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ENVIRONMENTAL ASSESSMENT SUMMARY PROJECT AREA The Big Sandy River Unit of the Colorado River Basin Salinity Control Program is predominantly an agricultural area containing about 1,227,500 acres. This area, consisting of portions of three counties-- Fremont, Sublette, and Sweetwater--is primarily utilized by livestock, both cattle and sheep. Located within the approximate center of the area is the Eden Valley Irrigation Project, the subject of this salinity study by the U.S. Department of Agriculture (USDA). Irrigation was introduced in the valley in 1886 when settlers were issued permits to divert water from the Big Sandy River. Due to project actions beginning with the Eden Irrigation and Land Company in the early 1900's, the Rock Springs Land and Water Company and the Wyoming Land and Water Company in the 1930's, and the existing Eden Valley Irrigation and Drainage District, there has been an increase of irrigated land in Eden Valley to the present 15,700 acres of the 18,370 water-righted acres. During 1962-1973, the Eden Valley irrigation project served an average of 84 farms, averaging 173 acres per farm. Total farm population is 279 people; 79 of the 84 operators currently are part-time farmers. STUDY AUTHORITY Under the Colorado River Basin Salinity Control Act (Public Law 93-320), the USDA's Soil Conservation Service (SCS) had the responsibility for studying the effects of on-farm irrigation improvements to reduce salinity loadings to the Colorado River. Under the same authority, Interior's U.S. Bureau of Reclamation (BR) is currently studying off-farm conveyance and storage system improvements and their effects on salinity contributions to the Colorado River. In carrying out this study, basic data was gathered from the BR, United States Geological Survey, Bureau of Land Management (BLM), Wyoming State Engineer's Office, Water Resource Research Institute of Wyoming, and the Eden Valley Irrigation and Drainage District. SCS conducted special surveys and investigations to obtain other necessary data for this study. These included discussions with farmers to inventory present irrigation practices, stream and drainage flow measurements, and compiling data on the soils and geology of Eden Valley. PROBLEMS Using this basic data and a data base period of 1960 through 1977, it was determined that the Big Sandy River contributed about 149,180 tons of salt annually into the Green River and the Colorado River system. Most of the salt is leached from the underlying Bridger and Green River marine shale formations and carried to the Big Sandy River by underground aquifers. Excessive irrigation on highly permeable soils and irrigation delivery system seepage provide water for deep OJ1L5~ 1