I have tried Lafforthun's tray idea, It works but takes for ever to set up. I work coastal muck bottoms, Instead of a dam break in the middle of the dam, make your dam break on the side where you have some bottom. Beaver don't care where the leak is they just need to stop it. Sometimes I make two or three breaks. The one in the middle I wont set (soft) the two on the edges get a trap and a castor set on the dam about three feet from the dam break. Quick and deadly