67 



Top width and wetted perimeter were proportionately less affected by 

 changes in discharge than were the other characteristics in pools, runs 

 and riffles of the Hildreth Section. In pools, mean velocity and cross 

 sectional area were most affected by changes in discharge, followed in 

 order by mean depth and maximum depth. In runs, cross sectional area 

 was most affected, followed in order by mean velocity, mean depth, and 

 maximum depth. The order for riffles was cross sectional area, mean 

 depth, maximum depth, and mean velocity. 



Mean physical and hydraulic values in percent of the mean values at 

 1000 cfs for all 20 cross sections in the Hildreth Section are compared 

 in Figure 17. Mean values for the various characteristics increase as 

 streamflow increases, but the rate of gain becomes less as the volume 

 of flow fills the river channel. Visual inspection of Figure 17 shows 

 that the rate of loss for all of the measured characteristics was greatly 

 accelerated at flows less than approximately 200 cfs. For all water 

 types combined, cross sectional area was most affected by changes in 

 discharge, while top width and wetted perimeter were least affected. 



Cross sectional area, an index of the total amount of living space 

 available for fish, was greatly affected by changes in discharge in all 

 water types. Based on the mean values at bank full flow (1000 cfs), 

 mean cross sectional area in pools, runs, and riffles in the Hildreth 

 Section is reduced by 50% at flows of approximately 275, 311, and 380 

 cfs, respectively (Appendix Table 32). Average daily flows as low as 

 57 cfs (October 16, 1974) were recorded in the Hildreth Section during 

 this study. At this flow, mean cross sectional area in pools, runs, 

 and riffles was approximately 25, 18, and 14%, respectively, of the 

 mean values at bank full flow. 



