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(Valdez et al. 1982,.Osmundson and Kaeding 1989) and thus may be one of the most important stretches.of the upper Colorado River for this rapidly declining species., Attempts to recover the populations of both the Colorado squawfish and razorback sucker will require identification of the causes of their con- tinued decline, followed by management efforts aimed at removing or amel- iorating, in whatever manner feasible, those factors identified as detri- mental. Importance of Flow Regimes Most variables within the fishes riverine environment are strongly influ- enced by the timing, duration and magnitude of river flows. Flow regimes shape the gross physical structure of the river, such as channel morpholo- gy and substrate type; this structure in turn determines the quantity and quality of various habitat types available for use by fish. The magnitude of flow also influences the properties of the medium in which the fish live, the water itself: its quality, velocity, depth, temperature, turbid- ity, etc. In addition, the fishes biotic environment is also greatly influenced by flows. Just as squawfish and razorback sucker have physical habitat preferences and limitations, so do the other species of aquatic life with which they interact, either favorably or unfavorably. Densities of food items, predators, and competitors of the rare fish increase or decrease depending on how well the flow regime suits their species-specif- ic needs. Modern man's alteration of natural flow regimes An the Colorado River basin has no doubt had a;profound effect_on the environment,of the indige- nous aquatic species there. Though flows in the Colorado have always 2