Most storms which occur in the study areas appear to be general in nature 

 and deposit rain over a relatively large area. This is clearly shown by 

 a composite discharge hydrograph for a representative period, for I^aybeso 

 Creek, Harris River, and Indian Creek (figo 8). The watersheds of these 

 3 streams cover an area of about 56 square miles. It will be seen that 

 their discharge rises and falls nearly simultaneously, in minor as well as 

 in major variation, indicating that all three adjacent watersheds receive 

 precipitation diiring all, or most, storms which are recorded at Mollis, 

 The total amount and intensity of precipitation received, however, 

 probably varies from watershed to watershed. 



Pattern of streamflow 



Rate and pattern of stream discharge strongly influences both the 

 physical and the biotic elements of the stream. An increase in rate of 

 discharge may have detrimental effects on the spawning facilities of the 

 streams as water velocity is a critical factor in the movement of stream- 

 bed material. ^\nien water velocity is doubled, its cutting pov;er is 

 increased about 4 times; its carrying power is increased about 32 times. 

 Considerable quantities of gravel may be shifted from one position to 

 another, carried downstream, or deposited on dry land when flood waters 

 overflow their banks. Streambank cutting may result in the addition of 

 considerable amoxmts of erosion material in the stream channel. Movement 

 of streambed material during the period when salmon eggs are present in 

 the stream may be detrimental to them. 



Shifting gravel may destroy the eggs either by removing their protective 

 cover, by abrasion, or by smothering due to deposition of erosion material 

 on top of the redds. In addition, it may be physically impossible for 

 spawning salmon to use otherwise desirable spawning beds becaiose of high 

 water velocity. It has been estimated that the optimiim water velocity for 

 spawning pink salmon is about 2 cubic feet per second. It has been noted 

 in this study that in stream sections where velocity is too great, spawning 

 is very light or entirely lacking. 



The small base flow of strear.is is a further indication that detention 

 storage is not great. Ground-water soon drains from the steeply sloping 

 watersheds. The combination of steep slopes, heavy precipitation and 

 small water-holding capacity of the watersheds results in a fairly 

 unstable stream discharge pattern, especially in streams whose water- 

 sheds do not contain sizeable lakes o On streams without lakes, the 

 discharge responds quickly to rainfall intensity and stream level 

 fluctuates quite rapidly within relatively short periods of time. The 

 flashy nature of storm runoff is shown by an analysis of a storm which 

 deposited 1.25 inches of rain within 16 hours on the Maybeso Creek 

 drainage. The creek rose from a height of 0.8 feet to a height of 

 2.6 feet within a few hours after the first rain fell on the watershed. 

 Additional evidence of the highly fluctuating nature of streamflow 

 is found in an examination of the ratios between high and low stream 

 flow stages. This ratio varies between 42:1 and 152:1 in the three 

 streams, I4aybeso, Harris, and Indian. In general, stream discharge 



26 



