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Scientists find the Colorado River was blighted by a worse drought in the 2nd century

A new study shows that, more than a thousand years ago, the Colorado River Basin had an even worse drought than the one currently plaguing the region.

(CN) — While the current drought afflicting the Colorado River Basin is the worst since federal scientists began keeping records, a new study using paleoclimatic data discovers it is not the worst drought in the region’s recent geological history. 

Researchers at the Bureau of Reclamation published the study Thursday in Geophysical Research Letters, a peer-reviewed geoscience journal. They used paleoclimatic evidence like tree rings and indicators in bogs and caves to reconstruct stream flows; the researchers found evidence of a devastating drought that struck the Colorado River Basin in the second century. 

“Previous studies have been limited to the past 1,200 years, but a limited number of paleo records of moisture variability date back 2,000 years,” said Subhrendu Gangopadhyay, lead author and principal engineer for the Water Resources Engineering and Management Group at the Bureau of Reclamation. “While there has been research showing extended dry periods in the southwest back to the eighth century, this reconstruction of the Colorado River extends nearly 800 years further into the past.”

The researchers used indicators in bogs, caves and sedimentary layers near lakes to flesh out their picture of the climate thousands of years ago. Tree rings have helped scientists create the Palmer Drought Severity Index, which shows regional fluctuations in climate and weather, but they are limited to about 1,200 years because of the life cycle of trees in the area. 

Trees in California live for as much as 3,000 years, but the Rocky Mountain juniper typically has a much shorter lifespan — though the oldest is believed to be about 1,800 years old.

“Tree-ring records are sparse back to the second century,” said Connie Woodhouse, a professor at the University of Arizona and a study co-author. “However, this extreme drought event is also documented in paleoclimatic data from lakes, bogs and caves.”

The drought Woodhouse is talking about found that stream flows on the Colorado River and its tributaries were at about 68% of average flows. The current drought, which is stretching into its 22nd year, is at about 84% of its average flows. 

Average flow baselines were established by taking the flows from 1906 to 2021 and coming up with the average. The experiment was carried out at Lees Ferry in Glen Canyon National Park, where there is a stream flow monitoring system. 

With data from tree rings and other indicators, scientists benefited from a longer historical record to compare current droughts and other patterns against.

The data will be made public so that managers of water districts up and down the Colorado River Basin can use it to make preparations necessary to their particular situations.

“The results of this work can provide water managers with an increased understanding of the range of flow variability in the Colorado River,” added Gangopadhyay. “It should provide information to help water managers plan for even more persistent and severe droughts than previously considered.”

The current drought is the worst in more than a thousand years, and has prompted unprecedented actions by water officials, including transferring water from Wyoming to bolster the sinking surface level at Lake Powell.

The Colorado River Basin supplies water to more than 40 million people and provides sustenance to 2.5 million acres of croplands in the American West.

Drought has persisted in the region for two decades, with water managers and scientists increasingly laying part of the blame on human-caused climate change, which they say causes significant changes in long-term weather patterns. 

From 2000 to the present, the Colorado River Basin has experienced its driest period on record, which has measured the river for more than 100 years. At the same time, the population of the American Southwest has continued to grow at an accelerated pace.

Arizona grew 11% since 2010, according to the latest U.S. Census. Nevada grew by 15% and Utah, at 18%, has grown the most of any state.

Lake Powell stands at less than 25% of its storage capacity. In California — also been beset by dry skies and scant precipitation — the largest reservoir, Shasta, stands at 40% capacity. 

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