LAND WATERS STREAMS 



145 



its bottom. This is much less than the surface velocity, or even 

 the average velocity. 



The particles of fine sediment, such as silt and mud, are fre- 

 quently carried by streams quite above their bottoms, as shown by 

 (the muddiness of many streams. Most particles of mud are small 

 bits of mineral matter, the specific gravity of which is between 



two and three times that of water. _ 



| Why do they not sink through the 

 ; \\-ater and come to rest at the bottom 

 of the stream? 



A particle of sediment in running 

 vater is subject to two principal 

 brces, that of the current which 

 :ends to move it nearly horizontally 

 own stream, and that of gravity 

 which tends to carry it to the bed of 

 :he stream. As a result of these two 

 'orces, the particle tends to move in 

 he general direction which repre- 

 sents the resultant of these forces 

 'Fig. 109) . If a river were the simple 



Fig. 109. Diagram to illustrate 

 the relative strength of the 

 two forces acting on a particle 

 in suspension. The arrows 

 represented by full lines show 

 the relative strength of the 

 two forces when the stream's 

 velocity is about 5 miles per 

 hour. No account is taken in 

 the diagram of the viscosity of 

 the water, or of the accelera- 

 tion of velocity of fall. 



straightforward current which it is popularly thought to be, a 

 Darticle in suspension would reach its bottom in the time it would 

 ,ake to sink through an equal depth of still water; for the descent 

 ;vould be none the less certain and scarcely less prompt because 



. HO. Diagram to illustrate the effect of irregularities, a and b, in a 

 stream's bed, on the current striking them. 



)f the forward movement of the water. The current would simply 

 a factor in determining the position of the particle when it 

 eached the bottom, not the time of reaching it. Very fine particles, 

 ike those of clay, sink less readily than coarser ones, like grains of 



