56 HYDROLOGY 



minimum flow and the possibilities of storage for increasing the 

 same in order to safely develop the enterprise. 



Factors Affecting Stream Flow. It was previously shown how 

 absorption and the natural storage of underground waters had a 

 very important bearing on the regularity of the stream flow, these 

 waters being the main source of supply during the dry season. 

 It was also shown how vegetation and heavy forests will inter- 

 pose an appreciable time element in the run-off. In addition 

 there are several other factors which may delay the same. So 

 for example, where snow and ice form to considerable depths, a 

 large part of the precipitation may be stored for weeks or months. 

 On the other hand, the effect of an abnormally dry or wet season 

 may extend beyond a single year; since it somewhat affects the 

 conditions of the ground during the next year, so that a succession 

 of dry or wet years may disturb the expected relations of run-off to 

 rainfall producing unexpected drought or flood. 



Most watersheds have some natural storage features tending 

 to equalize the stream-flow as compared with the rainfall. In the 

 northern part of the United States most watersheds have distinct 

 periods in the water year as distinguished from the calendar year. 

 These are usually classified into storing, growing and replenishing. 

 Beginning about the first of December water begins to accumulate 

 in the form of snow, ice, or in the soil, and for months there is an 

 increasing storage. With the beginning of spring the storage 

 period terminates, and the growing period begins, during which 

 moisture is absorbed. By harvest time vegetation has ceased 

 to absorb moisture and it usually tends to replenish the ground 

 until the end of the fall. That these periods have great effects on 

 run-off can readily be appreciated and how great the effects may 

 be can well be judged from the typical figures in table XXIII. 



The curves in Fig. 17 indicate graphically the approximate 

 relations for this area, and will show that for the same watershed 

 the percentage run-off increases with increasing rainfall. 



Lakes, ponds and swamps are, of course, of great value in 

 regulating the stream-flow, and very frequently broad rivers have 

 storage possibilities not readily appreciated at first. In localities 

 where there is a pronounced dry season extending over several 

 months' time, water-power plants have been built in which it is 

 regularly proposed to store water for six months at a time, thus 

 enabling the average daily output of the plant to be increased 



