Regional hydrologic synchrony in the Southeastern United States: Insights from 916 years of reconstructed streamflow

Streamflow variability is a critical component of water availability across the South Atlantic-Gulf (SAG) water resource region of the United States, yet long-term coherence among basins remains poorly understood. We developed independent May–July streamflow reconstructions for the Roanoke River (South Atlantic), Pascagoula River (southern Mississippi Basin), and St. Johns River (northern and central Florida) spanning 1100–2015 CE. Each reconstruction is highly skillful (RE = 0.39–0.62; CE = 0.39–0.62) and explains 51–63% of observed variance. Across the 916-year record, only five droughts affected all three basins simultaneously, yet three occurred since 2000 (2006, 2007, 2011). These 21st-century droughts were broader and more spatially coherent than comparable events in 1491 and 1587. Basin-to-basin comparisons reveal shared low-flow years were most frequent between the Pascagoula and St. Johns Rivers (15 events), followed by Roanoke–Pascagoula (12) and Roanoke–St. Johns (9). When the St. Johns River experienced low [high] flows, the Pascagoula River had a 40% [48%] likelihood of concurrent extremes—the highest regional coherence observed. Low-flow events for individual basins lasted 2–3 years on average, with the longest drought persisting 26 years on the St. Johns (1459 CE). For both the Pascagoula and St. Johns River, return intervals based on observational streamflow records underestimated the recurrence frequency of extreme events like the early-2000s low flow event by 100–500 years. In contrast, observationally based return intervals on the Roanoke overestimate the length of time between the driest events, indicating severe droughts are more likely to occur than previously thought. These findings illustrate the risk of multi-basin droughts in the SAG region, particularly for the closely linked Pascagoula and St. Johns basins. Future drought planning and water-resource management must account not only for drought in individual basins, but the complex effects of synchronous hydrologic drought across the region.