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Show ities encourage, and incomes permit more abundant use of dairy products. For the entire watershed, about one-third of the cropland is in clean tilled crops. The forest types range from the pine-hardwoods of the Coastal Plain, through the various mixtures of Appalachian and northern hardwoods, to small areas of red spruce on some of the high elevations along the western border. About 70 percent of the commercial timber is hardwood and 30 percent is softwood. About 46 percent of the forest land is farm wood lots, 35 percent private nonfarm, 11 per- cent national forest, 2 percent other Federal owner- ship, and 6 percent State and county-owned. Not more than one-third of the forest area can be said to contribute adequately to the protection and productivity of the Potomac watershed. Simi- larly, much, of the cropland and most of the pasture are providing less watershed protection and yielding less net farm income than could be attained and maintained under better farm and forest land management. The farms of the basin average about 150 acres, but nearly one-third have less than 40 acres per farm, with correspondingly low incomes, especially on the shale soils. The farms having the highest incomes ar-e the limestone, soil-conserving crop farms. It is the small, uneconomic farms that con- tribute most to the runoff and erosion problem and are lesst able to participate in conservation activities unless incomes can be supplemented by nonfarm forest and industrial employment. Because of the current low production on much of the forest land, the basin is an importer of lum- ber and other forest products. If properly managed the forests are thought to be capable of supplying all local needs for domestic lumber and have a sur- plus for shipment outside the basin. Saw timber growth now averages about 35 board feet per acre per year, wliile the potential annual growth is esti- mated to be at least 175 board feet. An apprecia- tion of this fact offers an opportunity to com- munities, who through their local officials and business establishments can initiate their own pro- grams. In addition to low productivity, lack of proper management results in increased flood run- off from forested areas and in a reduction of volume of water available as underground flow during dry periods. Susceptib ility of the basin soils to erosion.- The soils of the basin can be divided generally into six major gr-oups according to their parent material 592 or physiographic situation: shales, sandstones, lime- stone, Piedmont, mountain, and alluvial. The shale soils, occurring on rolling and hilly lands, are generally very shallow, unproductive, or of low water-holding capacity, and highly erodible if without adequate forest protection. The sand- stone soils, more deeply weathered and of coarser texture than the shale, respond well to manage- ment on the gentler slopes, absorb rainfall more readily, and are the least erodible of all groups. The agriculturally favorable limestone soils are the most fertile of the upland soils. Though subject to severe erosion, they respond readily to proper treatment. The rolling Piedmont soils of crystal- line rock origin are comparable in fertility to the limestone. They too are very erodible. The rough, stony mountain soils of the Blue Ridge and Allegheny Plateau are practically all in forest. Al- though only slightly erodible, their capacity for water storage is low so that the maintenance of a well-developed forest cover and humus layer is essential for maximum reduction of storm runoff. The level, fertile, alluvial soils, occupying the flood plains, present no problems of runoff or erosion, but are subject to damage by deposition, swamping, bank cutting, and scour. The lack of proper watershed management prac- tices on erodible soils has resulted in serious erosion problems in many parts of the basin, and in par- ticular the Shenandoah and Monocacy River drain- age areas. Approximately three-fourths of the cropland and one-third of the grassland of the basin are subject to severe erosion. While the most serious loss from erosion is the depletion of the fertility of the soil itself, the quality of stream waters also suffers. Sediment interferes with self-purifi- cation, causes increasing costs and difficulties in the purification of water supplies, requires additional dredging in the navigation channels, and destroys oyster beds. Relation of land management to flood dam- ages.-Flood damages in the tributary and upper headwater areas of the Potomac are, on the average, equal to damages occurring from major floods on the main stem. These tributary damages result from rain storms of two types. The general storms which give rise to major floods on the main stem also produce floods on the tributaries. In addition, there are frequent local storms of high intensity, the area affected by each such storm being small but the damage locally great. An example of the latter type of storm occurred in Virginia, June 17 and 18, 1949. The storm had two centers, one on the watershed |
