How close is the drain to the shore?(are you sure its beavers clogging it or just their floating debris). I recently saw a show of beavers as nusiances. It talked about building beaver proof fences around road culverts. They are attracted to the sound of running water and have an instinct to try and stop it. They also like building at perpendicular angles therefore fence built like a triangle not square to try and deter damming stream. I think the show was on Discovery channel.
