No, the dams don't hold flood water back

Every time we have flooding on the Mississippi I hear people complain that the Corps of Engineers is holding water back behind the dams to reduce flooding downriver, making local problems upriver worse.

Not true.

The lock and dam system on the Mississippi is designed to maintain minimum water levels during a drought.  The dams have no effect on flooding and were never intended to be used for flood control.  As the river rises, the gates are lifted out of the water and the river rolls on through.  During floods, you could run a motorboat right through the dam's gates (if you were crazy enough) because there's really no drop in river level anymore.

For whatever reason, this complaint circulates most frequently on the St. Croix River, where water levels are affected up to a bit north of Stillwater by Lock/Dam 3 at Red Wing.  Lock/Dam 3 maintains the river level at 675.0 feet above sea level--thanks to the dam, the river won't drop below that.  The dam effectively guarantees the river at Stillwater will be about five feet above the natural low water that would otherwise occur during a drought if the dam weren't there.  But most of the time the river is much higher than "flat pool" (675.0) and that's caused by flow from upstream--and sometimes backwater effect from a higher-flowing Mississippi--not by anything that's happening at the dam.