TRANSPORTATION 



77 



of line sediment, such as silt and mud, are 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. Yet they do not sink through the water and 

 come to rest at the bottom. 



A particle of sediment in running water is subject to two prin- 

 cipal forces, that of the current 

 which tends to move it nearly hori- 

 zontally down stream, and that of 

 gravity which tends to carry it to 

 the bed of the stream. As a result, 

 the particle tends to move in the 

 direction which represents the re- 

 sultant of these forces (Fig. 66). If 

 a river were the simple straightfor- 

 ward current which it is popularly 

 thought to be, a particle in suspen- 

 sion would reach its bottom in the 

 time it would take to sink through 

 an equal depth of still water; for the 

 descent would be none the less cer- 



Fig. 66. Diagram to illustrate 

 the relative strength of the two 

 forces acting on a particle in sus- 

 pension. 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 acceleration of 

 velocity of fall. 



tain and scarcely less prompt because of the forward movement of 

 the water. The current would simply be a factor in determining 

 the position of the particle when it reached the bottom, not the 

 time of reaching it. Very fine particles, like those of clay, sink less 

 readily than coarser ones, because the former expose larger surfaces, 

 relative to their mass, to the water through which they sink. But 

 even such particles, unless of extraordinary fineness, would pres- 

 ently reach the bottom if acted on only by a horizontal current and 

 gravity. Since even sediment which is not of exceeding fineness is 

 kept in suspension, it is clear that some other factor is involved. 

 This is found, in part at least, in the subordinate upward currents 

 in a stream. 



Where a bowlder occurs in the bed of a stream (Fig. 67) a part 

 of the water which strikes it is forced up over it. If there are many 

 bowlders, the process is repeated frequently, and the number of 

 upward currents is great. Any roughness will serve the same pur- 

 pose, and every stream's bed is rough to a greater or less extent. 

 Roughnesses at the sides of a channel start currents which How 



