If a fish enters an irrigation ditch it encounters the screen, and is routed back to the river. The large paddles rotate to keep the screens clean. Unscreened ditches kill fish. Improperly screened ditches injure fish. Better survival rate in Idaho allows more adult fish to return from the ocean.
State law requires screens in endangered species habitats for salmon ind steelhead.
In waters harboring fish of special concerns like bull trout or native cutthroat trout.