Understanding how aircraft fly and their efficiency is much about how air moves over the surfaces; water is just another fluid so I inquired on a model aviation listserve as to what was going on. I received an interesting link to a video that was a news segment where they did a test to demonstrate the phenomenon I had witnessed. In doing further research using “dangers of low head dams” will give plenty of results. Low head normally refers to a dam that is 10 feet high or less, the danger is the recirculating current found behind the dam. Many people have lost their lives by getting too close to this powerful current and having their boat pulled under, the warning signs below dams are there for a good reason.
In the following diagrams I show how as the water is squeezed going over the dam the speed increases, it then goes downward with great force over the back of the dam. The fast moving current hits the bottom and then pushes upward to the surface but is pulled back towards the fast moving current over the dam; it is a continuous cycle of recirculating water.
Video of Bottles Trapped Behind Low Head Dam