Controls on channel planform changes in rivers of the Atlantic Coastal Plain: Findings from the Congaree river valley (South Carolina, USA)

Rivers shaped the landscape of the Atlantic Coastal Plain during the last tens of thousands years. However, their spatial and temporal evolution is understudied. We conducted geophysical, geological and remote sensing surveys in the Congaree River valley, characterized by numerous traces of palaeochannels preserved in the floodplain. The age of the former channels was determined by AMS radiocarbon dating. The main goal of our study was to identify factors controlling the river planform changes and compare the findings with other rivers of the Atlantic Coastal Plain. We found that the Congaree floodplain was shaped by single-thread channels characterized by formation of levees and floodbasins at least 34,000 cal. BP. Large-scale meanders were active between 16,000 and 12,000 cal. BP. They were replaced by meandering channel belts shifting by avulsions between 12,000 and 3,000 cal. BP. Compound meanders have been formed during the last 3000 years. Traces of these channel planforms are also present in other rivers of the Atlantic Coastal Plain. The abandonment of the largescale meanders was caused by increase in climate humidity, changes in vegetation cover, and inundation of the coastal plain by the ocean level rise, resulting in base-level rise. The avulsions of meander belts in the Early and Middle Holocene were formed in conditions of a humid climate, a series of flooding periods, and continued baselevel rise. A relatively drier climate in the Late Holocene caused a change in the meanders' evolution from the channel belts shifting by avulsions to compound meanders evolving by cutoffs. The last 200 years were dominated by a huge delivery and deposition of fines, caused by deforestation and land-use changes in the river valleys.