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Water Quality Spanish explorers named the Colorado River for its ruddy color. Because the arid Colorado Plateau is sparsely vegetated, the force ot the water along the steep gradient causes erosion. The composition of the rock formations in the basin contributes to this process and consequently, the Colorado carries more silt than most rivers. Because much of the Colorado Plateau region was once covered by an inland sea. a virtually limitless amount of marine- created, salt-bearing shale underlies the topsoil in the plateau. Intiow of water and salts to Lake Powell approach a three-la-two relationship - approximately 11 million acre-feet of water and 7.5 million tons of salt annually flow into Lake Powell. Salinity content increases as the Colorado flows downstream. Natural leaching of salts from the basin's saline soils and geologic formations, and return flows from agriculture and other human activities increase the river's salinity. Where the river meets Imperial Dam, the water contains about 2,000 pounds of salts per acre-foot. compared with 300 pounds per acre-foot in California's Sacramento River. 711e Im:r:est rn'('rse osmosis plafll in the \\'orld, the Yuma Desalting PlaHt has 1/01 hel'n operated in l1eG/'ly fil'() years. The loss of water through evaporation, municipal, industrial and agricultural use, concentrates the salt in the remaining water, creating higher salinity concentration levels. High salinity can damage soils and crops and corrode pumps, household plumbing and machinery. Highly saline water is not suitable for municipal water, industrial, agricultural uses or groundwater recharge without extra - and more expensive - treatment to remove minerals. For several years, the water's increasing salinity has been considered a serious problem along the lower Colorado River. A 1973 U.S.-Mexico agreement - Minute 242 to the Mexican Water Treaty - requires I!IQ '3"11 II .IIZI . III . n - ---H!1Ie:PX~~_-:~:~ ~~.~ m~~I\""~ ~ - .~,~~,.;lI:o'" ~1'Llil:.__ - ~~~::",-~--., .!!!! -. 1:1 IlI"J'I'~- - ~ _,0:;;;:, ... . . _... 1i"Il' - ~~J-'-~" 'rr - s.t~_ ~ .~-"'>,.."".... "J" :~~---- ":~~t--.- . ..i.!t.~~ _~- _g......-' -----~---'u~~::.~r=' the United States to maintain the quality ot the water delivered to Mexico. In 1974, Congress passed the Colorado River Basin Salinity Control Act. The act directed the secretary of the Interior to initiate programs to assure compli- ance with the water quality commitment made to Mexico and to control the salinity concentration of the Colorado River in the United States. (The act was amended in 1995 to further authorize the Bureau to incorporate and manage a basin-wide approach to salinity control. The secretary of Agriculture also is directed to carry out salinity control measures under the Environmental Quality Incentives Program.) Also in 1974, the seven basin states formed the Colorado River Basin Salinity Control Forum which established water quality standards tor salinity under U.S. EPA regulations. based on the EPA' interpreta- tion of the 1972 amendments to the Clean Water Act. The most controversial salinity control measure is a desalination plant constructed near Yuma, Ariz. The plant was built to desalt drainage return flows from Welton-Mohawk Irrigation and Drainage District prior to the water returning to the Colorado River. The $250 million federal plant was completed in 1992 and is intended to annually reclaim 80.000 acre-feet of irrigation drainage water from about 60,000 acres of farmland east of Yuma. As planned, once most of the salt has been removed, the treated water would be blended back into the lower Colorado River upstream of Morelos Dam, complying with Minute 242 by reducing the water's salinity content before it is delivered to Mexico. Critics of the plant contend it would have been less expensive to buyout the irrigation district whose agricultural return flows were increasing the salinity content of the Colorado River or, at least, retire the salliest farmland in the area rather than building the plant. Operating costs for the plant - the largest reverse osmosis facility in the world - are projected to be in excess of $20 million a year. The desalination plant operated at one-third its capacity nearly three months after it began opera- tion in 1992. Operation continued until flooding on the Gila River washed out the facility delivering irrigation water to the plant. According to the Bureau, the plant remains on stand.by at a cost of $6 million per year. The plant can remain on stand-by primarily because wafer conserved by fhe lining ot the Coachella Canal (about 132.000 acre-feet annually)