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San Juan rivers and a small stretch of the mainstem Colorado <br />River below Cataract Canyon. Impoundments in the Sub-basin are <br />Navajo Reservoir on the San Juan River in New Mexico and Lake <br />Powell on the Colorado River in Utah. Of the tributary rivers, <br />the San Juan is the longest, provides the highest discharge, and <br />is the southern-most major tributary in the Upper Basin (U.S. <br />Geological Survey 1985, 1985a). <br />The Colorado River Basin has the highest percentage (87%) of <br />endemic primary riverine fishes of any North American river basin <br />(Miller 1959). A total of 55 forms has been reported from the <br />Upper Colorado River Basin, of which 13 are native and eight are <br />endemic (Tyus et al. 1982). Among these native forms, four are <br />federally listed as endangered and two are federal Notice of <br />Review as candidates for listing. Those "target" species <br />receiving the most study are the Colorado squawfish <br />(Ptychocheilus lucius), species in the genus Gila (bonytail, G. <br />ele ans, humpback chub, G. cypha, and roundtail chub, G. <br />robusta), and the razorback sucker (Xyrauchen texanus). Target <br />species, as defined in this report, are the five species listed <br />in the preceding sentence. While considerable research has been <br />conducted over the last decade on some of these fishes in the <br />Green and Upper Mainstem Colorado sub-basins, little attention <br />has been given to them or other rare fishes-in the San Juan Sub- <br />basin. <br />The New Mexico portion of the San Juan sub-basin consists of <br />the San Juan, Navajo, Los Pinos, Animas, La Plata, and Mancos <br />rivers and Navajo Reservoir (Figure 2). Although Jordan (1891) <br />3