Variable ice dam to stop the flooding destruction in the spring(Mississippi River). The ice dam would be upstream where it's safe to backup the river. Hydropowered freezing units. The water only needs to be cooled a few degrees and slowed down to freeze. The freezing sections would insulate against the coldness being pulled down stream (better freezing ability). Which would close up the river flow and be strong enough to hold back the water until it is safe to continue its flow.