Large floods in the southwestern United States in relation to late- Holocene climatic variations

Abstract

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 fine-grained flood deposits used to reconstruct these flood histories selectively preserve evidence of only the largest events. Large floods were frequent on rivers throughout the region from 4.8-3.6 ka (¹⁴C yrs). In contrast, the period from 3.4-2.2 ka is marked by a significant decrease in the number of large floods on virtually all of the rivers. The frequency of large floods increased after about 2.2 ka, with particularly prominent peaks around 1 ka and after 500 yrs BP, separated by a sharp decrease between 600 and 800 yrs BP. The storms that generate large floods (≥ 10-year) in this region fall into three general categories: 1) winter North Pacific extratropical storms, 2) late-summer and fall storms that draw on moisture from recurved Pacific tropical cyclones, and 3) summer storms, mainly convective thunderstorms. Winter storms and tropical cyclones are associated with the very largest floods on the rivers with paleoflood records, and are the most probable causes of the late-Holocene paleofloods. 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-mb heights and monthly sea-level pressure indicate that the winter floods are associated with a low-pressure anomaly off the California coast and a high-pressure anomaly over the Aleutians or Gulf of Alaska. Shifts in the locations of the low- and high-pressure anomalies over the North Pacific appear to control which subregions of the southwestern U.S. experience floods. Composite 700-mb anomalies during tropical cyclone floods show a similar pattern, with a variation to a blocking high-pressure anomaly in the central North Pacific and a low-pressure anomaly over the western U.S. There is a strong connection between the negative phase of the Southern Oscillation Index and the large floods associated with winter storms and tropical cyclones. Over the last 5000 years, the episodes of more frequent large floods coincide with cool, wet, neoglacial periods. Warm periods are times of dramatic decreases in the number of paleofloods. Although the floods record individual extreme storms, they cluster in times of generally moister conditions in the region. A clear positive relationship exists between floods and low-frequency variations in El Niño over the last 1000 years. Warm coastal sea-surface temperatures indicative of El Niño-like conditions are associated with more frequent large floods over at least the last 2000 years. The paleoflood records demonstrate centennial-scale variations in the conditions conducive to the occurrence of large floods in this region.