Sediment transported by Georgia streams

Abstract

A reconnaissance investigation of the sediment transported by selected Georgia streams during the period December 1957 to June 1959 was made to provide a general understanding of the physical quality of stream water in Georgia and to supply facts needed in planning more detailed work. The investigation was made by studying the variation of sediment concentration and sediment load with stream discharge at 33 sites and by relating the available data to topographic, geologic, climatic, and soil conditions in the State. In the Blue Ridge Mountains area of northern Georgia the great relief, moderately heavy precipitation, fast runoff, and loamy soils cause sediment concentrations and sediment loads which are above average for the State. During periods of moderate to low streamflow, the concentration of suspended sediment ranges from 1 to 25 ppm (parts per million). After heavy rainfall, sediment concentration increases rapidly as water discharge rises, and occasionally exceeds 1,000 ppm before decreasing again. The concentration may reach a maximum and decrease before the discharge peak is reached. A major part of the annual sediment load can be carried during a short period of time because of the great increase in both water discharge and sediment concentration during floods. The lower Coastal Plain differs from the mountainous areas in several respects. The topography is gently rolling to almost level, precipitation and runoff are less than average for the State, and topsoils generally consist of hard and loamy sand. Concentration of suspended sediment in streamflow commonly ranges from 1 to 20 ppm during periods of low to moderate discharge and increases to 15 to 60 ppm at high discharge. Because of the small increase in concentration with increasing stream discharge, the sediment load varies approximately in proportion to the discharge. The sediment characteristics of streams in the Piedmont, the Valley and Ridge area. and the upper Coastal Plain are intermediate .between those of the Blue Ridge area and the lower Coastal Plain. Comparison of suspended load with estimated bed load in a few Georgia streams suggests th.at bed load is less than 20 percent of the suspended load for most streams. Factors which appear to be most important in causing variation in sediment yield in Georgia are topographic relief, soil texture, and location of dams. Variations in other factors such as precipitation, runoff, covering vegetation, drainage area, and channel types serve to modify the effects of the major factors. In general, Georgia stream water is of good quality. Water of some streams is of exceptionally fine quality and contains less than 30 ppm combined dissolved and suspended solids during at least 90 percent of the time. Knowledge of the nature and cause of variation in water quality will permit the most effective use of Georgia streams.