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The PHABSIM methodology allows for two a~roaches to study site <br />selection--critical reach and representative reach (Trihey 1979>. A <br />critical reach is an area that is extremely important for the continued <br />survival of a species. Determining a critical reach requires a rather <br />extensive knowledge of the stream hydrology and channel geometry in <br />addition to species-specific life history requirements. Given the limited <br />knowledge of hydrologic and biologic interactions controlling the fish <br />populations, the critical reach approach was considered inappropriate <br />method upon which to select a site. A representative reach on the other <br />hand depicts the stream as a whole rather than concentrating on one <br />specific area. It is intended to incorporate all the different variations <br />which occur within the sec~nent of the stream and indicates the importance <br />of hydrologic and geomorphic gradations within a watershed for determining <br />macrohabitat conditions. <br />3. Field Measurements <br />Channel geometry and hydraulics <br />Field data collection for PHABSIM is based upon measurements of <br />certain parameters at multiple stream cross-sections according to <br />techniques described by Trihey and Wegner (1981). Site specific data. <br />essential for successful use of a hydraulic simulation model is obtained <br />at a min~~n of four cross sections referenced to a common bench mark to <br />characterize the general cross-sectional shape and longitudinal streambed <br />profile. The depths and velocities measured at each cross section <br />represent the average characteristics of the stream hydraulics within an <br />imaginary cell extending half the distance to the next upstream and <br />downstream transects and half the distance to each adjacent vertical. <br />7 <br />