The Atchafalaya River Avulsion - control of Nature or control of dredging?

The Atchafalaya River (AR) - the primary distributary of the Mississippi River (MR) - increased the proportion of water it gathered from the MR from 17% in 1910 to 30% in 1950. This increase has been attributed primarily to natural causes such as the steeper slope to the Gulf of Mexico down the AR. Avulsion has always been considered natural and inevitable, facilitating the construction of control structures to regulate discharge partitioning. However, the increase can also be explained by the extensive dredging (1.7 x 107 m3 removed) and subsequent erosion (1.7 x 108 m3) in the AR between 1932 and 1948. We develop a 1-D backwater flow model to resolve bifurcation partitioning between the MR and AR, and apply it to bathymetry data collected by the US Army Corps of Engineers. Models comparing pre-dredging, proposed dredging, and post-dredging channels suggest that dredging can explain 77% of the increased discharge fraction down the AR between 1910 and 1950. These results highlight that channel deepening is more important than relative slopes for determining channel bifurcation partitioning and stability.