A 1000-YEAR PALEOFLOOD RECORD FROM THE TENNESSEE RIVER NEAR DECATUR, TENNESSEE

The importance of paleoflood records in risk evaluation for dams and nuclear power plants led the Electric Power Research Institute (EPRI), Nuclear Regulatory Commission, and Tennessee Valley Authority (TVA) to host several workshops and provide support for research projects investigating methods for identifying paleoflood signatures in sediments. Universities in Tennessee, Alabama, Kentucky, Texas, and Georgia, as well as the USGS, investigated paleoflood deposits in the Tennessee River Valley. Our team at the University of Tennessee focused on identification and characterization of flood deposits between Forks of the River just upstream of Knoxville and the upper portion of the Chickamauga reservoir near Dayton, Tennessee. One of the most impressive paleoflood exposures in this section of the Tennessee River is PF 1, located just downstream of Watts Bar dam. This 3.6 m profile spans ca. 1000 years and consists of 15 stratified paleoflood deposits, each overlain by a buried soil horizon. The buried soils mark periods of floodplain stability and have higher organic and silt contents than the paleoflood deposits, which are 60-90% sand. The profile includes fining upward sequences, but the trend across the entire deposit is of sediment coarsening up profile, which may indicate an increase in flood velocities through time. Radiocarbon dating of charcoal in buried soil (10 samples) and flood (1) layers indicates 6 floods from ca. 1000-1400 CE, and 9 since ca. 1400 CE. Buried soil horizons express immature pedogenic characteristics that suggest short intervals of landscape stability between individual flood episodes, consistent with the radiocarbon results. However, micromorphological analyses reveal the presence of some clay cutans, indicating that some of these features can form within a few centuries. The 2-sigma calibrated age ranges of many of our radiocarbon dates overlap, and the calibrated range for our uppermost date spans over 300 years, highlighting the shortcomings of radiocarbon dating for historic flood deposits. OSL dating could potentially provide more precise age estimates. Additionally, we are testing whether the presence of PAHs or spheroidal carbonaceous particles in flood deposits and soils can indicate whether palefloods postdate the period of motorized transport on the Tennessee River.