Regional variations in small-basin floods in the United States

This paper presents a reconnaissance study of regional flood patterns in the United States, focusing on peak discharges at several recurrence intervals and characteristics of flood-causing rainfall. Because of an interest in flash floods, attention was restricted to watersheds between 10 and 200 km² in area. Data were obtained from 130 stream gaging stations with a consistent 30-year period of record and from reports documenting 90 exceptional floods occurring mostly on ungaged watersheds. Peak discharges vary considerably within local regions. Roughly 60% of the local variability can be explained by watershed characteristics, but watershed area is not a reliable predictor of peak discharge within the narrow range of watershed sizes examined. On a continental scale the spatial patterns of the median and 25-year floods are similar. In both cases a concentration of large floods is found in the southeastern Great Plains and parts of the southeast. In the west, north, and northeast, floods tend to be small, but large floods still occur in scattered locations. The pattern and seasonality of the exceptional floods, which are presumed to have relatively long recurrence intervals, are different from the pattern of median and 25-year floods. The largest of the exceptional floods are concentrated in the central and southern Great Plains during May and June. They occur farther west (and several months later) than the largest median floods. Exceptional floods occurring in the semiarid west were caused by as little as 5-10 cm of rain in 30-60 min, whereas in humid areas most of the exceptional floods resulted from 13-32 cm of rain in 1-12 hours.