LAND WATERS STREAMS 



113 



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

 run-off at any given time 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 con- 

 centrated 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 



Fig. 72. A gully developed by a single shower. (Blackwelder.) 



have permanent streams. These may be small (creeks or brooks) 

 or large (rivers) . Streams which flow but part of the time, as after 

 a rain-storm, during 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 depressions in which water runs only 

 after showers are called gullies if they are very small (Fig. 72), or 

 ravines if somewhat larger. Gullies and ravines are but small val- 

 leys, and just as the tiny streamlets unite to form creeks and these 

 to form rivers, so the little gullies in which the smallest temporary 



