30 J. L. Greenleaf — The Hydrology of the Mississijppi, 



Another item of interest, if not of importance, is the amount 

 of the annual rainfall and its distribution. Although the flow 

 of the streams, or " run off " as it is sometimes called, depends 

 upon the rainfall, yet it bears by no means a direct and simple 

 relation to the latter. The percentage varies with the nature 

 of the country, the climate, and the season of heaviest rainfall, 

 from eighty or ninety down to less than five per cent. Merely 



Fig. 1. 



to suggest some of the causes of its variation, the following 

 may be mentioned. A steep and rocky region will rapidly 

 pour most of its rainfall into the stream. A level country, 

 over which the water settles and flows off slowly, may expend 

 nearly all in evaporation. A sandy soil absorbs the rain like a 

 sponge as fast as it falls, to feed the never failing springs, and 

 consequently gives a high ratio of flow ; or, on the contrary, 

 the water may sink too deep for that, and possibly make its 

 first appearance in some far distant valley belonging to another 

 watershed. A winter rain storm will perhaps all find its way 



