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. . . {) :~~ ~;: tl l : ~ U v ... ~) 'f J R2CROSS The CWCB's R2CROSS methodology (Espegren 1996) is an example of a habitat-retention, standard-setting model. The R2CROSS methodology uses field data collected at a single stream transect to model instream hydraulic parameters at unobserved streamflows. R2CROSS is based on an engineering formula called Manning's equation. Manning's equation is most applicable when used in riffle stream habitat types where streamflow is controlled by channel geometry rather than a downstream, flow-control point. R2CROSS instream flow recommendations are based on the assumption that riffles are the most critical stream habitat-type and that preserving riffle habitats will also protect other aquatic stream habitats; i.e., pools and runs (Nehring 1979). Nehring (1979) identified average depth (xd ), average velocity (xv), and percent wetted perimeter (% WP) as important indices of coldwater fish habitat (Table 2). The xd and %WP criteria are functions of stream top width and grassline-to-grassline wetted perimeter, respectively. A constant X of 1 ft/sec is recommended for all streams. The DOW has determined that these three v parameters are good indices of flow-related stream habitat quality and that maintaining these parameters at adequate levels across riffle habitat-types will also maintain adequate aquatic habitat in pools and runs for most life stages of coldwater fish species and aquatic invertebrates. These three critical hydraulic parameters are predicted within an R2CROSS staging table at various levels of discharge (Espegren 1996). Nehring concluded that year-round instream flow recommendations for coldwater fish species should set at the streamflow which maintains two of the three criteria presented in Table 2. Based on Nehring's findings, CDOW routinely recommended single, year round flow amounts until the mid 1980's. With the passage of Senate Bill 414 in 1981 and in response to water availability concerns, CDOW biologists began developing "seasonal, split-flow" recommendations in order to match seasonal changes in the biologic needs of the fish with the natural variability in the annual stream hydrograph. This practice has evolved into CDOW's current practice of recommending summer instream flows which meet all three hydraulic criteria and winter flows that meet two of the three criteria (Skinner pers. comm.). Aquatic biologists may elect to increase or decrease the resulting flow recommendations based upon biologic judgement and expertise with regard to stream conditions, species composition, and aquatic habitat quality. R2CROSS may be an equally appropriate tool for developing biologic instream flow recommendations on cool and warmwater stream segments when the habitat requirements of the aquatic species assemblages that inhabit these stream reaches can be expressed in terms of the hydraulic parameters estimated in the R2CROSS staging table; i.e., wetted perimeter, velocity, and depth, etc. 6