PIERRE, S.D. — The U.S. Army Corps of Engineers say 2019 could mark the second-highest runoff along the upper Missouri River basin on record, second only to 2011's infamous floods, if current levels continue.
In testimony given to the U.S. Senate's Subcommittee on Superfund, Waste, Management and Regulatory Oversight on Wednesday, Aug. 28, USACE Brigadier General Peter Helmlinger said 2019 has already marked the third-highest runoff on record at 45.3 million acre-feet as of July 31. Once August's projected runoff total is calculated, he said that could be as great as 52.9 maf. 2011's runoff was 61 maf.
The intense runoff is due to this year's extreme weather: Helmlinger said March's bomb cyclone dumped two to four inches of rain across the plains, which were already covered in snowpack that held another two to eight inches of snow water equivalent. On top of already wet soil, Helmlinger said 40 stream gauges in the impacted area set records for stage and discharge within 48 hours of the storm.
Helmlinger said the bomb cyclone forced the Corps to release higher-than-average amounts of water from "rapidly rising pools" at the Oahe and Fort Randall reservoirs, located near Pierre and Platte, respectively.
The flooding didn't stop after March's bomb cyclone. Helmlinger told legislators Wednesday, "The flooding that has occurred and continues to occur on the lower Missouri River is not the result of a single event, but rather a series of events."
Come May, Helmlinger said South Dakota saw rainfall equaling three- to five-times higher than the historic average, keeping the James and Big Sioux Rivers near or above flood stage for most of the month. Runoff into the Oahe and Fort Randall reservoirs during this time was 500% and 950% of the historic average, respectively, and the Corps released higher-than-average amounts of water from the reservoirs.
Then June came and brought with it mountain snowpack melt, which began to fill Montana's Fort Peck and North Dakota's Garrison reservoirs, and above-average precipitation continued into the summer, adding to the runoff. By July, runoff in the upper Missouri River basin was 213% of its historic average.
Now, five months after the initial bomb cyclone in March, runoff levels continue to exceed forecasts. If this continues, Helmlinger said the Corps' currently release of 70,000 cubic feet per second will continue into September in order to brace the system for the quickly approaching 2020 flood season.
In response to the system-wide flooding, Helmlinger said the Corps implemented three phases: closing critical levee breaches in order to protect infrastructure; repairing damaged levees; and working with tribal, state and local partners to reduce future flood risks.
Helmlinger concluded, "(T)hroughout the year, the Corps plans to continue to reach out to the people who live and work in these river communities and their representatives to provide technical assistance and help them plan their future actions."
Wednesday's field hearing, chaired by South Dakota's Republican U.S. Sen. Mike Rounds in North Sioux City, fell on the same day that Republican Gov. Kristi Noem requested two additional disaster declarations be granted for damages in South Dakota due to severe summer weather. President Donald Trump signed off on a disaster declaration after March and April's severe storm, which caused an estimated $46 million in damage statewide.