LAND WATERS STREAMS 115 



erosion. The rate of flow is determined by several conditions, 

 especially (1) the gradient (slope), (2) the amount and especially 

 the depth of the water, and (3) the amount of sediment (load) it 

 is carrying. The higher the gradient, the deeper the water, and 

 the less the load it is carrying, the faster it flows. If it flows off 

 in a sheet, as on a smooth surface, the depth of the water is slight, 

 and the flow is not very swift (unless the slope is very steep), and 

 the wear is correspondingly slight. Such wear is sometimes called 

 sheet erosion. 



Beginning of a valley. 1 . If the slope of the surface is not uni- 

 form, the effect is very different. If there is, for example, a slight 

 depression near the base of the slope (Fig. 75), more of the descending 



Fig. 74. Figure to illustrate the steepening of a slope by general sheet 

 wash; ab would be reduced to ac and to ad successively, if the debris 

 washed down were not deposited at the base. The deposition of debris 

 at the base would modify the lower part of the slope. 



water flows through it than over other parts of the surface, and 

 the greater volume of water following the depression would give 

 it greater velocity. Greater velocity would cause greater erosion, 

 and greater erosion would deepen the depression. The immediate 

 result would be a gully or wash (Fig. 72) . So soon as the gully is 

 started, it tends to concentrate drainage in itself still more, and it 

 is thereby enlarged. The water which enters it from the sides 

 widens it; that which enters at its head lengthens i^ by causing 

 its upper end to advance up the slope; and all which flows through 

 it, deepens it. The enlarged gully will gather more water to itself, 

 and, as before, increased volume means increased velocity, and in- 

 creased velocity, increased erosion. As the gully grows, therefore, 

 its increased size becomes the occasion of still further growth. 

 Continued growth transforms the gully into a ravine, though there 



