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May 1 - 31, 2011
As floods along the Mississippi River began to recede, waters rose along the Missouri River. Extending across 10 U.S. states, the Missouri drains nearly one-sixth of the area of the United States. In June 2011, above Sioux City, Iowa, the Missouri River Basin experienced the highest runoff for any single month since the U.S. Army Corps of Engineers (USACE) began keeping detailed records in 1898.
Multiple factors contributed to the heavy runoff into the Missouri River Basin, the USACE explained, including heavy snowpack on the plains, near-record snowpack in the western U.S. mountains, and rainfall. This color-coded map shows rainfall anomalies for May 2011, compared to the 1998–2010 average. Below-average percentages are brown, and above-average percentages are blue. Rivers are thin blue lines.
Blue dominates most of the western United States in this color-coded map, testifying to the unusually high rainfall in May 2011. The USACE characterized the rainfall into the upper Missouri River Basin in mid- to late May as a critical factor in subsequent runoff. In May 2011, eastern Montana received 300 to 400 percent of normal rainfall, with some locations receiving more than a year’s average rainfall in just two weeks. Parts of North and South Dakota and Wyoming received more than 200 percent of normal rainfall. In some places, heavy rain continued into June.
By June 2011, the heavy rain had translated into heavy runoff. That month, the USACE reported, runoff into the Missouri River Basin above Sioux City totaled 13.8 million acre feet. (An acre foot is the amount of water required to cover one acre with one foot of water.) It was enough water, the USACE explained, to fill Memorial Stadium in Lincoln, Nebraska, every five minutes throughout the month of June.
The previous record monthly runoff for the Missouri River Basin above Sioux City was set in April 1952, at 13.2 million acre feet. In 2011, the combined runoff for May and June was 24.3 million acre feet, just below the normal annual runoff total of 24.8 million acre feet.
Not all of the western United States was soggy in May 2011. In this image, a brown patch of below-average rainfall extends from southern Colorado and Kansas into New Mexico and Texas. This region experienced deepening drought as the summer of 2011 progressed.
This image is based on data from the Multisatellite Precipitation Analysis produced at Goddard Space Flight Center, which estimates rainfall by combining measurements from many satellites and calibrating them using rainfall measurements from the Tropical Rainfall Measuring Mission (TRMM) satellite.