NOAA/WDS Paleoclimatology - Middle Mississippi River Valley 4,000-year Sediment Data as Paleoflood Record

This study presents a ~4,000-year paleoflood reconstruction from South Horseshoe Lake (SHL), an oxbow lake in the middle Mississippi River Valley (between the confluence of the Missouri and Ohio rivers) , developed using sediment accumulation rates and high-resolution particle-size analysis. The SHL record provides the longest continuous terrestrial archive of Mississippi River flooding to date, extending millennia beyond the instrumental period. Variations in silt and very fine sand fractions are used to identify discrete flood events and infer changes in flood magnitude, while sedimentation rates reflect longer-term shifts in flood frequency and floodplain connectivity. Our results indicate that the interval between ~4,000 and 2,000 years before present was characterized by elevated flood frequency and repeated extreme events, driven by enhanced Gulf of Mexico sea-surface temperatures and intensified moisture transport from the Gulf into the midcontinent. A comparison between the SHL paleoflood record and sediment cores from the Gulf of Mexico Orca Basin provides the first terrestrial support that the previously inferred Mid-to-Late Holocene “megaflood” deposits identified in the Orca Basin sediment record represent basin-wide hydrologic events. Flood frequency and magnitude declined after ~2,000 years before present and remained relatively subdued through the Medieval Climate Anomaly and Little Ice Age. Recent increases in sedimentation rates and flood indicators show a shift toward hydroclimatic conditions analogous to the Mid-to-Late Holocene transition, suggesting that ongoing warming and land-use change may result in flood frequencies and magnitudes exceeding those of the preindustrial Late Holocene.