The Flashiest Watersheds in the Contiguous United States

Abstract

The authors identify the flashiest watersheds in the contiguous United States based on frequency of discharge peaks exceeding 1 m3 s−1 km−2. The entire digitized record of USGS instantaneous discharge data is used for all stream gauging stations with over 10 years of data. Using the 1 m3 s−1 km−2 threshold, the flashiest basins in the contiguous United States are located in urban areas along a swath of states from the south-central United States to the mid-Atlantic and in mountainous areas of the West Coast, especially the Pacific Northwest. The authors focus on small watersheds to identify the flashiest cities and states across the country and find Tulsa, Oklahoma; Baltimore, Maryland; and St. Louis, Missouri, to be the flashiest cities in the contiguous United States. Thunderstorms are major agents for peak-over-threshold flood events east of the Rocky Mountains, and tropical cyclones play a secondary role, especially in the Southeast. West Coast flood events are associated with winter storms. Flooding west of and within the Rockies is linked to steeply sloped terrain and compact watersheds. East of the Rockies, urban areas dominate flashy watersheds. The authors find that watersheds northeast (downwind) of city centers are flashier than other urban watersheds, consistent with the downwind maximum in rainfall found in many urban regions. They examine anomalous flood response in the Illinois–Missouri region; St. Louis is among the flashiest cities in the United States, while Chicago is among the least flashy. Their flashiness map is compared with other measures of flooding, including flood damage and National Weather Service flash flood reports.

Predictors of urban variable source area: a cross‐sectional analysis of urbanized catchments in the United States

“…Watersheds with lower average slopes are expected to exhibit more variability in the saturated zone and from subsurface throughflow, which result in a VSA response (Dunne et al ., ). Smaller basins are likely to exhibit flashier hydrological response, which may be associated with reduced VSA effects (Smith and Smith, ). The meteorological control included was the average total annual precipitation in the watershed.…”

“…Developed open space in the National Land Cover Dataset is defined as development that is less than 20% impervious, so these areas could still contain roads and drainage infrastructure that increase hydraulic connectivity. Urban pervious surfaces could have very little storage because of compaction and localized subsurface saturation because of lawn watering and leakage and therefore lead to saturation overflow conditions even during very small events (Bhaskar and Welty, ; Lerner, ; Smith and Smith, ). Although this process is physically more similar to Dunne's VSA concept of saturation overland flow, if storage is minimal, the hydrological response at this level of analysis is indistinguishable from Hortonian overland flow.…”

“…Contrary to commonly held beliefs about limiting imperviousness of development in order to avoid negative changes in hydrologic regime, studies indicate that developed open space can also have limited ability to prevent flashy response. Reasons for this include the limited infiltrative capacity of compacted soils (Smith and Smith, ), high proportion of runoff response attributed to shallow subsurface flow under residential lawns (Wigmosta and Burges, ), subsurface saturation due to leaky water distribution infrastructure (Lerner, ) and decreased evapotranspiration associated with vegetation change (Bhaskar et al ., ). Figure shows an adaptation of the VSA to include urban run‐on from impervious areas and other potential sources of impacts to soil saturation in urbanized catchments (Miles and Band, ).…”

Regional Variation in Gravel Riverbed Mobility, Controlled by Hydrologic Regime and Sediment Supply

“…The West Coast is characterized by high erosion rates (average long-term erosion rate = 336 mm/kyr, and has a Mediterranean climate with dry, warm summers and mild, wet winters (Hirschboeck, 1991). Rivers in the Mid-Atlantic Appalachians and along the West Coast tend to experience a substantial number of "flashy" storms (Smith & Smith, 2015). Floods result from early autumn tropical cyclones, wintertime "nor'easters," and, occasionally, intense summer thunderstorms (Hirschboeck, 1991).…”

Spatial Analysis of High-Resolution Radar Rainfall and Citizen-Reported Flash Flood Data in Ultra-Urban New York City

“…Many studies have detailed the increase in flood volumes and discharge peaks associated with increasing impervious area (for example, [3][4][5][6][7]). A national study found that the flashiest watersheds in the country are urban [8]. The U.S. is urbanizing as the population continues to move into cities and suburbs.…”