The Salton Sea Transboundary Watershed (USGS Hydrologic Unit 18100200) is the Priority Watershed in the Colorado River Basin Region. It encompasses one-third of the region (about 8,360 square miles) and contains five (out of a total of six) of the Region's impaired surface waterbodies. Most of the watershed is in Imperial County. The watershed has been identified as a Category I (impaired) Watershed under the 1997 California Unified Watershed Assessment (UWA). The California UWA was developed and implemented in response to the Clean Water Action Plan released by President William Clinton and Vice-President Albert Gore on February 19, 1998. The UWA was a collaborative process between the State and the United States Environmental Protection Agency (USEPA) and was developed to guide allocation of new federal resources for watershed protection.
Major waterbodies in the watershed include the Salton Sea, the New River, the Alamo River, and the Imperial Valley Agricultural Drains and the Coachella Valley Stormwater Channel.
The New River originates in Mexico. It flows approximately 20 miles through the City of Mexicali, Mexico, crosses the International Boundary, continues through the City of Calexico in the United States, and travels northward about 60 miles until it empties into the Salton Sea. Its flow at the International Boundary is about 150 to 200 cubic feet per second (cfs). (108,400 to 145,000 acre-feet per year (AFY)). The New River carries urban runoff, untreated and partially treated municipal wastes, untreated and partially treated industrial wastes, and agricultural runoff from the Mexicali Valley, Mexico across the International Boundary into the United States. In addition, the River carries urban runoff, agricultural runoff, treated industrial wastes, and treated, disinfected and non-disinfected domestic wastes from the Imperial Valley. It also carries approximately 6 to 11 cfs (4,350 to 7,970 AFY) of treated wastewater from point sources in Imperial Valley. The New River flow at the Salton Sea is about 600 cfs (430,000 AFY).
The Alamo River originates approximately two miles south of the International Boundary with Mexico, and flows northward across the border for about 50 miles until it empties into the Salton Sea. The Alamo River is dominated by agricultural return flows from Imperial Valley. It also carries approximately 15 to 27 cfs (10,867 to 19,200 AFY) of treated wastewater from point sources in Imperial Valley. Its flow at the International Boundary is three to five cfs (2100 to 3620 AFY), whereas at its delta with the Salton Sea is about 800 to 1000 cfs (600,000 to 800,000 AFY).
Imperial Valley Agricultural Drains
The Ag Drain system comprises over 1,450 miles of surface drains, which discharge into the Alamo and New Rivers and the Salton Sea [2.11]. The Ag Drains primarily carry agricultural runoff from the Imperial Valley. Agricultural discharges in the Imperial Valley average about 830,000 acre-feet/year. Of this amount, approximately 36 percent is tailwater, 33 percent is seepage, and 30 percent is tilewater. The resulting mix of tailwater, tilewater, and seepage contains pesticides, nutrients, selenium, and silt in amounts that violate water quality standards.
Coachella Valley Stormwater Channel
The Coachella Valley Stormwater Channel flows southward though the Coachella Valley and discharges to the Salton Sea. The Coachella Valley Stormwater Channel flows include stormwater, agricultural discharges, and treated municipal and aquaculture wastewater.