Islands of trees were stranded in the flooded Missouri River (top) west of Rockport, Missouri, on October 2, 2011, while farmland remained underwater (bottom right). Photo © Lee Valley, Inc.
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The Missouri is the continent's longest river. It trickles out of the mountains in Montana and flows east and south toward the Mississippi. Its waters are essential to the basin's farmers and ranchers, who raise 46% of the wheat, 22% of the corn, and 34% of the cattle in the United States.
The river is also home to a system of dams, levees, and spillways meant to insulate people living in the basin from the river's extremes. But in 2011, they couldn't keep the Missouri completely under control.
Beginning in late spring and continuing through the summer, floodwater spilled over the Missouri's banks into fields and towns, reaching up to five miles from the river's normal edge. The waters finally started to recede this fall, giving the basin's residents their first glimpse at the $2 billion or more in damages from this slow-moving catastrophe.
The Actors
Like most rivers fed by mountain snow, the Missouri experiences a surge every spring. So why was the 2011 spring flood so extreme? The lead actors were cold temperatures and heavy snowfall throughout the winter, followed by heavy rains during late spring.
A large swath of territory in Montana, North Dakota, and South Dakota also experienced unusually cold temperatures from March through May. This caused snow to linger on the ground longer than usual at low elevations.
(Click tabs to change months.) Temperatures in March 2011 were unusually cool (dark blue) across Montana and North & South Dakota. Data from NOAA's Climate Prediction Center. Maps by Hunter Allen & Richard Rivera.
Temperatures in March 2011 were unusually cool (dark blue) across Montana and North & South Dakota. Data from NOAA's Climate Prediction Center. Maps by Hunter Allen & Richard Rivera.
In April 2011, unusually cool temperatures overtook the rest of the basin and most of the western U.S. Data from NOAA's Climate Prediction Center. Maps by Hunter Allen & Richard Rivera.
Temperatures remained several degrees below average in the Northern Rockies and the Great Plains in May 2011. Data from NOAA's Climate Prediction Center. Maps by Hunter Allen & Richard Rivera.
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"Usually when someone talks about snowpack in the basin, we assume they're talking about mountain snowpack," said Jesse Aber, of Montana's Department of Natural Resources and Conservation. "This year we had two snowpacks: we had mountain and prairie."
(Click tabs to change dates.) A photo-like satellite image from April 20, 2010, shows the headwaters of the Missouri River in Montana under more typical spring conditions: snow in the mountains, but not on the prairie. NASA image by the LANCE/EOSDIS MODIS Rapid Response Team.
A photo-like satellite image from April 20, 2010, shows the headwaters of the Missouri River in Montana under more typical spring conditions: snow in the mountains, but not on the prairie. NASA image by the LANCE/EOSDIS MODIS Rapid Response Team.
Following an unusually cool and snowy winter, both mountains and prairies in the Missouri River headwaters of Montana were covered in white in a photo-like satellite image from April 23, 2011. NASA image by the LANCE/EOSDIS MODIS Rapid Response Team.
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Not only did the snow linger longer than usual, there was more of it than usual. As of May 1, areas around the headwaters of the river had anywhere from 130% to more than 180% of the average annual snowpack on the ground compared to the period from 1971-2000.
And then came the rain.
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