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e . '. ,..... ,-'-'" . l . .....,. ...~.' I. ~J i during the remainder of the year. A continuous flow would exist throughout the year, except for the month of July when no flow would exist under monthly average, maximum, or minimum conditions. Mancos River Return flow from the full service land on the ute Mountain Ute Indian Reservation wouid increase the flow of the Mancos River from a point 20 miles downstream of the town of Mancos by a year-round average of 8 cfs or 21 percent. Water Quality , Groundwater Material for the following discussion on water quality, except where noted, has been taken from the Animas-La Plata Project Definite Plan Report (1979) and fron the Animas-La Plata Project Final Environmental statement (1980). Existina Conditions Animas River All four rivers in the project area have their head-waters in or near a heavily mineralized region of the Colorado Rockies north of the project area. Heavy metal pollution is a water quality problem, particularly in the Animas River. At the Animas River headwaters, leached minerals and acid water from natural sources and abcmdoned and existing mining operations enter the river. High concentrations of iron, manganese, copper, lead, zinc, silver, cadmium, mercury, and arsenic along with a low pH cause the headwaters of the Animas River to be nearly devoid of fish and aquatic life and unusable for domestic and agricultural purposes. By the time the river reaches Durango, however, aeration caused by the stream's continual turbulent action and the high-quality inflow from tributaries have acted to improve the river's overall quality. Increases in pH and alkalinity cause the metals to be precipitated as oxides and hydroxides. The city of Durango withdraws raw water from the river during late summer to supplement its main supply from the Florida River. While the total concentration of several heavy metals in the Animas River may periodically exceed the drinking water standards, most of the metals are in a state of suspension and can be removed by settling and filtration treatment. The concentrations of nitrogen and phosphorus (nutrients) and the number of coliform organisms found in samples taken from the Animas River at and just upstream of Durango indicate upstream domestic and agricultural influence on the water quality. The nutrients have not caused obvious in stream or use problems, but domestic water supplies using this water must be disinfected to eliminate the coliform organisms. Proceeding downstream to Aztec and Farmington, further increases in nutrients are noticeable. 10