The hydrology of riparian wetlands worldwide has been altered extensively owing to the construction and operation of dams. We developed a model for the Roanoke River floodplain (United States) to simulate flood extent and duration based on a power law correlation between inundation area A, as mapped from synthetic aperture radar (SAR) imagery, and river discharge Q. Model fit was 0.955 for the upper portion of the study area and 0.789 for the tidally influenced lower section. We then compared hydroperiod simulated for predam (1912--1949) and postdam (1965--1995) periods. Topographically wet areas are now flooded longer than before damming, and dry areas are now drier. Similarly, hydrologically wet years experience longer floods, whereas the driest years are drier. Most importantly, spring hydroperiod regimes are now wetter than prior to damming. Our results suggest that the intermediate zone of the hydrologic gradient has been squeezed to either wetter or drier conditions. The model presented represents a simple but effective empirical method to simulate hydroperiod regimes at the landscape scale in large lowland systems where the data necessary to develop more complex physical models are not available.