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Muddy Creek. Chemical composition is similar at sites 31
<br />and 32 except that concentrations of dissolved sodium and
<br />sulfate are higher at site 31. Hams Fork and Muddy Creek
<br />dilute the dissolved-solids concentration downstream from
<br />site 31, resulting in a flow-weighted mean of 580 mg/L at
<br />site 32 during 1955-83 (table 7). The predominant ions are
<br />calcium and bicarbonate during the high-flow season (May
<br />and June) and sodium and sulfate during the low-flow season.
<br />Annual monotonic-trend analyses indicated a signifi-
<br />cant increase in flow-adjusted concentration of 3.1 mg/L per
<br />year (table 8). This represents a 16-percent increase in the
<br />median annual flow-adjusted concentration during the period
<br />of record.
<br />Henrys Fork near Manila, Utah (site 33)
<br />Henrys Fork, like Blacks Fork upstream from site 31,
<br />drains the northern slopes of the Uinta Mountains. However,
<br />it does not have large tracts of irrigated agriculture. The
<br />streamflow hydrograph for site 33 (table 3, pl. 1) shows a
<br />modest snowmelt-runoff peak, which has a long period of
<br />sustained base flow (fig. 15G). Mean annual flow-weighted
<br />dissolved-solids concentration averaged 616 mg/L (table 7);
<br />the predominant ions are calcium and sulfate throughout the
<br />year. The water of Henrys Fork has a larger proportion of
<br />dissolved calcium and magnesium than the water of Blacks
<br />Fork and has a smaller proportion of dissolved sodium and
<br />chloride.
<br />Annual monotonic-trend analyses indicated highly
<br />significant decreases in dissolved-solids concentration and
<br />flow-adjusted concentration during 1955-83. Median annual
<br />concentration decreased by 9.4 mg/L per year, and median
<br />annual flow-adjusted concentration decreased by 5.0 mg/L
<br />per year (table 8). These trends represent a 41-percent
<br />decrease in median annual concentration and a 20-percent
<br />decrease in median annual flow-adjusted concentration dur-
<br />ing the period of record.
<br />Green River near Greendale, Utah (site 34)
<br />Site 34 (table 3, pl. 1) is 0.5 mi downstream from Flam-
<br />ing Gorge Reservoir. Flow of the river has been completely
<br />Figure 15. Mean daily streamflow at selected-sites in upper
<br />Green subregion of Green region. A, Site 22, Green River at War-
<br />ren Bridge, near Daniel, Wyo. B, Site 23, New Fork River near
<br />Big Piney, Wyo. C, Site 24, Green River near La Barge, Wyo.,
<br />and site 25, Green River below Fontenelle Reservoir, Wyo. D,
<br />Site 26, Little Sandy Creek above Eden, Wyo., site 27, Big Sandy
<br />Creek below Eden, Wyo., and site 28, Big Sandy River at Gasson
<br />Bridge, near Eden, Wyo. E, Site 29, Bitter Creek above Salt Wells
<br />Creek, near Salt Wells, Wyo. F, Site 30, Green River near Green
<br />River, Wyo. G, Site 31, Blacks Fork near Lyman, Wyo., site 32,
<br />Blacks Fork near Little America, Wyo., and site 33, Henrys Fork
<br />near Manila, Utah.
<br />controlled since filling of the reservoir began in November
<br />1962. Prior to November 1962, the snowmelt-runoff peak
<br />was larger and somewhat earlier than the peak upstream at
<br />site 20 because of snowmelt inputs from the Blacks Fork and
<br />Henrys Fork basins. Since the initial filling period, seasonal
<br />variability has decreased (fig. 17). Mean daily streamflow
<br />is almost constant throughout the year; slight increases oc-
<br />cur during late summer when excess storage is released and
<br />during the winter when the power demand is greater.
<br />Releases generally range between 1,000 and 4,500 ft3/s.
<br />Large releases occurred during 1983 and 1984, when the
<br />spillway was used to prevent overflow of the dam. Dissolved-
<br />solids concentration at site 34 is remarkably constant, and
<br />calcium and sulfate are the predominant ions throughout the
<br />year. Prior to streamflow regulation, calcium and bicarbonate
<br />predominated during the high-flow season, and calcium,
<br />sodium, and sulfate predominated during the low-flow
<br />season.
<br />The period of record was divided into a preintervention
<br />period (1957-62) and a postintervention period (1965-83),
<br />based on the initial filling of Flaming Gorge Reservoir. An-
<br />nual step trends were highly significant for dissolved-solids
<br />concentration (112 mg/L) and significant for dissolved-solids
<br />load (346,000 tons). These trends represent a 29-percent in-
<br />crease from the preintervention median concentration and
<br />40,000
<br />6w
<br />LL 20,000
<br />LLI
<br />LL W
<br />2It
<br /><u
<br />w Q Age
<br />0
<br />1- z
<br />(n -
<br />-20,000
<br />O 150,000
<br />J Cf)
<br />0 ZO 100,000
<br />w ? 50,000
<br />J Q
<br />0 Q 0
<br />SL)o
<br />Q -50,000
<br />to 50
<br />O Z(n
<br />02 25
<br />oPQ¢
<br />(1) < cc H o
<br />W l,- J J
<br />>zJQ 25
<br />O U a
<br />(n Z 50
<br />(n O -
<br />Q U 75
<br />0 N D J F M A M J J A S ANNUAL
<br />MONTH
<br />EXPLANATION
<br />Highly significant (p !cO.01)
<br />® Significant (0.01< P <_ 0.05)
<br />® Marginally significant (0.05<p50.10)
<br />Figure 16. Step trends at site 30 (Green River near Green River,
<br />Wyo.) from 1952-63 to 1964-83.
<br />Upper Green Subregion 39
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