HISTORIC ALLUVIAL SEDIMENTATION AND ALLOSTRATIGRAPHY OF THE SOUTH FORK OF THE BROAD RIVER, NORTHEAST GEORGIA

The arrival of widespread European-American agricultural practices (circa A.D. 1820) to the southern Piedmont introduced rapid changes in land use that significantly altered many drainage basins and alluvial systems. Rapid historic alluvial sedimentation caused by agriculturally-induced erosion is clearly expressed in the South Fork of the Broad River in northeast Georgia. Stratigraphic sampling of 34 sites indicates that the average thickness of historic sediment stored within the valley bottoms is 0.89 m. A volumetric analysis using Geographic Information Systems (Arc View 3.2 and Erdas IMAGINE 8.3) indicates that early historic (A.D. 1820 - A.D. 1940) sediment yields were greater than 1,000 tonnes/km^2/yr. The rapid sedimentation and floodplain aggradation resulted in a stratigraphic framework related to historic land use change. Allostratigraphic units are assigned, which are based on unconformable boundaries, landscape morphology, and relative pedogenic development. The allounits are classified as either “prehistoric” (prior to circa A.D. 1820) or “historic” (after circa A.D. 1820). The historic facies typically consist of graded sequences of reddish (7.5YR 4/4 to 10YR 4/4) loam to sandy loam. The lower boundary of the historical allounit typically is distinguished by a change in lithology and infrequently by an underlying paleosol. In addition to texture, color, and stratigraphic position, radiocarbon dates and organic carbon content are used as accompanying classifying factors. This approach provides a context for past sediment yields that can be used to better understand present-day sediment yield estimates. This is important background data for researchers and managers who are concerned with assessments of the Total Maximum Daily Load (TMDL) of sediment. Thus, the combination of GIS applications and allostratigraphy offers a useful approach for sediment budget analysis in areas that have experienced rapid sedimentation due to land use change.