PALEOFLOOD HISTORIES FROM TWO SINKHOLE LAKES IN THE CHOCTAWHATCHEE RIVER FLOODPLAIN IN FLORIDA

Historic flooding in the Choctawhatchee River floodplain in northern Florida has resulted in devastating consequences, including the costly destruction of homes, businesses, and farm lands. Long flood records preserved in natural archives provide essential context for accurate flood prediction. We investigated the history of large floods in northern Florida using ²¹⁰Pb, ¹³⁷Cs, and ¹⁴C-dated sediment cores from Henry Lee Pond and Red Bug Pond. To identify flood deposits, we measured loss-on-ignition, coarse silt and sand contents using wet sieving techniques, and x-ray fluorescence-derived elemental abundances. We used Lidar-based elevation data to determine the flood stage elevations at which each lake is susceptible to flood deposition. Flood stages must exceed 1.6 and 2.6 m above the action stage, the high-water stage at which action is taken for flood mitigation preparation, for flood waters to reach Red Bug Pond and Henry Lee Pond, respectively. We utilized the elevation difference between these sites to estimate relative magnitudes of paleofloods. Floods that are likely to deposit sediment in these sites have magnitudes of at least major flood stage (> 0.9 m above action stage). Comparison of the sediment flood proxy data with USGS stream gage data reveal that major historical floods form cm-scale sand deposits and elevated titanium concentrations in Red Bug Pond sediments. Only a single flood, in 1929, was large enough to flood Henry Lee Pond, which coincides with a several cm-thick sand bed. Prior to 4.5 thousand years ago (ka), large time gaps between age control points, over 1 m of sand deposition, and an erosional contact indicates a ~4 kyr-long depositional hiatus at Red Bug Pond. The Henry Lee Pond record documented a large magnitude flood ~5.5 ka and lacks evidence for large floods between 5.5 and 7.7 ka. Several deposits of large floods are preserved in the Red Bug Pond and Henry Lee Pond sediment cores between 2.5 and 4.5 ka. Six floods occurred during the past 2 kyrs in roughly 400-year intervals and were preserved in the Red Bug Pond basins, but not Henry Lee Pond, indicating that the magnitudes of these paleofloods were smaller relative to the previous two millennia. Our new records demonstrate substantial Holocene variability in the frequency and magnitude of floods at the millennial timescale.