Nuisance Beaver Dam Removal
As far down as you can work, pound a pipe through from one side to the other. A 4 inch pipe is ideal but as thick as a dam is you may need to use several 1-2 inch pipes. The water will flow through the pipes and the beavers will not figure out a solution.
Eventually (weeks or months) the pipe may clog with debris on the upstream side so be prepared to wade or swim in to unplug the pipe. For this reason mark the locations of the pipes.