Latest Corps Forecast Predicts Below Normal Runoff Into Missouri River Mainstem Reservoirs
By Matt Schauer

Posted: May 4, 2012


Below is a news release describing runoff conditions in the Missouri Basin. As the new release states, we are looking at slightly lower than normal runoff into fort Peck assuming we receive normal rainfall. The snowpack in the Yellowstone Basin is well below normal and certainly could have an effect on Fort Peck lake levels down the road. This summer it shouldn’t though. Fort Peck should have good water levels through the summer.

John Daggett

Omaha, Neb. – Warm, dry weather during March and April resulted in below normal runoff into the Missouri River mainstem reservoir system, managed by the U.S. Army Corps of Engineers Water Management Division, nearly a year after the upper basin experienced a flood of record with historic runoff and record releases from the reservoirs.

Runoff above Sioux City, Iowa in April was 63 percent of normal, bringing in 1.8 million acre feet (MAF) of runoff compared to a normal amount of 2.9 MAF. When coupled with the long-term forecasts for summer precipitation in the upper basin, the result is a below normal runoff forecast for calendar year 2012.

“Conditions have changed dramatically from last year, especially in regards to mountain snowpack,” said Jody Farhat, Chief of the Water Management Division. “While the risk of flooding due to mountain snowpack runoff is low, the risk of localized flooding from rainfall-driven events is normal.”

Mountain snowpack above Fort Peck peaked at 97 percent of normal on April 9, while snowpack in the Fort Peck to Garrison reach peaked at 88 percent of normal on March 22. Snowpack normally peaks around April 15. As of May 1, snowpack in the reach above Fort Peck was 72 percent of the normal April 15 peak; snowpack in the Fort Peck to Garrison reach was 66 percent of the normal peak. Last year mountain snowpack peaked on May 2 at 140 percent of normal above Fort Peck and 136 percent of normal in the Fort Peck to Garrison reach.

View mountain snowpack graphic here.

The current forecast for 2012 calls for 21.6 MAF of runoff above Sioux City, 87 percent of normal. A normal runoff year typically brings in 24.8 MAF of water. Runoff for the 2011 calendar year totaled 61.0 MAF, 246 percent of normal and the highest amount in the Corps’ 114 years of detailed record-keeping.

On May 1, system storage was 56.8 MAF, at the base of the 16.3 MAF annual flood control and multiple use zone. The annual flood control pool is the desired operating zone for the system because it allows the Corps to serve all eight congressionally authorized purposes: water supply, irrigation, navigation, hydropower, recreation, water quality control, flood control and fish and wildlife.

Reservoir Forecasts

Gavins Point releases averaged 26,200 cubic feet per second in April, and were reduced from 27,000 cfs to 25,000 cfs today in response to downstream rainfall. Releases are expected to reach 30,000 cfs next week to prevent the endangered least tern and threatened piping plover from nesting on low sandbars that would be flooded later this summer when higher releases are needed to provide navigation flows The nesting season runs from May to late August. The reservoir’s elevation is currently 1206.6 feet mean sea level. It is expected to return to elevation 1206 by the end of May.

Releases from Gavins Point will be reduced to zero for eight hours on May 9 to assess damages of the spillway slabs. Releases will be stepped down from the current rate of 25,000 cfs over several hours, and then returned to pre-assessment levels. Releases have not been reduced to zero since the early years following the construction of Gavins Point Dam. The lowest releases to com e out of the reservoir during the last two decades took place during the 1993 flood when releases ranged from 6,000 to 9,000 cfs for about a month.

Fort Randall releases averaged 23,400 during April. Releases in May will be adjusted as necessary to maintain Gavins Point near elevation 1206. The reservoir ended the month near elevation 1354.3, 1.5 feet higher than the previous month. It is expected to increase by 0.7 feet to 1355.2 by the end of the month.

Big Bend releases averaged 23,000 in April. They are expected to average 26,300 cfs during the month of May. The reservoir will remain within its normal elevation range of 1420 to 1421 feet during the month.

Oahe releases averaged 24,800 during the month of April. During the month of May, releases are expected to average 25,600. The reservoir ended the month at elevation 1606.3 feet, up 0.5 feet from the previous month. It will continue a gradually rise during May, ending the month 0.3 feet higher. Average elevation for this time of year is 1604.1 feet.

Garrison releases ranged from 25,000 to 28,000 cfs in April. Releases were adjusted to keep the Oahe reservoir rising during the forage fish spawn. Releases are currently at 26,000 cfs and are expected to remain near that rate through May. The reservoir ended April at elevation 1835.8, down 2.2 feet from the month before. It is forecast to end May near its current elevation of 1835.8. The average elevation for this time of year is 1833 feet.

Fort Peck releases averaged 6,900 cfs during April. Releases were increased from 7,500 cfs near the end of April to 10,000 cfs in early May. Releases will remain at that rate during May. The reservoir ended the month at elevation 2235.8 feet, up 0.9 feet from the previous month. It is expected to end May at 2236.3 feet, up 0.5 feet from April’s end of month elev ation. Average elevation for this time of year is 2228.5 feet.

The reservoir releases and elevations discussed above should not be assumed to be definitive. Additional heavy precipitation or lack of precipitation in the basin could cause adjustments to the reservoir release rates.

The six mainstem power plants generated 822 million kilowatt hours of electricity in April. Typical power generation for the month of April is 684 million kWh. The power plants are projected to generate 10 billion kWh hours during calendar year 2012, which is near the long-term average.

To view the detailed three-week release forecast for the mainstem dams click here

Communicating with the basin

As part of efforts to communicate more frequently and more broadly with stakeholders in the Miss ouri River basin during 2012, the Corps began holding twice monthly informational conference calls in January. The eighth call of the year is scheduled for Friday, May 4. Calls are geared toward Congressional delegations, Tribes, state, county, local officials and the media.

Audio files of the call can be heard here. The call is also offered as a free podcast in iTunes. Subscribe here. Or, simply run a search for Missouri River Basin Water Management in iTunes.

The general format of each call includes an update from the National Oceanic and Atmospheric Administration’s Climate Prediction Center on the long-range temperature and precipitation outlooks for the Missouri River Basin. The Missouri River Basin Water Management Division follows with a report of the current basin conditions, forecast inflows, and current and forecast reservoir release plans. The Corps’ Omaha and Kansas City Districts provide an update on the status of levee and other repairs following the Flood of 2011. A question and answer opportunity follows the reports. The public is also encouraged to continue following the Corps on its social media sites to remain apprised of the most recent information and latest updates.