Flood peak distributions for the eastern United States

Annual maximum peak discharge time series from 572 stations with a record of at least 75 years in the eastern United States are used to examine flood peak distributions from a regional perspective. The central issues of this study are (1) "mixtures" of flood peak distributions, (2) upper tail properties of flood peaks, (3) scaling properties of flood peaks, (4) spatial heterogeneities of flood peak distributions, and (5) temporal nonstationarities of annual flood peaks. Landfalling tropical cyclones are an important element of flood peak distributions throughout the eastern United States, but their relative importance in the "mixture" of annual flood peaks varies widely, and abruptly, in space over the region. Winter-spring extratropical systems and warm season thunderstorm systems also introduce distinct flood peak populations, with spatially varying control of flood frequency distributions over the eastern United States. We examine abrupt changes in the mean and variance of flood peak distributions through change point analyses and temporal trends in the flood peak records through nonparametric tests. Abrupt changes, rather than slowly varying trends, are typically responsible for nonstationarities in annual flood peak records in the eastern United States, and detected change points are often linked to regulation of river basins. Trend analyses for the 572 eastern United States gaging stations provide little evidence at this point (2009) for increasing flood peak distributions associated with human-induced climate change. Estimates of the location, scale, and shape parameters of the generalized extreme value (GEV) distribution provide a framework for examining scaling properties of flood peaks and upper tail properties of flood distributions. It is shown that anomalously large values of the GEV shape parameter estimates are linked to the role of tropical cyclones in controlling the upper tail of flood distributions. Scaling analyses of flood peaks highlight the heterogeneities in flood magnitudes over the region with maxima in scaled flood magnitudes in the high-elevation Appalachian Mountains and minima in the low-gradient Coastal Plain.