Reservoirs are by far the most significant human activities that are imposing hydrologic alterations, specifically related to extreme flow conditions. This study presents the effects of reservoir regulation on flood seasonality in different hydrologic and climate settings across the contiguous United States. The data employed consists of reservoir information from the National Inventory of Dams (NID) and Global Reservoir and Dam (GRanD) database along with USGS stream flow data for pre- and post-impoundment periods. A new flood seasonality index was developed with circular statistics to reveal any significant shifts in flood timing between pre- and post-impoundments periods at each USGS station. Reservoir Impact Index (RII) was developed as a function of storage capacity and mean annual streamflow to quantify the regulation effects of reservoirs on flood seasonality. Process understanding of how reservoir regulation affects flow seasonality was analyzed based on RII using simple but physically-based reservoir models with different degrees of complexity, e.g., simple linear and hedging models. Results indicate that the shift in seasonality of annual maximum flood (AMF) at downstream generally increases with increasing RII, given that reservoir has enough storage to regulate the flood. The process modeling results also imply that reservoir state prior to the occurrence of AMF, antecedent climatic patterns and catchment state affect the shift in AMF arrival at downstream. These findings will help improve the ability to examine issues connected to flood frequency characteristics including nutrient delivery, sediment load and stream temperature shifts at downstream of dams.