Sediirent losses from forested uplands 

 are usually modest (2.5 cm or 1 inch/ 

 16,000 years; Soil Conservation Service 

 1977). However, the losses after the 

 forest is removed can be quite substantial 

 (Table 2). When forest cover was reduced 

 from 80% to 20% in the Potomac Basin, 

 sediment loading increased eight times 

 (Patrick 1972). The Soil Conservation 

 Service (1977) reported losses from crop- 

 lands (some of which were once forested 

 floodpTain) of 38.4 tons/acre/year of top- 

 soil to the Obion-Forked Deer River (TN). 



It is difficult to distinguish wheth- 

 er sediments are derived from natural or 

 culturally accelerated sources (Strahler 

 1956). Several investigators have at- 

 tempted to estimate losses from the up- 

 lands by measuring the thickness of sedi- 

 ment layers in coastal floodplains. Soils 

 surveys indicate a loss of 15.2 cm (6 

 inches) of topsoil from the South Carolina 

 Piedmont in 150 years. Between 1910 and 

 1934, one Georgia Piedmont watershed lost 

 218 tons/km2/year, but by 1974 this rate 

 was reduced by 86% (30 tons/km^/year) 

 (Meade and Trimble 1974). Happ (1945) 

 concluded an average Piedmont upland soil 

 loss of 9.4 cm (3.7 inches) since earliest 

 settlement. 



Although agriculture has heavily 

 accelerated the loss of soil from uplands, 

 90% of the sediment from accelerated Pied- 

 mont erosion remains on hill slopes and in 

 stream bottoms (Trimble 1979). In fact, 

 the composition of alluvial sediments and 

 their rate of deposition in some flood- 

 plains do not reflect a marked change in 

 rate due to agriculture. In South Caro- 

 lina, both Coastal Plain rivers (black- 



water) and rivers originating in the 

 Piedmont (alluvial) have three terraces 

 (Pleistocene) above the present floodplain 

 that are similar in type of landform, 

 slope, particle size, and composition of 

 sediments to those of the present (Holo- 

 cene) floodplain (Thorn 1967). For example, 

 the quartz sands of the present point bars 

 of the Little Pee Dee River (a blackwater 

 stream) are similar to those of the higher 

 terraces. In the Great Pee Dee River (a 

 Piedmont, or alluvial, stream), the three 

 older terraces also have the same composi- 

 tion of silts and sands as does the pres- 

 ent floodplain. 



Water and sediment supply are not 

 continuous but result from discrete cli- 

 matic events (Harvey et al. 1979). The 

 largest portion of the total load of many 

 rivers is carried by high flows on the 

 average of once or twice a year. As 

 flow variability increases and as size of 

 watershed decreases, a larger percentage 

 of sediments is carried by less frequent 

 flows. In many basins 90% of sediment is 

 moved during floods recurring at least 

 once every 5 years (Wolman and Miller 

 1960). Piedmont streams carry 10 times 

 the sediment of Coastal Plain streams at 

 the same discharge rate during floods 

 (Meade 1976). 



Slope and Meandering 



River systems are remarkably dynamic. 

 Changes of slope (elevational gradient) 

 which cause rivers to flow can be due to 

 (1) crustal uplifting or downwarping 

 responses of the coast to the removal of 

 the Pleistocene ice mass, (2) scour or 

 erosion which steepens headwaters, or (3) 



Table 2. Comparative sediment losses and land-use practices 

 (Happ et al. 1940). 



Land use 

 practices 



Sediment loss 

 (tons lost/acre/year) 



Oak forest 



Bermuda grass 



Cotton (contour plowed) 



Cotton (down slope plowed) 



Barren abandoned field 



0.05 



0.19 



69.33 



195.10 



159.70 



