Much-above average runoff in the upper Missouri River basin (above Sioux City, Iowa) extended into May following widespread and heavy rainfall in South Dakota and Nebraska. Additionally, widespread and heavy rainfall in the lower basin, particularly in Kansas, has led to large tributary and Missouri River flows downstream of the six main stem reservoirs on the Missouri River.
May runoff in the upper basin was 8.9 million acre feet (MAF), which is 267 percent of average. The average May runoff is 3.3 MAF. May runoff was the second highest on record, only surpassed by 2011’s 9.2 MAF. Runoff in the Fort Randall Dam to Gavins Point Dam reach was 1.4 MAF, which is more than the average annual runoff for that reach.