The Free Press, Mankato, MN

December 12, 2011

Day 4: Glacier carved out deep river valley 12,000 years ago

By Tim Krohn
Free Press Staff Writer

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You can blame Warren for a lot of the sediment washing into the Minnesota River today.



River Warren that is.



The beautiful Minnesota River Valley is deep and wide — but because it was carved so deep, the streambanks, ravines and bluffs are prone to sloughing into the river when water comes pouring in.



The deeply cut valley was the work of what is known as the pre­historic River



Warren. Creation of the valley started about 12,000 years ago as the last glaciers in this area melted and created the massive Lake Agassiz that cov­ered parts of the Dakotas and northern Minnesota, up through central Canada to Hudson Bay. The lake — 400 feet deep in places — was bigger than all the present Great Lakes combined.



That big lake drained in various directions over time, but geologists say something cata­clysmic happened about 11,000 years ago.

Lake Agassiz broke open near the present day Browns Valley on the Dakota/Minnesota border and began sending torrents of water toward present- day Mankato where it hit limestone bedrock that sent the water heading toward what is now the Twin Cities.



River Warren drained Lake Agassiz for thousands of years.



(The prehistoric river got its name from Gen. G. K. Warren who, in 1868, while looking for railroad routes studied the river valley and first explained how it was created.)
 
After Lake Agassiz drained, the flow of water in the Minnesota River became a relative trickle in the bottom of the deep valley.



Besides creating appealing but erosive bluffs, River Warren also made the river valley a dreamscape for geologists who can easily study some of the oldest rock formations in the world that were exposed by the deep cut into the earth.