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measured have a combined annual streamflow of 77,000
<br />acre-ft. Assuming a mean dissolved-solids concentration of
<br />1,300 mg/L, the combined annual dissolved-solids load at
<br />these sites may be estimated as 136,000 tons. Therefore, the
<br />annual dissolved-solids load in measured flow from the Grand
<br />Valley is about 439,000 tons. The total load of dissolved
<br />solids from the Grand Valley irrigation area is larger because
<br />of contributions from ungaged irrigation-return flows and
<br />from seepage directly into the Colorado River. Estimates of
<br />the mean annual dissolved-solids load from the Grand Valley
<br />unit include 497,000 tons, of which 441,000 tons was
<br />attributed to agriculture (Iorns and others, 1965), and
<br />580,000 tons (U.S. Bureau of Reclamation, 1985a).
<br />Colorado River near Colorado-Utah State line (site 18)
<br />The streamflow hydrograph for site 18 (table 3, pl. 1)
<br />depicts the cumulative effect of transbasin exports, irrigation-
<br />return flow, and reservoir regulation (fig. 12C). The period
<br />after 1966 depicts a decreased snowmelt-runoff peak and
<br />increased fall and winter flow, mainly due to storage in and
<br />releases from the large reservoirs on the Gunnison River.
<br />Mean annual streamflow averaged about 4.3 million acre-ft
<br />and dissolved-solids load averaged about 3.4 million tons
<br />(table 5). During the snowmelt season, calcium, sulfate, and
<br />bicarbonate are the prodominant ions. During the low-flow
<br />season, calcium, sodium, and sulfate predominate. Increased
<br />dissolved-solids concentration during September and Octo-
<br />ber, when the proportion of dissolved sulfate is largest,
<br />indicates the presence of irrigation-return flow. Proportions
<br />of dissolved sodium and chloride, which are more indicative
<br />of natural base-flow contributions, are largest from January
<br />through March. The period of record (1962-83) includes four
<br />years of data prior to completion of Blue Mesa Dam.
<br />Annual monotonic-trend analysis indicated a marginally
<br />significant decrease in median annual dissolved-solids con-
<br />centration of 11.2 mg/L per year and a highly significant
<br />decrease in median annual flow-adjusted concentration of 4.8
<br />mg/L per year (table 6). During the period of record, these
<br />trends represent a 34-percent decrease in median annual
<br />dissolved-solids concentration and a 16-percent decrease in
<br />median annual flow-adjusted concentration. Flow-adjusted
<br />concentration of every constituent except dissolved chloride
<br />decreased significantly. These trends correspond with the
<br />decreases detected after 1965 in the Gunnison subregion,
<br />which does not contribute large quantities of dissolved
<br />chloride to the Colorado River.
<br />San Miguel River at Uravan, Colo. (site 19)
<br />The streamflow hydrograph for site 19 (table 3, pl. 1)
<br />shows a flattening of the snowmelt-runoff peak caused by
<br />diversions for irrigation (fig. 12D). Chemical composition
<br />of dissolved solids is predominantly dissolved sulfate (fig.
<br />7). Dissolved sodium and chloride are present only in minor
<br />proportions. Calcium, bicarbonate, and sulfate are the
<br />predominant ions during the snowmelt season, and calcium
<br />and sulfate are predominant in base flow. Although a large
<br />area is irrigated upstream from the site, the area is not
<br />underlain by Mancos Shale; therefore, the dissolved-solids
<br />concentration at site 19 is not high and averaged about 330
<br />mg/L during 1974-79 (table 5).
<br />Dolores River near Cisco, Utah (site 20)
<br />The streamflow hydrograph for site 20 (table 3, pl. 1)
<br />is similar to the hydrograph for site 19 (fig. 12E). Streamflow
<br />at site 20 is depleted by irrigation in the San Miguel River
<br />basin and by diversions to the Montezuma Valley. The mean
<br />annual concentrations of major constituents is about the same
<br />as that for site 19, except for much higher concentrations
<br />of dissolved sodium and chloride. Calcium and bicarbonate
<br />are the predominant ions during the snowmelt season.
<br />Sodium and chloride are predominant during low streamflow.
<br />Mean monthly dissolved-solids concentration often exceeds
<br />2,000 mg/L.
<br />Of the 362,000 tons of dissolved solids contributed
<br />annually by the Dolores River, excluding the San Miguel
<br />River, 223,000 tons is dissolved sodium and chloride. Almost
<br />all of this contribution may be attributed to dissolution from
<br />salt anticlines, primarily where the Dolores River crosses
<br />Paradox Valley. Other estimates of the mean annual
<br />dissolved-solids load contributed by Paradox Valley are
<br />200,000 tons (U.S. Forest Service and others, 1976) and
<br />205,000 tons (U.S. Department of the Interior, 1985). A
<br />minor but concentrated source of salt loading to the Dolores
<br />River is Salt Creek, which drains the Sinbad Valley anticline.
<br />The low-flow dissolved-solids concentration at a site on Salt
<br />Creek near Gateway, Colo., is 40,000 mg/L, of which 92
<br />percent is dissolved sodium and chloride. The Dolores River
<br />contributes 8.7 percent of the dissolved sodium load and 18.6
<br />percent of the dissolved chloride load leaving the Upper
<br />Colorado River Basin.
<br />Annual monotonic-trend analysis for the period of
<br />record, 1952-83, indicated highly significant increases in
<br />median annual dissolved-solids load of 4,990 tons/yr and
<br />median annual flow-adjusted concentration of 4.7 mg/L per
<br />year (table 6). These trends represent a 46-percent increase
<br />in median annual dissolved-solids load and a 24-percent in-
<br />crease in median annual flow-adjusted concentration during
<br />the 32-year period. The increase in flow-adjusted concen-
<br />tration primarily consisted of sodium and chloride, indicating
<br />the source might be loading from the salt anticlines. Testing
<br />of brine wells in the Paradox Valley during 1978-83 may
<br />have contributed to this increase in loading, because the
<br />brine, which was pumped into storage ponds, seeped into
<br />the Dolores River. Climatic variation also may have been
<br />a factor. The 2 years with the highest annual streamflow and
<br />dissolved-solids loads (1972 and 1983) occurred during the
<br />last 11 years of the 33-year period of record. The 2 years
<br />with the highest dissolved-solids concentrations and very low
<br />annual streamflow (1977 and 1981) also occurred toward the
<br />end of the period of record.
<br />Lower Colorado Subregion 31
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