Effects of climate, regulation, and urbanization on historical flood trends in the United States

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. To help attribute causes of historical peak-flow changes, it is important to separate basins by characteristics that have different effects 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 amounts of reservoir storage). Although many significant peak-flow magnitude trends were found in the last century across the conterminous United States, trend magnitude and direction vary by region and basin type. In general, there was a low percentage of significant increases and decreases for minimally altered basins while regulated basins had many significant decreases and the limited number of urbanized basins with long-term records (concentrated in the Northeast and Midwest) had many significant increases. For all basins regardless of type, parts of the Northeast quadrant of the U.S. had high densities of basins with large increases and parts of the Southwest quadrant had high densities of basins with large 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. It is important to consider basin-specific changes such as historical reservoir additions and urbanization when computing future design floods for bridges and other hydraulic infrastructure.