" Dynamics of Land Loss and Land Building on the Mississippi River Delta"
Dr. Douglas A. Edmonds, Associate Professor
Department of Earth and Atmospheric Sciences, Indiana University
The world’s river deltas are disappearing into the ocean from anthropogenic changes to sediment supply, river flow, subsidence, and sea level rise. The Mississippi Delta serves as a poignant example—since 1930 delta area has decreased by ~30% at the rate of one football field every hour. This threatened delta supports both productive ecosystems and densely populated cities and needs to be restored. Successful restoration depends on defining and reversing the processes leading to land loss.
In this talk, I use composite remote sensing imagery to define that land loss on the Mississippi River delta since 1984 mostly occurs from interior marsh conversion to water or marsh edge erosion by waves. Spatial autocorrelation suggests edge erosion is a significant process and accounts for up to 40% of land loss. Understanding marsh edge-erosion dynamics, particularly on ponds, is critical because positive feedbacks can lead to marsh collapse. By tracking pond changes over the last 30 years, I show a striking consistency between pond expansion direction and the dominant wind direction. Modeling shows that wind-generated waves are causing edge erosion. Expansion rate increases rapidly for ponds wider than 300 m suggesting the wide ponds are unstable.
Reversing land loss processes is a complex goal that requires re-establishing sediment flux to the marsh to halt pond expansion and reinitiating the natural processes of deltaic land building. Most restoration plans advocate for using sediment diversions to create new deltaic land in critical areas. Sediment diversions should be optimized to create sustainable deltaic land. I show using morphodynamic modeling that land building, in deltas like the Mississippi, is the result of mouth bar formation and river avulsion. The relative time scales of these processes determine the deltaic shape, channel number, and marsh area. These results suggest that sediment diversions can be designed to create certain kinds of deltaic land.