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.
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