Hydroelectric dams are a ubiquitous part of the Pacific Northwest landscape. But what do they mean for salmon and steelhead?
In Washington, Oregon, Idaho and California hundreds of dams obstruct salmon passage. In the Columbia system alone, more than 50% of the historic habitat of salmon has been lost to dam construction. Grand Coulee and Hells Canyon have blocked salmon from huge swaths of the Columbia and Snake basins, lesser known dams in many tributaries have in combined to eliminate a hundreds of miles of productive spawning and rearing habitat. With the exception of the 45 miles of free flowing river in the Hanford reach the Columbia and Snake below Lewiston have been reduced in their entirety to a series of slack water impoundments by a series of 13 mainstem dams.
While fish passage facilities such as fish ladders exist at many dams, the changes brought about by dam constructed cannot be alleviated simply by passing fish above them. Dams fundamentally alter the movement of water, sediment and fish through river systems, slowing and warming the water, impeding the upstream and downstream movement of fish and starving the river downstream of gravel and wood.
In the slow moving warm water behind dams, native and non-native predators thrive, preying heavily on migrating juveniles, and estimates are that between 3% and 15% of juvenile salmon die at each dam. Historically these fish were carried to sea on the spring freshet, but with dams impeding the natural flow of the rivers fish now must expend tremendous amounts of energy swimming to sea, arriving at the ocean weeks later than they once did. Dams also impede the upstream migration of returning adults. During the peak of summer, water temperatures in heavily dammed watersheds often rise above 70 degrees Fahrenheit, temperatures that are lethal to salmon and steelhead. Under these conditions salmon and steelhead are subject to high pre-pawn mortality and survivors are forced to cease their migration and seek out cold water refuges.
All told dams have blocked thousands of miles of salmon habitat in the region but the tide is starting to turn. For the first time in American history dams are being removed faster than they are being constructed and high profile dam removal projects on the Elwha, White Salmon, Sandy, and Rouge are a demonstrating the benefit of dam removal. A tentative agreement is in place to remove four dams on the Klamath by 2020. The wave of dam removals that is sweeping through the Northwest is ushering in hope for a century of dam removal, one that must include the removal of the four lower Snake River dams. These four dams are greatest impediments to salmon recovery in the Columbia system, and biologists agree that removing these dams is the best way to restore abundant populations of wild salmon in the pristine tributaries of the Snake.