THE WORK OF RUNNING WATER. 123 



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 sedi- 

 ment offers the water a greater surface on which to work. 



CONDITIONS AFFECTING THE RATE OF EROSION. 



In considering the rate of erosion, both the work of the stream 

 in its valley and that of the general run-off are to be considered. 

 The conditions which favor the most rapid erosion in a stream's channel 

 are not necessarily those which determine most rapid degradation in 

 the ba^in outside of the valley. 



The Influence of Declivity. 



In general the greater the declivity the more rapid the rate of 

 erosion, whether in the stream's channel or on the slopes above it. 

 The truth of this conclusion is illustrated by the great erosive power 

 of swift streams as compared with slow ones. 



It does not follow, however, that high declivity favors each element 

 of erosion. The effect of declivity on weathering is far from simple. 

 For example, great declivity, by allowing more of the rainfall to flow 

 off over the surface, and by causing it to floAv off more promptly, re- 

 stricts the work of solution, and therefore of decomposition, both 

 at the surface and beneath it. High declivity is also unfavorable to 

 the growth of vegetation, and so to the wedge-work of roots. On the 

 other hand, a given amount of wedge-work of roots and ice is more 

 effective where the slope is steep than where it is gentle, for such ma- 

 terials as are loosened descend the slopes more readily. The prompt 

 removal of weathered materials, by exposing fresh surfaces of rock, 

 accelerates weathering. The total amount of weathering may there- 

 fore not be diminished by the increase of slope, even though certain 

 of its processes are hindered. 



The effect of high decli^'ity on transportation, the second element 

 of erosion, is too patent to need explanation. 



Corrasion likewise is favored by high declivity, for the abrasive 

 power of a stream increases as the square of its velocity. With cor- 

 rasive power increased, corrasion T\ill also be increased if the water 



