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r emphasis placed on the careful development and testing of habitat suitability criteria used in PHABSIM is a recognition that the microhabitat requirements by some (not all) species are flexible and can be modified by species interactions such as competition and predation. The habitat time series is based on the fact that decisions related to habitat and water manage- ment must address the temporal variability of riverine environments. Habitat bottlenecks are difficult to identify without an abundance of hy- drologic-, habitat-, and population-related data. Nevertheless, the existence of habitat bottlenecks has been demonstrated in cold- and cool-water stream environments. Studies in community ecol- THE INSTREAM FLOW INCREMENTAL METHOLDOLOGY 21 ogy provide evidence that they also exist in warm- water streams. Therefore, the idea of using a habitat time series to relieve potential habitat bottlenecks, avoid exacerbating them, or (rarely) amplify them is a reasonable approach to the problem of temporal variability. A more funda- mental argument for employing the habitat time series is that temporal evaluations are routine in the water management disciplines. The habitat time series allows the presentation of biological information in a format that is familiar to water managers and engineers. IFIM is unique among habitat assessment tools in that it fosters the simultaneous examination of habitat variability over time and space.