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I I I I I I I I I I I I I I I I I I I OVERVIEW The Colorado River basin encompasses about 244,000 square miles and parts of seven states. Commonly called the Colorado River system, or often just the Colorado River, this drainage basin consists of a network of streams and rivers, many of which originate in high mountainous areas where their waters are clear and cold. During the warm-weather months these rivers are appreciably warmed as they flow downstream. Erosion of the arid lands of the drainage adds large amounts of soil to the rivers. The red color thus imparted to the Colorado prompted the early Spanish explorers to name the river Rio Colorado--the Red River. The Colorado River basin became separated from neighboring river drainages relatively early in the geologic history of the North American continent. The fishes of the Colorado River have thus long been isolated from fishes that live elsewhere in North America, and evolution has resulted in many endemic species--species that are found nowhere else. Eighty-seven percent of the native fishes of the Colorado River system are endemic species (MHller 1958). Beginning in the late 1800's and continuing today, man's activities have affected the ecosystem of the Colorado River basin. Among these activities are the impoundment and alteration of river flows by dams and other water- regulation structures, and the introduction of additional fish species to the rivers of the basin (Miller 1961, ~nckley and Deacon 1968, Holden and Stalnaker 1975a). Most fishery scientists believe the populations of native fishes of the Colorado River have declined appreciably because of man-induced changes in the aquatic environment (Mnller 1961, Minckley and Deacon 1968, Holden and Stalnaker 1975a). However, there is little historic information that could be used to show precisely how much the populations have declined. The limited work conducted by early fishery scientists in the late 1800's was not comprehensive and was restricted mainly to the few accessible river areas near roads. The fish communities of many of the rivers were essentially unknown until after the mid Twentieth Century, perhaps decades after they had begun to undergo man-induced change. Despite this lack of definitive information on the historic fish community, there is no doubt that the habitat of many native Colorado River fishes has been markedly altered since the arrival of the white settler. The region where these changes are most clearly evident is the lower Colorado River basin. Here, major tributary streams such as the Salt and Gila rivers now have large reaches that are dry much of the year; these river reaches formerly were habitat for native fishes (Minckley and Deacon 1968, Minckley 1973). Dramatic changes are also evident in the Colorado River of the lower basin, where the placement of dams has created a series of reservoirs and intervening cold tailwaters--habitats unsuitable for most native fishes but suitable for many introduced species. The endangered fishes of concern in the upper Colorado River basin are extirpated from the lower basin, or occur there in relatively limited populations. Four native fishes that inhabit the upper Colorado River basin are considered by most fishery scientists as threatened with extinction: the Colorado squawfish Ptvchocheilus lucius, humpback chub Gila cypha, bony tail Gila eleqans, and the razorback sucker Xvrauchen texanus. The squawfish, 1