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San Juan Region The San Juan region consists of the San Juan River drainage basin and the large and area downstream from the confluence of the Green and Colorado Rivers. Most of the flow of the San Juan River originates on the western slope of the Rocky Mountains in Colorado. Streamflow contributed to the Colorado River in this region originates almost en- tirely in the San Juan River basin. The San Juan region contains 35 percent of the total drainage area of the Upper Colorado River Basin and contributes 16 percent of the streamflow and 14 percent of the dissolved-solids load. Lake Powell extends upstream to within about 20 mi of the con- fluence of the Green River, and it has inundated many features in the region, including the mouth of the San Juan River and the former town of Hite, Utah. Except for minor inputs from the Paria River, the flow of the Colorado River leaving the Upper Colorado River Basin is completely con- trolled by Glen Canyon Dam and Lake Powell. During 1966-80, the mean annual streamflow of the Colorado River at Lees Ferry, Ariz. (site 69) (table 3, pl. 1), was about 8.8 million acre-ft, mean annual flow-weighted dissolved-solids concentration was 564 mg/L, and the dissolved-solids load was about 6.7 million tons (table 9). Data from 13 sites were evaluated for the San Juan region (pl. 1). Mean annual streamflows, dissolved-solids concentrations and loads, and major-constituent loads were determined at each of these sites (table 9). Chemical com- position of streamflow for selected sites is shown in figure 21. Selected monotonic trend-analysis results are listed in table 10. The San Juan region was divided into two sub- regions: the San Juan and the main stem (pl. 1). San Juan Subregion The San Juan subregion consists of the entire San Juan River basin. Large tracts of land are irrigated along the San Juan and Animas Rivers in the vicinity of the towns of Farmington, Bloomfield, and Fruitland, N. Mex. Navajo Dam and Reservoir is the largest water-storage project in the San Juan region. Completed in 1963 and having a capacity of about 1.7 million acre-ft (table 2), Navajo Reservoir is the third largest reservoir in the Upper Colorado River Basin. The Navajo Indian Irrigation Project diverts water from the reservoir to supply Navajo Indian lands east of the Chaco River. Delivery of water to the Navajo Indian Irrigation Project began in 1976. Delivery during 1984 totaled 120,000 acre-ft, about one-half of the estimated depletion at full development. Several diversions export water from the San Juan River basin to the Rio Grande basin. The largest is Azotea Tunnel, which has exported about 107,000 acre-ft/yr from the Navajo River and Rio Blanco drainages since 1971 (table 1), as part of the San Juan-Chama Project. Vallecito Reser- voir, Lemon Reservoir, and Electra Lake provide water storage in the Los Pinos and Animas River drainages. Large volumes of irrigation water are imported to the McElmo Creek drainage from the Dolores River through the Montezuma Tunnel. Vallecito Creek near Bayfield, Colo. (site 60) Streamflow in the headwaters of the San Juan drainage generally has low dissolved-solids concentrations. Site 60 (table 3, pl. 1) has very large mean annual runoff, averag- ing 26.8 in., and very low mean annual flow-weighted dissolved-solids concentration, averaging 34 mg/L (table 9). The site is upstream from Vallecito Reservoir, and seasonal streamflow consists of a broad snowmelt-runoff peak fol- lowed by a gradual recession period and very little stream- flow during the winter months (fig. 22A). Calcium and bicarbonate are the predominant ions throughout the year. Analyses of annual monotonic trends indicated small but significant decreases in median annual dissolved-solids concentration of 0.3 mg/L per year and in median annual flow-adjusted concentration of 0.3 mg/L per year (table 10). These decreases represent a 17-percent change in the median annual concentration and in the median annual flow-adjusted concentration during the 21 years of record. Most of this change results from a decrease in dissolved sulfate. The cause of this decrease is not known. San Juan River near Archuleta, N. Mex. (site 61) Site 61 (table 3, pl. 1) is 7 mi downstream from Navajo Dam. About one-half of the total flow of the San Juan River enters the river upstream from this site. Streamflow has been completely regulated since June 1962. Although the initial filling period of the reservoir ended in June 1964, storage did not exceed 1 million acre-ft of its 1.7 million acre-ft capacity until 1968. The effect of reservoir storage and releases on streamflow is shown in figure 22B. Prior to 1962, streamflow was dominated by snowmelt runoff; after the initial reservoir filling, streamflow was almost constant. Although the seasonal variability in dissolved-solids con- centration has been greatly decreased, the mean annual dissolved-solids concentration has not changed between the two periods. From 1964 to 1983, mean annual flow-weighted dissolved-solids concentration averaged 166 mg/L (table 9). Calcium and bicarbonate are the predominant ions throughout the year. The period of record was divided into a preintervention period (1956-61) and a postintervention period (1964-83) (table 4), based on the initial filling of Navajo Reservoir. No annual step trends were statistically significant. There is no evidence of leaching or mineral precipitation in the reservoir. Monthly step trends in streamflow and dissolved- solids concentration and load indicate the decrease in seasonal variability after the initial filling of the reservoir (fig. 23A). Annual monotonic-trend analyses of the postinter- vention period indicated a marginally significant decrease in median annual dissolved-solids concentration of 1.1 mg/L San Juan Subregion 49