Spatial variations in the timing of annual floods in the southeastern United States

Cluster analysis of data from 806 USGS gaging stations in the southeastern US was used to investigate spatial variations in the timing of the annual flood. Three primary regions were identified with distinctly different seasonal flood regimes: the Gulf States Region, the Florida Region, and the Carolina Region. The cluster membership within each region shows a high degree of spatial homogeneity, with >90% of the stations belonging to one dominant cluster. The Gulf States Region (Tennessee, Mississippi, Alabama, and Georgia) displayed the strongest seasonality in the study area, with a unimodal flood regime dominated by late winter/early spring flooding. Almost 71% of the annual floods occur from January though April with the highest monthly frequency occurring in March. The northern half of the Gulf States Region experiences a higher proportion of these floods in winter, while spring floods dominate in the southern half of the region. Along a 100 km wide belt bordering the Gulf Coast (coastal Mississippi, Alabama, and the panhandle of Florida) and extending into southeastern Georgia, the dominant late winter/early spring mode characteristic of the rest of the Gulf States Region is coupled with an increased incidence of summer and fall flooding (peaking in September). The flood regime in the Florida Region (peninsular Florida) is strongly bimodal, displaying a dominant September mode and a weak March mode. About 54% of the annual floods occur from August through October. The overall pattern in the Carolina Region (Virginia, North Carolina, and South Carolina) is characterized by a more uniform monthly distribution of annual floods than elsewhere in the study area. Nevertheless, the highest monthly frequencies occur in winter and spring (51% from January through April), while a secondary period of higher monthly frequencies occurs in August through October. Drainage area has little effect on regional patterns; however, the smaller watersheds within each region generally display higher annual flood frequencies during the summer and fall.