414 A NATIONAL PLAN FOR AMERICAN FORESTRY 



are comparatively rapid. Flood stages are often reached in a few days 

 and subsidence occurs just as quickly when the bulk of the surface 

 water has drained away. 



CHARACTER AND CAUSES OF EROSION 



While erosion problems are confined to the upland portions of the 

 drainage basin they are intimately related to the flood and stream-flow 

 problems of the lowlands since both are quite largely the product of 

 uncontrolled surface run-off. That the uplands of the basin are espe- 

 cially susceptible to erosion is shown by the results of surveys made in 

 1930 by the Southern Forest Experiment Station. These reveal that 

 28 percent of the entire upland area in northern Mississippi is denuded 

 land, barren of cover and actively eroding. In 4 counties more 

 than 30 percent of the total area is eroding, while in 2 more than 

 40 percent is so classed. An additional area of 27 percent of the 

 region is abandoned land, the abandonment of which in part was 

 caused by soil impoverishment. These data apply specifically to the 

 upland watersheds of the streamy tributary to the Yazoo River, but 

 the widespread and destructive soil destruction found in this portion 

 of the Mississippi silt loam uplands characterizes a large part of the 

 entire region. 



A mere quantitative estimate of erosion, however, gives little indi- 

 cation of the real seriousness of the problem . Much of the erosion is of 

 a peculiarly destructive type. Washes 20 to 30 feet in depth are 

 common and occasionally gullies nearly 100 feet in depth are found. 

 Such erosion has made these silt uplands and their counterpart in the 

 upper Mississippi drainage, a region of outstanding watershed and 

 erosion problems. 



A combination of factors is responsible for the serious erosion and 

 flood problems of the region. The precipitation is heavy and aver- 

 ages, for the region, between 40 and 55 inches, more than half of it 

 falling when the cultivated lands are bare of cover. Most of it comes 

 as numerous rains, frequently torrential in character. According to 

 United States Weather Bureau records, there have been about 35 

 days annually over a 20-year period with rainfall of one fourth inch 

 to 1 inch and about 4 days with rains of 2 inches and over. A maxi- 

 mum precipitation of 4 inches an hour and 9 inches in 24 hours is 

 reported. 



The soils of the region are highly erosible and consist mainly of 

 silt loams and clay loams derived from loess. When protected by 

 forest or other vegetative cover these soils do not erode easily and are 

 capable of absorbing large quantities of rainfall. When bared, how- 

 ever, they wash badly. The silty soils are frequently underlain at 

 depth of a few feet by unconsolidated sands and other incoherent 

 materials. Once a gully has cut through the surface loams and 

 exposed these unstable strata, erosion proceeds on a gigantic scale, 

 and is extremely difficult to control. 



The unwise use of these uplands for agriculture is responsible for 

 practically all of the soil wastage that has occurred. The cultivation 

 of cotton and corn to the exclusion of other crops leaves the soils 

 exposed to the action of the elements during much of the year. Even 

 on moderate slopes the soil losses from the cultivated fields of the 

 region are enormous. Forest Service studies at Holly Springs, Miss., 



