Pleistocene continental ice sheets covered most of Canada and reached south into the Montana plains. There, the ice dammed north-flowing rivers and shifted their courses from the Labrador Sea to the Gulf of Mexico. The pre-Pleistocene upper Missouri and Yellowstone rivers had previously flowed into the Amazon-scale "Bell River" of Canada in the area now filled by Hudson Bay. This paleo-river was named for the Canadian geologist who first recognized it in 1895, after a career mapping Pleistocene landscapes of the Canadian Shield. For 60 million years, until it was defeated by continental glaciation, the Bell River flowed through Hudson Strait to a great delta in the Labrador Sea. The stratigraphic record of the delta links the Bell River to its headwaters in Canada and the US. The upper Missouri and Yellowstone tributaries may have had reached the Great Basin and Colorado Plateau before those headwaters were cut off in Pliocene time by the Yellowstone hotspot and by Rocky Mountain tectonics. 

In Canada, a 3 km thickness of Pleistocene ice and glacial drift buried the Bell River basin. Glacial ice streamed down Hudson Strait to the Labrador Sea, where it deeply scoured Pliocene sediments of the 9-km thick beds of the Bell River delta. In Montana, a series of ice-marginal lakes filled the valleys of the north-flowing rivers. The lakes overflowed into a long lake-and-river system that carved the Pleistocene channel of the Missouri River across Montana. Where it met the glacial front, the Missouri River filled Glacial Lake Great Falls to a depth of 200 m. The Missouri abruptly dropped 300 m from lacustrine clays down a series of five waterfalls over hard sandstone beds of the Kootenai Formation. Downstream, the diverted river followed the Wisconsin-stage gorge of the Missouri River Breaks, where the river ran 400 m below the pre-Pleistocene land surface. A preserved Illinoian-stage diversion channel, 160 km long, 3 km wide, and 160 m deep shows that the Illinoian glacial front reached up to 60 km to the south of the Wisconsin glacial front, although the diversion channels mostly coincide. Interplay between the Montana rivers and pre-Illinoian continental ice sheets is not clear. The rivers may emptied into long systems of ice-marginal lakes across Canada before the Pleistocene glaciers crossed into Montana.