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WATER QUALITY ASSESSMENT The San Juan River Basin Overview , The San Juan Basin is separated from the Dolores Basin by a low divide to the north. The Continental Divide separates the San Juan f om the Rio Grande Basin to the east. The San Juan Basin drains approx mately 5,800 square ~les in Colorado. ~ The presence of heavy metals in ome streams and high levels of dissolved solids, sediment and nutrients in others indicate the presence of highly mineralized soils and varyi g degrees of mining and agricultural activities on the land which they d in. Point source dischargers are predo~nantly from municipal waste t [atment plants and mining operations. . Nine main tributary streams drain the Colorado portion of th San Juan Basin. These streams are: the San Juan, Piedra, Los Pinos, Animas, Florida, La Plata, Mancos, Dolores Rivers and Me Elmo Creek. Peak runoff for these streams occurs between April and June, and lowflo occur during December and January. However, low flows in the area south of Highway 160 typically occur in late summer due to irrigation diversions. During the winter months low temperatures generally preclude runoff. Any flows during this period are contributed by the tributary aq ifers which have stored water from the previous season's precipitation. Low flow periods in streams are important since a.stream's capacity t assimilate waste loads is severely diminished during these periods. Lo flow data is used by the Water Quality Control Division in recommendin stream standards to the Water Quality Control Commission and in det rmining the effluent limitations for discharges to psrticular stream se ents. Water Quality in the San Juan Basin Tributaries The San Juan River The San Juan River originates high on the Continental Divide Springs and drains the easternmost section of the Colorado p Basin. Overall, this river exhibits good water quality and relatively unaffected by man's activities. In the lower reaches of the San Juan, the water's temperatur and September sometimes exceeds the 600 limit which is set t cold water fisheries. The Rio Blanco River, which joins the below Pagosa Springs has warm water flows (750) which, in pa accounts for the temperature rise in the San Juan. The Rio temperature probably results from natural warming and reduce Throughout the river, substantial populations of pollution i organisms are found, which indicates a healthy stream. Many fish species, such as a cut throut, rainbow and brook trout, in large numbers. -4- . above Pagosa rtion of the ppears to be in August protect San Juan t, lanco's flows. tolerant of the finer are present ,