The Effects of impoundments on salinity in the Colorado River

The increase in salinity of our western rivers has been identified as one of the most serious water quality problems in the nation. This is of special concern in the Colorado River where salinity has increased from pristine levels estimated at 380 mg/1 to present-day levels of 825 mg/1 at Imperial Dam. Flow depletions, associated with decreased runoff and increased evaporation and diversions, coupled with high salt loading from natural and man-created sources are considered the primary causes for rising salinity in the river. The urban and agricultural development projected to occur in the basin through this century could deplete flows by an additional 2 million acre-feet (2.5 x 109 m3 )/yr. Salinity models indicate that depletions of this magnitude will elevate total dissolved solids concentrations (TDS) to 1150 mg/1 at Imperial Dam. Since this would have an enormous economic impact on municipal and agricultural water uses, salinity control programs are being implemented in the basin to maintain TDS at or below the 1972 levels. Historical data for the Colorado River, however, indicate that TDS concentrations are not increasing as rapidly as the models predict. Despite the extensive development and large flow depletions that have already occurred in the basin, TDS concentrations in Grand Canyon and below Hoover Dam have not changed appreciably since monitoring began. Water quality monitoring has recently shown that TDS concentrations throughout the Lower Colorado River Basin have been decreasing since 1972. This is thought to be a transient phenomenon caused by changes in flow patterns, salt routing or possibly inundation of saline sources in the Upper Colorado River Basin following completion of Lake Powell and other impoundments during the 1960s. This might also reflect more permanent reductions in TDS due to changes in chemical processes operating in the impoundments. The U.S. Geological Survey (USGS) has monitored ion and TDS concentrations in the inflows and outflow of Lake Mead and Lake Powell since early impoundment. The purpose here is to present results of our analysis of the USGS salinity data and describe how these large impoundments have historically influenced ion and TDS concentrations in the Colorado River. The implications of these findings are discussed relative to current efforts to control salinity in the Colorado River Basin