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24 BIOLOGICAL REPORT 24 new rhithron food web, including the valuable trout fishery, was severely damaged by the epi- sodic side flows that occurred during summer 1991 and 1992, when the regulated flows were at or near the 300 cfs minimum. Benthos and fish were smothered by fine sediments, a situation that has persisted owing to the lack of a spring flush to clean the substratum (my observation). Recent experimental flows to help determine flow recommendations for endangered fishes in the Gunnison River reached 4,000 cfs in 1992 but were insufficient to rearrange alluvium entrained in the river channel (Elliott and Parker 1992). Because of the interactive effects of (1) a lack of spring peaks or other flushing flows, (2) an ex- tended period of minimum flow (both 1 and 2 due to drought and regulation), (3) warmer tempera- tures associated with low flows, and (4) episodic loading of the channel from ephemeral side flows, the position of the discontinuity moved upstream during 1991-92, and side channels and eddies filled with fine sediments and vegetation. Today, the riparian corridor of the river is densely vege- tated, and surface water and groundwater ex- change with critically important backwater sys- tems (e.g., Fig. 2) has been altered or lost (Stanford and Ward 1992b). The food web in the lower part of the Gunnison Gorge remains im- paired owing to persistent fine sediments in and on the substratum, which prevents establishment of a productive biofilm and restricts attachment sites for zoobenthos. The Gunnison River case history illustrates a classic response of a stream to regulation. Similar results have been recorded elsewhere (e.g., Petts 1986; Stanford and Hauer 1992). An upstream discontinuity exists on the Colorado River (Voelz and Ward 1991) and the Green River (Pearson and Franklin 1968; Pearson et al. 1968), although the latter is significantly reset toward predam pota- mon conditions by the Yampa River (Annear and Neuhold 1983). Conclusions Based on Review of the Ecological Literature Pertaining to the Endangered Fishes and the Regulation of Flow The endangered fishes remain relatively rare in the Upper Colorado River Basin as a conse- quence of stream regulation and possibly pre- dation and other interactions with nonnative fishes. Recruitment of adults has not been clearly demonstrated for any of the species, but age structure of squawfish suggests adult re- cruitment is occurring (i.e., larvae, YOY, juve- niles, and adults are collected each year in the Upper Colorado River Basin, although all age classes often are not observed in the same river segments). Clear evidence for adult recruit- ment is lacking for the other species. In recent years gravid razorback sucker and humpback chub were collected during the spawning sea- son at a few sites, and a few YOY were col- lected. Bonytail chub seem to be extirpated. 2. The distribution, relative abundance, and some important physical habitat preferences of squawfish, humpback chub, and razorback sucker (in that order) are reasonably well known (Fig. 6) and documented in peer-re- viewed literature. However, only the life his- tory of squawfish is fairly well understood. Important aspects of the life history and habi- tat preferences for humpback chub and razor- back sucker remain to be documented. Much of what is known about the life history and popu- lation dynamics of humpback chub is based on unpublished studies in the Grand Canyon, which may or may not be relevant to the Upper Basin (e.g., no population in the Upper Basin is known to migrate into a tributary to spawn, as occurs in the Little Colorado River within the Grand Canyon; Larry Crist, personal com- munication). Detailed information about spawning and rearing is lacking for humpback chub and razorback sucker throughout the Up- per Basin, and virtually nothing is known about bonytail chub. Moreover, accurate esti- mates of annual population size are problem- atic for all of the fishes (Tyus 1992), and mark- recapture studies using the new transponder tag technology are warranted. On the other hand, a great deal more is known about the distribution and abundance of the fishes, ex- cept bonytail chub, than is known about the influences of river hydraulics, sediment trans- port, and riparian controls on the food web that supports the fishes. In other words, the data on which current flow recommendations are based primarily describe the distribution and abundance of the fishes, not the ecosystem- level processes and responses that determine productivity. 3. Strong linkages between trophic (food web) and geomorphic attributes of the Upper Colorado River Basin ecosystem are variable in time and space. For example, algae (periphyton) and