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W l--'" ........ CD Missouri, Platte, Arkansas, Canadian and Rio Grande Rivers which all flow into mOre humid regions. The increased discharge, of course, dilutes and decreases the salinity of these rivers. The Upper Colorado River Basin Much of the research on diffuse-source salinity has been done in the Upper Colorado River Basin, which is defined as the area (276,000 km2) drained by the Colorado River above Lee's Ferry, Arizona (Fig. l-3). Most of the water in the Upper Colorado River 8asin originates from spr ing snow melt in the high country of the Rocky Mountains. At the high elevations outcrops of Precambrian crystalline rocks produce relatively insoluble weathering products, which are characterized by low solute and sediment production. However, at the lower elevations the upper Colorado River 8asin is underlain by Paleozoic to Recent sedimentary:rocks, including thick marine shales. Water derived from such outcrops in semiarid areas has a high salt content. The quality of surface water in the Upper Colorado River Basin varies considerably (Fig. 1-4). The high elevations yield high quality runoff, whereas the lowlands yield more saline runoff. Solute and sediment concentrations in the mountains are approximately one order of magnitude lower than at the lower elevations (Fig. 1-5). Saline springs occur in several parts of the basin, particularly at mountain fronts and in association with salt structures (e.g. within Paradox Valley). The major point sources (such as the Dotsero-Glenwood Springs) have been identified in the Upper 8asin (e.g. U.S. Bureau of Reclamation, 1976). In addition, saline groundwater movement, recharge, and yields have been documented (e.g. Schneider, 1975; Warner and Heimes, 1979). In contrast, shallow, subsurface movement or interflow contributions to salinity have not been studied in detail. However, McWhorter and Skogerboe (1979) maintain that this contribution is minimal and that saline interflow occurs only under restricted conditions. More has been written on salinity due to irrigation (Evans, Walker, and Skogerboe, 1981) than on any other single water quality topic because as much as one-third of the solute load of the Colorado River, approximately a million ton/yr at Imperial Dam, California, is attributed to irrigation (Strand, 8oesch, and Kruse, 1981). 7 }- , ," i! 'I $ :1' "1 .~. t : "1 .,1i I I " i "'1 '., ~i l ) .. li ';~ , :! 'i , I " ~ '. "