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Sun River Project
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Sun River Project History (711 KB) (pdf)
General Description| Plan| Development| Benefits
General Description

The Sun River Project is composed of the Greenfields and Fort Shaw Divisions in central Montana, west of the city of Great Falls. Principal features are Gibson Dam and Reservoir, Willow Creek Dam and Reservoir, Pishkun Dikes and Reservoir, Sun River Diversion Dam, Fort Shaw Diversion Dam, and nine canal systems.

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The project uses the waters of the Sun River and its tributaries, stored and regulated by Gibson, Pishkun, and Willow Creek Reservoirs, for irrigating about 93,000 acres of land lying along the Sun River. Water stored in Gibson Reservoir is released into the river for diversion downstream into the Pishkun Supply Canal, Willow Creek Feeder Canal, or the Fort Shaw Canal. The Pishkun Supply Canal, heading at Sun River Diversion Dam, conveys water to Pishkun or Willow Creek Reservoirs through the Willow Creek Feeder Canal, which stems from the Pishkun Supply Canal and empties into a natural channel to the reservoir. Water released from Pishkun Reservoir enters the Sun River Slope Canal, which branches into several main canals for distribution to about 81,000 acres in the Greenfields Division. Storage in Willow Creek Reservoir is returned to the Sun River. Water for 10,150 acres in the Fort Shaw Division is diverted directly from the river into the Fort Shaw Canal.

Facility Descriptions

Gibson Dam, a major structure of the project, is on the Sun River, 70 miles west of Great Falls. It is a concrete arch dam 199 feet high and contains 167,500 cubic yards of concrete. A drop inlet spillway located just upstream from the north end of the dam has a capacity of 30,000 cubic feet per second. The entrance to the shaft and 29.5-foot-diameter tunnel is controlled by six 34- by 12-foot radial gates. Outlet structures for the dam are two 72-inch-diameter semisteel-lined conduits through the base of the dam, each equipped with a 60-inch jet-flow valve. The maximum capacity of the outlets is 3,050 cubic feet per second. Gibson Reservoir has a total capacity of 99,100 acre-feet.

Pishkun Reservoir is an offstream storage reservoir, about 15 miles northeast of Gibson Dam, and has a capacity of 46,700 acre-feet. The reservoir is formed by eight earthfill dikes with heights ranging from 10 to 50 feet and an overall length of 9,050 feet. The outlet for the reservoir is a 12-foot-diameter, horseshoe shaped, concrete conduit through Dike No. 4. The downstream canel has a maximum capacity of 1,600 cubic feet per second.Capacity of outlet works is 1850 cfs at elevation 4371.2, there is no spillway for the reservoir.

Willow Creek Dam is an earthfill structure on Willow Creek about 15 miles southeast of Gibson Dam. In addition to storing water from Willow Creek, the reservoir is fed from the Sun River through the Willow Creek Feeder Canal. The structure is 93 feet high, has a crest length of 650 feet, and contains 275,000 cubic yards of material. An open spillway channel 700 feet wide at the ground surface has a capacity of 10,000 cubic feet per second. The outlet works tunnel runs through the right abutment. The reservoir has a capacity of 32,300 acre-feet of water.

The Sun River Diversion Dam, located 3 miles downstream from Gibson Dam, is a concrete arch structure with a structural height of 132 feet and a crest length of 261 feet. The dam contains 6,500 cubic yards of concrete. It is equipped with an overflow crest for a spillway. The outlet works tunnel runs through the canal wall of the right abutment. The capacity of the outlet works feeding the Pishkun Supply Canal is 1,400 cubic feet per second.

Pishkun Supply Canal

The Pishkun Supply Canal extends 12.1 miles from the Sun River Diversion Dam to the Pishkun Reservoir. A few hundred feet below the diversion dam, the canal crosses Sun River through a 1,400-cubic-foot-per-second capacity monolithic siphon 700 feet long. The canal flows through two tunnels, 980 feet and 2,280 feet long, and numerous drain and control structures after it crosses Sun River.

Willow Creek Feeder Canal

Stemming from Pishkun Supply Canal a short distance below the river diversion, the Willow Creek Feeder Canal has a maximum capacity of 500 cubic feet per second and is 7.3 miles long to the point where it enters a natural channel to Willow Creek Reservoir.

The Fort Shaw Canal inlet is on the main channel of the Sun River immediately upstream from the Fort Shaw Diversion Dam. The flow into the canal is regulated by a concrete headworks. The canal has a capacity of 225 cubic feet per second and is 12.1 miles long. It includes a monolithic siphon over Simms Creek and several drops and control structures. This canal supplies water for the entire Fort Shaw Division through approximately 85 miles of laterals.

Sun River Slope Canal System

This system furnishes water for the Greenfields Division. Sun River Slope and Spring Valley Canals combined extend 39.2 miles from Pishkun Reservoir to a drop at Early settlers developed lands between the Sun and Fairfield. The diversion capacity of the Sun River Canal is 1,600 cubic feet per second. Three major drops and various control structures and lateral turnouts are a part of the canals. Greenfields Main Canal heads at the end of Spring Valley Canal and extends 27.4 miles northeast. It has an initial capacity of 1,200 cubic feet per second at its terminus. Greenfields South Canal is supplied by the Greenfields Main Canal at a point about 2 miles below the start of the main canal. The initial capacity is 425 cubic feet per second and the length is 14.6 miles. Mill Coulee Canal is supplied from the Greenfields South Canal. The initial capacity is 200 cubic feet per second and the length is 10.7 miles.

Operating Agencies

The Fort Shaw Division is operated by the Fort Shaw Irrigation District. The Greenfields Division is operated by the Greenfields Irrigation District.

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In 1870, Colonel John Gibbon marched the Seventh U.S. Infantry into the Sun River Valley and took station at Fort Shaw, which had been established to protect the freighters, travelers, and miners against the Indians.

Gold had been discovered in Alder Gulch, and miners, adventurers, and trappers were coming into the new country. Boats ran up the Missouri River to Fort Benton, and from there supplies were freighted into the placer diggings.

Soon after Colonel Gibbon's command settled down to routine garrison duty, company gardens were established. The post adjutant was directed to survey and build a ditch from Sun River to irrigate the post gardens located on the bench southwest of the fort. Thus began the first irrigation district, then considered a marvel in the engineering field and thought to be the largest ditch in the country. The capacity was about 15 cubic feet per second.

Early settlers developed lands between the Sun and Teton Rivers for grazing. Studies were made by the Geological Survey during the latter part of the 19th century. Local interests formed several corporations to develop irrigation within the area, and some progress was made with private irrigation.


In 1902, a number of settlers filed homestead and desert claims on Greenfield Bench and organized the Kilraven Cooperative Canal Company to irrigate the lands. They partially excavated 4 of the 14 miles of canal necessary to deliver the water; then, in 1903, they abandoned the work. On October 17, 1903, the newly organized Reclamation Service withdrew from entry all the lands pertaining to the cooperative company`s system to incorporate them into the Sun River Project. The settlers holding desert entries on these lands claimed an extension of time to make proof on their entries, based on the premise that withdrawal of these lands would preclude other settlers from taking up land and helping with the construction of the cooperative company`s system. Extensions of time were granted in most cases from year to year. In the winter of 1905-1906, the people of Choteau, Great Falls, and vicinity, realizing the importance of the Sun River Project and sent a joint committee to Washington to urge the Director and the Secretary to approve the project and make an early allotment of funds for beginning the work. When the bill opening the Fort Shaw lands to settlement was passed by the Congress, these lands were selected as the first unit of the Sun River Project.


The project was authorized by the Secretary of the Interior on February 26, 1906, in accordance with the act Of June 17, 1902 (Ch. 1093, 32 Stat. 388, 43 U.S.C. 391).


Construction work on the Fort Shaw Division began in May 1907, and the bulk of the work was completed by July 1908. The first water was delivered to the division lands in 1909. Construction of the Greenfields Division began in 1913; the first water was made available in 1920. The main storage dam, Gibson, was constructed during 1926-1929. Since that time, construction work has been of a minor nature.

Rehabilitation and Betterment

Modification of Gibson Dam to ensure safe passage of an inflow design flood has been completed. The modifications allow the dam to be safely overtopped by a water depth of 12 feet. A rehabilitation and betterment program on the Greenfields Division irrigation facilities is scheduled to be completed in 1985. This program includes lining portions of the main canals and laterals; replacement of several open laterals with buried pipe; installation of automatic and telemetric equipment for control of water regulating facilities at Gibson and Pishkun Dams and at storage points on the irrigation system; and repairing, updating, and replacing of various structures and measuring devices.

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The land is divided into farms. Principal crops are malting barley, alfalfa hay, grass hay, and irrigated pasture.


These reservoirs provide excellent recreational opportunities. Gibson Reservoir offers fair fishing for rainbow, cutthroat, and brown trout, Pishkun Reservoir offers major game fishing for kokanee salmon, northern pike, and rainbow trout, and Willow Creek Reservoir offers fishing for rainbow trout and kokanee salmon. Swimming, camping, and picnicking are enjoyed during the summer months.

Flood Control

Gibson Reservoir has provided an accumulated $3,044,000 in flood control benefits between 1950 and 999.

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Last updated: Dec 26, 2012