Hydrologic intensification has heightened flood risk in the coastal Mid-Atlantic United States. The Delaware River Basin (DRB) is characterized by diverse topography from mountains to coastal plains, and experiences a variety of flood events that vary in mechanism and timing across its subbasins. This complexity makes it challenging to understand connections between upstream subbasin-scale floods and downstream flood severity at the Delaware Estuary inlet. Under a warmer climate, we expect that intensified rain and reduced snowpack will uniquely affect flood characteristics of individual subbasins. The key processes affecting the riverine component of estuarine flooding, which aggregates these subbasin-level impacts, are yet to be identified. Using multi-decadal streamflow simulations of the DRB generated by a process-based hydrological model, we investigate flood spatial coherence, which we define as the number of subbasins flooding simultaneously during an estuary inlet flood, and how it changes under climate warming scenarios. We find that the most severe floods at the inlet occur when DRB subbasins exhibit high flood coherence. Warming conditions amplify flood magnitude and increase flood coherence across the DRB. Two dimensions of flood coherence stand out in terms of response to warming. First, there is a marked increase in flood coherence (and magnitude) in late summer, a period historically characterized by low coherence, high-magnitude floods. Second, snow-dominated subbasins contribute the most to increased flood coherence overall, and this effect increases with flood magnitude at the estuary inlet. We conclude that the impact of climate warming on future DRB flood risk may be controlled foremost by a shift from high-frequency, low-severity floods toward lower-frequency, high-severity floods within historically snow-dominated subbasins, punctuated by increased basin-wide flood coherence during the most severe late-summer tropical cyclone-driven floods.