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Yellowtail Unit
Photo of Yellowtail Dam and Reservoir
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Pick Sloan Missouri Basin Program
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Yellowtail Unit History (55 KB) (pdf)
General Description| Plan| Development| Benefits

General Description

The Yellowtail Unit in south-central Montana is a multipurpose development providing irrigation water, flood control, and power generation. Facilities consist of Yellowtail Dam and Bighorn Lake on the Bighorn River, Yellowtail Powerplant at the toe of the dam, Yellowtail Afterbay Dam a short distance downstream, and related structures.

The Crow Indian Reservation, spreading over about 3,500 square miles, encompasses the damsite, a portion of the reservoir area, and about two-thirds of the area of the potential Hardin Unit. The Hardin Unit is proposed to use Yellowtail storage for irrigation.

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Yellowtail Dam, at the mouth of Bighorn Canyon, impounds flows of the Bighorn River for multipurpose use. Bighorn Lake is about 72 miles long at maximum water surface elevation, 66 miles long at the top of joint-use storage, and extends into the Bighorn Basin of Wyoming. The reservoir is confined in the canyon for most of its length. The widely varying releases from the powerplant are regulated by Yellowtail Afterbay Dam, constructed 2.2 miles downstream from Yellowtail Dam. The afterbay minimizes downstream fluctuations in the Bighorn River.

Facility Descriptions
Hardin Unit - Potential

Irrigation storage in Bighorn Lake would be used principally by the potential Hardin Unit, which would provide for irrigation of 42,600 acres of land that require full irrigation service and 950 acres that require a supplemental water supply.

The 862 cubic feet per second of water needed for the Hardin Unit would be diverted through the left abutment of Yellowtail Dam into the Grapevine Tunnel, which would carry the water 0.9 mile to the Grapevine Penstock. The penstock would deliver 70 cubic feet per second to the Fort Smith Canal and 792 cubic feet per second to the Campbell Pumping Plant.

The tunnel and penstock would be designed to operate under pressure to take advantage of reservoir head on hydraulic turbines that would be used for the pumps. The pumping plant, planned to be of the outdoor type, would contain two units capable of lifting 239 cubic feet per second of water at maximum head to the Campbell Canal. Turbine discharges from the pumping plant would go directly into the Hardin Canal, and all of the 553 cubic feet per second discharged would be needed to irrigate land on Hardin Bench.

The irrigation system would involve 76 miles of canals and 100 miles of laterals, ranging in capacity from 553 to 70 cubic feet per second.

Yellowtail Dam and Bighorn Lake

Yellowtail Dam is a concrete thin-arch structure with a structural height of 525 feet, a crest length of 1,480 feet, and a volume of 1,545,664 cubic yards of concrete.

The spillway, in the left abutment of the dam, consists of an unlined inlet channel, an intake structure controlled by two 25- by 64.4-foot radial gates, a concrete-lined tunnel transition, a concrete-lined tunnel which ranges in diameter from 40.5 to 32 feet, and a stilling basin. Discharge capacity for the spillway is 92,000 cubic feet per second at maximum water surface elevation 3660.0. The outlet works consists of an irrigation outlet and an evacuation outlet, each with 84-inch-diameter outlet pipes and controlled by 84-inch ring-follower gates. Both outlets discharge into a stilling basin at the right of the powerplant at the toe of the dam.

A 9.5-foot-diameter tunnel intake is included in the left abutment of the dam for the Grapevine Tunnel, controlled by a 7.86- by 15.03-foot fixed-wheel gate. The complete Grapevine Tunnel is to be constructed as a feature of the Hardin Unit.

Bighorn Lake has a total capacity of 1,328,360 acre-feet, including the exclusive flood control capacity of 258,331 acre-feet, an active conservation capacity of 336,103 acre-feet, and an inactive/dead storage capacity of 477,576 acre-feet. At an elevation of 3657.0 ft, the reservoir has a surface area of 17,300 acres.

Yellowtail Powerplant

Four 12-foot-diameter penstocks through the dam supply water to four 87,500 horsepower, Francis-type hydraulic turbines, each driving a 62,500-kilowatt generator. The powerplant structure is at the toe of Yellowtail Dam.

Yellowtail Afterbay Dam

The afterbay pool is formed by Yellowtail Afterbay Dam, constructed on the Bighorn River 2.2 miles downstream from Yellowtail Dam. The dam is a 72-foot-high concrete gravity structure with embankment wings. The afterbay dam has a crest length of 1,360 feet and consists of 21,600 cubic yards of concrete and 162,000 cubic yards of earth material and riprap.

Yellowtail Afterbay Reservoir has a capacity of 3,140 acre-feet. Discharges are used to provide a uniform daily flow into Bighorn River, leveling the peaking power discharges from Yellowtail Powerplant.

The spillway consists of an ogee crest controlled by five 30 by 13.5-foot radial gates. The sluiceway is controlled by three 120- by 96-inch cast-iron slide gates with automatic controls.

Headworks for the existing Bighorn Canal, adjacent to the sluiceway, are controlled by two 120- by 96-inch cast-iron slide gates with automatic controls. Diversion capacity for the canal handworks is 750 cubic feet per second. Construction of the afterbay dam required removal of the existing handworks and the Bighorn Indian Diversion Dam midway between Yellowtail Dam and Yellowtail Afterbay Dam.

Operating Agencies

Operations are the responsibility of the Bureau of Reclamation; the unit was transferred to operation and maintenance on August 1, 1967.

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The early history of the area is closely identified with that of the Crow Indian Reservation, which, as established under the ratified treaty of 1868, had an area of about 38 million acres. By an act of Congress, the reservation was defined as a triangular area lying between the 107th meridian, the Yellowstone River, and the southern boundary of Montana.

The Indians were primarily interested in hunting, trapping, and fishing. Encouraged by overzealous fur buyers, the game resources were rapidly diminished by hunters, and the Indians gradually retreated toward the mountains, leaving large areas of both grazing and arable land available for occupation and use. Because of the retreat, the size of the reservation was successively reduced in 1880, 1890, and 1904.

During 1900-1910, many farmers came into the area under the Homestead Act of 1862, which encouraged the establishment of 160-acre farms.

About 1900, the Indian Service, foreseeing that fish and wildlife resources would become inadequate for the needs of the Crow Indian Reservation, began the establishment of farmland allotments among the Indians and instituted a program designed to encourage their interest in agriculture. Pending such use, much agricultural land was leased to settlers, and by 1910 practically all desirable lands were included in operating farms or ranches. Many Indian allotments were sold as they became patented. This practice was greatly accelerated after passage of the Crow Act in 1920, which provided for allotment of all reservation lands except mountainous areas and certain specified withholdings. The reservation now includes about 2,055,600 acres of the Indian land and 179,200 acres of land owned by others.


First investigations by the Reclamation Service were made during 1903-1905 to study feasibility of making a gravity diversion near the Yellowtail damsite to a canal system along the west side of the river. Investigations started in 1913, and a detailed report dated October 24, 1917, recommended construction of a gravity arch dam of rubble concrete about 480 feet high, a powerplant with 500 miles of transmission lines, and 62 miles of highline canals to irrigate 60,000 acres of benchland. Bureau of Reclamation investigations of the Yellowtail Dam and Bighorn Lake sites in 1939-42 were later summarized and published in Senate Document 191, 78th Congress, 2nd session, in which a low dam at the Yellowtail site and one at the Kane site were proposed to be operated in conjunction with a total installed capacity of 105,000 kilowatts.

The definite plan report, dated January 1950, was approved by the Commissioner of Reclamation on November 10, 1950. The report substantiated conclusions that a single high dam at the Yellowtail site would be more economical than the two smaller dams.


Construction of Yellowtail Dam 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 and House Document 475, as revised and coordinated by Senate Document 247, 78th Congress, 2nd session.)


Construction began on Yellowtail Dam and Powerplant in May 1961 and was completed in December 1967; construction of Yellowtail Afterbay Dam was started in April 1964 and completed in November 1966. Power units 3 and 4 began operation in August 1966, followed by unit 2 in October 1966 and unit I in November 1966.

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Bighorn Lake is the key feature of the entire irrigation development in the Lower Bighorn Basin, but the Hardin Unit will distribute water to basin lands. The area known locally as the Hardin Bench lies 100 feet or more above the Bighorn River and will be served by gravity. About 12,700 acres located on higher benches will be served by a pumping plant about 1 mile below Yellowtail Dam. The principal cash crops are anticipated to be sugar beets, malting barley, and beans. Hay, small grains, and corn will probably be grown in appropriate quantities to supply feed for livestock and to provide a suitable rotation for soil management.

Recreation, Fish & Wildlife

Bighorn Lake, because of its outstanding scenic character and excellent fishing, offers many recreation opportunities for southern Montana and northern Wyoming residents, as well as for vacationing tourists. Bighorn Canyon National Recreation Area is a lesser known treasure waiting to be discovered. It boasts breath-taking scenery, countless varieties of wildlife, and abundant recreational opportunities, such as boating, fishing, ice fishing, camping, and hiking.

Hydoelectric Power

Electric energy produced at the 250,000-kilowatt Yellowtail Powerplant is marketed through the transmission facilities of the Western Area Power Administration, Department of Energy.  Four 12-foot-diameter penstocks through the dam supply water to four 87,500 horsepower, Frais-type hydraulic turbines, each driving a 62,500-kilowatt generator. The powerplant structure is at the toe of Yellowtail Dam.

Flood Control

Controlled flows of the Bighorn River result in extensive flood control benefits downstream. Bighorn Lake has an exclusive flood control capacity of 258,331 acre-feet and a surcharge capacity of 52,829 acre-feet for a total flood capacity of 311,160 acre-feet and, as of 1998, it is estimated that the reservoir has reduced flood damages by $90.4 million since its construction in 1966.

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Last updated: Jul 20, 2012