THE MAGNITUDE OF CLIMATICALLY TUNED EXTREME INLAND FLOODING REDUCED IN THE SOUTHEASTERN US

The segregate effects of natural climate change and anthropogenic disturbance on extreme inland flooding are poorly understood. Investigating long-term proxy-based reconstructions of extreme flooding is one of the keys to resolving this issue. Here we present a ~200 yr history of extreme flooding of the Great Pee Dee River (GPDR) in South Carolina to examine climatic and anthropogenic control on the magnitude and frequency of extreme floods.

A ~2 m piston core from an oxbow lake was analyzed using Bayesian End-Member Modelling to develop a grain-size proxy for extreme flooding of the GPDR. Core chronology was established through a combination of ²¹⁰Pb, ¹³⁷Cs, ¹⁴C, and OSL derived data. Our grain-size proxy successfully identified all major floods of the GPDR recorded at a close USGS gage station and showed a strong correlation (r²=0.7) with the magnitude of floods in the instrumental record. Our reconstructed frequency of major floods has a multi-decadal signal that is positively correlated with multi-decadal oscillation of sea-surface temperature in the north Pacific (PDO). PDO is regarded as an extratropical response of the low-frequency signal of El Niño-Southern Oscillation (ENSO). Winter El Niño conditions enhance subtropical jets extending across the southern tier of the US, which increases extratropical cyclone and frontal system activities and, consequently, the probability of extreme flooding in our study area. This reconstruction shows that the magnitude of extreme flood has reduced dramatically during the past 70 years in comparison with the late 19^th to early 20^th century, strongly corroborating a limited number of instrumental records of the Savannah River and the Congaree River in the southeastern US. This decrease of magnitude is due to the interaction of climate change, land-use modification, and river damming. Our result indicates that the magnitude of extreme flooding in major rivers decreased despite an increase in frequency and intensity of extreme precipitation in the southeastern US over the last century, suggesting a complex interplay between extreme inland flooding, human interference and heavy precipitation.