An even worse drought 2,000 years ago

Colorado River reservoirs are far closer to empty than full. Now comes evidence of a worse drought long, long ago

by Allen Best

The Colorado River Basin has suffered a handful of extended, deep droughts. We’re in one of them. But as bad as the current drought is, leaving reservoirs far more empty than full, new evidence has emerged of an even worse drought. It occurred 2,000 years ago.

“The new findings should “help water managers plan for even more persistent and severe droughts than previously considered,” said Subhrendu Gangopadhyay, the lead author of the study that was published in Geophysical Research Letters. Gangopadhyay is principal engineer for the Water Resources Engineer and Management Group at the Bureau of Reclamation.

The definition of average used by the team of researchers was the average of flows recorded at Lees Ferry since 1906. This location below Glen Canyon Dam is the official dividing line between the lower Colorado River Basin and the upper basin. The latter is where nearly all of the river flows originate, more than half in Colorado.

The new research finds that compared to the current 20-year drought in the Colorado River, with only 84% of average water flow, it was surpassed by a 22-year period in the second century, when the average water flow was 68% of average.

Paleoclimatologists have long known of severe droughts in the Colorado River. One occurred in the late 16th century, about the time Spanish colonists were staking claims in the Southwest, and others occurred midway through the 12th century, and again in the late 13th century, about the time the ancestral Pueblo were vacating cliff dwellings in Mesa Verde.

This new study stretches the record deeper into the past.

“This new finding suggests that the range of natural hydroclimatic variability in the Colorado River is broader than previously recognized, setting a new bar for worst-case scenario from natural variability alone,” the study concludes.

In other words, Mother Nature could deliver even worse.

That’s not even including the effect of artificial heating of the atmosphere caused by accumulating greenhouse gases. Previous studies have calculated that a third to a half of the reduced precipitation is due to global warming.

Paleoclimatologists have a variety of tools for establishing precipitation of past centuries. Tree rings reflect growing conditions, especially precipitation. Wider bands correspond with more moisture, narrower rings less.

These tree ring studies have been catalogued at many areas. For example, one of the researchers in the current study, Connie Woodhouse, then affiliated with the University of Colorado at Boulder but now with the University of Arizona, has studied Douglas fir trees near Eagle among many other places.

Prominent in this study was research conducted in the San Juan Mountains southwest of Alamosa, near the former mining site of Summitville. It is not in the Colorado River Basin but it does reflect the climate in the San Juan Mountains, which provides a tributary for the Colorado River. That particular site showed a severe drought in the second century, the driest in the last 2,250 years.

For this study, tree rings were not enough. There were just a few fragments. “Tree-ring records are sparse back in the second century,” said Woodhouse. “However, this extreme drought event is also documented in paleoclimatic data from lakes, bogs, and caves.”

Researchers also used statistical method called grid-point reconstructions.

The take-away, once again, is that the natural drought could lift from the Colorado River Basin next year. Or it could deepen.

As for the aridification caused by greenhouse gases in the atmosphere, we’re likely stuck with that even if a miracle occurs and the world figures out how to stop the production of carbon dioxide and other gases.

Top: Lees Ferry, located 15 miles downstream of Glen Canyon Dam is the dividing line between the upper and lower Colorado River basins. Photo/Allen Best

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