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I I I I I I I I I I I I I I I I I I I STUDY AREA The San Juan River is the southern-most major tributary of the Upper Colorado River Basin. It drains portions of New Mexico, Colorado, Arizona, and Utah (Figure 1). The San Juan River originates in the San Juan mountains of southern Colorado and enters the Colorado River at what is now Lake Powell. Tributaries include the Animas, La Plata, and Mancos rivers as well as many smaller tributaries such as Montezuma and McElmo Creeks (Figure 2). Navajo Dam, completed in 1962, near Archuleta, New Mexico regulates the flow of the San Juan River. As is typical of tailwaters below large dams, the river below Navajo Dam is too clear and cold to support populations of endangered fish. These cold waters extend some 43 miles downstream to Farmington, New Mexico. Therefore the native-fish study area begins at Farmington and ends at Clay Hills Landing near Lake Powell, 177 river miles (RM) downstream. For data analysis, the study area was stratified into seven river reaches delineated by towns or major man-made structures along the river that are easily identifiable. The seven river reaches are: Farmington, NM to Hogback Diversion (RM 179.0 to 158.6), Hogback Diversion (NM) to Shiprock, NM (RM 158.6 to 147.9), Shiprock, NM to Four Corners Bridge (RM 147.9 to 119.2), Four Corners Bridge to Aneth, UT (RM 119.2 to 101.0), Aneth to Bluff, UT (RM 101.0 to 76.4), Bluff to Mexican Hat, UT (RM 76.4 to 53.0), and Mexican Hat, UT to Clay Hills Boat Landing, UT (RM 53.0 to 2.9). Data will not be available for the Mexican Hat to Clay Hills reach until after the 1993 field season. The system of river miles, places RM 0.0 at Paiute Farms Marina where the San Juan River joins Lake Powell, and proceeds upstream to Navajo Dam (RM 221.8). Within the study area, there are five man-made diversion structures. These structures, located between Farmington, NM and the Utah state line may act as potential barriers to fish passage at certain flows (Figure 3). Of these, the concrete weir downstream of Fruitland, NM probably is the greatest obstacle to fish passage. There are no water diversion structures on the San Juan River in either Colorado or Utah. Starting at Navajo Dam the river gradient is moderate and the channel begins to form secondary and occasional tertiary channels. Between Fruitland, NM (RM 166) and Four Corners Bridge (RM 119.2) the channel forms numerous secondary and tertiary channels in response to the moderate average gradient and local alluvial geology. It is in this river reach that a geomorphologically unique area of the river known as the "Mixer" is located. The Mixer extends from RM 133.4 to 129.8. The river channel in these 3.6 miles of river has been relatively stable over the historic record with little variation in the degree of channel braiding. Certain areas are, however, locally dynamic. The habitat is complex with numerous channels always present. The locally dynamic areas contribute to this complexity. Between Four Corners and Aneth, braiding becomes somewhat reduced, mainly in response to a change in geology and topography, although the river gradient remains about the same as the Hogback to Four Corners reach. From Aneth to just below Bluff, braiding increases somewhat, but not to the same extent as seen above Four Corners. About ten miles below Bluff, the river becomes canyon-bound and remains a single channel with a slope roughly equal to the average slope of the river between Navajo Dam and Lake Powell. 2