For the first decade of the century, the Upper Missouri River Basin was the driest it’s been in 1,200 years, even more parched than during the disastrous Dust Bowl of the 1930s, a new study says.

The drop in water level at the mouth of the Missouri — the country’s longest river — was due to rising temperatures linked to climate change that reduced the amount of snowfall in the Rocky Mountains in Montana and North Dakota, scientists found.

The basin has continued to experience droughts this decade — in 2012, 2013 and 2017 — but their severity in comparison with historic drought is unknown. The Turn-Of-The-Century Drought” study, published Monday in the Proceedings of the National Academy of Sciences, focused only on the 10 years after 2000.

“In terms of the most severe flow deficits, the driest years of the Turn-Of-The-Century-Drought in the [Upper Missouri River Basin] appear unmatched over the last 1,200 years,” the study said. “Only a single event in the late 13th century rivaled the greatest deficits of this most recent event.”

Researchers familiar with drought of this magnitude in the dry Southwest were surprised to find it in the Midwest. The Missouri River winds 2,300 miles from western Montana to St. Louis, where it joins the Mississippi River. It runs through the Dakotas and past Nebraska, Iowa, Kansas and Missouri, where it supports farming in several states and provides fresh water for dozens of municipalities.

“We don’t tend to think of the upper Missouri region as being as drought-threatened as a region like the Southwest United States,” said Erika Wise, an associate professor in the geography department at the University of North Carolina at Chapel Hill and study co-author.

“These findings show that the upper Missouri Basin is reflecting some of the same changes that we see elsewhere across North America, including the increased occurrence of hot drought" that’s more severe than usual, Wise said.

The study is the latest to show how human-influenced climate change threatens to reshape the landscape by making naturally occurring drought far more severe.

A study published last month in the journal Science found that a vast region of the western United States — California, Arizona and New Mexico north to Oregon and Idaho — is already in the grips of the first climate change-induced megadrought.

Scientists at NASA and Columbia University had predicted such a drought would start sometime around 2030, but last month’s study showed that the phenomenon is already here. “The megadrought era seems to be reemerging, but for a different reason than the [past] megadroughts,” said Park Williams, the study’s lead author and a researcher at the Lamont-Doherty Earth Observatory at Columbia University.

Prolonged drought can disrupt agriculture and hurt economies, the researchers said. It affects dams that manage water resources and slows commercial traffic in rivers. On top of that, it harms marine life that must cope with lower water levels and animals such as waterfowl that rely on fish to survive. Runoff from the mountains to the Upper Missouri started decreasing in the 1950s, then dramatically declined between 1980 and 2000, the study said.

“Future warming is anticipated to cause increasingly severe droughts by enhancing water deficits that could prove challenging for water management,” the Missouri River study said. Historically, the Upper Missouri accounts for about 30 percent of the total flow at the mouth of the Missouri and will diminish over time, the authors said.

“The watershed is a critical source of water for the region — supporting megafarms, hydropower, tourism, and healthy ecosystems,” one author said. Declines in stream flow will be felt downstream on farms from Iowa to Missouri, especially if the southern part of the Missouri River experiences drought when the northern portion is dry.

“In the UMRB, what we’re really worried about is a future of snow droughts,” Wise said. “Snowpack in the Rocky Mountains is very sensitive to warming temperatures. Snow provides the water for stream flow to the Upper Missouri, and we’ve designed our agriculture and infrastructure around expectations that this water will be provided at a certain pace over a certain part of the year.”

Wise said a team of nearly 20 government and academic researchers knew the Upper Missouri was unusually dry between 2000 and 2010, but they set out to determine whether that level of drought was normal for the basin of creeks, streams and other waters that feed the main river.

Instrumental records could tell them what happened only in the past 100 years, so the researchers relied heavily on tree ring data, where rings are wide during periods of normal to high moisture and narrow when it’s low. Connie Woodhouse, a University of Arizona professor and study co-author, oversaw that area of research.

“These data provided us a detailed record of stream flow for every year since 800 A.D.,” said Woodhouse, who sampled trees throughout the watershed, many of them far from the river, to determine how they were affected by precipitation over time. Records allow scientists to examine the rings of trees both living and dead.

“I didn’t set out expecting to find that warming temperatures are impacting stream flow in the UMRB in the same way that they are impacting the Upper Colorado River Basin,” said Woodhouse, who has also studied that river basin. For one thing, the Upper Missouri has a more complex topography than the Upper Colorado, “so I didn’t necessarily expect to find this same result, but we did!”

Writing in a commentary that accompanied the study, Jonathan Overpeck, a climate scientists at the University of Michigan, and Bradley Udall, a scientist at Colorado State University, said drought in the Upper Missouri “mirrors the change occurring in the Southwest, where rivers provide the only large sustainable water supply to the region and over 40 million water users, yet flows have already declined significantly since just the late 20th century.”

In the West, rising temperatures brought on by heat-trapping emissions from human activity “is also contributing to drier soil, widespread tree death, and more severe wildfires,” as recent events in California show, the two wrote. “Greater aridity is redefining the West in many ways, and the costs to human and natural systems will only increase as we let the warming continue.”