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STUDY AREA r The San Juan River originates at an elevation exceeding 3,048 m (10,000 ft) in the La Plata and San Juan mountain ranges of southwestern Colorado. From there the river flows 31 miles (50 km) southwest, enters New Mexico and joins its first major tributary, the Piedra River, and eight miles (13 km) downstream the Los Pinos River merges with the San Juan River. Approximately three miles (5 km) below this confluence, the natural flow of the river is interrupted by Navajo Reservoir, a 1.7 million acre-ft (2.096 x 109 m3) mainstream storage unit (U.S. Geological Survey 1985a). Downstream from the hypolimnetic release of Navajo Dam, the San Juan River flows approximately 109 miles (175 km) in New Mexico, passing through the towns of Archuleta, Blanco, Bloomfield, Farmington, Fruitland, and Shiprock, and makes a north turning arc and exits the state near Four Corners (Figure 2). The San Juan River, as it courses through New Mexico, is supplemented by waters from Gobernador, Chaco, and Red washes, Canyon Largo,'and the Animas, La Plata, and Mancos rivers (Figure 3). Near Four Corners, the river turns northwest, cuts across the extreme southwestern corner of Colorado and enters Utah. From the Colorado-Utah state line, the San Juan River flows 136 miles (219 km) across southern Utah and empties into Lake Powell on the Colorado River. In Utah, the San Juan River flows past the towns of Aneth, Montezuma Creek, Bluff, and Mexican Hat (Figure 2) and is fed by McElmo, Montezuma, Desert, and Chinle creeks, and Butler, Cottonwood, Comb, and Oljeto washes (Figure 3). The Animas and Mancos rivers (New Mexico) and Montezuma Creek (Utah) were the largest perennial tributaries of the San Juan River within the study area. Flows Historic flows in the San Juan River were highly variable, but discharge in the San Juan river between Navajo Reservoir and Farmington is now largely controlled and stabilized by Navajo Dam. From Navajo Reservoir downstream to Lake Powell, the river is supplemented by perennial and ephemeral tributary rivers, arroyos and washes, and diverted for use by industry, municipalities, and agricultural irrigation. Mean annual discharge of the San Juan River from 1913 to 1985 was 2,394 cubic feet/second (cfs) (67.8 m3/s) at Farmington (gauge # 09365000) and 2,574 cfs (72.9 m3/s) at the Bluff gauge (1914 to 1986, gauge # 09374500) (U.S. Geological Survey 1985a, 1985c). High flow values of 68,008 cfs (1,926 m3/s) and 69,986 cfs (1,982 m3/s) have been recorded at the Farmington and Bluff gauges, respectively. Low flow values at these two stations were 14 cfs (0.4 m3/s) (Farmington) and 1 cfs (0.03 m3/s) (Bluff). While it appears that discharge in the San Juan River declined following 7