Where Are the Flashiest Watersheds in the United States? Exploring the Spatial Patterns of Frequent Flash Flooding Across the Contiguous United States

We identify the flashiest watersheds in the contiguous United States based on the frequency of 1 m3 km-2 s-1 discharge peaks. The entire digital record of USGS instantaneous data is used to determine the number of discharge peaks over this threshold per year of data for all USGS gages with more than ten years of instantaneous data. We find that the flashiest basins in the contiguous United States are located in urban areas along a swath of states from the southern Midwest to the Mid-Atlantic as well as in mountainous regions along the west coast. We focus on small urban watersheds to identify the flashiest cities and states across the country. The USGS GAGESII dataset along with lightning data are used for hydrologic, hydraulic, land use and climate data to attempt to explain differences in flashiness across the watersheds. We focus particularly on the Illinois region to investigate how St. Louis is among the flashiest cities in the United Stated while Chicago is among the least flashy. Our flashiness map is compared with other measures of flooding such as flood damage and flash flood reports to see the impact of these frequent small flood events.