WSP06951

• File Number: 8273.300
• Description: Colorado River Basin Salinity Control - Federal Agencies - USGS
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 1/1/1985
• Author: USGS
• Title: Ground-Water Contribution to the Salinity of the Upper Colorado River Basin
• Water Supply Pro - Doc Type: Report/Study

northern and eastern parts of the subbasin, the Mesaverde Group is in upland
areas and the Mancos Shale at lower altitudes. In some areas the Dakota
Sandstone is locally present.

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Five sampling sites were selected in the lower Gunnison River subbasin
(figs. 10 and 14; table 3). The dissolved-solids concentrations in Tongue
Creek (site 34) were 1,760 mg/L and in Roubideau Creek (site 35) were
1,600 mg/L. These relatively large values are probably a result of the
erosion of outcrops of Mancos Shale in the drainages ,of these creeks.
Escalante Creek (site 36) had dissolved-solids concentrations of 382 mg/L,
and East Creek (site 37) had dissolved-solids concentrations of 808 mg/L.
These creeks drain areas underlain primarily by the Dakota Sandstone and the
Morrison Formation.

The dissolved-solids concentration in the Gunnison River near Grand
Junction, Colo., (site 38), adjusted for the effects of Blue Mesa and Morrow
Point Reservoirs, was 938 mg/L at an adjusted discharge of 784 ft3/s. At
site 38, comparison of the measured dissolved-solids concentration,
1,080 mg/L, with the historical mean for December, January, and February for
water years 1976 and 1977 from a water-quality station at this site, shows
the measured value to be about 80 percent greater than the mean value of
600 mg/L. Measured flow, 680 ft3/s, was about 29 percent of the historical
base-flow average of 2,357 ft3/s. The large difference between measured data
and mean average data at this site is most likely a result of regulation of
flow by Blue Mesa and Morrow Point Reservoirs.

The lower Gunnison River subbasin contributed an estimated base-flow salt
load of 265,000 ton/yr at a discharge of about 99 ft3/s. Measured tributary
inflow accounted for an estimated 73,000 ton/yr of the base-flow salt load and
for 51 ft3/s of the estimated base-flow discharge. The remaining estimated
base-flow salt load of 92,000 ton/yr is probably produced by unmeasured
tributary inflow, residual irrigation return flow, and channel erosion of the
Mancos Shale by the Gunnison River.

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Salt-load distribution

The adjusted estimated base-flow salt load of the Gunnison subregion was
724,000 ton/yr, using an adjusted base-flow discharge of 784 ft3/s (fig. 14).
These figures were adjusted to account for the effects of Blue Mesa and Morrow
Point Reservoirs. The areal distribution of the sources of base-flow salt
load and discharge for the Gunnison River basin is as follows: The East and
Taylor Rivers, Tomichi Creek, and upper Gunnison River subbasins produce about
9 percent of the estimated base-flow salt load and about 56 percent of the
estimated base-flow discharge; the North Fork Gunnison River subbasin produces
about 10 percent of the estimated base-flow salt load and about 8 percent of
the estimated base-flow discharge; the Uncompahgre River subbasin produces
about 45 percent of the estimated base-flow salt load and about 23 percent of
the estimated base-flow discharge; and the lower Gunnison River subbasin
produces about 37 percent of the estimated base-flow salt load and about
13 percent of the estimated base-flow discharge.

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