Using Sedimentary Deposits from Oxbow Lakes for Reconstructing Floods and Anthropogenic Impacts on the Mississippi Delta
Liviu Giosan, Geology & Geophysics Jeff Donnelly, Geology & Geophysics
The Mississippi River Delta is one of the largest and most productive coastal ecosystems in North America. From energy, to fisheries, to navigation, the richness of this ecosystem has contributed to the U.S. economy for 300 years. Currently, the delta is in danger of drowning due to the combined influence of subsidence, sea-level rise and reduced sediment loads. While the influence of subsidence and sea-level changes on delta evolution have been studied in detail, the problem of sediment delivery to the delta has largely been ignored. Most of the sediment reaching the delta is delivered during floods, such as the 2011 extreme event. Unfortunately no flood reconstruction is available to date beyond very limited instrumental measurements of the last century. Similar lack of data is common for other deltas around the world. We propose to develop a methodology for reconstructing flood frequency and changes in sediment load types for delta-building rivers based on Mississippi oxbow lakes. Our reconstruction will allow for a quantitative analysis of the impact of climate change and/or human intervention on floodplain maintenance and provides the main motivation for our project.
Over the last two centuries, engineered structures such as levees were built along the lower Mississippi river for navigation and flood control. The levees isolated the river from its delta, while dams, dikes, and other changes on the Mississippi River and its tributaries diminished sediment supply by about 50%. When the delta-sustaining water and sediment of the river no longer flowed over the delta, wetland loss rapidly accelerated. More than 1,800 square miles of delta have been lost in the past 100 years. If much of the sediment delivered by the Mississippi during floods is directed to and trapped on the floodplain, restoration of Mississippi delta is possible even at the current high rates of sea-level rise. Previous work shows that oxbow lakes are ideal recorders of river floods. Oxbow lakes are numerous and easily accessible along the Mississippi lower valley and can be used to gain a deeper understanding on how the river works to build and maintain its delta. We propose to survey and core several such lakes in order to study the source and frequencies of flood layers and their link to the known history of climate changes and human activities over the last 3000 years. We envision that data from this pilot project will lead to a larger externally funded study to provide a state-of-the-art understanding of the sedimentary regime of a large river as it reaches the coast and constructs its delta. Development of local collaborations such as the proposed river observatory between WHOI and Tulane will be a significant objective of the project.