DEFORESTATION AND FLOODS IN NORTHERN CHINA 893 



ability of the forest to retard the melting of snow is clue chiefly to the 

 shade and protection given by the trees and the underbrush, and eath 

 gradual thawing (this would be the case if the mountains in North 

 China climate were well wooded) will render it possible for streams 

 to be slowly fed; hence a more even stream-tlow. 



• II. Forests and Soil Erosion. — When water rushes down a bare 

 slope it possesses great mechanical power. It loosens the soil and 

 carries it downhill. It makes gullies and ravines and causes land- 

 slips. • The soil brought down not only renders the fertile land below 

 valueless, but also, as brought out in previous paragraphs, goes to the 

 silting up of streams. 



It is generally understood, however, that of all the vegetable covers, 

 forests arc most efficient in protecting slopes from the erosive action 

 of water. This is because, first, the roots of the trees hold the soil 

 firmly in place and so increase its resistance to erosive action ; second, 

 the crowns of the trees protect the earth from the violence of beating 

 rain and intercept a considerable portion of it ; and. third, the velocitv 

 of the moving water is checked, because of the reduced amount of the 

 run-ofif; hence the erosive force of the run-off itself is minimized. 

 On the whole we may say that on a forested slope a series of obstacles 

 is ahvays present to oppose the movement of the water and reduce its 

 velocity and force, and consequently its erosive action. 



The importance of a forest as an effective agency for protecting the 

 soil from erosion has been recognized for centuries in Europe. The 

 so-called "protection forests" in France, Switzerland, and Austria 

 were created with the express purpose of protecting mountains and 

 hills and of preventing communities from being impoverished by floods 

 and torrents which destroy and silt over fertile lands at the foot of the 

 mountains. 1 listory has shown time and again that wherever extensive 

 deforestation has taken place the consequence has been the gradual 

 formation «if a series of torrents, the abandonment of farms, the rapid 

 silting up of river channels, and frequent visitations by floods. To 

 remedy these evils great eft'orts have to be made to reafTorest the 

 denuded areas. France has experienced this, and millions of dollar.- 

 have been spent in the work of reforestation. 



111. I-orrsIs (lud I'loods. — From their relation to stream-flow and >()il 

 erosion, we can readily see the relation of forests to floods. On all 

 (lemided mountains a heavy rain is generally followed by the formation 

 of a system of gullies. These gullies begin a short distance below tin 

 divide and then form lines of least resistance to the passage of water. 

 As these gullies or furrows extend down the slope they join lu-iHi- 



