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Watershed Characteristics

A. General watershed characteristics

Watershed Area: 198,400 acres
Project Area: 45,238 acres
Critical Area: 28,159 acres

The Rock Creek RCWP project is located in south central Idaho and covers 45,238 acres within a 198,400-acre watershed (Map). Rock Creek is located in Cassia and Twin Falls counties within the Snake River Basin/High Desert ecoregion. Its headwaters are in the Sawtooth National Forest in western Cassia County. Rock Creek flows northwest approximately 42 miles through Twin Falls County to the Snake River confluence north of the city of Twin Falls. About 25 miles of Rock Creek were involved in the RCWP project area (Maret et al., 1991).

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Map of Rock Creek Idaho Watershed, Subwatersheds, and Monitoring Stations

The primary agricultural activities in the project area are irrigated pasture and cropland and rangeland. Irrigation is required for crop production because annual rainfall is low (9 inches per year). Irrigation water diverted from the Snake River is delivered to the farms through a network of canals and laterals (drains or ditches). Irrigation return flows eventually empty into Rock Creek, which discharges into the Snake River.


B. Sub-watersheds within the Project Area

The total land area in the project amounts to 45,238 acres lying in 10 sub-watersheds (Map 2).


C. Geology

The watershed is underlain by limestone, quartzite, shale, sandstone, granite, and metamorphosed sediments. The lower watershed (most of the farmed area) is underlain by deep layers of fractured basalt. This formation yields large supplies of ground water to the northeast.

Slopes range from nearly level to very steep on hill and mountain sides. The slope on the irrigated crop lands is predominately 0 to 2% with a range from 0 to 8%.

Elevations within the watershed range from 8,000 feet at the headwaters to 3,000 feet at the confluence with the Snake River (Maret et al., 1991).


D. Soils

Soils in the lower watershed are generally thin, light-colored, medium-textured surface soils and very strongly calcareous, silty subsoils. These highly productive, but highly erosive, soils vary in total depth and are underlain by fractured basalt. The deeper soils (40-60 inches to bedrock) are termed Portneuf silt loam, and the more shallow soils (1-20 inches to bedrock -- commonly located on old shield volcanoes which form much of the topography) are known as Trevino silt loam. Much of the area of the Rock Creek lower watershed was covered with 20-70 inches of loess soils before furrow irrigation (Maret et al., 1991).

The soil is suited to irrigated agricultural production because of high productivity and high infiltration rates. The soil is also highly erodible. The major crops grown in the area include dry commercial beans, wheat, sugarbeets, corn, and alfalfa. The row crops - beans, sugarbeets, and corn - present the greatest erosion hazard. Gravity irrigation is the prevalent irrigation method. Erosion in the irrigation furrows that traverse the fields creates the sediment and phosphorus problem in irrigation runoff (Walker et al., 1986).


E. Climate

The climate in the project area is semi-arid with moderately cold winters and warm summers. Annual precipitation averages about nine inches. Except in unusual years, precipitation runoff appears to be a minor factor in the annual sediment loading to the stream (Maret et al., 1991).

Average temperatures range from a low of 26-28 o F in January to a high of 72-74 o F in July. Wind usually comes out of the west. The area has about a 120-day frost-free growing season (Maret et al., 1991).


F. Land use

The Rock Creek watershed covers a total of 198,400 acres in the following categories (Maret et al., 1991):

Irrigated pasture and cropland - 51,900 acres (containing 28,159 designated critical acres) Rangeland - 109,000 acres
Woodland - 25,400 acres
Other uses (urban, roads, farmsteads) - 12,000 acres

Within the watershed lie approximately 350 farm units (Maret et al., 1991).

The primary crops produced are dry beans, dry peas, alfalfa, sugar beets, corn, small grain, potatoes, and pasture. Because of low annual precipitation, all crops are irrigated (Maret et al., 1991).

Major animal operations include 34 dairy farms (6,800 total animals/9,520 total animal units) and 21 beef cattle farms (6,300 total animals/6,300 total animal units) (Gale et al., 1993).