I've seen smarter beavers build MUCH shorter dams to back up MUCH larger bodies of water.
One dam was so large that it backed up a good-sized lake that we used to duck hunt on.
Very early one morning we went to the lake to go duck hunting and it was no longer there. A mining company with a mine not too far away had sent in a blaster who blew up the dam, coincidentally washing out a huge section of the mine road about 300 metres downstream from the dam.
Damn, that would have been fun to watch.

It took over a week of heavy equipment work to re-build the road.