I joined a department field trip to SE Missouri last weekend. There are some really pretty places, and interesting geology. 3 1/2 years ago we had driven to that area for a dat hike, but it was disappointing; we did not go to the best places. One place we stopped was Johnson's Shut Ins, which was scoured by a flood due to a dam collapse (see link) the dam was, and the new one is, on the top of a mountain (OK, hill I suppose). It serves as a giant battery: water is pumped up at night and when power is needed during the day, the water is dropped through a turbine. The flood scoured everything across about 200 or more m wide and maybe 30 m high. Where it levels out, at the state park, where there might have been a lot of people had it been daytime and not winter, is a strewn field of boulders carried by the flood, between about 1 and 10 tons I would guess.
Chris

• Taum Sauk dam and scour