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I
<br />INTRODUCTION
<br />The Colorado River basin drains portions of seven western
<br />states (Wyoming, Colorado,.Utah, Nevada, New Mexico, Arizona, and
<br />California) and approximately 8% of the land surface area of the
<br />United States (Bishop and Porcella 1980). It is unsurpassed as a
<br />source of water for consumptive use, but has a very minor
<br />discharge compared to that of eastern rivers (Pillsbury 1981).
<br />To assist in the administration of water management programs, the
<br />U.S. Congress divided the Colorado River basin into roughly equal
<br />upper and lower segments in 1928 (Figure 1).
<br />The Upper Colorado River basin, which extends 549 miles (885
<br />km) from north to south and 349 miles (563 km) from east to west,
<br />drains 283,600 km2 (109,498 mil) in southwestern Wyoming, western
<br />Colorado, eastern Utah, northwestern New Mexico, and northeastern
<br />Arizona (Iorns et al. 1965). The primary lotic systems of this
<br />section are the Colorado, Green, Yampa, White, Gunnison, Dolores,
<br />and San Juan rivers. Several authors have further sub-divided
<br />the Upper Colorado River Basin into three major hydrologic sub-
<br />basins: Upper Mainstem Colorado, Green, and San Juan (Iorns et
<br />al. 1965, Joseph et al. 1977, Carlson and Carlson 1982).
<br />The San Juan River Sub-basin is the second largest of the
<br />Upper Colorado River Basin sub-divisions and drains 99,200 km2
<br />(38,300 mil) in Colorado, Utah, New Mexico, and Arizona. The
<br />primary riverine components in this sub-basin are the Dirty
<br />Devil, Escalante, and San Juan rivers and a small stretch of the
<br />mainstem Colorado River below Cataract Canyon. Major
<br />impoundments of the sub-basin are Navajo Reservoir on the San
<br />Juan River in New Mexico and Lake Powell on the Colorado River in
<br />Utah.
<br />The San Juan River drainage is composed of the San Juan,
<br />Navajo, Los Pinos, Animas, La Plata, and Mancos rivers, Montezuma
<br />Creek, several arroyos and washes, Navajo Reservoir, and the San
<br />Juan River arm of Lake Powell. Of the San Juan River Sub-basin
<br />tributaries, the San Juan River is the longest and has the
<br />largest discharge. In addition, it is the southern-most major
<br />tributary in the Upper Colorado River basin (U.S. Geological
<br />Survey 1985a, 1985b).
<br />The Colorado River basin has the highest percentage (870) 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 native forms, four are
<br />federally listed as endangered (humpback chub, Gila cypha,
<br />bonytail, Gila eleaans, Colorado squawfish, Ptychocheilus lucius,
<br />and Kendall Warm Springs dace, Rhinichthys osculus thermalis) and
<br />razorback sucker, Xyrauchen texanus, is a federal Notice of
<br />Review candidate for listing (U.S. Department of the Interior
<br />1989). Roundtail chub, Gila robusta, is listed as endangered
<br />(Group 2) by the state of New Mexico (New Mexico Department of
<br />Game and Fish 1985). The species currently receiving the most
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