This video demonstrates the extreme danger of going over a low head dam (weir). The extremely powerful recirculation backwash at the bottom of the dam can roll and hold a paddler until he or she drowns and keep a boat for a long time.
Escape may be possible by paddling (if you are still in the boat) or swimming to the side of the dam in hopes there is an eddy shoulder there. Otherwise, you can try to dive down into the deeper water, which will be flowing downstream. Some have advised taking off your PFD to try such a dive, but I think that would be a very last ditch, desperation tactic.
The preventative measures are to learn to recognize a low head dam horizon line, go ashore well above it, and always paddle in dangerous current water with other boaters who are skilled in and equipped for swift water rescues.
HERE is some literature from the ACA.
Escape may be possible by paddling (if you are still in the boat) or swimming to the side of the dam in hopes there is an eddy shoulder there. Otherwise, you can try to dive down into the deeper water, which will be flowing downstream. Some have advised taking off your PFD to try such a dive, but I think that would be a very last ditch, desperation tactic.
The preventative measures are to learn to recognize a low head dam horizon line, go ashore well above it, and always paddle in dangerous current water with other boaters who are skilled in and equipped for swift water rescues.
HERE is some literature from the ACA.