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Fish Loss: Solutions

An effective solution to fish loss can be seen in this canal diverting water from a tributary to the Salmon River in central Idaho.

Fish loss in canals clearly has a large impact on fisheries in the Wind River watershed, but what can be done about it?

One simple way to limit fish loss in canals is to practice “staged drawdown”, which involves the gradual lowering of canal flows when a canal is to be shut off. The gradual lowering of canal water levels encourages fish living in the canal to move up and back to the stream. This technique is inexpensive, and can allow some trapped fish to escape back into the stream. However, this technique is only works in canals which lack drop structures or other barriers to upcanal fish movement, and can only save the fish which are living in the upper reach of a canal at the end of an the irrigation season.

The most effective way to prevent fish entrapment in canals is the installation of fish screens at or near canal headgates. Sounds simple right? You just weld a screen across the headgate and off you go… Actually, effective fish screens are quite complicated and highly engineered, since they must be able to screen out juvenile as well as adult fish. To further complicate things, water flow velocity through fish screens must be relatively low to prevent small fish from becoming pinned against the screen and injured or killed. Finally, fish screens need to be self-cleaning to minimize inconvenience and operation costs.

Fish screen technology has been developed primarily in the Pacific Northwest, where endangered species like Chinook salmon and steelhead necessitate that all diversions in many drainages be screened. In Idaho, state law requires that fish screens be installed in all diversions from streams that are home to not just endangered salmon and steelhead, but also species of concern like bull trout and native cutthroat trout.

A rotating drum screens fish into an escape pipe and flushes them back into the river.

The most common screen design in use in Idaho are rotating drum screens (see photo at left). These screens are self cleaning, and all moving parts are powered by paddlewheels. Such screens are placed in canals below the actual diversion, and fish that reach them are screened into an escape pipe and flushed back into the river. These screens have proven to be reliable and effective. In fact, studies in the Salmon River system show that the average steelhead smolt passes unharmed through more than a dozen of these structures on its journey back to the Pacific Ocean. More information about the Idaho Fish and Game Department’s fish screen program can be found at: www.townstate.com/screenshop/

Fish screens can be expensive, but they help protect our valuable fisheries resources. Brendan McGinn notes: “Fish screens are much less costly when designed into a new or rebuilt irrigation diversion structure. We need to be proactive and start thinking about installing fish screens into all new structures now, rather than waiting until a local fish species is listed and then be forced to do it anyway.”