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SITE 13
<br />100,000
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<br />?z
<br />-200,000
<br />50,000
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<br />0
<br />1n
<br />LLI - -50,000
<br />J O
<br />C)o -100,000
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<br />-150,000
<br />cn 250
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<br />LU F J J -250
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<br />U -750
<br />O N D J F M A M J J A S ANNUAL
<br />MONTH
<br />EXPLANATION
<br />Highly significant (p <_0.01)
<br />® Significant (0.01< P<_ 0.05)
<br />® Marginally significant (0.05<p<_0.10)
<br />Figure 11. Step trends at site 13 (Gunnison River near Grand
<br />Junction, Colo.) from 1934-65 to 1966-83.
<br />monthly dissolved-solids concentration ranged from 970
<br />mg/L during May to 4,000 mg/L during January. As at site
<br />14, streamflow is artificially large because of the water
<br />imported into the local area.
<br />Reed Wash drains an area affected by the Grand Valley
<br />Unit of the Colorado River Basin Salinity Control Project.
<br />Construction of salinity-control features in Grand Valley
<br />began in 1980. The period of record at site 15 was not long
<br />enough for an annual step-trend analysis, but several annual
<br />monotonic trends were detected for 1974-83. Trends in-
<br />dicated significant increases in median annual streamflow
<br />of 1,620 acre-ft/yr (table 6) as a result of small increases
<br />in streamflow for September through November, possibly
<br />from extension of the period of application of irrigation
<br />water. Annual monotonic trends indicated a significant
<br />decrease in median annual dissolved-solids concentration of
<br />about 38 mg/L per year and a marginally significant decrease
<br />in median annual flow-adjusted concentration of 37 mg/L
<br />per year, as a result of decreases for August through January
<br />in the constituents that are dominant in irrigation-return flow
<br />(dissolved calcium, magnesium, and sulfate). A marginally
<br />significant decrease in median annual dissolved-solids load
<br />of 2,770 tons/yr also was detected. During the 10-year period
<br />of record, these trends represent a 26-percent increase in
<br />median annual streamflow, a 25-percent decrease in median
<br />annual dissolved-solids concentration, a 25-percent decrease
<br />in median annual flow-adjusted concentration, and a
<br />19-percent decrease in median annual dissolved-solids load.
<br />Mack Wash near Mack, Colo. (site 16)
<br />area was authorized, the goal of which was to remove
<br />180,000 tons of dissolved solids per year from the Dolores
<br />River.
<br />Adobe Creek near Fruita, Colo. (site 14)
<br />Streamflow at site 14, like streamflow at sites 15, 16,
<br />and 17 (table 3, pl. 1), is composed almost entirely of
<br />irrigation-return flows and excess irrigation waters from the
<br />Gran d Valley agricultural area. During 1974-80, mean an-
<br />nual flow-adjusted dissolved-solids concentration averaged
<br />1,220 mg/L (table 5). Calcium and sulfate are the predomi-
<br />nant ions throughout the year. Sodium and magnesium con-
<br />stitute a large fraction of the cation composition from
<br />November through March. Dissolved sulfate comprises about
<br />49 percent of the dissolved-solids load. This composition is
<br />typical of agricultural return flow from areas underlain by
<br />gypsiferous shale.
<br />Reed Wash near Loma, Colo. (site 15)
<br />The streamflow hydrograph for site 15 (table 3, pl. 1)
<br />during 1974-83 shows the effect of irrigation waters in the
<br />streams that drain the Grand Valley agricultural area (fig.
<br />12A). Irrigation waters return to the stream system rather
<br />quickly, but accumulate large dissolved-solids loads. Mean
<br />Site 16 (table 3, pl. 1) is just downstream from Highline
<br />Lake and upstream from most of the Grand Valley irrigated
<br />area. Therefore, it has a lower dissolved-solids concentra-
<br />tion, and its streamflow hydrograph indicated the pattern of
<br />releases from Highline Lake.
<br />Salt Creek near Mack, Colo. (site 17)
<br />Most of the drainage basin of Salt Creek is outside the
<br />irrigated area of Grand Valley, but the streamflow hydro-
<br />graph for site 17 (table 3, pl. 1) indicates that most of the
<br />water is from irrigation-return flow (fig. 12B). Because Mack
<br />Wash discharges into Salt Creek, streamflow at site 17 is
<br />affected by reservoir releases from Highline Lake, upstream
<br />from site 16. During 1974-83, mean annual flow-weighted
<br />dissolved-solids concentration at site 17 averaged 1,360 mg/L
<br />(table 5). Annual monotonic-trend analyses indicated a
<br />significant increase in median annual streamflow of 2,220
<br />acre-ft/yr (table 6), resulting from increased monthly
<br />streamflow during August. This trend represents a 40-percent
<br />increase in median annual streamflow for the 10-year period.
<br />Although the volumes of streamflow at sites 14, 15,
<br />16, and 17 in the Grand Valley are different, the mean annual
<br />dissolved-solids concentrations and the seasonal fluctuations
<br />of streamflow and dissolved-solids concentration are similar.
<br />Calcium and sulfate are the predominant ions during every
<br />Lower Colorado Subregion 29
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