Response of extreme floods in the southwestern United States to climatic variations in the late Holocene

A regional synthesis of paleoflood chronologies on rivers in Arizona and southern Utah reveals that the largest floods over the last 5000 years cluster into distinct time periods that are related to regional and global climatic fluctuations. The flood chronologies were constructed using fine-grained slackwater deposits that accumulate in protected areas along the margins of bedrock canyons and selectively preserve evidence of the largest events. High-magnitude floods were frequent on rivers throughout the region from 5000 to 3600 C-14 yrs BP (dendrocalibrated age = 3800-2200 BC) and increased again after 2200 BP (400 BC), with particularly prominent peaks in magnitude and frequency around 1100-900 BP (AD 900-1100) and after 500 yrs BP (AD 1400). In contrast, the periods 3600-2200 BP (2200-400 BC) and 800-600 yrs BP (1200-1400 AD) are marked by sharp decreases in the occurrence of large floods on these rivers. In the modern record, storms that generate large floods (greater than or equal to 10-year) in the region fall into three categories: (1) winter North Pacific frontal storms; (2) late-summer and fall storms that draw in moisture from recurved Pacific tropical cyclones; and (3) summer storms, mainly convective thunderstorms. Winter storms and tropical cyclones are associated with the most severe floods on the rivers in this study, and are the most probable causes of the paleofloods over the last 5000 years. Floods from both winter storms and tropical cyclones occur when deep mid-latitude troughs steer storm systems into the region. Composite anomaly maps of daily 700-mbar heights indicate that these floods are associated with a low-pressure anomaly off the California coast and a high-pressure anomaly over the Aleutians or Gulf of Alaska. A strong connection exists between the negative phase of the Southern Oscillation Index (often associated with El Nino conditions) and the large floods associated with winter storms and tropical cyclones. The paleoflood records confirm the existence of centennial-scale variations in the conditions conducive to the occurrence of extreme floods and flood-generating storms in this region. The episodes with an increased frequency of high-magnitude floods coincide with periods of cool, wet climate in the western U.S., whereas warm intervals, such as the Medieval Warm Period, are times of dramatic decreases in the number of large floods. A positive relationship between the paleofloods and long-term variations in the frequency of El Nino events is evident over the last 1000 years. This relationship continues over at least the last 3000 years with warm coastal sea-surface temperatures indicative of El Nino-like conditions.