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. CHAPTl':R II ~EED FOR ACTION w ~ 00 :..' Colorado River Basin Salinity . In the Colorado River Basin, ~alt pickup from the Glenwoorl-f)otsero Springs Unit and other sourceFl has resulted in the water '1u~lity of the Colorado River deteriorating over the long term as riverflows have heen developed for beneficial use, . At its headwaters in the mountatns of north-central Colorado, the Colorado River has a salinity concentration of about ')0 milligrams per liter (mg/L). The concentr.:ttion progressively increases downstream as it re"lult. of water diversions And salt contrihutions from a variety of sources, In jQ83. the averagp total dissolved solids (TOS) concentrA- tion was ."\bout 710 mg/L at Imperial Dam, the- last major div~rsion point of the river in the United State~. In 20 of the la~t 21 years, avera~e annual TDS exceeded 800 mg/L, with a maximum of 896 m~/L in }CJ70. Unless control measures are undertaken, the concentrations are €1(pecterl to in- crease as the Colorado River Basin States continue to develop their compact-apportionerl waters,l/ Water with salinity concentrations of 1,000 m~/L or less is gen- erally considered to be satisfactory for irrlgatin~ most crops, al- though concentrations of 500 mg/L can have detrimental effects on salt-sensitive crops, dependitlg on the chemical constituents of the ....ater. On land with good drainage, water exceerling 1,000 mg/L can he used for crops with high salt tolerance... According to Seconrlary Drinking Water Standard~ published by thE> F.nvirnnmE>ntal Protection Agency (EPA 1974), the saUnity concentration of public drinking water should be less than 500 mg/L. The salinity of the river results from two ~ener",l c311ses--salt loading and salt concentration, I. Salt loading is the addition of salt to the river from such sources as erosion of saline ~oil ~aterial~, irriga- tion return flows, and saline springs ~nd wel1~. T~e 3ver- age annual satt load of the river is estimated at ahout q million tons below Hoover Dam, 260 rniles upstream from Imperial Dam. 1/ The waters of the Colorado River were divided between the Upper and Lower Basins by the Colorado River Compact of 1922. The waters were subsequently apportioned among the Upper Basin State<; (Colorado, i'.lew ~exico, Utah, and Wyoming) by the Upper Colorado River Compact of 1948, and a'!long the Lower Basin States (Arizona, California, and Sevada) by the 1964 Supreme Court Decree, Arizona vs. California. . 7