Primo -
Can you not set some of their favorite wood out in that shallow area by the spillway, set up shop across the way providing good cover, and pick it off this way?
You would build your blind days in advance, when you have the trap at the spillway, then remove the trap and sit behind / in your blind. With one in the chamber ready to fly and the beaver on the wood you've set out, that should give you time to squeeze the trigger. If you're set up on a bipod or tripod then you're just sitting quiet as can be and no movement other than squeezing the trigger.
What is the distance from "across the way" wherever you would set up, be it opposite of spillway or behind it?
