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Probable Effects of the Proposed Sulphur Gulch
<br />Reservoir on Colorado River Quantity and Quality near
<br />Grand Junction, Colorado
<br />By M.J. Friedel
<br />Abstract
<br />A 16,000 acre-foot reservoir is proposed to be located
<br />about 25 miles east of Grand Junction, Colorado, on a tributary
<br />of the Colorado River that drains the Sulphur Gulch watershed
<br />between De Beque and Cameo, Colorado. The Sulphur Gulch
<br />Reservoir, which would be filled by pumping water from the
<br />Colorado River, is intended to provide the Colorado River with
<br />at least 5,412.5 acre-feet of water during low-flow conditions to
<br />meet the East Slope's portion of the 10,825 acre-feet of water
<br />required under the December 20, 1999, Final Programmatic
<br />Biological Opinion for the Upper Colorado River. The reservoir
<br />also may provide additional water in the low-flow period and as
<br />much as 10,000 acre-feet of water to supplement peak flows
<br />when flows in the Colorado River are between 12,900 and
<br />26,600 cubic feet per second. For this study, an annual stochas-
<br />tic mixing model with a daily time step and 1,500 Monte Carlo
<br />trials were used to evaluate the probable effect that reservoir
<br />operations may have on water quality in the Colorado River at
<br />the Government Highline Canal and the Grand Valley Irrigation
<br />Canal.
<br />Simulations of the divertible flow (ambient background
<br />streamflow), after taking into account demands of downstream
<br />water rights, indicate that divertible flow will range from
<br />621,860 acre-feet of water in the driest year to 4,822,732 acre-
<br />feet of water in the wettest year. Because of pumping limita-
<br />tions, pumpable flow (amount of streamflow available after
<br />considering divertible flow and subsequent pumping con-
<br />straints) will be less than divertible flow. Assuming a pumping
<br />capacity of 150 cubic feet per second and year round pumping,
<br />except during reservoir release periods, the simulations indicate
<br />that there is sufficient streamflow to fill a 16,000 acre-feet res-
<br />ervoir 100 percent of the time. Simulated pumpable flows in the
<br />driest year are 91,669 acre-feet and 109,500 acre-feet in the
<br />wettest year. Simulations of carryover storage together with
<br />year-round pumping indicate that there is generally sufficient
<br />pumpable flow available to refill the reservoir to capacity each
<br />year following peak-flow releases of as much as 10,000 acre-
<br />feet and low-flow releases of 5,412.5 acre-feet of water.
<br />It is assumed that at least 5,412.5 acre-feet of stored water
<br />will be released during low-flow conditions irrespective of the
<br />hydrologic condition. Simulations indicate that peak-flow
<br />release conditions (flows between 12,900 and 26,600 cubic feet
<br />per second) to allow release of 10,000 acre-feet of stored water
<br />in the spring will occur only about 50 percent of the time. Under
<br />typical (5 of 10 years) to moderately dry (3 of 10 years) hydro-
<br />logic conditions, the duration of the peak-flow conditions will
<br />not allow the fu1110,000 acre-feet to be released from storage to
<br />supplement peak flows. During moderate to extremely dry (2 of
<br />10 years) hydrologic conditions, the peak-flow release condi-
<br />tions will not occur, and there will be no opportunity to release
<br />water from storage to supplement peak flows.
<br />In general, the simulated daily background dissolved-
<br />solids concentrations (salinity) increase due to the reservoir
<br />releases as hydrologic conditions go from wet to dry at the
<br />Government Highline Canal. For example, the simulated
<br />median concentrations during the low-flow period range from
<br />417 milligrams per liter (wet year) to 723 milligrams per liter
<br />(dry year), whereas the simulated median concentrations
<br />observed during the peak-flow period range from 114 milli-
<br />grams per liter (wet year) to 698 milligrams per liter (dry year).
<br />Background concentration values at the Grand Valley Irrigation
<br />Canal are generally only a few percent less than those at the
<br />Government Highline Canal except during dry years.
<br />Low-flow reservoir releases of 5,412.5 acre-feet and
<br />10,825 acre-feet were simulated fora 30-day period in
<br />September, and low-flow releases of 5,412.5 acre-feet were
<br />simulated fora 78-day period in the months of August through
<br />October. In general, these low-flow releases resulted in changes
<br />to salinity concentrations ranging from slight decreases to
<br />slight increases in dissolved-solids concentrations over the
<br />range of hydrologic conditions simulated. Low-flow releases of
<br />5,412.5 acre-feet of water over the 78-day period resulted in
<br />percentage increases in salinity greater than the measurement
<br />error for salinity in fewer than 10 percent of the driest years sim-
<br />ulated. Low-flow releases of 5,412.5 acre-feet of water over the
<br />30-day period coupled with peak-flow releases of as much as
<br />10,000 acre-feet of water also resulted in percentage increases
<br />in salinity greater than the measurement error for dissolved-
<br />solids in fewer than 10 percent of the driest years simulated.
<br />Observed trends in stream dissolved-solids concentrations at
<br />the Grand Valley Irrigation Canal are similar to observations of
<br />simulated dissolved-solids concentrations change at the
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