Channel-floodplain interaction on meandering rivers

Channel migration along meandering rivers is associated with an exchange of sediment between channel and floodplain. The exchange is not necessarily an even one, with imbalances possible locally and over extended reaches. Locally, the imbalances occur because (a) eroding banks are generally higher than the tops of adjacent point bars and (b) eroding banks are generally at the outside of bends and thus longer than adjacent point bars. Both imbalances can be compensated by deposition elsewhere in the floodplain and do not necessarily imply net export of sediment. The magnitudes of the imbalances along several U.S. rivers are approximated based on remotely sensed data. They generally each represent ∼10% of gross cut bank erosion.; Local disequilibrium in channel/floodplain sediment exchange is incorporated into a theoretical framework for modeling floodplain evolution. The theory includes independent models for floodplain deposition and for the net re-supply of sediment at eroding banks. A mechanism for tracking suspended sediment tracer material is included. A stabilizing feedback between overbank deposition, flood duration, and floodplain elevation results in the evolution toward a geomorphic steady state associated with a reach-wide balance in sediment exchange. The theory is applied to the case of the upper Clark Fork River, Montana, where it simulates the basic features of the system's response to historic fluvial disposal of mining waste. Key features of the simulated response include rapid short-term floodplain aggradation followed by the slow re-supply of mine-related material to the channel, in agreement with observations.; A longer time-scale model for valley infilling that accounts for the net transfer of sediment to a floodplain is applied to the evolution of the Fly River system, Papua New Guinea, over the period since the last glacial maximum (LGM). The model describes a simple network consisting of the lower Fly River and its two major tributaries, the middle Fly and Strickland Rivers. Rapid aggradation on the lower Fly since LGM results in a marked decrease in the middle Fly's bankfull slope immediately above the confluence. Simulations imply that at LGM, the lower Fly passed through a steep region not far upstream from the low-stand delta.