U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005–3031

April 2005

Simulated Effects of the Proposed Sulphur Gulch Reservoir 

Operations on Colorado River Quantity and Quality

By M.J. Friedel

Printed on recycled paper

Introduction 

The U.S. Geological Survey (USGS), 

in cooperation with the Northern Colorado Water Conservancy District (Northern) and Denver Water (Denver), began 

a study in 2001 of the amount of water 

available for storage in the proposed 

Sulphur Gulch reservoir and the effects 

that reservoir operations might have on 

dissolved-solids concentrations (herein 

referred to as “salinity”) in the Colorado 

River under four reservoir-operating 

scenarios. The proposed 16,000-acrefoot reservoir would be located about 

25 miles east of Grand Junction, Colo., 

on a tributary of the Colorado River 

that drains the Sulphur Gulch watershed 

between De Beque and Cameo, Colo. 

(fig. 1). Denver and Northern are considering building the reservoir to augment 

streamflows by as much as 300 cubic 

feet per second (ft3/s) during critical 

low-flow conditions in the fall, and possibly also during the peak spring runoff, 

to increase flows for endangered fish 

as recommended by the U.S. Fish and 

Wildlife Service (1999). Water would be 

pumped from the Colorado River, when 

available, to fill the reservoir at a rate of 

150 ft3/s, during times that releases are 

not occurring. 

This fact sheet summarizes a detailed 

report of the study (Friedel, 2005), 

including an overview of the modeling 

approach and simulated effects of reservoir operations on the amount of water 

available for storage in the proposed 

Sulphur Gulch reservoir and salinity 

of the Colorado River at the Government Highline Canal and Grand Valley 

Irrigation Canal diversions. In the study, 

a stochastic model was used that takes 

into account the random and uncertain 

nature of the quantity and salinity of 

hydrologic variables (such as streamflow, 

runoff, evaporation, canal diversions, and 

reservoir storage) and provides results as 

a probable range of values for the hydrologic variables and salinity that would 

result from the reservoir operations. The 

model provides an understanding of the 

likely effects of operation of the reservoir that accounts for the day-to-day and 

year-to-year uncertainty and variability in 

Colorado River streamflow and salinity, 

quantity and salinity of runoff from the 

Sulphur Gulch watershed, and evaporation from the proposed reservoir. 

Description of Model 

The model uses linked hydrology and 

water-quality components that account 

for water rights and incorporate the 

random variability and uncertainty of 

hydrologic variables and salinity. The 

hydrology component of the model is 

used to compute probable flows at key 

hydrologic points in the study reach that 

are used by the water-quality component. 

The water-quality component of the 

model computes the probable salinity at 

points where water enters or leaves the 

study reach. The model incorporates the 

day-to-day and year-to-year variability 

and uncertainty in the actual measurement of hydrologic variables and salinity 

and computes the distribution of probable changes in hydrologic variables and 

Figure 1. Location of study area between De Beque and Palisade, Colorado.
salinity using the Monte Carlo method 

(Kalos and Whitlock, 1986). 

Overall, the model is a simplified 

representation of the Colorado River–

Sulphur Gulch system as shown in 

figure 2. Salinity is the water-quality 

variable used in the model. Salinity is 

considered to be conservative; that is, the 

dissolved solids that make up salinity are 

expected to remain in the dissolved form 

as water is mixed and evaporated. The 

estimated salinity of water that would 

be released to the Colorado River from 

Sulphur Gulch reservoir is the result of 

mixing water pumped to Sulphur Gulch 

reservoir from the Colorado River with 

runoff to the reservoir from the Sulphur 

Gulch watershed. Increases in salinity 

due to evaporation from the reservoir 

surface are included in the estimate.

Streamflow Quantity Simulations

The model was used to understand the 

quantity of water available for storage in 

and release from Sulphur Gulch reservoir 

based on a set of four reservoir-operation 

scenarios (table 1). In simulation of these 

scenarios, pumping as much as 150 ft3/s 

of Colorado River water to Sulphur 

Gulch reservoir was allowed throughout 

the year, except during reservoir-release 

periods. Pumpage was limited by water 

availability, taking into account demands 

of downstream water rights. The initial 

simulations included estimating the 

amount of annual divertible flow, annual 

pumpable flow, storage by season, and 

storage by day. Carryover of storage in 

Sulphur Gulch reservoir from year to 

year was included in all model simulations. Simulations of divertible flow, the 

amount of water available for diversion 

from the Colorado River to Sulphur 

Gulch reservoir after taking into account 

demands of downstream water rights 

indicated that divertible flow would 

range from 621,860 acre-feet of water 

in the driest year to 4,822,732 acre-feet 

of water in the wettest year. Because 

of the planned pumping capacity of 

150 ft3/s, pumpable flow (the amount of 

streamflow available after considering 

the pumping constraints on divertible 

flow) would be less than divertible flow. 

However, the pumpable flow still would 

be sufficient to fill a 16,000-acre-foot 

reservoir every year.

Reservoir Operation Scenarios

The proposed reservoir is planned 

to provide (1) at least 5,412.5 acre-feet 

per year of water to the Colorado River 

during low-flow conditions to meet the 

East Slope’s portion of the 10,825 acrefeet of water required under the December 20, 1999, Final Programmatic 

Biological Opinion (PBO) for endangered fish in the Upper Colorado River 

(U.S. Fish and Wildlife Service, 1999), 

and (2) as much as 10,000 acre-feet per 

year to enhance high streamflows (when 

streamflows are in the range of 12,900 to 

26,600 ft3/s) to scour the riverbed surface 

and replicate the natural hydrograph. 

Table 1 summarizes the four reservoirrelease scenarios that were simulated. 

Water-Quality Simulations

Water-quality simulations were 

conducted to understand the probable 

effects that reservoir pump-and-release 

operations would have on Colorado 

River salinity at the downstream diversion points of the Government Highline 

Canal and Grand Valley Irrigation Canal. 

This analysis began with a review of the 

simulated salinity profile at the Cameo 

streamflow gage prior to the beginning 

of any reservoir operations (background 

Table 1. Summary of reservoir-release scenarios simulated.

Reservoir-release 

scenario and

reservoir-release 

scenario number

Low-flow release High-flow release

Amount of water

released

(acre-feet)

Period

Duration of

release

(days)

Amount of water

released

(acre-feet)

Period

Duration of

release

(days)

1 5,412.5 September 1–30 30 0 N/A N/A

2 5,412.5 August 15–

October 31 78 0 N/A N/A

3 10,825 September 1–30 30 0 N/A N/A

4 5,412.5 September 1–30 30 10,000 May–June Variable

Figure 2. Schematic of the primary study reach hydrologic components.
salinity). An example of one of many 

likely annual background salinity profiles 

(fig. 3) indicates that the general shape of 

the profile has high and relatively stable 

concentrations in winter, rapidly declining concentrations during spring runoff, 

and slowly rising concentrations after 

the snowmelt-runoff peak in the summer 

and continuing into the fall. The simulated background salinity for a range of 

hydrologic conditions at the Government 

Highline Canal during low-flow and 

high-flow release periods is shown in 

figure 4. The background salinity during 

both release periods is larger in dry years 

than in wet years, and the range of probable salinity (the difference between minimum and maximum values) also is larger 

in dry years than in wet years. The simulated background salinity at the Grand 

Valley Irrigation Canal is very similar to 

that shown for the Government Highline 

Canal in figure 4 during both release 

periods. The simulated background salinity provides a basis to compare simulated 

probable salinity at the two diversions 

with Sulphur Gulch reservoir operations 

in place. As a point of reference, the 

changes were compared to the 3 percent 

margin of error in the measurement of 

salinity (Fishman and Friedman, 1989).

Based on simulations of reservoir 

operations, probable percent changes 

in salinity at the Government Highline 

Canal and Grand Valley Irrigation Canal 

diversions were determined and are 

shown for reservoir release scenario 4 in 

figure 5. In general, low-flow releases 

resulted in probable changes to salinity 

ranging from slight decreases (less than 

3 percent) to slight increases (less than 

3 percent) over the range of hydrologic 

conditions simulated, except for maximum probable salinity changes at the 

Grand Valley Irrigation Canal during the 

driest 10 percent of years simulated. Similar patterns of percent change in probable salinity occurred for reservoir release 

scenarios 1–3, although the changes for 

maximum probable salinity were greater 

than 3 percent higher during some hydrologic conditions. During the high-flow 

release period, the probable changes in 

salinity were less than 10 percent except 

for maximum probable salinity increases 

at the Government Highline Canal during 

the driest 10 percent of years simulated. 

Although maximum changes sometimes 

exceeded the 3 percent measurement 

error for all release scenarios, median 

changes greater than the 3 percent measurement error only occurred in the single 

driest of 1,500 simulations for release 

scenario 3. Also, maximum probable 

salinity changes during the driest years 

at the Government Highline Canal are 

generally less than those at the Grand 

Valley Irrigation Canal during the lowflow release period, whereas maximum 

probable salinity changes during the 

driest years at the Government Highline 

Canal are generally greater than those at 

the Grand Valley Irrigation Canal during 

the high-flow release period. 

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Figure 3. Stochastic simulation of salinity in the Colorado River (09095500) near Cameo, Colorado.

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Figure 4. Probability distributions of simulated background salinity at the Government Highline Canal during (A) the low-flow release period, and

(B) the high-flow release period. Probability values of 0, 50, and 100 percent represent the wettest, typical, and driest hydrologic conditions simulated.
Conclusions 

Simulations of the Colorado River–

Sulphur Gulch system with year-round 

pumping of Colorado River water to the 

proposed Sulphur Gulch reservoir and 

carryover storage in the reservoir indicate 

that there is sufficient flow available in 

the Colorado River to refill the proposed 

Sulphur Gulch reservoir to capacity 

each year. This holds true even following peak-flow releases of as much as 

10,000 acre-feet and low-flow releases of 

5,412.5 acre-feet of water regardless of 

the hydrologic condition and with taking 

downstream water rights into account, 

given the proposed pumpage and release 

capacities of the reservoir. Simulations 

also indicate that, with the exception of 

the single driest of 1,500 simulations, 

the probable annual median change in 

salinity caused by the proposed operation of Sulphur Gulch reservoir is less 

than the measurement error for salinity at 

the diversion points of the Government 

Highline Canal and Grand Valley 

Irrigation Canal, although maximum 

probable changes in salinity could be 

larger than the measurement error during 

some hydrologic conditions. Therefore, 

based on this analysis, operations of the 

proposed Sulphur Gulch reservoir would 

have little effect on water quality at the 

Government Highline Canal and Grand 

Valley Irrigation diversions under most 

hydrologic conditions. 

References Cited

Fishman, M.J., and Friedman, L.C., eds., 

1989, Methods for determination of 

inorganic substances in water and 

fluvial sediments: U.S. Geological 

Survey Techniques of Water-Resources 

Investigations, book 5, chap. A1, 

566 p.

Friedel, M.J., 2005, Stochastic modeling of the effects that the proposed 

Sulphur Gulch reservoir may have on 

Colorado River quantity and quality 

near Grand Junction, Colorado: U.S. 

Geological Survey Science Investigations Report 2004–5253, 54 p.

Kalos, M.H., and Whitlock, P. A., 1986, 

Monte Carlo methods: New York, John 

Wiley and Sons, Inc., 186 p.

U.S. Fish and Wildlife Service, 1999, 

Final programmatic biological opinion 

for Bureau of Reclamation’s operations 

and depletions, other depletions, and 

funding and implementation of recovery program actions in the Upper Colorado River above the confluence with 

the Gunnison River, December 1999, 

(http://www.r6.fws.gov/crrip/

biological.htm).

For More Information

Information on technical reports and 

hydrologic data related to Sulphur Gulch 

can be obtained from:

Director,

Colorado Water Science Center

Water Resources Discipline

U.S. Geological Survey

P.O. Box 25046, MS 415

Lakewood, CO 80225

303–236–4882

http://co.water.usgs.gov

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Figure 5. Probability distributions of simulated percent change in salinity following Sulphur Gulch reservoir operations scenario 4 at the Government Highline Canal and Grand Valley Irrigation Canal diversions during the (A) low-flow, and (B) high-flow release periods. Probabilities of 0, 50, 

and 100 percent represent the wettest, typical, and driest hydrologic conditions.