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Extreme Paleofloods of the Southwestern United States
Ely, L.L. 1997. Response of extreme floods in the southwestern United States to climatic variations in the late Holocene. Geomorphology 19: 175-201.

What was done
In an attempt to determine the environmental origins of extreme flooding events throughout the southwestern United States, according to the author, "paleoflood records from nineteen rivers in Arizona and southern Utah, including over 150 radiocarbon dates and evidence of over 250 flood deposits, were combined to identify regional variations in the frequency of extreme floods," which information "was then compared with paleoclimatic data to determine how the temporal and spatial patterns in the occurrence of floods reflect the prevailing climate."

What was learned
Ely reports that "long-term variations in the frequency of extreme floods over the Holocene are related to changes in the climate and prevailing large-scale atmospheric circulation patterns that affect the conditions conducive to extreme flood-generating storms in each region," which changes, in her view, "are very plausibly related to global-scale changes in the climate system." With respect to the Colorado River watershed, for example, which integrates a large portion of the interior western United States, she writes that "the largest floods tend to be from spring snowmelt after winters of heavy snow accumulation in the mountains of Utah, western Colorado, and northern New Mexico," such as occurred with the "cluster of floods from 5 to 3.6 ka," which occurred in conjunction with "glacial advances in mountain ranges throughout the western United States" during the "cool, wet period immediately following the warm mid-Holocene."

The frequency of extreme floods also increased during the early and middle portions of the first millennium AD, many of which coincided "with glacial advances and cool, moist conditions both in the western U.S. and globally." Then came a "sharp drop in the frequency of large floods in the southwest from AD 1100-1300," which corresponded, in her words, "to the widespread Medieval Warm Period, which was first noted in European historical records." With the advent of the Little Ice Age, however, there was another "substantial jump in the number of floods in the southwestern U.S.," which was "associated with a switch to glacial advances, high lake levels, and cooler, wetter conditions."

What it means
In distilling her findings down to a single succinct statement, and speaking specifically of the southwestern United States, Ely states that "global warm periods, such as the Medieval Warm Period, are times of dramatic decreases in the number of high-magnitude floods in this region."

Reviewed 4 August 2010