Like many U.S. lakes and reservoirs, Lake Powell has an active problem with zebra and quagga mussels, invasive bivalve species originating in eastern Europe. Mussels are most commonly transferred from lake to lake attached to the hulls, and inside the bilge area of boats. Lake users are required by law to clean, drain and dry their vessels, both before and after taking a trip to Lake Powell. Mussel infestations tend to clog the hydroelectric intakes at the Glen Canyon Dam, as well as the propellers and exhaust pipes of boats, requiring expensive de-contamination. However, their impact on the lake ecology appears to be low, or even beneficial due to their providing a food source for fish.