The Upper Missouri River Basin is rocking into record runoff territory.
According to the update on Thursday, June 6, from the U.S. Army Corps of Engineers’ honchos based in Omaha who manage the Missouri Basin north of Sioux City, the runoff during May across the sprawling Basin was the highest since 1898, except for 2011, the year of the big flood.
In May, runoff in the Upper Basin totaled 8.9 million acre-feet (MAF), which was 2.67 times the normal May runoff of 3.3 MAF and surpassed in recorded history only by 9.2 MAF in 2011.
(An acre-foot is the water it takes to cover one acre with one foot of water; 325,851 gallons, about what 10 Americans might use in a year.)
It was the widespread above-average rains in South Dakota and Nebraska this spring that are the mainsprings of the way-above-average runoff, according to the Army Corps.
The big May runoff after big March and April runoff figures increased the Army Corps’ forecast for 2019 total runoff in the Upper Basin to 50 MAF, which would be the second highest total runoff in 121 years of record-keeping, again surpassed only by 2011, when total runoff was 61 MAF.
The second-highest so far is 49 MAF in 1997. Last year holds third place: 42.1 MAF of runoff in 2018.
Meanwhile, bad flooding continues in the Lower Basin of the Missouri, which is doing more and more damage in this historic flooding year. But it also limits what the Upper Basin flood managers can do with to much water in these parts: they can’t just send it downriver as fast as they can.
So they are looking at other options.
The Army Corps officials said Thursday they are talking with the U.S. Bureau of Reclamation about managing flood control storage at certain Bureau projects in Wyoming and Montana in ways that willo ease the water pressure on the big reservoirs on the Missouri’s main stem, including Lake Oahe at Pierre.
The mountain snow pack has not been a concern this year, being at about 104 percent of average when it peaked April 18 in the Fort Peck “reach” in Montana and on April 17 in the reach from North Dakota’s Garrison Dam north and west to Fort Peck.
But the plains runoff and the big rains since spring came have filled the dam reservoirs.
Lake Oahe was up to 1618.8 feet above mean sea level on May 31, a gain of 2.4 feet in May; Fort Randall gained 6.7 feet in its reservoir level last month.
The big water has meant a big month of energy: 1025 million kWh generated in the Missouri’s six mainstem dams filled with turbines, 130 percent of a normal May output.
The power plants are projected to generate 13.3 billion kWh of electricity this year, 141 percent of the normal annual output of 9.4 billion kWh, the Army Corps reported on Thursday.
Lake Oahe was storing 22.4 MAF on May 31, 1.27 percent of the May storage average from 1967-2018.
Releases last month at Oahe Dam averaged a rate of 35,900 cubic feet per second (cfs). But they’ve gone up to about 50,000 cfs the past couple days and will stay there for awhile, the Army Corps says.
The elevation of the Oahe reservoir has fallen since May 31 to 1618.2 feet above sea level, 1.2 foot into the top zone reserved for “exclusive flood control,” and 1.8 feet from the top of the top. But that level is slated to keep falling to 1617 feet by June 20, which is the bottom of the exclusive flood control zone. Releases are slated to remain at 50,000 cfs until about June 25 when they are slated to be increased slightly. Inflows into Oahe are remain around 30,00 cfs, then begin rising about June 13 to hit about 50,000 cfs on June 25. It means for the next three weeks or so, the Army Corps plans to keep releases well above the projected inflows into Oahe.
The Army Corps flood managers also are upping the public relations levels: On Thursday they announced they are “reformatting” the monthly water management calls to be weekly calls, including a briefing from the National Weather Service, with updates on the Corps’ latest plans and projections; all because of the “ongoing flooding in the lower basin.”
The call is for members of Congress and their aides, for tribal leaders as well as state, county and city government officials, and flood districts, as well as the news reporters. Each call, including the one held Thursday, June 6, will be recorded and made available at www.dvidshub.net/unit/usace-nwd.