Capture Timescale of an Uncontrolled Mississippi–Atchafalaya Bifurcation with Future Lower River Strategy Implications

Andrus, T.M. and Bentley, S.J., 2023. Capture timescale of an uncontrolled Mississippi–Atchafayala bifurcation with future lower river strategy implications. Journal of Coastal Research, 39(6), 1001–1020. Charlotte (North Carolina), ISSN 0749-0208. The most recent and currently active delta lobe of the Mississippi River (MR) is the Atchafalaya–Wax Lake lobe, which was initiated approximately 400 years ago as a result of MR stream capture by the Atchafalaya River (AR). This capture process accelerated in the early to mid-1900s but further development was prevented by construction and operation of the Old River Control Structure (ORCS) complex in 1962. Because the initial research to warrant the construction of the ORCS was undertaken nearly 70 years ago, this study revisits the basis of the resulting predictions and evaluates how both branches could evolve moving forward both with and without ORCS controls. This study uses the Delft3D software suite to retest the capture hypothesis by accounting for complex hydrodynamic interactions from the MR/AR bifurcation to the Gulf of Mexico. A 150-year model duration was used to adequately characterize bifurcation evolution, stream capture life cycle, and long-term river management concerns. Results of modeled flow evolution with the ORCS removed indicate a rapid period of channel adjustment followed by a long and gradual increase in flow ranges and magnitudes in the AR similar to those currently in the MR. In this hypothetical scenario, the AR captures 50% of the MR very quickly (∼17 y) to become the dominant stream and then the capture rate begins to decelerate. One-hundred-fifty-year results with ORCS controls remaining unchanged reveal increased sedimentation, rising stages, and decreased stream power in the MR. Conversely, AR results indicate channel deepening, stage lowering, and increased sediment-transport efficiency. These trends suggest that although the flow split at the MR/AR bifurcation continues to be regulated by the ORCS, the geologic evolution toward capture gradually continues. This study presents a long-term view of an uncontrolled MR/AR avulsion, and capture results reiterate the ORCS's importance to socioeconomic factors such as flood control and navigation. However, results also reveal long-term sustainability concerns for the MR system associated with current ORCS flow regulation. Predictive modeling tools such as those developed in this study could be used to optimize ORCS management strategies and potentially enhance ecosystem restoration plans.