Widespread and heavy rainfall in the Missouri River basin above Sioux City, IA resulted in another month of much above average runoff. Precipitation during September was more than 200 percent of normal in eastern Montana, much of North Dakota, portions of South Dakota and northern Nebraska. As a result, September runoff into the upper basin above Sioux City, was nearly twice the record runoff, which was recorded in 1986.
Runoff in the Gavins Point to Sioux City reach was more than 16 times the long-term average and more than twice the previous record. Runoff in the Fort Randall to Gavins Point reach was more than four times average and almost twice the previous record. Runoff between Oahe and Fort Randall was over 12 times average and set a new record. Runoff between Garrison and Oahe was more than four times average. Finally, runoff between Fort Peck and Garrison was over two times average and is the second highest runoff of record, and Fort Peck was 1.5 times average.
The 2019 upper basin runoff forecast is 61.0 million acre-feet (MAF). If realized, this runoff total would equal the highest runoff in 121 years of record-keeping, 2011 (61.0 MAF). The January-September observed runoff (53.6 MAF) has already exceeded the second highest runoff in 121 years of record-keeping, 49.0 MAF observed in 1997, with three months still remaining.
“In response to the increased upstream runoff, releases from Gavins Point Dam have been increased to 80,000 cfs. This release rate is more than twice the average release for this time of the year,” said John Remus, chief of the U.S. Army Corps of Engineers Missouri River Basin Water Management Division.
The Missouri River mainstem reservoir systemstorage was 64.0 MAF as of Oct. 1, occupying 7.9 MAF of the 16.3 MAF flood control zone. All three of the upper three reservoirs — Fort Peck, Garrison, and Oahe — have fallen out of their exclusive flood control zones but remain high.
“As a result of the high reservoir levels and the forecast above-average runoff for the remainder of the fall, releases from all system projects will be much above average through November, to evacuate all stored flood waters prior to the start of the 2020 runoff season,” Remus said. “We are monitoring the situation very closely and will make any necessary adjustments. Failure to evacuate the stored flood water will lead to increased risk of flooding in 2020.”