CHAPTER XXV 

 EROSION AND SEDIMENTATION 



307. Questions for Discussion. 1. Brush is sometimes thrown into 

 a gully to check washing. How does it accomplish this? 2. Why are 

 hillside farms sometimes terraced? 3. It is sometimes reported that 

 floods have increased in the Mississippi Valley as the country has be- 

 come settled and forests have been cleared away. How might removal 

 of forests affect floods? 4. How do you account for the fact that the 

 soil, at least at the surface, is usually more moist in forests than in 

 adjacent fields? 5. Why is the soil of lowlands, such as bottom land 

 along rivers, usually more fertile than upland soil? 6. Does the con- 

 tinual but slight loss of soil from a field represent a money loss? Why? 

 7. Why is it desirable to maintain trees on steep slopes? 8. May the 

 deforestation of steep hillsides do injury to owners of other property ? 

 How ? 9. What are the chief regions in your country which illustrate 

 loss of soil by erosion? 10. What is being done to prevent this loss? 

 11. What efforts are being made in your state to develop a permanent 

 forestry policy ? Is this being done partly with reference to the state's 

 soil or water needs, or for a supply of timber ? 



308. Removal of the soil erosion. When the formation of 

 soils was being discussed we spoke of the action of water 

 upon rocks, gravel, and sand. We have but to look at a 

 large river (fig. 146) or a gutter stream to get an idea of the 

 power of water to carry soils. A study of a surface stream 

 that is flowing from a field or garden will tell a good many 

 things about erosion. First, the heaviest particles of soil that 

 are moved are carried only by the strongest part of the cur- 

 rent. This part of the current may also carry all finer grades 

 of soil material, but the slowest pail of the current may be 

 so very slow as to carry only the finest particles. If a dam 

 is made across a very small stream the heaviest materials are 

 dropped where the current is first slackened, and some very 



304 



