A NATIONAL PLAN FOE AMERICAN FORESTRY 1489 



Gullies planted to black locust have become stabilized within a period 

 of a few years following planting, and the use of this species for con- 

 trol purposes is becoming general in sections such as western Tennes- 

 see, where its success has been amply demonstrated. 



Forest cover exerts its control over erosion in a number of ways. 

 Litter, the organic cover of the soil in the forests, prevents packing 

 of the surface such as occurs on bare ground exposed to the weather. 

 It adds organic matter to the soil, keeping it porous and receptive to 

 the percolation of moisture. This reduces surface run-off and retards 

 the erosive action of such surface run-off as may occur. 



The results from a group of experiments carried out by Lowdermilk 

 in California illustrate the effectiveness in erosion control of a forest 

 cover produced by planting and of brush cover. The destruction of 

 cover by fire resulted in the removal of soil by erosion at the rate of 

 4 cubic yards per acre from bare plots in one rainy season, whereas 

 mere traces of eroded material came from undisturbed plots. Studies 

 of forest soils in tanks, wherein litter cover was left on some and 

 burned from others, showed that, under both artificial and natural 

 rains, erosion from the burned plots was as great as 2,300 times that 

 from the unburned when torrential rains were applied to the areas. 



In preliminary observations on erosion plots in northern Mississippi 

 one quarter of the annual rainfall carried away eroded material at 

 the rate of 109 pounds per acre from a plot in a 20-year-old black 

 locust-osage orange plantation, whereas 38,000 to 44,000 pounds per 

 acre came from plots on an abandoned field and on cultivated land. 



Denuded or barren watersheds are not satisfactory as a source of 

 urban water supplies. Denudation encourages excessive silting of 

 reservoirs or storage basins. The city of Raleigh, N.C., provides a 

 typical illustration. The city has a municipal water supply which 

 in 1914 consisted of an artificial lake, 77 acres in extent, and a water- 

 shed of 5.8 square miles. In 1914, when the dam was built, the lake 

 had a capacity of 160 million gallons. Subsequently a large portion 

 of the north side of the watershed was deforested by an agency not 

 under the control of the city, and the capacity of the reservoir had 

 decreased by 1932, through silting, to 100 million gallons. Another 

 reservoir above the first was constructed in 1923, 136 acres in extent 

 and draining a watershed of 6.83 square miles. This watershed is 

 largely wooded, and silting has been negligible. The impounded 

 water is much clearer after periods of heavy rain than that in the 

 lower lake, which takes on the decided yellow-brown color of silt. 

 The inference is reasonably clear that increase of forest cover on the 

 lower watershed by forestation would, within a short time, materially 

 reduce silting. 



Forestation measures to control erosion have widespread applica- 

 tion. Large areas of eroded land not capable of producing other 

 worth-while crops are suitable to forest growth and, once the forest 

 is established, the erosion cure is permanent provided reasonable 

 protection and management is given. 



STREAM-FLOW REGULATION 



The forestation of barren and unproductive watersheds can play a 

 big part in securing regularity of stream flow. The desirability of 

 such .regulation, particularly to reduce peak flows or floods and to 

 increase low-water flows, is discussed more fully in the section already 



