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Study Site Selection Time and financial resources are seldom adequate to support the field work necessary to document stream flow-habitat relationships throughout an entire stream. Therefore, it is important to select study sites which are both characteristic of the stream, and capable of providing pertinent informa- tion, Either of two approaches to study-site selection can be utilized with the incremental methodology; (1) critical reach, and (2) representative reach. Under the critical reach concept the study site is selected on the basis of its restrictiveness, l.e., stream flow characteristics at the critical reach are limiting attainment of the full potential of the instream resource, Associated with the critical reach concept is acceptance of the assumption that adequate stream flow through the critical reach will provide for satis- factory stream flow conditions throughout the remainder of the stream, The critical reach concept implies that rather extensive knowledge of both the stream (hydrology, water-quality, channel geometry) and the instream resource (species composition, life history, passage requirements) exists, One must be satisfied that conditions at the selected study site(s) are, ln actuality, limiting the instream resources potential. It should also be recognized that critical reaches only provide information specific to a particular set of.questions; thus little opportunity would exist to utilize the critical reach data base to address questions pertaining to other instream uses. A fisheries manager might select a critical reach on the basis of migra- tion blockages, overwintering areas, or essential spawning and rearing habitat, In the case of endangered species, critical reaches might be selected on the basis of a unique combination of microhabitat conditions which , 28