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<br />1 <br /> <br />002716 <br /> <br />Opportunities for Ecologica1lmprovement Along the Lower Colorado River <br />Mark Briggs and Steve Cornelius <br />7/24/97 <br /> <br />Lower Colorado River water is projected to become progressively more saline- due to a <br />variety of human-related activities. The Environmental Protection Agency (EP A) <br />estimated that increased salinity concentrations in the riyer have been principally <br />caused by out-of-basin exports, irrigation, and reservoir evaporation - accounting for <br />three percent, 37 percent, and 12 percent, respectively, of the increased salinity <br />concentrations that occurred between 1944 and 1988 (Colorado River Basin Salinity <br />Control Forum 1990). <br /> <br />The Delta <br />Prior to the construction of Glenn Canyon and Hoover Dams, Colorado River water <br />continually reached the delta and the Sea of Cortez, providing nutrients and estuarine <br />habitat for a plethora of marine life. During this time, the silt and water that the river <br />brought to the delta were critical in sustaining dense wetland plant communities that <br />are estimated to have contained 200 to 400 species (Ezcurra et a!. 1988 cited in Glenn et <br />a!. 1996). The area occupied by the delta prior to dam construction is estimated at over <br />780,000 hectaress (almost 2 million acres), and included two below sea level <br />depressions, the Salton Sea and the Laguna Salada (Sykes 1937). Tidal marsh, and <br />brackish and riparian ecosystems supported jaguar, beaver, and thousands of <br />migratory and resident waterfowl. In addition, over 20,000 Cocopah people prospered <br />by fishing, hunting, and gathering in the lush delta environment (Alvarez de Williams <br />[need date]). <br /> <br />Today, over 1 million hectares (2.5 million acres) of land in and surrounding the delta <br />has been converted to farmland. As a result of river impoundment and water <br />diversions, river water rarely flows all the way to the Sea of Cortez, altering the natural <br />salinity balance and decreasing the flow of nutrients that supports upper Sea of Cortez <br />fisheries (Glenn et al. 1996). In addition, reduced silt loads due to river impoundment <br />have actually sparked a period of erosion in the delta, rather than accretion (Thompson <br />1968). Therefore, the area occupied by the delta will probably decrease over time. <br /> <br />Although the 1944 treaty allocates to Mexico 10% of the lower Colorado River's base <br />flow during non-flood years, most of this water is diverted to the Canal Central for <br />agricultural irrigation in the Mexicali and San Luis districts of Mexico (Glenn et a!. <br />1996). As a result, mainstem water reaches delta wetlands only during times of high <br />flow. <br /> <br />The combination of river impoundment and diversions has had a devastating effect on <br />delta wetlands. In areas formerly dominated by Typha spp. and riparian forests of <br />cottonwoods or willows, a significant amount of the delta region south of the farmland <br />now consists of dry sand, mud, and salt flats dominated by salt cedar (Tamarix <br /> <br />.. According to a report developed by the Colorado River Basin Salinity Control Forum (1990), the <br />salinity concentration of Colorado River water below Parker Dam is projected to increase from 640 mg/I <br />in 1990 to over 850 mg/l by 2010. <br /> <br />Page 4 <br />