Legacy Sediments in Southeastern United States Coastal Plain Streams

Abstract

Soil erosion and sedimentation are natural processes that occur throughout the landscape. However, athropogenic disturbances may result in an increase in either of these processes, or both. Many studies reported the extensive sedimentation generated by the lack of conservative practices in the agricultural methods applied during the settlement of the American-Europeans (Trimble, 1974; Lowrance et al., 1989; Jackson et al., 2005). Yet, the effect of this sediment in the stream’s hydrology and geomorphology and the duration the alluvium may remain onsite is unknown. This research addressed the impacts of the disturbances caused by the early settlers on the hydrology and geomorphology of Coastal Plain streams and floodplains. The goals were to estimate legacy, mid-term (30 years), and current sedimentation, and discuss the implications of the sediment originated during the settlement on the streams’ ecology, hydrology, and geomorphology. The study area consisted of two watersheds at Fort Benning, Georgia: Bonham Creek and Sally Branch. Five different methodologies were used to address legacy, mid-term, and current sedimentation; and an additional simulation exercise using WEPP technology was applied. Research on the historical development of the study area showed increases in population from 1830 to 1930. Farmland area declined after 1910, indicating the end of the cotton-era in the South. Results from soil coring showed that the floodplains of the streams are evenly buried under an average 179 cm, with corresponding mass of 2,338 kg m-2, of sediment that originated during the settlement period. Organic carbon concentration within this sandy loam sediment averaged 25,854 mg kg-1. Morphologically, the streams showed a pattern of deep channel incision leaving the floodplain high and dry. The dendrogeomorphic data showed stability on the floodplains, with little sediment redistribution/scouring (0.4 cm yr-1) over the past 25 years. Results from our modeling exercise showed that WEPP did not accurately account for the sediment delivery to floodplains in the early settlement scenarios when compared to the field study estimates. In summary, there is evidence of the significant sedimentation the streams underwent during settlement, and that this sediment has affected stream ecology, hydrology, and geomorphology.