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~ have been very labor intensive. Recent advances in surveying techniques and computer capabilities allow for collection and processing of much larger databases. Also development of new tools for in stream flow and habitat availability relationships, i.e. two-dimensional flow models, may eliminate the need for microhabitat suitability curves used by IFIM. The goal of this project is to develop and validate a methodology for determining in stream flow recommendations for warm water fish communities in Colorado. The approach is to determine relationships between habitat availability and flow using a two- dimensional flow model to simulate meso-habitat diversity and abundance over a range of low flows on sections of three different rivers. Also fish population and species life history data will be collected within each of the study sites to provide habitat use and preference data. The data collected for this study is part of a larger study addressing the need to restore depleted flows on the Yampa River. (Anderson, Modde, Miller and Ivring, in press). Minimum flows and habitat for the aquatic community were determined using an "inflection point" methodology. This approach identified regulating properties of the channel that strongly influence habitat quality and quantity. For example when stream width is plotted as a function of flow the curve indicates how width increases as flow fills the channel. Typically, stream width quickly increases up to a certain level, but as flow continues to increase, depths and velocity increase at faster rates since stream width is approaching channel width. Most of this analysis focused on riffles because they are the habitat most sensitive to low flows. Also riffles have an important ecological function since they are strongly associated with macroinvertebrate communities and other forage speCIes. 2