308 



ELEMENTS OF GENERAL SCIENCE 



its load of silt, but when its rate of flow is reduced, it drops 

 part of its load. 



Not only does vegetation exert a strong influence upon 

 the deposit of silt but it also helps to prevent erosion. 

 The parts of plants above the soil retard the flow of water, 

 and the roots within the soil help to prevent the removal of 



FIG. 149. Deposit of soil by rivers 



The shaded portion in prehistoric times was a part of the Gulf of Mexico, but has 



been made into rich lands chiefly by deposit of material carried down by the 



rivers. Redrawn from Popular Science Monthly, 1916 



soil. In many places one may see instances in which water 

 has been turned aside and the soil left where plant roots 

 were well established. One of the most effective ways of 

 preventing erosion along hillsides and stream banks is by in- 

 troducing rapidly growing plants which help to hold the soil. 

 311. Deposit of soil from erosion. It is estimated that at 

 least one third of the sediment carried by streams of water 

 reaches the sea. The other two thirds is deposited in various 

 ways. A study of a stream bank will almost always show 



