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.. 64 P. B. HOLDEN The Co~orodo River> System--A ReguZated St!'eC1lTl The Colorado River system is a classic example of a regulated stream, not only because of the numerous mainstream dams, but also because most of its native fish fauna are obligate riverine species. The Colorado system starts as cold, clear mountain streams, but the mainstem and larger tributaries are very warm and turbid. Discussion will concentrate on four species, all "large, warm river" forms that have undergone drastic population reductions in the 20th Century. The Colorado squawfish (Ptyahoahei~u8 ~uaius) is North America's largest cyprinid, reportedly reaching 36 kg and 150 cm in length. It is piscivorous and was originally found in the mainstream Colorado River and all large tributaries. It reportedly migrates into trib- utaries to spawn (Miller, 1961), but recent evidence suggests that this may not be the case (Holden, 1977). It is presently considered endangered by the u.s. Department of the Interior (1977). The humpback chub (Gi~a aypha) is a rather bizarre minnow with a large nuchal hump, thin caudal peduncle, and large falcate fins, adaptions for life in swift water (Miller, 1946). It reaches lengths of 45 cm and was one of the last large species of North American freshwater fish to be discovered (Miller, 1946). The humpback chub is found only in several canyon areas in the upper Colorado Basin and Grand Canyon. It apparently prefers swift, deep water and seldom ventures out of these areas. It also is listed as an endangered species by the Department of the Interior (1977) . The bony tail chub (Gi~a eZegans) is closely related to the humpback chub, but lacks the abrupt nuchal hump. It was more wide- spread than the humpback chub, living in most of the mainstem and larger tributaries. It attains lengths of about 60 cm and is generally considered a swift-water form. It is presently protected by most states in the Colorado system and has been recommended for federal endangered listing. It is nearly extinct, with no known reproducing populations. The razorback sucker (Xyr>auohen texanus) is a large sucker with a distinct bony keel along its back, hence its common name. It had an original range similar to that of the bony tail chub. Earlier reports suggested that it migrated, apparently to spawn, but no recent evidence supports this hypothesis (Minckley, 1973). Regulation of the Colorado River began in the late 1800s, when water was diverted for irrigation in both the upper and lower basins. Regulation through mainstem dams began with the completion of Hoover Dam in 1935. At present, the lower Colorado Basin, from Lake Mead downstream, is a succession of reservoirs and dams