TRATs^SPORTATION BY EUNNING WATER. 15 



time of flood, is due to earthy matter suspended iu the water. These facts 

 are too obvious to ueed elaboration. 



SEDIMENTATION. 



For the present purpose it is not needful to go into an elaborate dis- 

 cussion of that difficult subject, the hydraulics of streams and other moving 

 waters. We have seen that an increase in velocity of curre.nt causes an 

 increase of transporting capacity proportioned to the sixth powers of the 

 velocities. A decrease in velocity causes, therefore, a proportionately 

 large decrease in carrying power. Now, the velocity of a stream depends, 

 assuming the force of gravity as constant, partly on degree of decli-sity 

 and partly on the friction to which it is subjected. The friction includes 

 the Adscosity of the water, the friction of the water and of the suspended 

 particles against the sides and bottom of the bed, and the friction of the 

 suspended particles against one another. In the case of currents containing 

 a large amount of solids in suspension, the friction resulting from the 

 presence of the suspended matter becomes so great, as compared with the 

 other sources of friction, that the velocity is determined chiefl}^ by the load 

 of sediment the stream has to carry. Any enlargement of the channel of 

 an ordinary stream, unless accompanied by a corresponding increase of 

 water supply, causes a slowing of the current. Conversely, a narrowing 

 of the channel acts like a partial dam; it increases the slope of the surface 

 and is accompanied by a more rapid flow. Any slowing of the current 

 will cause matter which could just be transported at the former velocity to 

 be thrown down. Such matter is called sediment, and the same term is 

 often applied to particles of solid matter while they are yet held in suspen- 

 sion. Aqueous sediment naturally settles in successive layers, and such 

 drift is said to be stratified. When the cim-ent is of uniform velocity, the 

 particles deposited are of uniform size. Upon this depends the sorting or 

 classifying power of water. 



One of the most common applications of these principles is seen when 

 a sediment-laden stream flows into a large body of rather still water, like 

 the sea or a lake. The currents are checked gradually, and there is a hori- 

 zontal assortment of sediment, the coarsest matter being deposited near the 

 mouth of the stream and the sediment becoming progressively finer as the 



