388 A NATIONAL PLAN FOR AMERICAN FORESTRY 



Glenn points out the immediate gullying of cleared slopes which, 

 even though in grass, wash down to the bare rock. Craig in Ken- 

 tucky, in the bulletin already cited, notes the use of land for corn 

 fields on slopes as steep as 75 percent, where, because of erosion, the 

 maximum limit of arability is 15 years. 



Even in lower country severe soil losses take place. In Hopkins 

 County in western Kentucky, a small reservoir and watershed of 

 2,340 acres were examined 65 for siltage after 20 years' time. Maxi- 

 mum differences in elevation in the watershed amounted to only 206 

 feet. The steeper slopes were wooded. Of the 930 acres farmed, 

 350 were in grass. Silting had taken place, however, at the average 

 rate of 3,534.6 cubic yards a year. The cultivated land was rolling 

 and had only 40 feet difference in elevation; yet the burden of silt, 

 almost entirely from the cultivated lands, amounted to 6 cubic yards 

 per acre per year. This is illustrative of the loss that can and does 

 occur from erosion of gentle or rolling arable lands. It does not 

 adequately portray the soil damage being done on rougher and 

 steeper lands. 



Glenn notes that in the process of land clearing in the mountains, 

 the soil frequently has been washed away and the area abandoned 

 before the land is completely cleared of the girdled forest. Adjacent 

 areas are then cleared and the process is repeated. Ayres and Ashe, 

 as already cited, estimated that 24 percent of the Appalachian Moun- 

 tain area has been cleared. The reclearing of abandoned areas has 

 helped to lower the net total cleared average. 



Not only does the land soon wash away when slopes are deforested 

 and exposed, but rainfall runs off down the stream courses in excessive 

 quantity instead of percolating into the soil. Leighton, by his studies 

 (already cited) on the three major branches of the Ohio River above 

 Wheeling, W. V a -> clearly proved the increase of run-off and the 

 progressive increase in flood occurrences on a drainage area the 

 deforestation of which had been constant and rapid for 30 years. He 

 states without qualification that 



the increase in flood tendency * * * is due by far the largest measure to 

 the denudation of forest areas. 



Run-off varies in different portions of the basin and is increasing in 

 proportion to the deterioration of the surface conditions. Humphreys 

 and Abbot 56 estimated in 1861 that the proportion of run-off to rain 

 in the Ohio basin is 24 percent. Measurements given in the 1911 

 report of the Pennsylvania Flood Commission show that for the 

 period 1899-1910 mean annual run-off above Pittsburgh varied from 

 40.0 to 71.7 percent of the rainfall, and that at Wheeling, W.Va., 

 for 1904-8 it was 58.9 percent. On the Allegheny at Aspinwall, Pa., 

 it was 66.4 for the years 1903-7. Recent measurements in Tennessee 

 reported by King indicate that 45 percent of the precipitation usually 

 runs off into streams of that State, and that in the "cloudburst" 

 causing the 1929 flood, measurements indicate that 91.5 to 97.3 per- 

 cent of the concentrated rainfall was immediately lost as run-off. 



88 Atkinson, J. B. Watershed of Loch Mary. The Bee, Earlington, Hopkins County, Ky., Mar. 

 11, 1909. 



6 Humphreys and Abbot, The Physics and Hydrology of the Mississippi River, Philadelphia, 1861; 

 see also Fuller, M. L., Underground waters of eastern United States. U.S. Geological Survey. Water 

 Supply Paper 114, 1905. 



