A 700-YEAR RECORD OF FLOODS AND STREAMFLOW ON THE LOWER MISSISSIPPI RIVER: INSIGHTS FROM A SEQUENCE OF OXBOW LAKE SEDIMENTS

The Mississippi River basin drains nearly half of the contiguous United States and is of critical economic importance due its use for shipping, agriculture, industry, and recreation. Flooding of the lower Mississippi River causes major disruptions to these industries and is economically costly. Our current understanding of flooding on this river comes predominantly from relatively short instrumental steam gage records (~100 years), and extending these records using paleofloods can be used to provide context to recent trends, reduce uncertainties in flood frequency analysis, and evaluate the ability of earth system models to simulate the sensitivity of streamflow to changes in climate. Here, we evaluate overbank deposits in a series of sediment cores from three different oxbow lakes that formed sequentially on the lower Mississippi River near Natchez, Mississippi to reconstruct shifts in flood frequency and magnitude over the last ~700 years. We use a combination of sediment characteristics, including grain size and organic content, to produce the longest continuous flood reconstruction on the lower Mississippi River. Using this record, we evaluate how flood regimes changed over the past millennium in response to changes in climate and river management, and use this to assess the response of Mississippi River flooding to moderate changes in climate. We integrate the paleoflood data with output from the Community Earth System Model (CESM) Last Millennium Ensemble (LME) to show that discharge on the lower Mississippi River decreased during the Medieval era (1000-1200 CE), a period of regionally warm and dry conditions. Consistency between our reconstructions and output from CESM demonstrate that streamflow of the lower Mississippi River is sensitive to relatively moderate changes in climate. Future work will continue to use historical and paleoflood data to validate model output, and narrow uncertainty in projections of Mississippi River streamflow in response to ongoing climate change.