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Probable Effects of the Proposed Sulphur Gulch Reservoir on Colorado River Quantity and Quality near Grand Junction, Colorado Structural features, including anticlines, domes, and faults, expose large sequences of strata. Several geologic units are major natural contributors of dissolved solids to streams. For example, shale formations that contain gypsum, calcite, dolo- mite and sodium-rich clay are the primary contributors of dis- solved solids (also called salinity). Dissolved solids such as sodium chloride, calcium bicarbonate, and calcium sulfate are transported concomitantly with ground water and surface water. About one-half of the salinity in the Colorado River is from nat- ural sources (U.S. Department of the Interior, 1994) such as weathering of geologic deposits and thermal springs (Butler and von Guerard, 1996). When streams come in contact with outcrops of sedimen- tary rocks, gypsum and calcite dissolve and salinity in the water increases. In the more arid climate at lower altitudes in the west- ern part of the basin, precipitation commonly is in the form of thunderstorms, and runoff from thunderstorms can deliver large loads of dissolved solids to streams and, therefore, salinity can increase. In addition, evaporation in semiarid and arid regions of the basin enhance the accumulation of salts on the soil and in reservoirs that can be delivered to streams. The presence of mineral hot springs upstream from the study areas also have an effect on salinity. The springs primarily are located in carbonate rock units in the area surrounding Glenwood Springs, Colorado. The mineral hot springs contribute about 15 percent of the total salinity annually to streams in the basin (U.S. Department of the Interior, 1994). In addition to saligity, selenium is present natu- rally in the shale bedrock of the basin upstream from Sulphur Gulch and is also in the surface water and ground water. Because selenium can be toxic to fish and other biota, knowl- edge of the occurrence and distribution of selenium in the study area is also of interest. Water Management and Use Irrigation, reservoir operations, interbasin water transfers, and power generation are the primary human activities that may affect salinity in the Upper Colorado River )aasin. In 1993, interbasin water transfers conveyed about 585,000 acre-ft of water (12 percent of the average annual streamflow in the basin) from the Upper Colorado River Basin to the South Platte, Rio Grande, and Arkansas River Basins (U.S. Department of the Interior, 1994). Interbasin water transfers generally occur near the stream headwaters, and the amount of streamflow diverted can be a substantial part of streamflow near these sources. Like- wise, streamflow diversions occur through the study reach for irrigation, power generation, and other purposes. By removing water from the system, streamflow diversions decrease the dilu- tion capacity of streams. In addition to providing drinking water, numerous basin reservoirs are used to regulate stream- flow in the Colorado River. Collectively, these reservoir opera- tions and streamflow diversions alter natural streamflow that may affect salinity and aquatic habitat of the streams. Land Use Rangeland and forest are the predominant land uses in the Southern Rocky•Mountain physiographic province (east), whereas agricultural land use predominates in the Colorado Plateaus physiographic province (west). Agricultural activities in the basin can cause increased levels of salinity that directly affect the surface- and ground-water quality and aquatic biota (Apodaca and others, 1996). For example, irrigation return flows can increase salinity in the surface and ground waters of the study area. In addition, partly because of reuse of irrigation water and leaching of bedrock, naturally occurring trace ele- ments such as selenium are present in water used for domestic and irrigation purposes (Apodaca and others, 1996). DESCRIPTION OF THE MODEL A stochastic modeling approach is used to quantify the effects that development and operation of the Sulphur Gulch Reservoir might have on daily streamflow and water-quality changes in the Colorado River. The basic approach involves development of a stochastic mixing model, stochastic model validation, and stochastic scenario modeling of reservoir oper- ations. In the following section, the conceptualization, parame- terization, and measurements used in the stochastic model development are described. Conceptualization The stochastic mixing model is composed of linked hydrology and water-quality submodels that incorporate ran- dom variability and uncertainty. The temporal variability and measurement uncertainty of model input is incorporated, and the distribution of probable results derived for changes in streamwater quality is determined by using the Monte Carlo method (Kalos and Whitlock, 1986). The surrogate for water quality in the stochastic mixing model is salinity, as indicated by measured instream dissolved-solids concentrations. Because salinity is considered to be conservative, the use of salinity is amenable to mixing without losing mass. Overall, the mixing model is a simplified representation of the Colorado River/ Sulphur Gulch system in which daily flows are added or subtracted, and concentrations are calculated based on the conservation of mass principle. In keeping with the conservation of mass principle, the mixing model accounts for flows and associated concentrations that are gained and lost in the Colorado River reach between the town of De Beque and the Palisade streamflow-gaging station. An overview of the study-area hydrology including cities, diversions, and location of return flow is presented in figure 2. The Palisade streamflow-gaging station is about 15 mi east of Grand Junction and is the upstream end of the so-called 15-mi reach. In the study reach, daily gains are attributed to streamflow that originates upstream from De Beque, releases