Evaluation of Instream Flow Methods and Determination

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14 e Multiple Transect (R-2 Cross) Method The minimum number of calibration flows (field discharge measurement) used was one and the minimum number of cross sections was two (Table 5). When only one calibration flow was used in the analysis, it was generally because the other calibration flows were too high or too low to be of any use in the analysis process, i.e., percent wetted perimeter, average depth, and average velocity at that particular flow never met the minimum levels established as the limiting factors. In all study areas at least one transect of three types of stream habitat (riffles, runs, pools) were selected to be included in the Multiple R-2 Cross Transect analysis. The reader is referred to Stalnaker and Arnette (l976) for concise definitions of these terms. Additional terms were added to more fully describe the particular stream transect being evaluated. These transects were selected because they were considered representative of the actual stream configuration in the area being evaluated. Each transect was analyzed according to (he criteria set forth in Table 3 for the "critical riffle" study transects together with the modifications in the criteria for run and pool habitats as set forth in Table 3-A. e Once the average velocities, average depths, percent wetted perimeters, and recommendations for instream flows were made for individual transects within a stream reach, the simple average was taken for each parameter, and the average recommended flow in cfs was used as the minimum flow for the Multiple Transect R-2 Cross analysis process (Table 5). For each transect analyzed, the parameters that first became limiting by falling below the minimum criteria are underlined in Table 5. In 50 out of 77 instances or 65% of the time average depth became the limiting factor first. Average velocity became limiting or co-limiting 13 times or 17% of the time and percent wetted perim- eter became limiting or co-limiting 14 times or l8% of the time. The recommended flows developed using this multiple transect analysis were very similar to the flows obtained using the single transect method. These results indicate that the extra time ex- pended on the multiple transect method is probably not worthwhile since there is no greater resolution or refinement in the minimum flow recommendation. The only advantage might be that the water courts may consider the recommendation more reliable since it was based on several different cross-sections over several types of stream habitat. e - "'1 ~ i!i ~ teet -"-J' ". ~. ,'i ;~ r~ " S -' ~~ -i J j -~ i I