I hate beavers.

the best way I have found to prevent beavers from stopping up culverts is to use an elbow on the water side and down but not glue it. Confuses them because the water is entering the pipe below surface. Say it is in a duck pond that you want to flood then roll the elbow up to the desired level but during off season leave elbow turned down. I am in excavating business and used this method with success for years.