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.