Construction on Yellowtail Dam and Power Plant began in May 1961 and was completed in December 1967; construction of the Afterbay Dam was started in April 1964 and was completed in November 1966. Operation of Units 3 and 4 began in August 1966, followed by Unit 2 in October 1966, and Unit 1 in November 1966. Units 1 and 2 (115-kV) are part of the Western Division of Pick-Sloan Missouri Basin Program, and Units 3 and 4 (230-kV) are part of the Eastern Division.
Construction of the Yellowtail Unit, part of the Pick-Sloan Missouri Basin Project Eastern and Western Divisions, was authorized by the Flood Control Act of December 22, 1944, Public Law 534, which approved the general comprehensive plan set forth in Senate document 191, as revised by Senate Document 247, 78 th Congress.
The Yellowtail Unit is a multi-purpose project which provides low cost power generation and makes an important contribution to the flood control, irrigation, and power supply in the Missouri Basin .
Yellowtail Power Plant, with an installed capacity of 287,496 kW, provides low cost power and supplies expanding power needs for residential and commercial use in a wide surrounding area. The widely varying releases from the power plant are regulated by the Yellowtail Afterbay Dam, constructed 2.2 miles downstream. The afterbay, with a capacity of 3140 acre-feet, minimizes downstream fluctuations in the Bighorn River by providing a uniform daily flow, leveling the peaking power discharges from the power plant. Yellowtail Power Plant is located at the downstream toe of the dam on the right abutment. Four 12-foot diameter penstocks embedded in the dam supply water to four 87,500 horsepower, vertical-shaft, Francis-type hydraulic turbines each driving a 62,500-kilowatt generator. Yellowtail's annual power generation over the last 10-years has averaged 974,400,000 kilowatt-hours. Yellowtail Dam, at the mouth of Bighorn Canyon, is a concrete structure rising 525 feet above the rock foundation and impounds flows of the Bighorn River for multi-purpose use. Bighorn Lake is about 72 miles long with a total capacity of 1,328,360 acre feet. Construction on Yellowtail Dam and Power Plant began in May 1961 and was completed in December 1967; construction of the Afterbay Dam was started in April 1964 and was completed in November 1966. Operation of Units 3 and 4 began in August 1966, followed by Unit 2 in October 1966, and Unit 1 in November 1966. Units 1 and 2 (115-kV) are part of the Western Division of Pick-Sloan Missouri Basin Program, and Units 3 and 4 (230-kV) are part of the Eastern Division. FY-2006 activities included: hollow jet valve condition assessment; power penstock fatigue analysis; Afterbay power line installation; triennial maintenance on generator step-up and station service transformers; Afterbay gate actuator replacement; replacement of the Afterbay 48-volt DC batteries; dam face and spillway tunnel inspections; maintenance of the Afterbay 13.8 kV switchyard supply breaker 4322; Afterbay control system upgrade; foundation drain cleaning; dam and power plant elevator modernization; Unit 3 and 4 annual maintenance; and Unit 2 quadrennial maintenance. Major activities planned in the near future include: completion of powerplant station service standby engine-generator addition; hollow-jet valve refurbishment; complete Afterbay gate actuator replacement; install new shoring in the right G&I tunnel; Afterbay control system upgrade and gate automation; and MAXIMO (CARMA) implementation. The Bighorn River Basin experienced its fifth consecutive year of drought conditions, resulting in below average power generation in FY2000 through FY2004. Responsibilities, in addition to the 250,000-kilowatt power plant, also include the operation and maintenance of two dams, two reservoirs, two switchyards, a Government camp and residential housing, two water and sewage systems, the Bighorn Canal headworks, and miles of road and fences.
PMA Service Area
Western Area Power Administration, Rocky Mountain Region and Upper Great Plains Region
Original Nameplate Capacity
Western Electricity Coordinating Council, Rocky Mountain Power Area and Mid-Continent Area Power Pool