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,penetration up the Yampa River documented during the present study, although humpback suckers were reported 18 km upstream at Castle Park (Fig. 1) during a previous investigation (P.B. Holden, unpublished data). Humpback suckers were also attempting to spawn along the shoreline of the gravel pit of the Wal- ter Walker Wildlife Area and on gravel bars in the Colorado River near Grand Junction, Colorado. Neither young of the year nor juvenile humpback suckers were captured at any location, despite many attempts. During the spawning period, water temperatures . in the Yampa River increased from 6 to 10 C and turbidities increased from 600 to 1000 Jackson Turbidity Units (JTU). The temperature of the Colorado River averaged 12 C during this time, and turbidities exceeded 1000 JTU. Temperatures in the gravel pit were approximately 17 C and tur- bidities were slightly more than 100 JTU. The spawning bars consisted of large cobble-sized rocks at depths of 0.5 to 1 meter, and water ve- locities averaged 1 m/s. The shoreline of the gravel pit at the Walter Walker Wildlife Area is composed of similar sized rocks and was often agi- tated by wave action. Douglas (1952)' described the spawning of humpback suckers in Lake Havasu on the lower Colorado River. However, spawning was not observed during the present investigation, primarily because of the high turbidity at most potential spawning locations. ,. Hybrids of toe flannelmouth sucker (Catostomus latipinnis) and the humpback sucker were observed during the present investigation and were also repor.ted in other studies (Hubbs and Miller 1953; Vanicek, Kramer and Franklin 1970; Holden and Stalnaker 1975a). The incidence of hybridization appears to be increasing (Holden and Stalnaker 1975a), as would be expected in an altered system where one fish is considerably more abundant than another closely related fish, both having similar reproductive requirements. We collected 8 x. texanus X C. latipinnis hybrids during 1974-76. Humpback suckers also appear to be hybridizing with the introduced Utah sucker (c. ardens)in the lower basin (Gustafson 1975), and these hybrids ~~y soon appear in the Upper Colorado River Basin. Humpback chub - This species was previously found in the Flaming Gorge basin of the upper Green River, but only a few (26) were captured in Dinosaur National Monument in 1968-7l--usually from eddies adjacent to fast currents (Holden 1973). Only recently discovered (Miller 1946), humpback chubs have never been known to be abun- dant. In 1973 the Colorado Division of Wildlife found a population of this species in the Colo- rado River in deep glides near the border between Colorado and Utah (G. Kidd, personal communica- tion). We collected 12 specimens, including pos- sible hybrids with other species of the genus Gila, from an eddy about 3.2 kID upstream from Castle Park in 1975, 5 on the Yampa near the cut- off channel to the Green at Echo Park in 1975, and 1 at Lily Park in 1976 (see locations in Fig. 1). Although our nets were set in eddies and were near or at the surface, they were adja- cent to or in water 4-5 meters deep. We postu- late that humpback chubs may be present in the denser pools of the Upper Colorado River system. ~r Bony tail chub - Although formerly abundant, I he numbers of this species have been drastically I reduced since the closure of Flaming Gorge Dam. i Vanicek (1967) found that the 1959, 1960 and 1961 year classes of bony tail chubs were more numer- ous than the closely related roundtail chubs (Gila robusta). Since then, the bony tail chub has virtually disappeared, while the roundtail chub remains rather common and in no apparent danger of extinction. Only a few bony tail chubs have been found recently (1968-71) in the lower Yampa and Green rivers in Dinosaur National Monu- ment and a few in Desolation Canyon and Canyon- lands National Park (Holden 1973). No.bonytail chub was collected in 1974-76 in Dinosaur Na- tional Monument. An occasional bony tail chub is collected in the lower Colorado River basin (Lake Mohave, Arizona; D.P. Toney, personal communica- tion). Indeed, this species appears close to extinction. PROBLEMS ASSOCIATED WITH DEFINING "CRITICAL HABITAT" In December 1975, the Colorado Squawfish Re- covery Team, composed of members from various federal and state agencies, held its first meet- ing in Las Vegas, Nevada. Several meetings have been held by the Team since then and a draft "Recovery PLan" for preservation of the Colorado squawfish has been prepared. One of the prob- lems that confronted the Team was defining "criti- cal habitat" for this species. The Endangered Species Act of 1973 -- P.L. 93- 205 (U.S. 93rd Congress, 1973) -- with amend- ments in the Federal Register requires that the "critical habitat" for endangered species be de- fined so that the potential effects of future alterations to ecosystems can be identified and alternatives sought that would minimize the im- pacts on fish and wildlife. The defining of "critical habitat" for aquatic organisms is not as simple as for terrestrial organisms. Water quality and quantity, in addi- tion to the physical environment, are important for aquatic organisms. Alterations (e.g., in water temperature, or dissolved oxygen) that oc- cur upstream can affect the aquatic organisms far downstream. In addition, the streamflow require- ments for the endemic fish in the Colorado River are not known. Ohmart, Deason, and Freeland (1975) showed that the backwater marsh habitat of the lower Colorado River, although never exten- sive, was extremely important for various species of.wildlife. Vanicek and Kramer (1969) reported that young squawfish and chubs were commonly cap- tured in backwater habitats in the upper basin. The natural flushing action of the spring runoff may be necessary to keep the~e habitats from be- coming filled with silt and organic material. The historic annual peak flows in the upper Colo- rado River have been reduced by reservoirs and may result in a reduction of the critical nursery areas that are used by young endemic fish. IMPORTANCE OF THE WATERS IN DINOSAUR NATIONAL MONUMENT FOR THE CONTINUED EXISTENCE OF ENDEMIC FISH The Yampa River may provide a refuge for some, if not all, of the large-river endemic fish that are now threatened or endangered. However, we believe its major contribution to the continued survival of threatened and endangered fish is its amelioration of the Green River below their con- fluence. Our concern is that any alteration of the Yampa River or its tributaries could have a serious negative impact on this ameliorating effect. Although some comprehensive plans (e.g., Water Resources Council 1971) have been made for the management of the upper Colorado River, fish and wildlife have not been considered adequately in these plans. Information needed for such planning is widely scattered in the literature but is being compiled for reference in future management (R.S. Wydoski, K. Gilbert, K.H. See- thaler, and C.W. McAda, in preparation) . 611 The rationale and moral obligation for man to protect threatened and endangered species has