Many studies have analyzed historical trends in annual peak flows in the United States because of the importance of flooding to bridges and other structures, and the concern that human influence may increase flooding. To help attribute causes of historical peak-flow changes, it is important to separate basins by characteristics that have different influences on peak flows. We analyzed historical trends by basin type: minimally altered basins, regulated basins (substantial reservoir storage but low urbanization), and urbanized basins (with low reservoir storage). Although many peak-flow magnitude changes were found in the last century across the conterminous United States, the trend magnitude and direction vary strongly by basin type and region. In general, there was a low percentage of significant increases and decreases for minimally altered basins while many regulated basins had significant decreases and the limited number of urbanized basins with long-term record showed a high percentage of increases. For urbanized basins, which are concentrated in the Northeast and Midwest, trend magnitude was significantly correlated with the amount of basin urbanization. For all basins regardless of type, parts of the Northeast quadrant of the U.S. had high concentrations of basins with large and significant increases while parts of the Southwest quadrant had high concentrations of basins with large and significant decreases. Basin regulation appears to have heavily influenced the decreasing trends in the Southwest quadrant; there were many large decreases for this basin type despite overall increases in heavy precipitation in this area. Changes over time in the number of 2-per-year and 1-per-5-year peaks over threshold are consistent with changes in the magnitude of annual peak flows.