DEFORESTATION AND EROSION 



By S. W. McCallie, State Geologist of Georgia 



THE configuration of the earth's surface as we see 

 it today is due largely to the erosive action of 

 running water. The hills, the ridges, and the mountains, 

 as well as the intervening valleys, owe their origin to this 

 destructive agent. 



l?ain as it falls on the surface of the earth is either 

 "evaix)rated, taken up by the soils, or runs of? from the 

 surface to swell the streams. Only that part of the rain- 

 fall last mentioned is discussed in this article as it alone 

 has direct bearing on the subject under consideration. 



The amount of surface run-off of rainfall of any 



cipitated mostly in the form of hard showers, and steep 

 hill slopes to accelerate the flow of water, thereby greatly 

 augmenting its erosive efTect. Illustrations of the erosive 

 effect of rainfall are here seen in a most striking manner 

 on every hand. Thousands of acres in this region within 

 the last few years have been made worthless for agricul- 

 tural purposes by the destructive agent of rain wash, as 

 a result of the removal of forests. 



Some idea of the destructive effect of rain wash may 

 be had by the study of the individual streams which drain 

 that region. The Savannah River, for instance, is a good 



IMMENSE ERODED GULLEY IN THE MOUNTAINOUS SECTION O F NORTH GEORGIA SHOWING HOW THE VEGETATION STOPS 



THE EROSIVE ACTION BY CHOKING THE GULLEY 



given region depends chiefly on three conditions, namely, 

 the rate at which the rain falls, the porosity of the soil, 

 and the slope of the surface. In other words, the 

 maximum erosive effect will take place when the rainfall 

 is in hard showers on steep hill slopes with only mod- 

 erately porous soils. On the other hand, in the case of 

 a long continuous, slowly, falling rain, extending through 

 hiany hours, the total amount of rainfall may be quite 

 large but the run-off will be small or negligible. 



.Vi)])lying the principles above enumerated to Georgia, 

 \vc tinfl tli:it surface erosion is most active in the north- 

 ern part f)f the state. Here we find a heavy rainfall pre- 



illustration. This river, together with its tributaries, 

 drain much of the mountain area of that part of the 

 state. The data which has been collected in the last 

 twenty years by the State Geological Survey and the 

 Federal Survey on the Savannah River shows that at 

 present it is carrying to the Atlantic Ocean annually 

 more than 2,50,000 tons of suspended matter. This 

 means, interpreted in car loads (fifty tons each) that 

 the river is carrying to the sea more than 135 car loads 

 of soil wash daily. 



This enormous wash from the drainage basin of the 

 Savannah River is how probably several times greater 



