Juvenile Downstream Passage on the West Coast
Migrating juvenile salmon and steelhead in the Columbia and Snake Rivers pass up to eight dams on their way to the ocean.
Migrating juvenile salmon and steelhead in the Columbia and Snake Rivers pass up to eight dams on their way to the ocean. The fish depend on current and as they approach a dam, the water slows and the current may be lost in the reservoir. The fish will actively search for a way downstream. This is when they are especially vulnerable to predators.
Some dam passage routes are safer than others. The safest way is usually a spillway, an opening to release water over the dam. Some spillways are equipped with movable weirs that make them easier for fish to find.
A large volume of water also passes through the dam's turbines, and this can be a dangerous and difficult course for juvenile fish. However, fish headed for the turbines may be guided into other bypass routes, such as gatewells, that will release them safely downstream. And for those going the turbine route, new technology has made turbine passage much safer.
At some dams, the fish are guided into floating surface collectors directly upstream. These collectors attract migrants into a holding tank until the fish are safely moved downstream by pipe or truck.
Juvenile Bypass Systems help fish avoid turbines
Juvenile bypass systems take advantage of existing dam structures to guide fish safely past dams. A long narrow slot above each turbine intake is known as a gatewell. This structure is only in use when a turbine is closed down for maintenance.
Scientists noticed that some juvenile fish swam up the gatewells rather than through the turbines. At first, fish were manually collected from the gatewells and released downstream. Eventually, automatic systems were installed to carry fish from the gatewells into pipes to carry them safely through the dam.
Surface Passage Structures in Dams
Spillways can be the safest route for juvenile fish to pass a dam, but the spillway may be difficult for the fish to find. Most salmon and steelhead juveniles like to stay in the top 20 feet of the water column. Spillway openings may be as much as 60 feet below the surface, too deep for the fish to use.
Beginning in 2001, some of the dams on the Columbia and Snake Rivers have been equipped with spillway weirs. These are movable gates on the upriver side. When juvenile fish are migrating, water flows over the top of the weir and out through the spill gate. Since fish do not have to dive down to find the spillway entrance, more fish are passed with less water spilled. And since the fish quickly find passage, they also may escape predators above the dam.
Floating Surface Collectors
Floating surface collectors are new routes past the dams. Water flowing into the entrance of the surface collector attracts juvenile fish. The fish enter and are held for transport around the dam by truck or barge, or guided into a pipe that safely carries them downstream of the dam.
The diagram below shows a bird's eye view of floating surface collectors.
New turbines are safer for fish
Turbines in the Columbia and Snake River dams are up to 23’ in diameter, and turn at around 70-80 rotations per minute. The flow through a single turbine is equivalent to that of a medium sized river, such as the Deschutes.
There is plenty of room for a 6" fish to pass through, but the route is not without danger. The fish risk being struck or pinched by the turbine blades or other equipment. And once past the blades, turbulence may directly injure or disorient the fish, making them easy prey for predators below the dam.
In the past, juvenile fish were frequently injured by the turbine route. Now, developments in turbine design have made it a safer route of passage. Fish friendly turbines are taking the place of aging turbines on the Snake and Columbia River dams. The diagrams below show the disadvantages of conventional turbines, and the new, improved replacement turbines.