Large floods and climate change in the southwestern United States

Floods can have a tremendous impact on water resources but their properties may fluctuate at frequencies that defy recognition in short-term instrumented records. Paleoflood deposits provide insight into this component of the hydrological system by preserving evidence of the largest discharges that have occurred on a given river over time periods that often exceed 1000 years. A regional paleoflood analysis currently in progress in the southwestern United States incorporates study sites on more than twenty rivers. Previous studies on rivers in central Arizona and southern Utah indicate periods of flooding around 800-1000 A.D., 1400-1600 A.D., the late 1800's and the present, separated by periods of relative quiescence. These consistent variations in the long-term flood series within a region suggest a hydroclimatic cause. As the paleoflood dataset from the Southwest becomes more complete, it will indicate variations in the spatial and temporal distributions of large floods in relation to past climatic conditions. The present flooding regime, while not particularly anomalous when viewed in the context of the entire paleoflood record, nevertheless does not contradict the possibility that we are moving into a period of climatic response to general global warming.