THE ROLE OF STREAMBANKS IN FLOODPLAIN SEDIMENT BUDGETS

As rivers migrate, sediment is supplied to the channel at rates that vary in both space and time. In many systems, the long-term reach-average exchange is probably roughly even. However, at the scale of individual river bends, floodplains are often net sediment sources even in reaches that are strongly depositional. This occurs because eroding channel bends usually a) elongate as they migrate, thereby reducing the net volume of the floodplain in the immediate vicinity and b) remove old portions of the floodplain that have had time to accumulate overbank sediment and are thus higher in elevation than newly deposited point bars. The local imbalance present at most bends can be made up at reach scale by a) deposition within abandoned channels such as oxbow lakes and b) overbank deposition on the floodplain. The local net flux of sediment from floodplain to channel was estimated using high resolution lidar topography and a series of historic aerial photographs along several lowland rivers in the eastern United States. Averaged across many active bends, the total local imbalance represents roughly 15 to 20% of cut-bank erosion. Significant local imbalances are observed even in systems that are strongly depositional at reach scale, highlighting the importance of including floodplain deposition when developing reach-scale sediment budgets. A loose coupling between models for overbank deposition and local net bank supply provides a mechanism for modeling the movement of fine-grained tracer sediment through an actively evolving channel/floodplain system. The feedback between local bank erosion and deposition history allows the floodplain to evolve toward a steady state sediment volume that depends on the water and sediment load supplied to the system.