One of the most significant features of the watershed is the Sun River Project, a large irrigation project which includes three storage reservoirs, two diversion dams, 131 miles of main canals, 562 miles of smaller side canals, and 265 miles of drain canals. In 1907, the U.S. Reclamation Service approved construction of the project that includes two different divisions: Greenfields and Fort Shaw, each with its own irrigation district. The Sun River Project totals 91,011 acres in size. Most of it lies north of the river in the 81,000-acre Greenfields Division, while the 10,000-acre Fort Shaw Division is south of the river. However, since Greenfields sometimes supplies an additional 3,169 acres with surplus (contract) water, the total area irrigated by the project is up to 94,000 acres.
The Sun River Project includes Gibson Dam and Reservoir, Diversion Dam, Pishkun Reservoir, Willow Creek Reservoir, Fort Shaw Diversion Dam, and nine canal systems. The principal crops are malting barley, alfalfa hay, grass hay, and irrigated pasture. By the mid-1990s, the Anheuser-Busch Company contracted approximately 60 percent of Greenfields' 81,000 acres for malting barley, which is used to make beer. View the schematic diagram to better understand the complex irrigation systems on the Sun River.
The length of the irrigation season is about 150 days and peak irrigation water demand occurs in July. The growing season averages 128 days between the last frost in May and the first freeze which usually occurs in mid-September. On average, this area receives about nine inches of precipitation during the irrigation season between April and September.
Gibson Dam is located 70 miles west of Great Falls and was built in 1926-1929. It is the main storage dam in the Sun River watershed. Gibson Dam is a large concrete structure 199 feet high and 960 feet long at the top. The spillway can pass a flow of 30,000 cubic feet per second of water. The outlet structures for the dam are two 6 foot-diameter pipes through the base of the dam with a maximum outflow capacity of 3,050 cubic feet per second. Gibson Dam was originally designed to produce power using two 6-foot diameter pipes through the south portion of the dam, but they were blocked after construction because it was believed the reservoir could not store enough water to provide a continuous supply of electrical power. However, there is currently a proposal to install turbines in the dam to generate electricity.
Gibson Reservoir can store about 96,000 acre-feet of water at normal full pool. The reservoir covers 1,296 surface acres when full and has 15 miles of steep shoreline in a forested mountainous area. The drainage area upstream from the dam is 575 square miles.
Gibson Dam is used to store water for irrigation. Consequently, its water level changes dramatically during a typical year. In most years the reservoir is completely filled in June by capturing snowmelt runoff. Then most of the water is released during the summer months. In an average year, the water level is about 75 feet below full pool by the end of the irrigation season in September. The water level in the reservoir builds slowly through the fall and winter and then rises dramatically during spring runoff in preparation for the next irrigation season.
Diversion Dam is located about 3.5 miles below Gibson Dam and was finished in 1915, 11 years before construction of Gibson Dam was even started. This dam was built on top of a natural waterfall and it creates a small reservoir known as Diversion Lake. The dam was built to divert water into the main water supply canal that provides water to the Greenfields Irrigation District and Willow Creek Reservoir. Diversion Dam is a concrete structure 132 feet high and 261 feet long at the top. Control gates near the south side of the dam allow water to enter a tunnel that lies underneath the main roadway. This tunnel is the beginning of the Pishkun Supply Canal. The tunnel is about Â¼-mile long and has a flow capacity of 1,400 cubic feet per second.
Pishkun Supply Canal extends 12.1 miles from Diversion Dam to Pishkun Reservoir and can carry up to 1,600 cubic feet per second of water. About Â½ mile below Diversion Dam, the canal crosses under the Sun River in a massive 700-foot long siphon tube. The Pishkun Supply Canal passes through two other tunnels on its way to Pishkun Reservoir. One tunnel is 980 feet long and the other is 2,280 feet long. The canal ends at Pishkun Reservoir and ultimately supplies irrigation water to the Greenfields Irrigation District farmland near Fairfield. The Pishkun Supply Canal was built between 1913-1920 and was enlarged during 1935-1938.
Pishkun Reservoir is the offstream storage reservoir for the Greenfields Division and is located approximately 15 miles northeast of Gibson Dam. The reservoir's total capacity is 46,700 acre-feet, but the useable capacity is 32,050 acre-feet. It covers 1,550 surface acres and has 13 miles of shoreline when full. The reservoir was constructed by building eight earthfill dikes 9,050 feet long and 10 to 50 feet high. It was originally constructed from 1914-1917 and was enlarged between 1930-1939. The outlet canal (Sun River Slope Canal) has a maximum capacity of 1,600 cubic feet per second, while the incoming canal (Pishkun Supply Canal) has a maximum capacity of only 1,400 cubic feet per second.
The water level in Pishkun Reservoir varies during the year. The reservoir is filled to within seven feet of full pool in the fall, after irrigation season, to protect fish screens on the outlet from ice damage. The reservoir is completely filled during spring runoff and is slowly drawn down to 10 feet below full pool during the summer irrigation season.
An extensive system of canals deliver water from Gibson and Pishkun Reservoirs to lands irrigated by the Greenfields Division of the Sun River Project. A major 39-mile long canal delivers water from Pishkun Reservoir to a concrete drop structure (a small waterfall built to control the slope of the canal) near the town of Fairfield. There are three major drop structures along the canal and several "turnouts", where water is directed into smaller ditches for irrigation. The upper 25 miles of this canal is referred to as the Sun River Slope Canal (capacity 1,600 cubic feet per second) and the lower 14 miles is known as the Spring Valley Canal (capacity 1,200 cubic feet per second).
The Greenfields Main Canal starts at the drop structure near Fairfield (the end of the Spring Valley Canal) and extends more than 25 miles to the northeast. The capacity of the Greenfields Main Canal is 1,200 cubic feet per second at the beginning but is reduced to 10 cubic feet per second at its end. The Greenfields South Canal branches off about two miles below the start of the Main Canal and heads east. The South Canal is about 17 miles long and has an initial capacity of 425 cubic feet per second.
The last of the major delivery canals on the Greenfields Division is the Mill Coulee Canal, which feeds from the Greenfields South Canal and travels east and south. It is 10.7 miles long with a capacity of 200 cubic feet per second. Construction of the Greenfields Division canal system began in 1913 and the first water was made available in 1920. Construction of the lateral canal distribution system was completed in 1936. Drainage canal systems installed under the supervision of the U.S. Bureau of Reclamation were completed in 1958.
The Willow Creek Feeder Canal branches off the Pishkun Supply Canal about Â½-mile from Diversion Dam on the Sun River. The feeder canal extends southeast about seven miles to a point where it empties into a small tributary of Willow Creek. Water is transported in this tributary for another seven miles before joining Willow Creek about a mile above Willow Creek Reservoir. The feeder canal has a maximum capacity of 500 cubic feet per second and has had major erosion problems. As a consequence, flows are generally limited to 125 cubic feet per second or less.
The earthfill Willow Creek Dam is located on Willow Creek approximately 15 miles southeast of Gibson Dam and five miles northwest of the town of Augusta. Willow Creek Reservoir has gentle sloping banks and is surrounded by native prairie near the foothills of the Rocky Mountain Front. The reservoir stores water from Willow Creek and also receives water from the Sun River Diversion Dam via the Willow Creek Feeder Canal.
Willow Creek Dam is 93 feet high and about 650 feet long at the top. The dam was built from 1907-1911 and it first stored water in 1916. The dam was raised by two feet in 1917 and by another 12 feet in 1941. It has an open spillway that can pass a flow of 10,000 cubic feet per second. The outlet from the dam can handle a flow up to 300 cubic feet per second. The outlet flows for about 2.5 miles to where it joins the Sun River across from the Floweree Canal, which supplies irrigation water to the Broken O Ranch. The reservoir covers 1,530 acres, and has 11 miles of shoreline and a useable capacity of 32,400 acre-feet of water. A major repair project was done in 1996 to fill a sinkhole in the dam.
If available, water is diverted to Willow Creek Reservoir in the fall and/or the spring. Water is released during the summer to supply downstream irrigation needs. The reservoir water level drops about 15 feet on average during irrigation season.
The diversion for the Fort Shaw Irrigation District is located on the Sun River about seven miles northwest of the town of Simms and about 43 miles downstream from Gibson Dam. The Fort Shaw Diversion Dam is a 400-foot wide rockfill structure. It contains about 3,000 cubic yards of rock and has a drop of nine feet. The concrete inlet has four 4-foot x 4-foot cast-iron automated gates that control the flow. The canal is 12.1 miles long, has a capacity of 225 cubic feet per second, and has a 1,565-foot-long siphon over Simms Creek. The system has about 85 miles of lateral canals that supply water to about 10,000 irrigated acres.
Construction on the Fort Shaw Division began in May 1907, and the bulk of the work was completed by July 1908. Construction of the siphon began in June 1907 and took a year to complete. The first water was delivered in 1909 and the canal was rehabilitated and improved in the mid-1980s.