A Tale of Two Cores: Investigating the Impact of Alluvial Flood Deposition on Flood Frequency and Magnitude Reconstruction

Abstract:

Extreme floods are underrepresented in stream gauge records. Sedimentological flood evidence (paleofloods) yield longer records, allowing extreme flood variation to be examined over several Holocene climate periods. But flood deposition and erosion can vary over alluvial surfaces, affecting the number and type of flood events preserved at a given site. This study aims to understand how spatial heterogeneity of flood deposition over an alluvial surface impacts sediment-based reconstructions of flood magnitude and frequency. We collected two sediment cores 500 meters apart from a natural levee at equal elevations along the middle Tennessee River in Alabama. We measured grain size from each core at a 1-cm resolution with laser granulometry. Optically stimulated luminescence dates revealed approximate age ranges of 50 – 5,500 calibrated years before present (yrs BP) for the downstream 3.5 m core (Big Oak) and 120 – 8,500 yrs BP for the upstream 4.18 m core (East Levee). We observed a buried soil at approximately 1.5 m in Big Oak and 0.75 m in East Levee with a minimum age of 2,000 yrs BP. East Levee contained a second buried soil with stronger development at 2.3 m with a minimum age of 5,000 yrs BP. Big Oak and East Levee sites contained 16 and 18 high magnitude paleofloods, respectively. Most of the Big Oak paleofloods (15) occurred in the last 2,000 years, while most of the East Levee paleofloods (11) occurred between 2,000 and 5,000 yrs BP. Four paleoflood events overlapped between sites. All of which were regionally identified Tennessee River floods that occurred in the last 2,000 years. This suggests East Levee captured less frequent flood deposition than Big Oak during this period, and only the largest floods left evidence of flood deposition. The difference in estimated flood magnitude was less than 10% for the four paleofloods found in both cores. These results suggest that the sediment cores contained distinct paleoflood records, but grain size magnitude estimation for a single flood event was reasonably consistent along the natural levee.