Do beaver dams prevent salmon from moving upstream?
Beavers and salmon have naturally co-existed in the Pacific Northwest for thousands of years. Beaver dams that prevent salmon from migrating are rare. In most cases, salmon will wait at the bottom of a dam until a heavy rain. When the flow of water over the top of the dam is higher, the fish will jump over the dam. Even juvenile trout and salmon just a few inches long can go over most beaver dams. Water flows over the top of the dam and across the sticks in the face of the dam, creating a fish ladder effect. Four inch long salmon have been observed scaling dams as high as six feet in this manner!
Should I knock down the beaver dam so the salmon can move upstream?
No. Removing beaver dams actually eliminates more salmon habitat than it helps. Beaver dams that prevent salmon from migrating are extremely rare. Beaver ponds are like nurseries to young salmon, providing them with food, shelter, calm water, and protection from floods. Coho salmon and cutthroat trout, in particular, rely on beaver habitat.
You should also consider the impact of removing a beaver dam to people. Some people who remove beaver dams drain the pond and are left with an unsightly mud-hole for years afterward. Draining a beaver pond has downstream impacts as well. The high flows of water created as the pond drains can cause severe erosion and deposit significant amounts of sediment on neighboring properties. You could be held liable for damage to downstream properties resulting from your removal of a beaver dam.
http://www1.co.snohomish.wa.us/Departments/Public_Works/Divisions/SWM/Services/Landowners/Streamside/FAQ/FAQ_Living_With_Beavers.htm
Snohomish County Public Works Jake Jacobsen, Steward
Beavers also provided an important function in creating and maintaining spawning and rearing habitat for fish, particularly salmon and steelhead, by creating pools that keep water clean by storing organic material and sediment. The dams also moderate periodic fluctuations in flows and allow for stable growth of important shoreline vegetation, which shades the water and provides cover for juvenile fish to hide under and insects for the fish to eat. In fact, deliberate overhunting of beavers contributed to the degradation salmon habitat throughout the Columbia Basin in the 1830s and 1840s. This was the first major human-caused impact on salmon and the beginning of their 150-year decline.
Northwest Power & Conservation Council http://www.nwcouncil.org/history/Beavers.asp