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. . CHAPTER tII OVERVIEW OF COLORADO RIVER BASIN SALINITY PROBLEM Basinwide Salinity The Colorado River (main stem) extends for 1,400 miles from its origin in Colorado until it reaches the Mexican border near Yuma,Arizona. The Colorado and its tributaries drain a primarily arid and semiarid river basin of 242,000 square miles in the south- western United States. The salt content of the river increases .downstream because its tributary streams flow over soils and rocks which, because of the arid climate, are less intensively weathered than soils in more humid regions, and because of flow into the Colorado River system from highly saline springs. Figure 3-1 ill us- trates 17 gaging station locations along the river and its tributaries and Table 3-1 shows 1974 salinity levels at these stations. The historic salinity of the Colorado River can be accounted for by such natural sources. Even today, nearly 65 percent of the measured salt load at Hoover Dam is from natural diffuse and point 1 sources. The diffuse sources include runoff of rain and snowmelt across saline lands. Point sources which contribute substantial surface or groundwater inflow of salts include Blue Springs, LaVerkin Springs, Littlefield Springs, Glenwood/Dotsero Springs, and Paradox Valley (see Figure 3-1). The saltiness of the river is naturally lU.S. Environmental Protection Agency, The Mineral Quality Problem in the Colorado River Basin, Summary Report, Washington, D.C.: Government Printing Office, 1971, pp. 15~16. The salt load data are historical from the 1942-1961 period, adjusted to 1960 conditions. DRI has further adjusted the percentages to delete salt loading caused by storage releases from Hoover Dam. 30 I I I ! Ij I I I , .1 I I i 1 ! i I , I ! I 143~ i i i I