchewan, which flooded southeastward into Lake Souris; the broad, shallow channels of this flood remain evident today and are partly occupied by the Upper Souris River and its major tributary, the Des Lacs River.
East of Minot, the Souris River now enters the former lake bed of Glacial Lake Souris, following it north and gradually eastward to merge with the Assiniboine River of southern Manitoba; Lake Souris, itself, eventually drained into Lake Agassiz, which contracted into Lake Winnepeg after drainage opened to the north. National Wildlife Refuges now line the Des Lacs and Souris Rivers which, as we saw last summer, may still flood across the Pleistocene channels and lake beds when a deep winter snowpack is followed by heavy spring rains.
