76 WORK OF RUNNING WATER 



is not the only one, and under some conditions erosion takes place 



without it. 



Transportation. A second element of erosion is transportation. 



The transportation of sediment is to be distinguished from the 



transportation of ma- 

 terials in solution. In 

 so far as mineral mat- 

 ter is dissolved, it be- 



Fig. 65. Diagram of a valley, the top of which is comes a nart o f 

 ten times the width of the stream. 



fluid of the stream. 



The quantity dissolved is too small to influence the mobility of the 

 water sensibly. 



The sediment transported by a stream is either rolled along its 

 bottom, or carried in suspension above the bottom. The coarser 

 materials (gravel and sand) are carried chiefly in the former posi- 

 tion, and the finer (silt and mud) largely in the latter. 



Transporting power and velocity. The transporting power of 

 lunning water depends* on its velocity. Swift streams have much 

 greater power of transportation than sluggish ones, but transpor- 

 tation does not always keep pace with transporting power. The 

 Niagara at its rapids is a stream of great transporting power, but 

 it carries little sediment, because there is little to be had. 



The velocity of a stream depends chiefly on three elements 

 its gradient, its volume, and its load. The higher the gradient, 

 the greater the volume, and the less the load, the greater the velo- 

 city. The relation between gradient and velocity is evident; that 

 between volume and velocity is illustrated by every stream in time 

 of flood, when its flow is greatly accelerated. The relation between 

 velocity and load is less obvious, but none the less definite. Every 

 particle of sediment carried by a stream makes a draught on its 

 energy, and energy expended in this way reduces the velocity. 

 A muddy stream is never so swift as a clear stream of the same size 

 would be, flowing in the same channel. 



How sediment is carried. Coarse materials, such as gravel- 

 stones, are rolled along the bottom of the swift streams which carry 

 them. Their movement is by the impact of the water. The same 

 is true to a large extent of sand grains. So far as concerns the 

 material rolled along the bottom, it is to be noted that a stream's 

 transporting power is dependent on the velocity of the water at 

 its bottom, which is much less than the velocity at the surface, and 

 less than the average velocity. 



