Riffles are the area of a river most affected by flow 

 reductions. It is reasoned that by maintaining adequate 

 physical conditions in riffles, more than adequate conditions 

 would also be maintained in pools and runs, areas normally 

 occupied by adult salmonids. 



The wetted perimeter curve for each riffle cross-section 

 was derived using the IFG-4 hydraulic simulation computer 

 program developed by the Cooperation Instream Flow Service 

 Group (Main, 1978) . The model was calibrated to field data 

 collected at the following flows: 



Subreach 



Madison (#1) 

 Madison (#3) 

 Beaverhead (#2) 

 Gallatin (#2) 

 Big Hole (#1) 



Calibration Flows (cfs) 



985 



Since well defined riffles are scarce in reach #1 of the Madison 

 River, cross-section #5, which transected a relatively shallow 

 area containing weed beds, was substituted. 



The IFG-4 program does not directly predict wetted peri- ^ 

 meter. The wetted perimeter for a flow of interest is approxi- 

 mated by having the program sum. all of the segment widths 

 having an average depth of at least 0.1 foot. The error asso- ' 

 ciated with this approximation is assumed to be negligible for 

 the relatively large, wide waterways the model was applied. 



Multiple Transect Method ._„^ 



The multiple transect method involved the use of the 

 wetted perimeter-discharge relationship for a composite of 

 four to seven channel cross-sections to derive instream flow 

 recommendations for each of the five reaches. Cross-sections 

 were generally placed within a single riffle-pool sequence 

 to sample several habitat types. The computed wetted peri- 

 meters for all of the cross-sections at each flow of interest 

 were averaged and the instream flow recommendation selected 

 at the inflection point on the plot of average wetted perimeter 

 versus discharge. 



Again, it is assumed that the capacity of a river to sus- 

 tain fish populations decreases proportionately with the de- 

 crease in the physical habitat. The average wetted perimeter 

 is assumed to provide an index of the physical condition of 

 the average habitat type within each river reach. 



