Swifter ourrent and deeper water over gravel on this type of riffle 

 renders it uneui table for spawning purposes. Examples of this con- 

 dition can be found between Lewis ton and Rush Creek, and between 

 Douglas City and Browns Creek* 



Expressed in terns of nests per oubic foot per second per mile 

 of stream, the results of the two spawning-bed surveys indicate 

 that »t water flows between 100 and 300 oubio feet per second, there 

 is a very slight decrease in water-use efficiency as flows become greater. 

 Since the water-use efficiency varies only slightly, there would be little 

 ourre in a line projected to show water-flow in relation to nesting 

 capacity* The change in efficiency between these two points should be 

 fairly constant because of tiie counteracting effects of variations in 

 nesting capacity with changes in river flow on the two types of riffles 

 (broad and narrow) already discussed* 



The curve shown in Figure 11 was derived by calculating a 

 theoretical number of nests for each rata of water flow* In this 

 oaloulatlon the number of nests was obtained by multiplying the ntmiber 

 of miles of stream by the flow in cubic feet per second and also by 

 an <effioi«iey factor* This factor was oemputed by subtracting from 

 the observed efficiency factor at 100 cubio feet per second the decrease 

 in efficiency from 100 oubic feet per second to the rate of flow in 

 question* The calculation is algebraically expressed in the follow- 

 ing fomulat 



H = M F 



where N is the number of the nests; M, the miles of stream; F, the river 

 flow; E-,, the water efficiency factor at the lowest flow measure; Eg, the 



water efficiency faetor at the highevt flow measured; L, the lowest flow 

 measured; n, the number of intervals used between the highest and lower 

 flow; and i, the site of the interval in cubic feet per second* 



Rains of short durations and variable intensity commonly occur 

 during the peak of the spawning period, but temporarily increased flows 

 caused by these rains oannot be considered in determiidng normal spawning 

 flows. To elimate errors that might result from these temporary fluctua- 

 tions, only the minimum flows recorded at Lewiston during the peak spawn- 

 ing period (November 1-15) over a period of 18 years are considered 

 (Table 18)* The average minimum flow during this period is 233 oubio 

 feet per second, but since the miniumu flow is below this average in 

 11 of the 18 years, the meditm flow must also be less than average* 

 Flows above 350 cubic feet per second might be slininated as erratic 

 since they result from unusual weather conditions and are widely separated 

 in their ooourrence* The average flow wilii these years excluded is 170 

 o.f.s., which is again greater than the minimum flows of more than half 



54 



