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35 <br />Ai = width of the ith section, and <br />n = total number of sections along the transect. <br />It should be noted that if both weighting factors are given <br />values of one or if one or both are equal to zero, then WUW is <br />equivalent to UW. The techniques used to specify weighting factors <br />have ranged from quantitative expressions based on the opinion of <br />fisheries experts to a probablistic function of fish presence (e.g., <br />Bovee and Cochnauer 1977). In all applications of habitat weighting <br />criteria, however, care should be taken to ensure that the <br />site-specific preferences of fish are addressed as accurately as <br />possible. If existing weighting functions originally developed in <br />other geographic areas are used in an instream flow study, <br />justification for their applicability to local fish populations should <br />be provided. <br />• Instream flow recommendations derived from usable width methods <br />are made by examination of the UW vs discharge curves. The minimum <br />acceptable flow for salmonid migration (passage) is that flow which <br />meets the minimum depth and maximum velocity criteria on at least 25% <br />of the total transect width and on a continuous portion equalling at <br />least 10% of the total width (Thompson 1972). An optimum spawning <br />flow provides suitable flow conditions, with respect to the selected <br />depth and velocity criteria, over the most gravel at critical spawning <br />transects. The discharge that provides suitable flow conditions over <br />80% of the gravel available at the optimum flow is the minimum <br />requirement for spawning. A major strength of the usable width <br />approaches is that flow recommendations can be made on a seasonal <br />basis with time-variable life stage requirements. An important <br />component of this type of instream flow assessment is the construction <br />of a periodicity chart which indicates the life stages present in each <br />season.