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Probable Effects of the Proposed Sulphur Gulch Reservoir on Colorado River Quantity and Quality <br />near Grand Junction, Colorado <br />Government Highline Canal, however, the magnitude of <br />percent and absolute change is less except under very dry <br />hydrologic conditions. <br />In addition to dissolved-solids concentration, understand- <br />ing instream changes in selenium concentration following res- <br />ervoir releases are of concern because selenium can be toxic to <br />fish and other biota. In general, instream selenium concentra- <br />tions are an order of magnitude greater in tributary creeks like <br />Sulphur Gulch (1 to 25 micrograms per liter) than in the Colo- <br />rado River (0.3 to 0.7 microgram per liter). Stochastic modeling <br />indicates that random sampling may result in a 1-percent and <br />35-percent chance, respectively, of exceeding Colorado <br />instream acute (18.4 micrograms per liter) and chronic <br />(4.6 micrograms per liter) water-quality standards in Sulphur <br />Gulch runoff. The lack of selenium in water pumped from the <br />Colorado River to storage likely will result in diluting reservoir <br />concentrations to respective levels ranged from 0.37 to <br />1.48 micrograms per liter under wet and dry hydrologic condi- <br />tions. Therefore, based on the simulations and inherent assump- <br />tions, selenium concentrations in the proposed reservoir are <br />expected to be less than the acute and chronic standards. <br />INTRODUCTION <br />The U.S. Geological Survey (USGS), in cooperation with <br />the Northern Colorado Water Conservancy District and Denver <br />Water, began a study in 2001 to evaluate the probable effects <br />that development and operation of the proposed Sulphur Gulch <br />Reservoir may have on instream quantity and quality changes in <br />the Colorado River. The proposed 16,000 acre-ft reservoir with <br />a 150 ft3/s pump and discharge capacity is to be located about <br />25 mi east of Grand Junction, Colorado, on a tributary of the <br />Colorado River that drains the Sulphur Gulch watershed <br />between De Beque and Cameo, Colorado (fig. 1). The proposed <br />Sulphur Gulch Reservoir is planned to provide the Colorado <br />River, at a point 25 mi upstream from Grand Junction (1) at <br />least 5,412.5 acre-ft of water during low-flow conditions to <br />meet the East Slope's portion of the 10,825 acre-ft of water <br />required under the December 20, 1999 Final Programmatic <br />Biological Opinion (PBO) for the upper Colorado River <br />(U.S. Fish and Wildlife Service, 1999), and (2) as much as <br />10,000 acre-ft to enhance the peak-flow when flows are in the <br />range of 12,900 to 26,600 ft3/s. <br />This report describes the stochastic modeling approach <br />and results of simulated daily reservoir operations on instream <br />Colorado River water quantity and quality at the Government <br />Highline and Grand Valley Irrigation Canals, and between <br />DeBeque and Palisade, Colorado. The use of a stochastic model <br />that takes into account the random nature of hydrologic and <br />water-quality variables is more suitable to provide simulated <br />estimates of salinity change as a result of reservoir operations, <br />because the temporal variability in daily Colorado River <br />streamflow and salinity is large, and values for runoff and <br />salinity from Sulphur Gulch and evaporation are uncertain. For <br />this reason, the specific objectives in this report are to: (1) <br />develop a stochastic mixing model that incorporates natural <br />variability and uncertainty for evaluation of water quantity and <br />quality (salinity and selenium) on a daily time step at locations <br />along the entire study reach; (2) quantify the effect of simulated <br />natural variability and uncertainty on probable changes in <br />stream quantity and quality subject to selected operational <br />pump and release activity; and (3) make an initial assessment of <br />the potential for selenium concentrations at the Sulphur Gulch <br />Reservoir. <br />DESCRIPTION OF STUDY AREA <br />The study area is in the western part of the Upper Colorado <br />River Basin and includes the Sulphur Gulch watershed and <br />includes a 10-mi reach of the Colorado River between De <br />Beque and a point 15 mi upstream (east) from Grand Junction <br />near Palisade and a 15-mi reach of concern between Palisade <br />and Grand Junction based on the PBO (fig. 1). The Sulphur <br />Gulch drainage area contributing runoff to the Colorado River <br />near De Beque is about 16 mil, whereas the Colorado River <br />Basin drainage area that contributes to runoff upstream from <br />De Beque is about 7,370 mil. In the Colorado River Basin, <br />physiography, climate, geology, and land use combine to affect <br />the quantity and quality of water resources. <br />Physiography and Climate <br />The Upper Colorado River Basin near the Continental <br />Divide contains a series of mountain ranges with elevations <br />ranging from 5,000 to more than 14,000 ft (NAND 88). The <br />middle parts of the basin consist of plateaus, ranging in eleva- <br />tion from about 3,100 to 11,000 ft that are semiarid and deeply <br />incised by canyons. Climate in the upper Colorado River Basin <br />is diverse because of these physiographic features, which <br />includes variations in elevation, latitude, and prevailing wind <br />patterns. Because of large differences in elevation between the <br />physiographic provinces, climate differs substantially between <br />the eastern and western parts of the basin. Mountainous areas <br />receive most of their precipitation as snow, whereas the lower <br />areas have dry winters and receive most of their precipitation <br />from intermittent summer thunderstorms. Although the moun- <br />tainous headwater areas of the basin receive a large quantity of <br />snow, most of the basin consists of semiarid or arid plains that <br />do not contribute substantially to annual streamflow. Plateaus <br />and high, intermontane basins typically have cold winters and <br />hot summers (Apodaca and others, 1996). <br />Geology <br />The geology of the Upper Colorado River Basin is diverse <br />and characterized predominantly by igneous and metamorphic <br />rocks in the high mountains and sedimentary rocks elsewhere. <br />