Wednesday, April 21, 2010

Braided Streams of Yore

The image above is from Dunn County - along the Lower Chippewa River in western Wisconsin. The dark squiggly lines in the farm fields are the remnants of braid bars. If you look at an image of the Lower Chippewa River today, you will note a distinctly meandering pattern. Why the change? Largely due to the input of sand and gravel from the big ice sheet about 20,000 years ago. All that shifting sand helped create an unstable river bed. Couple that with highly episodic flow and you have the main ingredients for a braided stream morphology, rather than a meandering one. Now that the glaciers have gone, the sediment supply is diminished, vegetation has helped stabilize the banks, and the river is cruising around a single, meandering channel.

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