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posed, including the Parachute Creek Member of the Green
<br />River Formation, which contains oil-shale deposits.
<br />Magnesium and calcium are the predominant cations during
<br />the high-streamflow season (April-June). Sodium and
<br />magnesium predominate during low streamflow. Bicarbonate
<br />is a predominant anion throughout the year. During the low-
<br />flow season sulfate also is predominant. Streamflow has a
<br />large snowmelt peak in May (fig. 8D); this occurs earlier
<br />than sites upstream, which are at higher elevations. Char-
<br />acteristics of streamflow are very similar to those at site 9
<br />(Roan Creek near DeBeque, Colo.).
<br />Colorado River near DeBeque, Colo. (site 8)
<br />Mean annual flow-weighted dissolved-solids concen-
<br />tration at site 8 (table 3, pl. 1) averaged about 395 mg/L,
<br />and mean annual dissolved-solids load was about 1.34 million
<br />tons during 1973-82 (table 5). The predominant ions are
<br />calcium and bicarbonate during the snowmelt season and
<br />sodium and chloride during the low-flow season. Large quan-
<br />tities of dissolved solids are added to the Colorado River in
<br />the reach between sites 5 and 8. No statistically significant
<br />annual monotonic trends were detected at this site.
<br />The reach of the Colorado River between the towns
<br />of Dotsero and Glenwood Springs, Colo., represents the
<br />largest single source of dissolved solids in the Upper Colo-
<br />rado River Basin. The dissolved-solids load contributed in
<br />that reach was estimated by subtracting mean annual
<br />dissolved-solids loads at sites 4, 6, and 7 from the mean
<br />annual dissolved-solids load at site 8 and further subtracting
<br />dissolved-solids loads from other known, small-stream in-
<br />flows that have an estimated dissolved-solids concentration
<br />of 600 mg/L. This resulted in an estimate of 475,000 tons/yr
<br />of dissolved solids contributed in the reach between Dotsero
<br />and Glenwood Springs, Colo. Seventy-three percent of this
<br />dissolved-solids load is dissolved sodium and chloride, which
<br />represents 17 percent of the dissolved-sodium and 38 per-
<br />cent of the dissolved-chloride loads leaving the Upper Colo-
<br />rado River Basin. Other estimates of the mean annual
<br />dissolved-solids load contributed by the thermal springs in-
<br />clude 440,000 tons (U.S. Department of the Interior, 1985),
<br />534,000 tons (Warner and others, 1985), and 476,000 tons
<br />(lorns and others, 1965).
<br />Roan Creek near DeBeque, Colo. (site 9)
<br />Streamflow characteristics at site 9 are similar to those
<br />at site 7. Both sites record runoff from Roan Mesa; however,
<br />streamflow and dissolved-solids load are greater at site 9
<br />because of its larger drainage area. Dissolved-solids concen-
<br />tration and chemical composition are similar at both sites.
<br />Colorado River near Cameo, Colo. (site 10)
<br />Streamflow hydrographs for site 10 (table 3, pl. 1)
<br />depict the cumulative effect of reservoirs and transbasin
<br />exports (fig. 8E). The snowmelt-runoff peak has decreased,
<br />and a slight increase in flow has occurred during the re-
<br />mainder of the year. A base flow of about 500 ft3/s is main-
<br />tained throughout the year. The effect of development before
<br />1935 is not depicted because of the lack of streamflow-gaging
<br />data. Chemical composition of streamflow is related to the
<br />primary sources of dissolved solids in the region. During
<br />the snowmelt season, calcium and bicarbonate are the
<br />predominant ions. During the base-flow period, when the
<br />thermal springs in the Glenwood Springs and Dotsero areas
<br />contribute a greater proportion of the streamflow, sodium
<br />and chloride are predominant.
<br />The period of record was divided into a preintervention
<br />period (1934-49) and a postintervention period (1950-83),
<br />based on the Alva B. Adams Tunnel/Lake Granby exports.
<br />The annual step trend in streamflow was about -385,000
<br />acre-ft, a marginally significant decrease of 13 percent from
<br />the preintervention median (table 4). This change was com-
<br />parable in magnitude to the decrease at site 5. In contrast
<br />to site 5, the annual step trend in dissolved-solids load was
<br />about -89,000 tons, a marginally significant decrease of 6
<br />percent, but dissolved-solids concentration did not change
<br />significantly. These differences apparently resulted from
<br />decreases in dissolved-solids concentration at site 10 during
<br />the base-flow period (September-March), which did not
<br />occur at site 5 (fig. 9). The decreases in dissolved-solids
<br />concentration may have been caused by the cumulative effect
<br />of upstream reservoirs, which also produced small, but
<br />significant, increases in streamflow at site 10 during the base-
<br />flow period.
<br />Annual monotonic-trend analysis for the preinterven-
<br />tion period indicated a marginally significant decrease in
<br />median annual dissolved-solids concentration of 5.8 mg/L
<br />per year (table 6). This trend represents a 21-percent decrease
<br />in median annual concentration during the 16-year period.
<br />Monthly trends indicated enhanced streamflow and corre-
<br />sponding decreases in dissolved-solids concentration during
<br />the low-flow season. Although no significant annual mono-
<br />tonic trends were indicated for the postintervention period,
<br />monthly trends again indicated increased streamflow and
<br />decreased dissolved-solids concentration during the low-flow
<br />season.
<br />Plateau Creek near Cameo, Colo. (site 11)
<br />The water at site 11 (table 3, pl. 1), which is drained
<br />from Grand Mesa along the southern side of the Colorado
<br />River, is less saline than that of Parachute and Roan Creeks
<br />(sites 7 and 9) and has a larger proportion of dissolved bicar-
<br />bonate. The mean annual flow-weighted dissolved-solids
<br />concentration is about 340 mg/L, and the mean annual
<br />streamflow is about 124,000 acre-ft (table 5). The site is
<br />downstream from many small reservoirs and about 25,000
<br />acres of irrigated farmland; its streamflow hydrograph is
<br />characterized by a late-May snowmelt peak and a well-sus-
<br />tained base flow (fig. 8F). The Tertiary Wasatch Formation
<br />and Cretaceous Ohio Creek Member of the Hunter Canyon
<br />Formation that underlie much of the drainage basin do not
<br />26 Characteristics and Trends of Streamflow and Dissolved Solids in the Colorado River Basin
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