Agricultural Drainage Assistance Program (ADAP)
Beavers build dams for protection and to create safe areas to access their food. When beaver dams are removed or notched, beavers will usually rebuild or repair them within 48 hours. Removing or notching beaver dams should be considered a short term solution to drainage problems caused by beaver dams and property owner can expect to remove or notch the dam every day or two for an extended period of time.
Trapping beaver is also an option and should be considered a medium term solution. Young beavers are forced out of their parent’s pond when they are about 18 months old. The young beavers seek out suitable beaver habitat that is unoccupied. If beavers are removed from suitable beaver habitat, a property owner can expect new beavers to move in within 6-36 months. Removing all beaver from an area can be difficult because, as some beaver are trapped and removed, the remaining beaver become more wary making them more difficult to trap. Washington Department of Fish and Wildlife licenses beaver trappers and maintains a list of licensed trappers (external link).
Long term drainage solutions to beaver dam related problems can sometimes be achieved with beaver deceivers or pond levelers (external Acrobat pdf). Beaver deceivers protect the inlet of a pipe or structure where it is easy for beaver to construct dams. A pond leveler is a pipe through a dam that allows the upstream water level to drop during periods of low flow.
A Hydraulic Project Approval (external link) issued by Washington Department of Fish and Wildlife (WDFW) is required to remove or notch a beaver dam. If the dam removal/notching is done by hand with hand tools, no King County permit is required. If machinery is used to remove/notch a beaver dam, a King County Grading Permit is required.