1. THE YELLOWTAIL UNIT. The Yellowtail unit is an integral part of the Missouri River Basin project. It is a multipurpose development to provide for irrigation, flood control, power generation, sediment retention, fishery and waterfowl resource improvement, recreation enhancement, and municipalindustrial water supply. Although no irrigation is directly included in the Yellowtail unit, a dependable and adequate supply of water for irrigation is provided through storage in the Yellowtail Reservoir for the proposed Hardin unit and for other future downstream irrigation developments. The Yellowtail unit consists of Yellowtail Dam and appurtenant works, the reservoir which has been named Bighorn Lake, a powerplant, a switchyard, a visitors' center, and an afterbay dam. The afterbay dam was constructed to provide a uniform daily discharge into the Bighorn River, leveling the peaking power discharges from Yellowtail Powerplant, and to provide for the diversion requirements to the existing Bighorn Canal.

Storage allocations in Bighorn Lake include 259,000 acre-feet of reservoir space solely for flood control and 250,000 acre-feet for joint use in flood control and conservation. The conservation portion of this joint-use storage capacity plus a specific allocation of 363,672 acre-feet for active conservation are used mainly for irrigation and power generation.

2. LOCATION. Yellowtail Dam is located near the mouth of Bighorn Canyon on the Bighorn River in sec. 18, T.6 S., R.31 E., Montana principal meridian, Big Horn County, Mont. (fig. 1). The site is within the Crow Indian Reservation, about 21 miles due north of the Montana-Wyoming State line and 45 miles by road southwest of Hardin, Mont. The powerplant is located immediately downstream from Yellowtail Dam, the visitors' center building is located near the right abutment of the dam along the access road to top of dam, and the switchyard is located on the hillside above the right abutment of the dam. When filled to elevation 3657.0, top of exclusive flood control, the reservoir extends a total of about 71 river miles through the entire length of Bighorn Canyon and onto the valley floor in the Bighorn Basin of Wyoming. The reservoir inundates an area of valley land several miles wide and extending about 11 miles south from the head of Bighorn Canyon. At elevation 3657.0 the reservoir reaches a point upstream about 15 miles south of the Montana-Wyoming State line. The afterbay dam is located about 2-1/4 miles downstream from Yellowtail Dam (fig. 2).

3. THE BIGHORN RIVER. The principal headwater tributaries of the Bighorn River originate in the Wind River Mountains in west-central Wyoming. These tributaries join to form the Wind River which flows northeastward through the Wind River Basin. The river continues through a low range of mountains by way of the Wind River Canyon to the Bighorn Basin. At the mouth of this canyon, the Wind River becomes the Bighorn River in name and flows northward through the Bighorn Basin along the foot of the Bighorn Mountains. Principal tributaries in this reach of the river are the Greybull and Shoshone Rivers which originate in the Absaroka Mountains to the west. The Bighorn River leaves the basin by way of the deep Bighorn Canyon and enters the Lower Bighorn Basin, then continues northeastward to join the Yellowstone River near Custer, Mont.