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INTRODUCTION Criteria are necessary for interpretation of any information base developed for a species, group of species, or recreational activity used to assess the impacts of altered streamflow regimes on instream habitats. As such, biological criteria are primarily aimed at those parameters affecting fish distribution-which are most directly related to streamflow: namely depth, velocity, temperature, and substrate. Cover, a habitat parameter of paramount importance to many species, is also indirectly related to streamflow. Cover may be incorporated into an instream flow assessment by evaluating the utility of available cover objects in reference to the flow parameters around them. The application of a methodology is dictated by the critera avail- able for the target species. Criteria are as much a constraint on streamflow assessments as are time, expense, manpower, and accuracy requirements. Fish species for which biological criteria are being developed can be roughly divided into five classes: i. Economic species sport and commercial fishes which are con- sidered important and desirable by the public, and are of importance to the objectives of the State and Federal management agencies. 2. Indicator species those species with narrow habitat toler- ances, inhabiting areas of streams which are particularly sensitive to changes in flow. It is assumed that if conditions remain suitable for the indicator species, all other resident species will continue to find suitable habitat there and species composition will remain constant. 3. Endangered or threatened species - these are species which may be locally abundant, but with a highly restricted distribution, or those which occupy much of their former range but in greatly reduced numbers. 4. Nongame species - species which may act in direct competi- tior with any of the other desirable species. 5. Forage species - this classification applies to those organ- isms occupying intermediate positions in the food chain, including both forage fish and aquatic invertebrates. 1