Flooding of the Great River during the Common Era: A Paleohydrological Record of High Magnitude Flood Events from the Central Mississippi River Valley

Abstract:

Streamflow characteristics are known to be sensitive to changes in climate, but few continuous records of flooding exist to evaluate the response of hydrological systems to centennial- and millennia-scale climate changes. Here, we present sedimentary records from two oxbow lakes (Horseshoe Lake and Grassy Lake, Illinois, USA) in the central Mississippi River valley (CMRV) that display abrupt shifts in sediment composition and particle-size consistent with deposition by floodwaters immediately following inundation of the floodplain. The sedimentary record at Horseshoe Lake begins ca. AD 100 and displays five major flood events, with four of these occurring after ca. AD 1100. Situated 200 km downstream, the record from Grassy Lake begins later, ca. AD 800, and also shows four major flood events after ca. AD 1100. An analysis of synchronicity using Bayesian age modelling software shows high likelihoods that the four overlapping flood events occurred at the same time, confirming that these events resulted from flooding of the Mississippi River. The most recent event we record at AD 1840 ± 50 corresponds to the AD 1844 flood, the largest flood by discharge (37 m³/s) measured by the gauging station at St. Louis, Missouri, indicating that our sedimentary records document high magnitude flood events.

Together, our two sedimentary records show a major shift in the frequency of high magnitude flooding in the central Mississippi River at ca. AD 1100. From AD 100 – AD 1100, only one relatively subtle flood event is recorded, but from AD 1100 – AD 1900, four high magnitude floods deposited distinctive sediment at both sites. The period of infrequent flooding corresponds to a time of agricultural intensification and population growth in the CMRV, while the entire region was abandoned when flood frequency increased. The pronounced shift in flood frequency we observe in our records at ca. AD 1100 begins during the Medieval Climate Anomaly (MCA; AD 950 – AD 1250), a period of warmer and drier conditions in the central USA, but persists through the Little Ice Age (LIA; AD 1400 – AD 1700), when cooler and wetter conditions prevailed in this region, suggesting that the occurrence of high magnitude floods may be controlled by seasonal climatic parameters that remain poorly represented in proxy-based reconstructions and modeled projections.