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East Bench Unit
Photo of Clark Canyon Dam and Reservoir
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Pick Sloan Missouri Basin Program
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East Bench Unit History (63KB) (pdf)
General Description| Plan| Development| Benefits

General Description

The East Bench Unit of the Pick-Sloan Missouri Basin Program is in southwestern Montana, along the Beaverhead River. The unit provides full irrigation service to 21,800 acres and supplemental irrigation service to 28,000 acres. Principal features include Clark Canyon Dam and Reservoir, Barretts Diversion Dam, East Bench Canal, and a system of laterals and drains.

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Clark Canyon Dam has been constructed at the head of the Beaverhead River to impound surplus flows of Horse Prairie Creek and Red Rock River, which join to form the Beaverhead River. Water stored at Clark Canyon Reservoir will be released into the Beaverhead River for downstream irrigation.

Barretts Diversion Dam, 11 miles below Clark Canyon Dam, diverts water from Beaverhead River to the East Bench Canal and the Canyon Canal. About 17,200 acres of irrigable land on East Bench are served through a system of laterals, and the remaining acreage on the bench is served directly from the East Bench Canal. The Canyon Canal is a private facility which supplies a portion of the formerly irrigated valley land. Drains and waterways have been built to intercept and convey excess water from the benchland to safe disposal points and eventually into the Beaverhead River.

Facility Descriptions
Clark Canyon Dam and Reservoir

Clark Canyon Dam is constructed at the head of the Beaverhead River. The zoned earthfill dam has a structural height of 147.5 feet, a crest length of 2,950 feet, and a volume of 1,970,000 cubic yards of material. The spillway consists of an approach channel, a concrete inlet channel, an ungated concrete crest, a concrete chute, a concrete stilling basin, and an outlet channel. The outlet works consist of an approach channel, a concrete intake structure, a concrete conduit, a gate chamber with four 3- by 6.5-foot high-pressure gates, two of which serve for emergency ahead of the regulating gates; a concrete access shaft and shaft house; and a concrete stilling basin. The outlet channel is shared by the outlet works and spillway.

Clark Canyon Reservoir has a total capacity of 257,152 acre-feet which includes an active capacity of 126,117 acre-feet, a joint use capacity of 50,436 acre-feet, and an exclusive flood control capacity of 79,090 acre-feet as well as dead storage and inactive storage capacities. The reservoir surface area is 5,903 acres.

Barretts Diversion Dam

Barretts Diversion Dam is a concrete, gated structure with embankment wings on the Beaverhead River 8 miles southwest of Dillon, Montana. The spillway capacity is 2,500 cubic feet per second, controlled by one 24- by 10-foot radial gate. The sluiceway is controlled by one 8-by 10-foot radial gate. The headworks capacity of East Bench Canal is 440 cubic feet per second, controlled by two 10- by 8-foot radial gates. The Canyon Canal headworks capacity is 200 cubic feet per second, controlled by one 10- by 8-foot radial gate. One 24-inch-diameter slide gate and one 36- by 30-inch slide gate, in series, control the discharge of water into the existing Rebich Ditch, which has a capacity of 12 cubic feet per second. A fish excluder is upstream of the canal headworks.

East Bench Canal heads at Barretts Diversion Dam and runs in a northeasterly direction for 44.2 miles. Initial capacity of the canal is 440 cubic feet per second.

Lateral and Drainage System

The lateral system has a total length of 61.1 miles. The drainage system has a total length of 16.7 miles.

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Cattle were first raised commercially in 1857 in the Beaverhead Valley and agricultural settlement began as early as 1862. The local market for farm produce and cattle at that time was restricted to miners and Army personnel. The cattle industry became well established by 1879, the date of completion of the first railroad to the vicinity. Before the railroad was built, overland cattle drives were made to Salt Lake City and other points to the south. Severe drought caused a setback in the cattle industry in 1886.

Farming was taken up in earnest in the early 1900`s. Settlement was encouraged, in part, as a result of vigorous campaigns by the railroads. The low rainfall and short growing season have tended to discourage farming operations, especially dry-farming. As a result, livestock ranching is the predominant activity.


The first comprehensive inventory of water resources relating to the unit was made by the War Department during 1928-1933. In the fall of 1938, the Bureau of Reclamation began investigations that ultimately led to a reconnaissance report for the Missouri River and its tributaries. Field work was started in May 1940, and a draft of the report was completed in May 1943. The final report was published in Senate Document 191 (78th Congress, 2d session). The East Bench Unit was included in the plan for the Pick-Sloan Missouri Basin Program (formerly Missouri River Basin Project) presented in this report. Following authorization of the Pick-Sloan Missouri River Basin Program, detailed and semi detailed investigations were begun throughout the basin. Investigations for the East Bench Unit were conducted in 1956.


The unit features were authorized by the Flood Control Acts of 1944 and 1946 (58 Stat. 887 and 60 Stat. 641).


The contract for the construction of Clark Canyon Dam was awarded in September 1961, and the structure was completed in 1964. Other project features were begun in 1961 and completed in 1963.

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Unit irrigation benefits consist of increased production of goods and services and improvements in the general welfare. Direct irrigation benefits consist of the increase in net farm income resulting from the use of the unit`s water

Recreation, Fish & Wildlife

Recreation on Clark Canyon Reservoir and Barrett's Diversion Dam, located on the Beaverhead River south of Dillon, Montana, is managed by the Bureau of Reclamation. Clark Canyon Reservoir is the site of Camp Fortunate, one of the more significant spots along the Lewis and Clark Trail. It was at Camp Fortunate that the Lewis and Clark expedition met the Limhi Shoshoni Tribe, and cached their canoes and a stash of supplies for the return trip. Sacagawea was reunited with her people here.

For specific information on recreation facilities please visit the individual facility pages, located above under Related Facilities.

Flood Control

Controlled flows of the Beaverhead River at its head result in extensive flood control benefits downstream. Clark Canyon Reservoir has an exclusive flood control capacity of 79,090 acre-feet, including a replacement storage capacity of 56,475 acre-feet allocated to assist with the flood and power operations of the Corps of Engineers Missouri River Main Stem System, and a surcharge capacity of 71,827 acre-feet for a total flood control capacity of 150,917 acre-feet. As of 1998, the reservoir has reduced flood damages by about $11.5 million.

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Last updated: Jul 20, 2012