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The 3,550 square miles of the Upper Chattahoochee River basin is an area of diverse physiographic and land-use char­ acteristics. The headwater areas are mountainous with steep, relatively narrow channels. Land in the headwater areas is heavily forested, but small towns and farms are common in the valleys of large streams. Downstream, the basin is char­ acterized by low hills and wider stream channels. Land in this part of the basin is also predominantly forested; how­ ever, large agricultural and urban areas are common. Urban land use is particularly intensive within the Atlanta Metro­ politan Area. Rates of sheet erosion were computed in nine watersheds using the Universal Soil Loss Equation. The dominant land use in each watershed ranged from forested to mostly urban. Computed average annual erosion yields ranged from 900 to 6,000 [(t/yr)/mr]. Erosion yields were greatest in watersheds with the largest percentages of agricultural and transitional land uses. The lowest yields occurred in highly urbanized watersheds. The sensitivity of average annual sheet erosion to timber harvesting was also evaluated with the Universal Soil Loss Equation. In general, post-harvest erosion yields were several orders of magnitude greater than computed pre-harvest yields in the same areas. Average annual suspended-sediment yields were calculated from measurements of sediment discharge from the same mine watersheds and ranged from about 300 to 800 [(t/ yr)/mi2]. Seodiment-discharges were greatest in urban water sheds and least in forested watersheds. A large part of the sediment discharged in urban streams was considered to be derived from stream-channel erosion. Unmeasured sediment discharge computed for four watersheds ranged from about 6 to 30 percent of the total annual sediment discharge. The impact of suspended sediment on the quality of streamflows was evaluated for 14 watersheds. In general, 60 per­ cent or more of the total annual discharge of trace metals and phosphorus was contributed by suspended sediment. Corresponding discharges of suspended nitrogen and organic carbon ranged from about 10 to 70 percent of total. Yields of suspended trace metals and nutrients from urban water­ sheds were consistently greater than corresponding yields from forested watersheds. Turbidity in basin streams in­ creased with increasing concentrations of suspended sedi­ ment. Such increases, in turn, decreased the aesthetic quality of the watercourse and the depth of light penetration into the water column. INTRODUCTION This investigation is one part of the U.S. Geologi­ cal Survey's intensive river quality assessment of the Upper Chattahoocheee River basin in Georgia (Cherry and others, 1976). In contrast to similiar studies in the Southeastern United States, this in­ vestigation examines erosion and sediment trans­ port in large watersheds. Relations are developed, albeit imperfectly, between rates of erosion and sediment discharge and characteristic land-use, soil, topographic, and climatic parameters. Such re­ lations, in turn, can be used by resource managers, hydrologists, and other earth scientists to better understand and accommodate the processes of ero­ sion and sediment discharge in the study area. ACKNOWLEDGMENTS Much of the collection and analysis of data for this study was accomplished by field observers and hydrologic field assistants employed by the U.S. Geological Survey. The field observers were partic­ ularly dedicated and helpful, and our thanks and acknowledgements are herein extended to Charles T. Satterfield of Dahlonega, Ga., and the Steven Wells family of Whitesburg, Ga. Equally notable were the efforts of our hydrologic field assistants; especially the contributions of Douglas F. duMas, Reid C. Webb, Michael D. Holcomb, David G. Pinholster, and Ronald Boyd. The help of these personnel and others is gratefully acknowledged. DESCRIPTION OF THE PROBLEM The periodic occurrence of large quantities of sediment in the Chattahoochee River and its tribu­ taries confronts the water-resource manager with a complex set of problems. Large-scale sedimentation aggrades stream channels, causing inundation of valuable bottom lands and increasing peak eleva­ tions of floods. Sediment deposition in reservoirs reduces their efficiency and useful life, and in some EROSION, SEDIMENT DISCHARGE, CHANNEL MORPHOLOGY, CHATTAHOOCHEE RIVER BASIN situations destroys the aesthetic and recreation potential of the impoundment. The sorbing capacity of sediments, especially clays and silts, provides a mechanism whereby nutrients, trace metals, and other chemical compounds are carried with the sedi­ ment into streams and reservoirs. High concentra­ tions of suspended sediment in a water supply in­ crease treatment costs and often restrict use of the water in certain industrial processes. Faced with these and a host of related problems the resource manager must decide on reasonable methods to control or accommodate erosion and the discharge of sediment to basin streams. Decisions to implement control methods should be based on in­ formation of sufficient quality and quantity to per­ mit such methods to be successful as well as eco­ nomically and environmentally feasible. Unfortu­ nately, comprehensive data relative to the processes of erosion and sediment discharge in the Upper Chattahoochee River basin have never been col­ lected. To collect and interpret such data and to detevmine causal relationships between sediment discharge and the source environment are the over­ all objectives of this study. To accomplish these ob­ jectives, data-collection sites were established on the Chattahoochee River and on selected tributary streams. Selection of each site was based, for the most part, on physiographic location, dominant watershed land use, and availability of streamflow record. Specific study objectives at each data-collec­ tion site included: (1) A description of sediment and sediment trans­ port characteristics in the stream channel. (2) A determination of average annual sediment discharge. (3) A determination of the chemical nature of suspended sediment and its contribution to stream quality. Study objectives relative to watersheds draining to selected data-collection sites included: (1) An evaluation of channel and watershed morphology using Strahler (1957) streamorder analyses. (2) A determination of average annual sheet ero­ sion using the Universal Soil Loss Equation. (3) The development of causal relations between stream quality, sediment discharge, land use, and other environmental factors. UNITS OF MEASUREMENT Data describing streamflow, lengths, and areas are defined or dimensioned in inch-pound units. Con­ centrations of suspended and dissolved constituents are defined in metric units and given in milligrams per liter (mg/L). Suspendedand dissolved-constituent discharges are expressed in terms of tons per year (t/yr). A list of inch-pound to metric con­ versions follows the "

u.s. geological survey professional papers/u.s. geological survey professional paper

Erosion, sediment discharge, and channel morphology in the upper Chattahoochee River basin, Georgia, with a discussion of the contribution of suspended sediment to stream quality