| Summary: | rugged valley walls. Generally, the wide segments trend west-east and the narrow segments trend north-south. The west-east segments of the Missouri River valley are wide because they coincide with much older valleys, some of which had already existed for a long time prior to glaciation. Old or mature river valleys tend to be broad with gentle slopes. Younger immature valleys are usually narrower with steeper sides. Most of the narrow, north-south segments of the valley formed in places where glaciers diverted rivers southward, forcing them to carve new valleys. For example, the 40-mile-long segment of the Missouri valley upstream from Garrison Darn is quite wide. This part of the valley, which is now flooded by Lake Sakakawea, was once the route of a river that flowed east to Riverdale, continued eastward past Turtle Lake and Mercer and flowed on into eastern North Dakota. Geologists sometimes refer to this east-flowing river as the "McLean River." The portion of the McLean River valley east of U.S. Highway 83 is buried beneath thick deposits of glacial sediment. It's still a somewhat lower area - a kind of sag through eastern McLean County. A number of lakes including Turtle Lake, Lake Williams, Brash Lake, and others are located in the sag. East of there the McLean River valley is so deeply buried beneath glacial deposits that it is virtually impossible to follow it drilling test holes to determine its location. Fortunately, we have drilled hundreds of such holes in our geologic studies of the glacial deposits and we have a good idea of the route the river followed into eastern North Dakota. Another west-east trending segment of the modern Missouri River valley is the one between Stanton and Washburn. This is an eastward continuation of the modern Knife River. Prior to glaciation, the Knife River flowed east past Washburn, turning slightly northeastward there. It joined the McLean River in eastern McLean County near the town of Mercer. The Knife-McLean River continued northeastward to the Devils Lake area, then north along the east side of the Turtle .Mountains into Canada (remember, all of this was before North Dakota was glaciated). When glaciers blocked the northerly flow of the rivers in these older valleys, the water, having no place to go, would pond up forming proglacial lakes. These lakes would eventually fill the valleys and overflow, cutting new valleys to the south along the glacier margins. When the McLean River was blocked by a glacier in the Riverdale area midway through the Ice Age, a large, proglacial lake formed ahead (to the west) of the ice in the valley of the McLean River. This was the "original" Lake Sakakawea, an early ice-dammed lake that predated the U.S. Army Corps of Engineers version of Garrison Dam by a few hundred thousand years. Eventually, the proglacial lake overflowed (there was no spillway) just about where Garrison Dam is today, and the resulting flood quickly carved a narrow trench southward to the Stanton area. But, at the same time this was happening, the Knife River valley was also flooded by glacial ice in the Washburn-Wilton area. The lake in the Knife River valley, in turn spilled southward into the Burnt Creek-Square Butte Creek drainage, carving a narrow trench from just south of Wilton to Bismarck-Mandan. It's possible these events took place quickly, kind of a domino effect, but we don't really know. 478 Chapter Ten - Natural History
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