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is a sample of the conditions found in a larger stream reach, which allows the extension of results obtained from the study area over the larger reach. In turn, the characteristics of the study area can have a profound effect on the ability to adequately simulate the hydraulics of the stream. The selection of a hydraulic simulation technique may be largely dictated by limitations imposed by the study area. This section describes two subjects as related to study area characteristics: 1. Site selection extens-ions into space as a function of method- ological approach. 2. Limitations of hydraulic simulation' models imposed by on-site conditions. EXTENSIONS INTO SPACE: METHODOLOGICAL APPROACH Critical Reach Concept Under the critical reach concept, the study site represents the type of area within the stream which is most sensitive to changes in flow and is assumed to be "critical" to the fish population by limiting the success of a particular life stage. This concept further assumes that if adequate flow is maintained through a critical reach, adequate habitat conditions will prevail for species throughout the entire stream. The critical reach approach requires more than casual experience with the dynamics of the fish population in a given river. If the limited life stage or critical reach cannot be clearly identified, one's reasoning and analysis may result in a questionable flow recommendation. For example, low recruitment of young of the year might be known to be limiting to the population of adults. A logical assumption might be that the availability of spawning area is the limiting factor. The critical reach would be selected over a heavily utilised spawning area. If spawning area is in fact the limiting factor to recruitment, the examination of flows over the spawning grounds would be entirely appro- priate. However, if the limiting factor for recruitment were inadequate habitat conditions for fry, a flow regime based on increasing spawning could actually increase fry mortality. This could result in an outcome. opposite of that desired. In this case either the assumption concerning maintenance of "noncritical" habitat was not valid, or the wrong type of site was chosen as a critical area. However, in many instances the field investigator will have suf- ficient information about a stream and its fish population to estimate the limiting factors and their associated critical reaches. For purposes of facilitating site selection and assessment, the critical reach should meet two basic criteria: yZ