80 WORK OF RUNNING WATER 



Not only is a slow stream more likely to have a flat, and therefore 

 a better chance to meander, but it is more likely to take advantage 

 of opportunities in this line, for a slow stream gets out of the way 

 for such obstacles as it may encounter, while a swift stream is 

 much more likely to get obstacles out of its way. 



Corrosion. In most cases the solution (and other chemical 

 changes) effected by a stream is much less important than its me- 

 chanical work. Only when conditions are unfavorable for the 

 latter is solution the chief factor in the excavation of a valley. This 

 may be the case where a stream's bed is over soluble rock, such as 

 limestone, and where the stream is clear, or its gradient so low that 

 its current is sluggish. The solvent power of water is not influenced 

 by the presence of sediment, though the presence of sediment offers 

 the water a greater surface on which to work. 



CONDITIONS AFFECTING THE RATE OF EROSION 



With a given amount of water, the declivity, the character of 

 the rock, and climate, are the principal factors influencing the rate 

 of erosion. 



Declivity. In general, the greater the slope the more rapid the 

 rate of erosion by running water, whether in the stream's channel or 

 on the slopes above. But high declivity does not favor every ele- 

 ment of erosion. It favors some phases of weathering and hinders 

 others, but it favors both transportation and corrasion. Both 

 corrasive power and transportive power increase rapidly with in- 

 crease of velocity, and under these circumstances, corrasion also 

 will be increased if the water has tools to work with, and trans- 

 portation will be increased if there is material which can be carried. 

 Since high declivity greatly increases both the transporting and the 

 corrading power of running water, and favors certain elements of 

 weathering, it is clear that its aggregate effect is to favor erosion. 



Rock. The physical constitution, the chemical composition, 

 and the structure of a rock formation influence the rate at which 

 it is broken up and carried away. 



Physical constitution. The constituents of clastic rocks may be 

 firmly or weakly cemented. The less the coherence the more ready 

 the disintegration, and the finer the particles the more easily they 

 are carried away. If the materials carried are harder than the bed 

 over which they pass, corrasion of the latter is favored. 



Chemical composition. Something also depends on the chemical 



