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$ are quite unrepresentative of the general riverine habitat type, With regard to instream recreation potential, a critical reach might be chosen on the basis of safety, access, or passage. The critical reach concept might also be used to evaluate such other instream concerns as: navigation, waste assimila- tion, or sediment transport. The representative reach concept reflects recognition of the importance of the structure and form of the entire stream in sustaining a particular instream resource, Application of the representative reach concept is appropriate when limited life history information is available on the target species, or when limiting stream channel conditions (critical reaches) cannot be identified with any degree of certainty, The representative reach concept 1S also the more appropriate approach for analysis of species interactions or complimentary instream uses, Two essentials of study site selection uS1ng the representative reach approach are homogenei ty and randomness. lnit i ally, the stream must be divided (stratified) into rather homogeneous segments based upon biological community structure, stream channel morphology, stream flow regime, and humar: activities, These stratified river segments are then sub-divided into popu;a- tions of candidate representative reaches by either implicit or explicit zonation techniques (Bovee and Milhous 1978), and three or four candidate reaches are randomly selected from each of the respective populations of candidate reaches, Following this office work using maps and aerial photos. an on-site inspection is made of the candidate reaches to confirm that they are generally representative of the river segment(s) being evaluated, The actual study site(s) is then chosen from among the three or four candidate reaches on the basis of access, manpower and financial resources, and the limitations and safety of field personnel, What must be kept foremost in mind 29