WORK OF RUNNING WATER 



snow; (3) streams are notably swollen after rains, and most after 

 heavy ones; and (4) many small streams which flow during wet 

 weather dry up in times of drought, while others shrink. It is 

 true that lakes, glaciers, and springs feed the rivers, but the lakes, 

 glaciers, and springs derive their supply of water from precipitation. 

 If the slope of a surface were perfectly even, the immediate 

 run-off (the water which flows off without sinking beneath the 

 surface) would flow in a sheet. There are slopes so smooth that 

 water runs off them in this way; but on most slopes, even those 

 which appear to be regular, there are small unevennesses, so that, 

 although the run-off may start as a sheet, it is soon concentrated 

 into rills and streamlets which follow the depressions. The smallest 

 streamlets unite to form larger ones, and the little rills, after many 

 unions with one another, reach valleys which have permanent streams. 

 Streams which flow but part of the time, as after a rainstorm, dur- 

 ing wet weather, or during but a part of the year, are temporary or 



intermittent streams. 

 Every permanent 

 stream and many 

 temporary ones flow 

 in depressions called 

 valleys. Valleys are 

 therefore about as 

 numerous as streams. 

 The very small de- 

 pressions in which 

 water runs after 

 showers only, are 

 called gullies if they 

 are very small (Fig. 

 40), or ravines if 

 somewhat larger. 

 Gullies and ravines 

 are but small valleys, 

 and just as the tiny 



Fig. 40. A gully developed by a single shower. 

 (Blackwelder.) 



streamlets unite to form creeks and these to form rivers, so the little 

 gullies, in which the smallest temporary streams flow, generally 

 unite to form wider and deeper ones (Fig. 41). These, in turn, 

 join one another and become ravines, which are but larger depres- 

 sions of the same sort, and ravines lead to valleys just as gullies 



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