Amplified Extreme Floods and Shifting Flood Mechanisms in the Delaware River Basin in Future Climates

Historical records in the Delaware River Basin reveal complex and spatially diverse flood generating mechanisms influenced by the region's mountains-to-plains gradients. This study focuses on predicting future flood hazards and understanding the underlying drivers of changes across the region. Using a process-based hydrological model, we analyzed the hydrometeorological condition of each historical and future flood event. For each event, at the subbasin scale, we identified the dominant flood generating mechanism, including snowmelt, rain-on-snow, short-duration rain, and long-duration rain. The rain-induced floods are further categorized based on the soil's Antecedent Moisture Condition (AMC) before the event, whether dry, normal, or wet. Our historical analysis suggests that rain-on-snow is the primary flood mechanism of the Upper Basin. Although most frequent, the magnitude of rain-on-snow floods is often less severe than short rain floods. In contrast, historical floods in the Lower Basin are primarily caused by short rain under normal AMC. Given the uncertainties in climate projections, we used an ensemble of future climate scenarios for flood projections. Despite variations in regional climate projections, coherent perspectives emerge: the region will shift toward a warmer, wetter climate, with a projected intensification of extreme floods. The Upper Basin is projected to experience a marked decrease in rain-on-snow floods, but a substantial increase in short rain floods with wet AMC. The largest increase in flood magnitude will be driven by short rains with wet AMC in the Upper Basin and by short rains with normal AMC in the Lower Basin. The Delaware River Basin spans a diverse landscape, from mountains to coastal plains. This geographic diversity makes the region susceptible to various flood events with distinct causes and timing. For example, in spring 2005, a rain-on-snow event accelerated the melting of an existing snowpack, leading to significant flooding. Then, in the summer of 2011, heavy rainfall from Hurricane Irene caused widespread inundation in the basin. This study focuses on predicting future flood hazards and understanding the underlying drivers of changes across the region. We found that, historically, the Upper Basin has mostly experienced floods caused by rain-on-snow, while the Lower Basin floods have been primarily driven by short, intense rainfalls. Intriguingly, even though rain-on-snow floods are most frequent in the Upper Basin, they are often less intense than floods caused by short rain, resulting in high-frequency, low-magnitude flood events. As climate models predict a shift toward a warmer and wetter regional climate, simulations suggest that the Upper Basin is likely to experience fewer rain-on-snow floods and more short rain-driven floods intensified by wetter pre-flood soil conditions. Despite variability in regional climate projections, a prevailing consensus suggests that the region's future floods will be more severe, predominantly driven by short, intense rainfalls. Historical floods in the Delaware River Basin are primarily from rain-on-snow at the Upper Basin and short, intense rain at the Lower Basin Future climate shifts the Upper Basin's primary flood mechanism from rain-on-snow to short, intense rain amplified by wetter pre-flood soils Despite climate projection uncertainties, consensus is future extreme floods, particularly from short rains, will intensify regionally