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Natural flood regimes provide a wide array of important ecological functions. Our goal is to assess the hydrologic impact of dams on flood flows throughout the United States. Regional regression models of the median annual 1-day maximum flow were developed as a function of natural watershed characteristics, dam storage, and population density. Most of the regressions have adjusted R2 values in excess of 0.80, and overall the models covered 78% of the area of the continental U.S. Alteration of flood flows is present in every region of the country, and is more severe west of the Mississippi and especially in the southern Great Plains, desert Southwest, and northern California. The percent of U.S. rivers with greater than a 25% reduction in the median annual flood is 55% for large rivers, 25% for medium rivers, and 10% for small rivers. The majority of freshwater ecoregions in the country have at least 10% of their rivers with 25% or greater alteration in all three river size classes. A simple model based on the ratio of dam storage to mean annual runoff was developed for assessing alteration in ungauged rivers, and was found to be generally useful for classifying rivers into categories of potential alteration. Overall, we document the alteration of natural flood flows across the U.S. in more detail than has been previously accomplished, and demonstrate the efficacy of multivariate regional regression models and other indicators for assessing hydrologic alteration. Copyright (C) 2010 John Wiley & Sons, Ltd.