A review of historical river flood records in the Delaware River Basin (DRB) reveals considerable variability in flood characteristics and flood seasonality across the basin, highlighting the complexity and diversity of underlying flood-producing mechanisms (FPM) that are influenced by the region’s mountains-to-plains gradients. Despite the critical implications for coastal flood adaptation development, future flood hazards and how FPM may change across the hydroclimatic gradients of DRB are not yet well understood. As part of the Integrated Coastal Modeling (ICoM) project, this study presents a flood attribution analysis approach that provides physical consistency between the upstream hydrometeorological processes and each consequential flood event through process-based modeling of the DRB hydrological system. Based on the times series of hydrometeorological variables from model input or simulations (e.g., rainfall, soil moisture, and snowmelt), each historic/future flood event over 30 years of period is attributed to a dominant FPM at each HUC8 basin of DRB among the following classes: snowmelt, rain-on-snow (ROS), short and long rainfall that are further classified by dry/wet antecedent soil moisture condition. The outcomes of this research suggest changing spatial patterns of flood vulnerability in future climate due to a shift of dominant FPM that is most pronounced for historically ROS-dominated areas. Changing soil moisture in response to changing climate also plays an important role in modulating rain-induced flood characteristics.