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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 23. Step trends at site 61 (San Juan River near Archuleta,
<br />N. Mex.) from 1956-61 to 1964-83.
<br />imported from the Dolores River basin. Although the
<br />chemical composition is similar to the water at site 66, the
<br />mean annual flow-weighted dissolved-solids concentration
<br />and load are much greater, averaging 2,210 mg/L and
<br />110,000 tons (table 9). The streamflow hydrograph shows
<br />the effect of irrigation diversions. A snowmelt-runoff peak
<br />is absent, and most of the streamflow seems to be composed
<br />of irrigation-return flows (fig. 22E). Dissolved-solids con-
<br />centration is almost constant throughout the year. Calcium,
<br />magnesium, and sulfate are the predominant ions. Sulfate
<br />is 63 percent of the dissolved-solids load.
<br />San Juan River near Bluff, Utah (site 68)
<br />Between sites 65 and 68 (table 3, pl. 1), the drainage
<br />area of the San Juan River almost doubles. Because most
<br />of the additional land is very arid, streamflow increases only
<br />slightly. However, mean annual dissolved-solids load in-
<br />creases by 374,000 tons (table 9). Excluding the measured
<br />inputs from the Mancos River (site 66) and McElmo Creek
<br />(site 67), which average 151,000 tons, dissolved-solids load
<br />in the reach increases by 223,000 tons/yr, 45 percent of
<br />which is dissolved sulfate. Mean annual streamflow at site
<br />68 has decreased since the filling of Navajo Reservoir. Deple-
<br />tions that began after 1961 include evaporation and increased
<br />storage in the reservoir, the Azotea Tunnel diversions,
<br />Navajo Indian Irrigation Project diversions, and several
<br />smaller uses downstream from Navajo Reservoir. Resultant
<br />changes in seasonal streamflow are shown in figure 22F.
<br />Seasonal variations in streamflow and dissolved solids have
<br />decreased, but inflows, primarily from the Animas River,
<br />result in a modest snowmelt-runoff peak during May and
<br />June. Flow-weighted dissolved-solids concentrations aver-
<br />aged 467 mg/L during 1964-83 (table 9). Since the filling
<br />of Navajo Reservoir, calcium and sulfate have been the
<br />predominant ions throughout the year. Because of decreases
<br />in seasonal variability, dissolved bicarbonate is no longer
<br />the major anion during the high-flow season.
<br />The period of record was divided into a preintervention
<br />period (1930-61) and a postintervention period (1964-83)
<br />(table 4), based on the initial filling of Navajo Reservoir.
<br />Annual step-trend analyses indicated a marginally significant
<br />increase in annual dissolved-solids concentration of 47 mg/L,
<br />which represents an 11-percent change from the preinterven-
<br />tion median concentration. Most of this increase was due to
<br />changes in dissolved sodium and sulfate concentrations.
<br />These constituents were added principally in the reach down-
<br />stream from the reservoir. A decrease in annual streamflow,
<br />although not statistically significant, limited the water
<br />available for dilution of dissolved solids at site 68.
<br />Annual monotonic-trend analyses indicated a signifi-
<br />cant decrease in median annual flow-adjusted concentration
<br />of 1.5 mg/L per year, which represents a 10-percent change
<br />from the preintervention median concentration. During the
<br />postintervention period, trends indicated a marginally signifi-
<br />cant decrease in median annual dissolved-solids concentra-
<br />tion of 7.1 mg/L per year and a significant decrease in median
<br />annual flow-adjusted concentration of 3.7 mg/L per year
<br />(table 10), mostly because of decreases in calcium, sodium,
<br />and sulfate. These decreases represent a 26-percent change
<br />in median annual concentration and a 14-percent change in
<br />median annual flow-adjusted concentration during the 20-year
<br />period.
<br />A second step-trend analysis, using 1968-83 as the
<br />postintervention period, indicated no significant annual
<br />trends. For several years after Navajo Reservoir began to
<br />fill, releases were small, supplying less water to dilute the
<br />dissolved solids contributed downstream from the reservoir.
<br />After 1968, the reservoir was mostly full, more water was
<br />released, and dissolved-solids concentrations decreased. The
<br />reservoir, once filled, has affected seasonal variability but
<br />has not affected long-term mean values at site 68.
<br />There are no additional sites on the San Juan River
<br />downstream from site 68. There are no perennial streams
<br />and virtually no irrigated agriculture in the reach, and
<br />dissolved-solids inputs probably are much smaller than those
<br />contributed in the reach between sites 65 and 68. Because
<br />of this, site 68 generally represents the entire input of the
<br />San Juan River to Lake Powell and the Colorado River.
<br />54 Characteristics and Trends of Streamflow and Dissolved Solids in the Colorado River Basin
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