DEVELOPING THE LONG VIEW OF THE MISSISSIPPI RIVER’S FLOOD HISTORY: HOLOCENE “MEGAFLOODS” TO YESTERDAY

Continued large increases in flood losses coupled with the increased likelihood of more extreme precipitation events related to climate change has led to the newly established Federal Flood Risk Management Standard. The new standard is set to replace the current regulatory one-percent-chance flood (i.e., 100-year flood) protection level standard for new federally funded projects. To inform the development of more robust exceedance probabilities on the main stem of the Mississippi River we have compiled flood discharges measurements at 14 long-term (>100 years) gaging stations along the Mississippi and its major tributaries. For the longer view (centuries to millennia) of the Mississippi River’s flood history, we also performed a detailed review of the Holocene paleoflood literature to quantify the magnitude of the largest floods after deglaciation to further inform flood risk assessment efforts.

For the nine gaging stations located above the confluence with the Ohio River, the 100-year flood has been exceeded between two to six times for each gage assessed since 1900 suggesting a review of the current flood frequency estimates is needed along these river segments. Official flood-flow frequencies for the Lower Mississippi River Valley have never been estimated because the flood risk assessment and associated mitigation efforts along this segment have been based on the “maximum probable flood” (MPF) which ranges from 70,800 m³/s near Cairo, Illinois up to 81,800 m³/s near Greenville, Mississippi. While the MPF has not been exceeded within the instrumental record, there have been at least four events during the period of record which have come within 20% of the MPF. The marine depositional record from the northern Gulf of Mexico suggests the occurrence of multiple mid- to late- Holocene “meagafloods.” It has been suggested the magnitude of discharge from these flood events were similar to the smaller late-Pleistocene to early Holocene deglacial floods (~100,000 m³/s average discharge). The return period of these “megafloods” has been estimated to be ~1,000 years. Floods of such a magnitude are unprecedented in the instrumental flood record and would substantially exceed the MPF and protection level of the flood mitigation works along the Lower Mississippi River.