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001921 BAUCH & SPAHR: SALINITY TRENDS IN SURFACE WATERS OF THE COLORADO RIVER BASIN 641 Upper Colorado River Basin ,-----..1 , . . . . ----r--- --....-----: : I . , 20 40 MILES I . I 20 40 ICJ\DMETERS EXPLANATION ~2 Dissolved-solids sampling and analysis sites Number designates site number listed in Table 1. 1'1 Salinity-control projects - - Physiographic Province' Boundary Fig. 1. Location of dissolved-solids sampling and analysis sites and salinity-control projects in the Upper Colorado River Basin in Colorado. salinity control, and water development in the Colorado River Basin (U.S. Department of the Interior, 1995). Dissolved solids in the Upper Colorado River Basin have been studied by Brennan and Grozier (1976), Mueller and Liebermann (1988), Mueller and asen (1988), and Nordlund and Liebermann (1990). Trends in dissolved-solids concentrations and loads have been reported by Kircher (1984), Kircher et a1. (1984), Moody and Mueller (1984), Mueller and Moody (1984), Lieber- mann et a1. (1989), and Butler (1996). A comprehensive accounting of repQrts dealing with salinity in the Upper Colorado River Basin in Colorado is in the bibliography by Bauch and Apodaca (1995). This study of salinity concentrations, loads, and trends in the Upper Colorado River Basin in Colorado is part of the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Program. This program is de- scribed in detail in Leahy et a1. (1993) and Gilliom et al. (1995). The Upper Colorado River Basin (UCaL) study unit, 1 of 60 study units in the NA WQA Program, is comprised of the Colorado River and its tributaries upstream from the Colorado-Utah stateline (Fig. 1) and is 1 of 16 study units that began investigations in October 1993. The UCaL study unit is described in Driver (1994), and the environmental setting of the unit is de- tailed in Apodaca et a1. (1996). In the headwaters area of the UCaL study unit, rocks of Precambrian and Tertiary ages are exposed. These rocks of predominantly granite and associated meta- morphic and volcanic materials are fairly resistant to weathering and the solvent action of water. Concentra- tions and loads of dissolved solids in the headwater areas typically are low, <100 mgIL (Liebermann et a1., 1989). In contrast, streams and tributaries in the middle and lower parts of the study unit are underlain by sedi- mentary rocks that were deposited in saline marine envi- ronments. Salts in these rocks, particularly the Upper Cretaceous Mancos Shale, are easily dissolved and car- ried into solution, resulting in high dissolved-solids con- centrations and loads in the water. The primary natural factor contributing to the dissolved-solids concentration of a stream reach is the presence or absence of soluble salts in the rock and soil substrate beneath and upstream from the site. The two main causes for an increase in dissolved solids are salt concentration and salt pickup, either natural or human induced. Salt concentration results from a consumptive use of water, the diversion of high-quality, low-salinity water out of the basin, and evapotranspira- tion and evaporation. No dissolved solids are added to the water, or removed in the case of evapotranspiration, but the dissolved-solids content increases because of less stream water being available for dilution. Salt pickup primarily occurs when dissolved solids are put into solu-