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Colorado squawfish persists in the Yampa and Green rivers, the upper Colorado River mainstream, and the lower San Juan River (Archer et al. 1985; Tyus et al. 1987; Meyer and Moretti 1988; Tyus 1989). The humpback chub is presently self-sustaining in the Yampa River and upper Colorado River (Archer et al. 1985; Karp and Tyus 1989). The razorback sucker persists in the lower Yampa and Green rivers, the mainstream Colorado River, and the lower San Juan River, but there is little indication of recruitment in these remnant populations (Tyus et aJ. 1986; Meyer and Moretti 1988; Lanigan and Tyus 1989; Tyus 1989). The bonytail chub is extremely rare in the upper Colorado River basin (Valdez and Clemmer 1982; V.S. Fish and Wildlife Service 1987). All four fishes have been extirpated in the Green River between Flaming Gorge Dam and the Yampa River confluence, due to loss of usable habitat following closure of the dam (Vanicek et aJ. 1970). However, the Yampa River supports all its native fish fauna (including self-sustaining populations of some of the rare species), contains much rare fish habitat, and contributes to the maintenance and availability of usable rare fish habitat in the downstream Green River. Our objective is to evaluate habitat use and streamflow needs of Colorado squawfish, bonytail chub, humpback chub, and razorback sucker in the Yampa River. Habitat requirements and factors limiting the distribution and abundance of each species are discussed by life history stage. Flow events considered essential to the survival of these four fishes in the Green River basin are identified for further quantification. Our intent is to describe flow needs of Colorado squawfish, bonytail and humpback chubs, and razorback sucker as indicated by their habitat use. We do not provide a quantification of these needs. Distribution, Abundance, and Habitat Use The distribution and abundance of fishes indigenous to the Yampa River have been studied since the early 1900's (Ellis 1914; Beckman 1952; Banks 1964; Vanicek et aJ. 1970; Holden and Stalnaker 1975). Early field studies provided much baseline information, although they were generally restricted in scope to seasonal fish surveys. More intensive, long-term sampling programs in the 1970's and 1980's (Prewitt et al. 1977; Seethaler 1978; Wick et aJ. 1979, 1982, 1985; McAda and Wydoski 1980; Miller et aJ. 1982; Tyus et aJ. 1982a, 1987: Hayues et aJ. 1984) have provided more quantitative approaches to the evaluation of fish habitat use and needs. In 1979, V.S. Fish and Wildlife Service (Service) developed standardized methods for studying the Colorado River fishes (Archer et aJ. 1980) and, in cooperation with the State of Colorado and the National Park Service, initiated fish studies in the Yampa River in fall 1980. The lower 198.4 km (Echo Park to near Williams Fork) was divided into eight relatively homogeneous river sections, using topographic and geologic maps, aerial surveys, and field reconnaissance, in an effort to evaluate habitat use of the rare fishes (Miller et al. 1982). The initial study area was extended to RK 224 because of angler-captured tag returns of Colorado squawfish from Craig, Colorado (see map opposite page 1). Data gaps identified in the initial studies (Miller et aJ. 1982) were evaluated in subsequent investigations using the same river stratification and fish sampling techniques. Habitat use data were compiled for all fish species captured by V.S. Fish and Wildlife Service and Colorado Division of Wildlife biologists (e.g., major habitat type, depth, velocity, and substrate information) using radiotelemetry, electrofishing, trammel netting, angling, and seining. Reproductive behavior of Colorado squawfish in the Yampa River was studied from 1981 to 1988 (Wick et aJ. 1983; Archer and Tyus 1984; Tyus et aJ. 1987; present study). Studies of winter habitat use by Colorado squawfish and general habitat use by humpback chub were initiated in the Yampa River in 1986 (Karp and Tyus 1989; Wick and Hawkins 1989). Razorback sucker spawning and winter habitat use have also been investigated (Miller et aJ. 1982; Valdez and Masslich 1989; V.S. Fish and Wildlife Service, unpublished data). The present report includes data from published sources and unpublished data from Service f1les in Vernal, V tah. In 1984, the Biology Subcommittee of the Vpper Colorado River Basin Coordinating Committee and the V.S. Fish and Wildlife Service identified river reaches considered important to the survival and recovery of the rare Colorado River fishes (V pper Colorado River Basin Coordinating Committee 1984; Archer et aJ. 1986). This information was more recently updated in the Recovery Implementation Program (V.S. Fish and Wildlife Service 1987). We provide additional information for further delineation of critical river reaches in the Yampa River. This information is summarized in Fig. 4 and discussed in the following sections. Colorado Squawfish Adult Adult Colorado squawfish are distributed in the mainstream Yampa River from its mouth upstream to Craig, Colorado (Fig. 4). The upper Yampa River (RK 81.6-198.4) is considered a concentration area for overwintering adults (Archer et aJ. 1986; Fig. 4), as evidenced by migration patterns of radio-tagged fish (Tyus et aJ. 1987) and abundance data (Miller et al. 1982; 4