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Green River hydrograph (Figure 3), thereby mitigating some of the adverse effects of flow regulation by Flaming Gorge Dam on the native fish fauna. Flows of the Yampa River, particularly spring runoff, may also enhance usable fish habitat by inhibiting the invasion and proliferation of introduced fishes that evolved in more mesic environments. Colorado squawfish, razorback sucker and other fishes depend on habitats in the Yampa and Green rivers for fulfillment of various life history requirements, and thus, the two rivers must be viewed as a single system when determining flow needs. The Green River basin supports the largest numbers of Colorado squawfish (Tyus 1990) and razorback suckers (Lanigan and Tyus 1989) in native riverine habitats. The humpback chub is presumed self-sustaining in DNM, but low numbers makes its status precarious (Karp and Tyus, in review). The persistence of the native fishes indicates that habitat conditions are suitable for certain life stages of Colorado squawfish, humpback chub, and razorback sucker, despite the proliferation of many non-native species. Population augmentation and study of razorback sucker and bonytail chub in the upper Green River are proceeding and will hopefully aid in their recovery. Flow needs of the rare fishes in the mainstream Green River are influenced by many factors including time of year, life history stage, and associated species. As indicated in Figure 9, reproductive activities of the Colorado squawfish, razorback sucker, and humpback chub are associated with spring and early summer hydrologic events. Additional alteration of these hydrologic events may affect migration of Colorado squawfish and razorback sucker, and spawning of Colorado squawfish, razorback sucker, humpback chub and other native fishes. Gradually declining summer flows and maintenance of low, stable flows in late summer and autumn are necessary for growth and survival of young Colorado squawfish and presumably young of the other rare native fishes. Abrupt fluctuations in surface water elevation in late summer to spring could strand Colorado squawfish (larvae and adults) and presumably other backwater inhabitants. Abnormally high releases from Flaming Gorge Dam are correlated with significant recruitment failure of Colorado squawfish. Stable winter flows through ice breakup provide stable shoreline habitats and reduce shoreline scouring. Stable flows are therefore important to overwinter survival of young and adults of native species. Very high winter flows may provide an undesirable winter refugium for some introduced fishes including channel catfish. Alteration of historic flow regimens may have directly impacted razorback sucker recruitment in the Green River basin, and its survival is imperiled. Humpback chub is rare in Whirlpool Canyon of the upper Green River, and its status and habitat needs in Desolation and Gray Canyons unknown. Studies of factors limiting the distribution and abundance of rare fishes in the Green River are complicated by the variability of the environment (e.g., seasonal fluctuations in discharge, temperature, food base, and species abundance) and logistical problems associated with studying fishes in large, turbid rivers. The determination of limiting factors for rare fishes is further complicated because of limited life history information. Habitat use of endangered fishes may only reflect temporary, seasonal, or marginal habitat availability. Thus, caution must be exercised when determining habitat needs and limiting factors for fishes in decline, and professional biological judgement must be recognized as critical in data interpretation. We stress the need for system-level cognizance and interpretation in evaluating factors that 25