Changes in sediment loads in rivers of the Atlantic drainage of the United States since 1900

Transcription

1 Changes in sediment loads in rivers of the Atlantic drainage of the United States since 1900 Robert H. Meade and Stanley W. Trimble Abstract. Changes in sediment loads in the Atlantic drainage can be related to construction of reservoirs and changes in land use. Sediment loads have decreased immediately downstream from reservoirs but the persistence of large loads at points farther downstream indicates that the river channels themselves are now being degraded. The decline of crop farming and the improvement of soil-conservation practices has also decreased the sediment yields, but this is reflected mainly in the sediment loads of the tributary streams; as yet, there has been no correspondingly marked decrease in the sediment loads in most of the main-stem rivers. Les différences des charges sédimentaires des fleuves dans l'écoulement vers l'atlantique en les Etats-Unis, depuis 1900 Résumé. Les différences des charges sédimentaires dans l'écoulement vers l'atlantique peuvent être liées à la construction des réservoirs et aux changements dans l'utilisation de la terre. Les charges sédimentaires ont précisément diminué en aval des réservoirs mais la persistance de grandes charges plus en aval encore indique que les lits eux-mêmes sont en cours de dégradation. La diminution des surfaces cultivées et les progrès dans la conservation du sol se traduisent également par une décroissance des charges sédimentaires, mais ce changement apparaît surtout dans les rivières tributaires. Jusqu'à présent, il n'existait pas une diminution similaire dans les charges sédimentaires de la plupart des fleuves. INTRODUCTION Suspended-sediment loads have been measured daily in some rivers of the Atlantic drainage of the United States since about These measurements can be compared with those of a stream-sediment survey made in 19? to show changes since that time. Changes in stream-sediment loads can be partially related to construction of reservoirs or to changes in land use. In the reconnaissance study made during the water year (Dole, 1909), the concentration of suspended sediment was measured by filtering 10-day composite samples that had been collected regularly at 19 stations in 13 of the principal river basins of the Atlantic drainage. Dole and Stabler (1909) used these data and the runoff records that were available to estimate the total suspended-sediment loads carried by the rivers. Before these and more recent estimates could be compared, it was necessary to revise Dole and Stabler's estimates to correct for procedures and assumptions that have subsequently been shown to cause errors. The largest error was caused by their assumption that the sediment loads per unit drainage area in the Piedmont and other upland regions (where most of the samples were collected) applied also to the Coastal Plain; later studies have shown that the sediment loads from the Coastal Plain are substantially smaller. Other smaller adjustments (usually 20 per cent or less) were applied to reflect differences in techniques of measuring and calculating suspended sediment or to extend runoff records that were too short in 1909 to give an accurate long-term average. Estimates of more recent sediment loads are based on regularly-collected data, most of which are tabulated in the US Geological Survey-Water Supply Paper series on 'Quality of Surface Waters of the United States'. These records, which are taken here as representative of present-day sediment loads, are largely from daily observa-

2 100 Robert H. Meade and Stanley W. Trimble FIGURE 1. Average annual loads of suspended sediment carried by rivers of Atlantic drainage of United States during years near 1910 (left) and 1970 (right). tions made over periods ranging from 1 to 21 years. The most obvious changes in sediment loads between then and now are in the southern part of the Atlantic drainage (Fig.l). On average, the large southern rivers now discharge at their mouths half or less of the sediment discharged 50 to 60 years ago. Farther north, the change is much less marked. CHANGES IN RIVER BASINS THAT SHOULD AFFECT SEDIMENT LOADS The two factors most likely to affect the amounts of sediment being carried by the main-stem rivers of the Atlantic seaboard are the construction of reservoirs and changes inland use. The numerous reservoirs that have been built (mainly for hydroelectric power) on the major streams since the early 1900s have interrupted the seaward flow of sediment. Figure 2 shows the decrease in turbidity of the Roanoke River at Roanoke Rapids, N.C. (drainage area 21,800 km 2 ) that followed the construction of three dams upstream. This effect has been repeated on many rivers. Sediment-load data collected in the late 1960s from the Santee River showed that the two reservoirs furthest downstream trapped about 85 per cent of the incoming load an amount similar to that which would be predicted from Brune's (1953) curve of trap efficiency

3 Changes in sediment loads in rivers Kerr Reservoir 1952 l , TURBIDITY (Jackson units) Roanoke Rapids Lake WATER DISCHARGE (km3/mo) H f l g 0 -» Wkmm. 0 2 o l Lake Gaston , FMAMJJASONC mêèërélo JFMAMJJÛSOND FIGURE 2. Decrease in turbidity of Roanoke River at Roanoke Rapids, N.C., following construction of three reservoirs upstream. versus capacity-inflow ratio. The pattern of land use has changed in several ways since European colonists arrived here in the seventeenth century, and this has apparently changed the pattern of sediment yield. In the rapidly urbanizing areas north of the James River, the cycle is characterized by Wolman (1967) as: a tenfold increase in river sediment as FIGURE 3. Maps showing decrease in row-crop agriculture in southern Piedmont, Each dot equals 1000 acres (405 ha).

4 102 Robert H. Meade and Stanley W. Trimble land was converted from forest to cropland, which began in the seventeenth century and reached a peak of sediment production around the year 1900; a period of somewhat reduced sediment production as farming declined in the area and tracts of former cropland reverted to woods or pasture; a short-lived but several hundredfold increase in sediment production on relatively small areas during the rapid urbanization that followed World War II; and, finally, a sharp reduction in yield as lawns were planted and areas covered with pavement. South of the James River, the landscape has remained more rural, but the patterns and practices of agriculture have changed. In the Piedmont region, which has long been the primary source of sediment in the South Atlantic rivers, row-crop farming and other types of erosive land use have decreased (Fig.3) while soil-conserving land uses such as forest and pasture have significantly increased (Trimble, 1973). At the same time, soil conservation practices have been widely applied to both cultivated and uncultivated land. As a result erosion has been much reduced; Fig.4 shows the changes in erosive land use and sediment yield in the drainage basin of the Lloyd Shoals Reservoir (3600 km 2 ). This is on one of the main tributaries of the Altamaha River and is fairly typical of the Piedmont of South Carolina, Georgia, and Alabama. The erosive land use for the watershed was calculated by determining the different types of vegetative covers and conservation practices and assigning numerical 'cover factors' related to rates of erosion (Soil Conservation Service, 1968). A composite factor for the entire basin was calculated for each decade from 1860 to The highest erosion rate was reached between 1900 and 1920 when the composite factor was 0.6, equivalent to 60 per cent of the area being in row crops with poor conservation practices. The sediment yield during was determined mainly from the deposition in Lloyd Shoals Reservoir and averaged 218 tons km~ 2 year -1. The present composite cover factor has been reduced to one-seventh of its former magnitude, and the sediment yield (as estimated from stream data) has been reduced correspondingly to about 30 tons km -2 year -1. FIGURE 4. Changes in land use and sediment yield, Lloyd Shoals watershed.

5 Changes in sediment loads in rivers 103 DISCREPANCIES BETWEEN EXPECTED AND OBSERVED CHANGES IN SEDIMENT LOADS The changes that might have been predicted from a knowledge of the distribution of reservoirs and the changes in land use are not altogether apparent in the sediment loads of many of the major rivers. Reservoirs trap substantial amounts of sediment, but the sediment loads a few miles downstream from a reservoir may show less decrease than expected. The effects of the reservoirs either have not yet been felt very far downstream or are being offset by increased degradation of the channels below the reservoirs. This is supported by a comparison of Dole's (1909) data for one of the two main tributaries of the Altamaha River at Macon, Georgia (drainage area 5800 km 2 ) with similar measurements taken 31 years later by Lamar (1944). In 1910, the Lloyd Shoals Reservoir was completed above Macon. Although the total sediment load measured at Macon was essentially the same in as in , the seasonal distribution of sediment concentration was markedly different. Before the construction of the reservoir, the highest concentrations were measured in the low-discharge months of summer, suggesting that upland erosion outside the river channel had the greatest effect on the concentration of suspended sediment in the stream. After the reservoir was completed, the suspended concentrations at Macon were more closely coupled to discharge events, suggesting that the channel itself was the principal source of suspended sediment. The effects of changes in land use are shown even less clearly than the effects of reservoirs in the sediment loads of the main-stem rivers. Decreases in erosive land use are now beginning to reduce suspended sediment in tributaries while some main- 'stem rivers remain essentially unchanged. Many of the headwater streams that were over-steepened during the period of extreme agricultural erosion and sedimentation have recently been degrading because of decreased upland sediment production (Trimble, 1971). An example is the Mauldin Millsite on a small tributary of the FIGURE 5. Cross sections showing aggradation and degradation of stream bed at Mauldin Millsite,

6 104 Robert H. Meade and Stanley W. Trimble Altamaha (Fig.5) which has gone through a cycle of burial and excavation since The material is being eroded from channels and banks to become 'sediment yield' at some point downstream. Elsewhere in the southern Piedmont, observers have noticed that larger streams tend to be more turbid than tributary creeks during high flows. CONCLUSION Although many reservoirs have been built and although land use has changed markedly on the Atlantic slope of the United States since 1900, the loads of suspended sediment carried by the main-stem rivers have not been reduced as much as might be expected. The reason seems to be that the main-stem rivers are still receiving sediment from their larger tributaries that accumulated during the years of accelerated erosion following the deforestation and farming of the land. The time required to clear the stream bottoms of this man-induced accumulation is rather long. The work of Gilbert (1917) on the movement of hydraulic mining debris through the Sacramento River basin of California and later work on the accumulation of this debris in San Francisco Bay (Smith, 1965), suggest that the time scale of the process seen here is of the order of a century or more. Apparently at least several more decades of augmented sediment loads can be anticipated even after the land use has stabilized. REFERENCES Brune, G. M. (1953) Trap efficiency of reservoirs. Trans. Amer. Geophys. Union 34, Dole, R. B. (1909) The quality of surface waters in the United States. Part I - Analyses of waters east of the one hundredth meridian. US Geol. Survey Water-Supply Paper 236, 123 pp. Dole, R. B. and Stabler, H. (1909) Denudation. US Geol. Survey Water-Supply Paper 234, Gilbert, G. K. (1917) Hydraulic-mining debris in the Sierra Nevada. US Geol. Survey Prof. Paper 105, 154 pp. Lamar, W. L. (1944) Chemical character of surface waters of Georgia. US Geol. Survey Water- Supply Paper 889-E, Smith, B. J. (1965) Sedimentation in the San Francisco Bay system. USDA Misc. Pub. 970, Soil Conservation Service (1968) Guide to sedimentation investigations, South Regional Service Area. US Soil Conservation Service Engineering and Watershed Planning Unit Technical Guide, No.12, 104 pp.: Ft. Worth, Texas. Trimble, S. W. (1971) Culturally Accelerated Sedimentation on the Middle Georgia Piedmont: US Soil Conservation Service, Fort Worth, Texas, 110 pp. Trimble, S. W. (1973) A geographic analysis of erosive land use in the Southern Piedmont. PhD. dissertation, University Georgia, Athens, Georgia, 176 pp. Wolman, M. G. (1967) A cycle of sedimentation and erosion in urban river channels. Geografiska Annuler 49A,