STREAMS A>s'D THEIR WORK 175 



erosion is more rapid tkan soil formation, the problem 

 of lessening soil erosion is one of the important tasks 

 of conservation. It takes about 10,000 years to form 

 one foot of residual soil; that is, soil that remains in 

 the place where it is formed. The methods generally 

 employed to prevent soil erosion are: first, deep cul- 

 tivation of the soil allows the water to penetrate the 

 soil and not to run off and carry the soil with it. 

 Second, in hilly countries, plowing at right angles to 

 the slope instead of up and down the slope saves the 

 soil. The furrows check and hold most of the sedi- 

 ment. Third, covering the soil with vegetation, as 

 growing crops, prevents erosion. Fourth, growing 

 grasses on the slopes tends to hold the soil. Fifth, 

 planting forests on the steep slopes checks soil move- 

 ment. Sixth, the building of terraces makes a series 

 of level plains. This problem of soil conservation is 

 an important one and should receive consideration by 

 every land owner. 



Stream development. A stream goes through three 

 stages of development : youth, maturity, and old age. 

 These terms, as related to streams, do not refer to 

 years but rather to stages of development. Each 

 stage has features that are characteristically its own. 

 A river in youth has some or all of the following char- 

 acteristics : 



First, a young river bed has a steep slope. Enough 

 time has not elapsed so that the stream has eroded 

 its bed to a gentle slope. 



