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Table 10. Selected annual monotonic-trend-analysis results-San Juan Region [Sites having less than 10 years of dissolved-solids data are not reported; n, number of years of dissolved-solids data; acre•ft/yr, acre-feet per year; mg/L, milligrams per liter; tons/yr, tons per year; HS, highly significant (p < 0.01); S, significant (0.01 (0.05 0.10)] Annual trend in dissolved solids Site Period Annual trend Flow-adjusted (table of in streamflow Concentration Load concentration 3, record Signif- Trend Per- Signif- Trend Per- Signif- Trend Per- Signif- Trend Per- plate (water n icance (acre- cents icance 1 icance (tons/ 1 icance 1 1) years) level ft/yr) level (mg/L) cent level yr) cent level (mg/L) cent 60 1963-83 21 -- -- -- S -0.3 17 -- -- -- S -0.3 17 61 1964-83 20 -- -- -- MS -1.1 12 -- -- -- -- -- -- 62 1955-83 29 -- -- -- -- -- -- -- 63 1962-82 21 -- - -- -- -- -- -- -- -- -- 65 1964-83 20 -- -- -- MS -3.7 20 -- -- -- S -2.7 15 68 1930-61 32 -- -- -- -- -- -- -- -- -- S -1.5 10 68 1964-83 20 -- -- -- MS -7.1 26 -- -- -- S -3.7 14 69 1942-62 21 -- -- -- S 7.3 33 -- -- -- S 3.3 14 69 1966-80 15 -- - -- -- -- -- -- 'Percent change during period of record. low-flow season. Excluding the Animas River inflow, about 65,000 tons/yr of dissolved solids is added to the San Juan River in the reach between Archuleta and Farmington, N. Mex., mostly as dissolved sulfate. Irrigation-return flows from agricultural lands possibly are the source of this in- crease. Because of the upstream regulation, dissolved-solids concentration does not vary greatly throughout the year. No statistically significant annual monotonic trends were detected for this site. Chaco River near Waterflow, N. Mex. (site 64) The Chaco River drains a large, but arid, region south of the San Juan River and discharges into the San Juan River just upstream from Shiprock, N. Mex. Site 64 (table 3, pl. 1) is about 5 mi downstream from the Four Corners Power- plant, a large, coal-fired utility that began operating in 1967. Coal is supplied from a large strip mine upstream, where Upper Cretaceous deposits in the Fruitland Formation are overlain by Kirtland Shale. Water is pumped from the San Juan River to Morgan Lake, beside the powerplant, where it is used for cooling purposes. Wastewater from the plant drains from several holding ponds into the Chaco River, creating a perennial stream downstream from the powerplant. Base flow is maintained at about 15 ft3/s. The remainder of the flow of the Chaco River is almost entirely from occa- sional, intense rainfall (fig. 22D). Mean annual flow- weighted dissolved-solids concentration averages 801 mg/L, and the mean dissolved-solids load is 33,000 tons/yr (table 9), 72 percent of which is sodium and sulfate. San Juan River at Shiprock, N. Mex. (site 65) Streamflow at site 65 (table 3, pl. 1) is about the same as at site 63, but dissolved-solids concentration and load are greater. Of the dissolved solids contributed in the reach, 74 percent is dissolved sodium and sulfate. During 1964-83, mean annual flow-weighted dissolved-solids concentration averaged 324 mg/L (table 9). Calcium, bicarbonate, and sulfate are the predominant ions during the high-flow season. Calcium and sulfate are predominant during the low-flow season. The proportion of dissolved sulfate in the dissolved- solids load increases from 32 percent at site 61 to 41 per- cent at site 63 and to 45 percent at site 65. The seasonal variability of streamflow and dissolved solids at site 65 markedly decreased after the closure of Navajo Reservoir. The period of record was divided into a preintervention period (1958-61) and a postintervention period (1964-83) based on the initial filling of Navajo Reservoir. Because of the short preintervention period, step trends were not evaluated nor were monotonic trends evaluated for that period. Annual monotonic-trend analyses for the postinter- vention period indicated marginally significant decreases in median annual dissolved-solids concentration of 3.7 mg/L per year and significant decreases in median annual flow- adjusted concentration of 2.7 mg/L per year (table 10). These decreases represent a 20-percent change in the median an- nual concentration and a 15-percent change in the median annual flow-adjusted concentration during the 20-year period. Mancos River near Cortez, Colo. (site 66) Site 66 (table 3, pl. 1) is downstream from about 12,000 acres of irrigated land underlain by Mancos Shale. Navajo Wash drains additional irrigation area and discharges into the Mancos River downstream from the site. Therefore, the Mancos River contributes substantially more dissolved solids than reported for this site. Snowmelt runoff from the moun- tain headwaters results in a hydrograph peak from April through June. Calcium and sulfate are predominant through- out the year. Magnesium also becomes predominant during 52 Characteristics and Trends of Streamflow and Dissolved Solids in the Colorado River Basin