During the winter 50 years ago the Mississippi river was allowed to drop extra low so that repairs could be done at a Lock & Dam 24 downstream. This is looking upstream from a rock dike shown below.
The dikes are water flow control structures used to maintain the channel where it needs to be...but boaters must be ever vigilant when they are covered by water.
If you look closely, you can see some of my family standing on this dike...it helps give you a better idea of the scale of this structure. So now you know that the bottom of the river is mostly sand and some mud!