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28 <br />or life stage in the affected stream. The staff gage, a graduated <br />stick set vertically into the water column, is established at a <br />location representative of this critical habitat. The stage (water <br />surface elevation) is read from the staff gage at several known flows. <br />An instream flow requirement can be determined by applying a <br />specified habitat criterion (e.g., minimum depth threshold) at the <br />gage location. If upstream control of the stream flow is available <br />and flow rates are known, this analysis can be as simple as raising or <br />lowering the flow until the habitat threshold is satisfied. However, <br />in many other situations, upstream controls will not be available or <br />flow rates will not be known. In these cases, data consisting of <br />discharge measurements and stage heights must be collected. To <br />extrapolate to an unobserved flow which satisfies the habitat <br />threshold, an empirical stage-discharge relationship is derived, <br />either graphically or by calibrating a power function equation: <br />H=aQb, <br />(2-1) <br />1 <br />where H = stage (water surface elevation read from staff gage), <br />Q = discharge, and <br />a, b = regression coefficients fitted to field data. <br />Wesche and Rechard (1980) reported that this type of staff-gage <br />analysis was used by the Bureau of Land Management. Personnel of the <br />U.S. Fish and Wildlife Service also experimented with this technique <br />at small hydroelectric sites in New England (G. Beckett, personal <br />communication). <br />2.4.3 R-2 Cross <br />The Region 2 office of the U.S. Forest Service in Lakeland, <br />Colorado, originally developed this analytical procedure for <br />predicting the hydraulic conditions which could be expected at <br />unobserved discharges. All calculations are based on the Manning _