There is lack of quantitative measurement and process-based understanding of human impacts on characteristic responses of hydrological systems, e.g., flood frequency curves (FFCs). Here a dimensionless Reservoir Impact Index (RII) is developed to quantify reservoir’s regulation capacity on streamflow in complex river systems. RII is applied to about 380 river stations over the contiguous United States, where the daily streamflow records can be split into two sufficiently long pre- and post-dam constructions periods. At each river station, the FFCs are constructed for the pre- and post-dam periods respectively, then compared to reflect the reservoir regulation effects. It is found that: 1) mean Annual Maximum Floods (MAFs) generally decrease with increasing RII; 2) FFC become steeper if RII is small enough, while becomes flatter once RII exceeds a threshold value. Three reservoir models with various complexities all suggest that this threshold value is closely related to reservoirs’ relative storage capacities.