at this inflection point. An example of a wetted perimeter-discharge curve 

 for a riffle cross-section is shown in Figure 1. 



Wetted perimeter was chosen because it was judged to be the single para- 

 meter most likely related to the amount of habitat available for adult trout 

 in the boulder and cobble strewn rivers of the study area. It was reasoned 

 that a wetted perimeter-habitat relationship could exist since wetted perimeter 

 is a "bottom" measurement and adult trout are primarily oriented to the river 

 bottom. It was assumed that once the rate of loss of wetted perimeter begins 

 to accelerate (at the inflection point on the wetted perimeter-discharge 

 curve), the loss of adult habitat is also accelerating. Trout habitat and 

 wetted perimeter relationships have not been documented in the literature 

 at present. This approach assumes such relationships exist. 



A riffle cross-section was chosen because riffles are the area of a 

 stream most affected by flow reductions. It was assumed that if a given flow 

 provided adequate adult habitat in riffles, more than adequate habitat would 

 also be provided in pools and runs, areas normally occupied by adult trout. 

 Riffle cross-sections would presumably provide more than a minimal adult 

 recommendation. 



The wetted perimeter-discharge curve for each riffle cross-section was 

 derived using a wetted perimeter predictive (WETP) computer program developed 

 by the MDFWP (Nelson, 1980). The WETP program uses two to ten sets of stage 

 measurements taken at different known discharges at each cross-section to 

 establish a least-squares fit of log-stage versus log-discharge. This 

 rating curve coupled with the cross-sectional profile is all that is needed 

 to predict the wetted perimeter for each flow of interest. In this study, 

 the WETP program was calibrated to field data collected at three to four 

 different flows for each riffle cross-section. 



Multiple Transect Method 



The multiple transect method involves the use of the wetted perimeter- 

 discharge relationship for a composite of four to seven cross-sections to 

 derive flow recommendations. Cross-sections were generally placed within a 

 single riffle-pool sequence to sample several habitat types. The computed 

 wetted perimeters for all of the cross-sections at each flow of interest were 

 averaged and the flow recommendation selected at the inflection point on the 

 plot of average wetted perimeter versus discharge. The wetted perimeter- 

 discharge curves were derived using the WETP computer program. The program 

 was calibrated to field data collected at three to four different flows for 

 each cross-section. An example of a wetted perimeter-discharge curve for a 

 composite of cross-sections is shown in Figure 2. 



As with the single transect method, wetted perimeter was chosen because 

 it was assumed to provide an index of the amount of adult trout habitat. It 

 was reasoned that, if proven reliable, multiple transect recommendations would 

 be more acceptable to the water courts since the recommendations are based on 

 several cross-sections encompassing various habitat types. 



