Flood Variability in the Western United States: Overview and Examples
Greater understanding of riverine flood hazards, and how they vary in space and time, is needed to protect lives, property, and infrastructure. To this end, the variability of floods within the wide-ranging climatological conditions of the contiguous United States west of the Mississippi River were assessed using the flood potential method. This procedure fosters the comparison, visualization, and communication of flood hazards, for highlighting the status of flooding given observational data from the nation’s streamgaging network, and how floods are changing due to such non-stationarity mechanisms as climate change. Within the study area, 117 zones of similar flood response were delineated using 4621 streamgaged watersheds with areas <3860 mi2. Explained variances of the regressions were high, with an average R2 = 0.91. The central tendency and variability of flood magnitudes experienced within each zone is an intrinsic characteristic, with exceedingly large floods systematically defined and ranked as extreme. The highest flood potential occurs in the southern Midwest and Texas, and along the West Coast, with the lowest flood potential in the Great Basin, Rocky Mountains, and northern Midwest. Extreme floods are not becoming larger or more frequent. However, some zonal areas (29%) are currently experiencing increasing trends in the magnitude or frequency of large floods, though magnitudes vary considerably more in space than in time. Results are presented through the Flood Potential Portal, a decision support system to explore flood variability at a full range of scales and predict flood discharge magnitudes using multiple methodologies. Examples are provided, including for the Yellowstone region floods of 2022 and Oroville Dam flood of 2017, to provide applications that use the methodology for enhancing knowledge of flood hazards.