Map showing a portion of the route of the preglacial “McLean River,” which flowed eastward through a broad valley that passed between Garrison and Riverdale, to the Turtle Lake area, and on into Sheridan County. When the McLean River valley was blocked by a glacier (to the east of the area shown on this map), a proglacial lake formed in the valley. When the lake overflowed southward from a point near Riverdale – at the site of the modern Garrison Dam – a narrow diversion trench was cut. The modern Missouri River flows through this diversion trench.
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