Reconstructing extreme floods in eastern North America using sedimentary records

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This article is part of the Fall 2019 issue of the Earth Science Matters Newsletter.

satellite image of Chesapeake Bay after Tropical Storm Lee (2011)

Sediment plume in Chesapeake Bay following Tropical Storm Lee, NASA https://earthobservatory.nasa.gov/images/52169/sediment-clouds-the-chesapeake-bay image from 13 September 2011. Sediment plumes, like the one seen here, form when large amounts of water quickly drain off the land. Eventually the sediment in the water column settles out to the bottom of the water body and gets buried and preserved in the geologic record. The mouth of the Susquehanna denoted by aqua highlighted “S”, the Potomac denoted by “P” and its outlet to the Atlantic Ocean with an “A”. The sediment core from this study was collected at the point labelled “2209”, and the stars identify Washington DC (DC), Richmond (R), and Annapolis (A). Figure adapted from Figure 1D in (Toomey et al. 2019).

(Credit: Michael Toomey, USGS. Public domain.)

 

 

 

The threats posed by extreme flooding to many U.S. cities, and nearby infrastructure, built along rivers are not well constrained due to limited data. For most rivers in the eastern United States, reliable streamgage measurements extend less than 100 years and often capture only a handful of the major floods recorded by historic sources (e.g. newspapers and other written accounts). In order to fill this critical gap, scientists can study geologic signatures of past flood events buried in coastal sediments. Geologic reconstructions of past floods can be used to: (1) assess the magnitude of these historic events, (2) extend records of flooding deeper into the past and (3) identify what drives changes in extreme precipitation on long timescales. At present, however, few such reconstructions exist for the eastern United States.

 

In a recent study, USGS scientists, along with university collaborators from the College of William and Mary, Woods Hole Oceanographic Institution, University of Rhode Island and Texas A&M, began to fill this knowledge gap by reconstructing extreme floods on the Susquehanna River over the past two thousand years. The Susquehanna has the largest watershed on the U.S. Eastern Seaboard. Sediment core MD99-2209, collected from the main stem of Chesapeake Bay (water depth = 26 meters) near Annapolis, Maryland by the research vessel Marion Dufresne in 1999, was analyzed for coarse-grained sediment layers thought to have been deposited during large floods.

 

Several such layers were identified downcore. The ages of these ‘paleo-events’ were determined using lead (210) during the past ~100 years and radiocarbon deeper in the core. Additionally, the abundance of ragweed pollen increased directly after Colonial land clearance, providing an additional age marker. The most recent ‘flood’ deposits found in this core can be attributed to hurricane Agnes (1972) and the Great Flood of 1936. However, many other coarse-grained layers in the sediment core predate robust historic records and are indicative of prehistoric floods on the Susquehanna between 1800–1500, 1300–1100, and 400–0 CE. A possible explanation for increased flood frequency during these intervals is that cooler conditions near Chesapeake Bay—relative to the tropical North Atlantic—produced favorable conditions for tropical cyclone development and landfall in the Susquehanna watershed. Ongoing work, elsewhere in Chesapeake Bay, aims to develop a complementary record of coastal inundation during intense hurricane strikes in order to further test this hypothesis.

The paper, “The Mighty Susquehanna—Extreme Floods in Eastern North America During the Past Two Millennia” was published in Geophysical Research Letters. It is available at: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018GL080890

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