Educational Series 7. Geologic History of Minnesota Rivers

Abstract

A glance at a glacial map of the Great Lakes region reveals that all of Minnesota was glaciated at some time, and all but the southeastern and southwestern corners were covered by the last ice sheet, which culminated about 20,000 years ago. Thus all the existing river courses in the state, except the Mississippi River below Hastings, are postglacial in origin. Older river courses were overridden by the ice sheet and buried by glacial deposits. During glaciation, meltwater streams were numerous-some of them flowed under the ice itself, and most of the valleys they cut were abandoned when the ice retreated. Further changes occurred during ice retreat, such as the formation of large glacial meltwater lakes immediately in front of the ice (proglacial lakes), which were drained by streams much larger than their postglacial successors. This account describes the nature of some of the preglacial, glacial, and postglacial rivers in Minnesota.