Exploring past floods helps scientists better understand what may happen with the next big flood.
A rich history of large late-Holocene Tennessee River floods is preserved in caves and alcoves throughout the Tennessee River Gorge area near Chattanooga, Tennessee. Preliminary stratigraphic analyses, coupled with geochronologic techniques, show evidence of at least four floods occurring in the last ~3,000 years with possible discharge estimates greater than or similar to the 1867 peak of record (460,000 ft3/s at Chattanooga, Tennessee). One of those floods may have occurred in the last 400 years, and has an estimated discharge at least twice the magnitude of the 1867 flood. At least 1–2 additional large floods with estimated peaks similar to the 1917 flood (341,000 ft3/s) may have occurred in the last ~3,000 years. In addition to flood evidence found in caves and alcoves, flood deposits preserved in exposed stratigraphy at Williams Island, an alluvial island at the head of the gorge, date to ~9,000 years. Determining accurate discharge estimates in this section of the river is difficult due to the backwater from the gorge constriction during high flows, but the flood records preserved here can be used to validate flood evidence downstream in the gorge, where the stable boundary and narrow valley provide more reliable discharge estimates. Stratigraphic records of past floods to reduce uncertainty in flood frequency analyses have been used extensively in the arid western United States, especially for floods with low annual exceedance probabilities. Preliminary results indicate that previously developed techniques to develop stratigraphic records of past floods can be successfully applied to reduce uncertainty in flood frequency analyses in the temperate eastern regions of the United States.
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