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INTRODUCTION Study Site Tusher Wash Diversion Dam is an 840-foot long dam that spans the width of the Green River approximately 8 river miles north of the town of Green River, Utah (Figure 1). This structure was built in 1906 and consisted of wood cribbing filled with rock. A concrete cap was poured on the 12 foot wide structure in 1936, and this configuration is still in place. The 8-foot high diversion dam was designed to force water into a canal on the west side of the river and past a water wheel on the east side of the river (Figure 2). The sluiceway on the east side of the river, which feeds the water wheel, is 11-feet wide and approximately 3-feet deep. The water wheel is in operating condition, but was only used for a few days during 1998 (personal observation). Another sluiceway is located near the center of the dam. This notch is about 150-feet long and 0.5-feet deep. Flow measurement devices are not used on any of the irrigation or power generation facilities. However, estimates suggest that approximately 715 cubic feet per second (cfs) of water from the Green River enters the canal system on the west side of the river, when all components of the system are operating at full capacity. After flowing approximately 2,500 feet down a canal that is locally known as the raceway, the water reaches the Thayn Power Plant and pump station. At this point, about 200 cfs is used to turn a turbine that pumps 35 cfs up hill to an irrigation canal (known as the 42 foot ditch or the Thayne ditch). The remaining water can be used to turn two additional turbines and fill another irrigation canal, known as the Green River Canal. The power plant has the capacity to pass 600 cfs through its turbines to produce electricity (Thayn and Thayn 1987). The power plant was run at full capacity until early in the summer of 1999. However, a recent court decision ruled that a maximum of 435 cfs can legally pass through the power plant and pumping station (Reed Harris, U.S. Fish and Wildlife Service, personal communication). At the present time, the Green River Canal is taking 80 cfs during the irrigation season (Keith Rose, U.S. Fish and Wildlife Service, personal communication). During recent years, when the power plant and pump station was running at full capacity, irrigation diversions amounted to 0% to 4.9% of the of the average monthly discharge of the Green River, while the total diversion at Tusher Wash Diversion Dam amounted to 5.9% to 29.3 % of the average monthly discharge of the Green River (Table 1). Native fish species, including the four endangered species of the Upper Colorado River Basin (Colorado pikeminnow (Prychocheilus lucius), razorback sucker (Xyrauchen texanus), humpback chub (Gila cypha) and bonytail (G. elegans)) have been captured both upstream and downstream from Tusher Wash Diversion Dam. Subadult and adult Colorado pikeminnow have been documented to move upstream and downstream past Tusher Wash Diversion Dam (Tyus et al. 1980; Tyus 1985; U.S. Fish and Wildlife Service 1998). However, in most cases, discharge of the Green River at the time the fish passed the dam is not known, since several months or years had passed between the times of capture and recapture. Little data has been collected in relation to movement of other native fish species past Tusher Wash Diversion Dam or on passage of any fish species during low flow periods. A limited study conducted in 1995 and 1996 showed no fish movement past Tusher Wash Diversion Dam (Utah Division of Wildlife Resources, unpublished data). However, this study only included three days of sampling and 139 marked fish. Therefore, the probability of documenting f sh moving past the dam was quite low.