DEVELOPING A LONG-TERM PERSPECTIVE OF FLOODING IN THE MISSISSIPPI RIVER BASIN USING FLUVIO-LACUSTRINE SEDIMENTS

Fluvial floods are a major environmental hazard with significant social and economic costs, but how major river systems will respond to projected climatic variability and change remains unclear. Given this uncertainty, a new generation of paleoflood records is being developed using the sedimentary records held in floodplain lakes and depressions to reconstruct the timing, frequency, magnitude, and provenance of flooding over centennial to millennial timescales. Here, I will discuss recent work on paleoflood records from the central Mississippi River, as well as on-going work that aims to create a basin-wide network of paleoflood records.

Particle-size analysis on sediment cores from two oxbow lakes adjacent to the central Mississippi River identified several layers with a distinctively uniform and fine silty texture that is consistent with deposition of the suspended load immediately following inundation of the floodplain during large overbank floods. An analysis of synchronicity using Bayesian age-depth modelling found high likelihoods that all overlapping flood events at the two sites occurred at the same times, confirming that these events resulted from flooding of the Mississippi River. These records show that floods comparable in stage to the largest floods in the instrumental record occurred about once a century for most of the last 2,000 years. However, large floods were suppressed from ca. AD 600-1200, corresponding with the Medieval Climate Anomaly (MCA), when central North America experienced enhanced aridity. It remains unclear whether the MCA is also associated with shifts in the frequency of flooding over other parts of the Mississippi Basin.

Additional sedimentary records from oxbow lakes adjacent to the lower Missouri, Ohio, and Mississippi Rivers are currently being developed to examine long-term spatiotemporal patterns of flooding across the Mississippi Basin. We are exploring the use of additional dating techniques to improve the precision of flood age estimates, as well as alternative proxies to identify floodwater deposits and determine their provenance. Ultimately, this network of paleoflood records will clarify the climatic conditions associated with enhanced flood frequencies in the Mississippi Basin, and improve predictions of hydrological variability for the 21^st century.