220 ELEMENTS OF GENERAL SCIENCE 



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

 soil. In many places one may see illustrations showing how 

 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. 



239. 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 the 

 formation of deposits of gravel, sand, and silt. In times of 

 overflow of the channel banks this deposit is large, but we 

 must remember that erosion is also large at such times. When 

 streams overflow, the velocity of the water is usually decreased 

 and much of the load of soil is dropped. In this way large 

 areas along the Mississippi River are annually replenished by 

 the richest soil. Great dangers attend such overflow, however, 

 and effort is made to prevent it by building strong dikes in 

 the attempt to hold the river within the channel. 



At the mouth of such great rivers as the Mississippi and 

 the Yangtse the accumulation of sediment has built up great 

 deltas. Much of the tremendous quantity of soil thus carried 

 into the sea goes far from shore and doubtless sooner or later 

 settles to the bottom to contribute its part to filling the ocean 

 basins. Since this sediment has been eroded from the high- 

 land areas, it is evident that its removal from one place and 

 deposition in another contributes slowly but surely to the 

 processes of leveling the earth's surface. 



In China part of the people near the mouth of the Yangtse 

 River flood their fields at a time when the river is heavily 

 laden with silt in order to replenish their soil. After the 

 silt is deposited, the water is drained away. Untold wealth 

 in soil is constantly being lost to the human race through 

 erosion, and the problem of maintaining the fertility of the 

 soil is annually made more difficult thereby. 



