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EXECUTIVE SUMMARY Tusher Wash Diversion Dam is located on the Green River near the town of Green River, Utah. This diversion dam supplies water to a water wheel, agravity-fed canal, a pumping station, and a power plant. Research was conducted in 1998 to determine 1) if the dam prevents upstream movement of subadult native fish; 2) if the canal entrains native fish; and, 3) if the dam needs to be modified to better accommodate native. fish. Data showed that native species are being entrained in the canal system. Larval Colorado pikeminnow were found in the canal system, but no razorback sucker larvae were captured. No subadult Colorado pikeminnow were captured in the canal system, but one hatchery-reared razorback sucker was found dead on a trash rack at the power plant. Other native species found in the canal system included flannelmouth sucker, bluehead sucker, speckled dace, and Gila species. Crayfish, northern leopard frogs, and several non-native fish species were also found in the canal system. Tagged fish were able to pass the dam at flows of approximately 4,820 cubic feet (137 cubic meters) per second; however, the discharge of the Green River drops below this level during average years, so it is unknown whether or not native f sh can pass the dam during low flow periods. Entrainment (and probable mortality) of native fish species into the irrigation canals and the power plant is an issue that needs to be addressed. Improvements to Tusher Wash Diversion Dam and its associated canal systems will be expensive to develop and maintain, so other options, such as reducing the amount of water that enters the canal system, should be considered as potential ways to reduce native fish mortality and improve fish passage. LIST OF KEY WORDS Tusher Wash Diversion Dam, Green River, Colorado pikeminnow, razorback sucker, flannelmouth sucker, bluehead sucker, subadult, larvae, fish movement, native species, Green River Canal, Thayn Power Plant, Thayne Ditch vi