Ancient "Megadroughts" Struck U.S. West, Could Happen A

Richard A. Lovett
for National Geographic News
May 24, 2007

Much of the western U.S. may be headed into a prolonged dry spell—a "perfect drought," scientists say, that could persist for generations.

The West already has been dry for six years and is looking to be dry again in 2007, said Glen Macdonald, an ecology professor at the University of California, Los Angeles (UCLA).

(Related: "U.S. Southwest Drought Could Be Start of New Dust Bowl" [April 5, 2007].)

But that's nothing compared to what has happened in the region in the past, according to Macdonald and other scientists.

In a study published today in the journal Geophysical Research Letters, a team from Arizona and Colorado found that the Southwest suffered a six-decade megadrought from 1118 to 1179.

For 62 years mountain snows—one of the area's main sources of water—were frequently diminished, reducing the river's flow during the heart of the drought by an average of 15 percent.

And for an extended period there were no high flows at all, said Connie Woodhouse, a study co-author from the University of Arizona in Tucson.

This is grim news for today's Westerners, who rely on wetter years interspersed through a drought to fill reservoirs, the scientists said.

Water Data From Dead Trees

The new findings came from a study of growth rings in trees from the upper Colorado River dating back to A.D. 762. These rings indicate year-by-year moisture conditions that can be used to estimate long-ago river flows.

Prior studies hadn't gone this far back in the history of the Colorado River headwaters because there aren't enough living old trees to analyze.

But Woodhouse's team discovered that there are lots of ancient logs, stumps, and standing dead trees that can provide data.

Continued on Next Page >>




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