Upper St. Anthony Falls Lock and Dam, Minneapolis, MN. (Photo: US Army Corps of Engineers)
The only major waterfall on the Mississippi is in Minneapolis: St. Anthony Falls. The power of the falls brought life to General Mills, Pillsbury, Gold Medal Flour and Betty Crocker in Minneapolis. But it was originally in St. Paul. The falls moved 15 miles upstream in the last 12 thousand years (fast in geological time). The river has a fragile bed — the falls once flowed over a bed of limestone, and then shale, and then really soft sandstone that you could scratch away with your fingers.
In the mid 1800s, millers began harnessing the power of St. Anthony Falls. One miller had the idea to tunnel under the falls to an island he bought upstream to harness more power. As he tunneled out the river bed, the falls started collapsing into the river. To stop the rapidly-speeding geologic clock, the Army Corps of Engineers built a 36-feet-deep wall in 1937 underneath the Mississippi river at St. Anthony Falls.