Hydrologists have traditionally assumed that the annual maximum flood process at a location is independent and identically distributed. While nonstationarities in the flood process due to land use changes have long been recognized, it is only recently becoming clear that structured interannual, interdecadal, and longer time variations in planetary climate impart the temporal structure to the flood frequency process at flood control system design and operation timescales. The influence of anthropogenic climate change on the nature of floods is also an issue of societal concern. Here we focus on (1) a diagnosis of variations in the frequency of floods that are synchronous with low-frequency climate state and (2) an exploration of limiting flood probability distributions implied by a long simulation of a model of the El Ni¿o/Southern Oscillation. Implications for flood risk analysis are discussed. ¿ 2001 American Geophysical Union