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Lake Sakakawea rising, holding back water: Corps of Engineers aims to alleviate downstream flooding

Water backed up behind Garrison Dam is now expected to surpass earlier projections but will remain below spillway level. The U.S. Army Corps of Engineers cites downstream flooding as the reason for cutting back on the amount of water being released through Garrison Dam. Kim Fundingsland / Minot Daily News

It is a juggling act and a calculated gamble.

The U.S. Army Corps of Engineers has increased releases into the Missouri River out of Gavins Point Reservoir in Nebraska while holding back water in upstream reservoirs that are approaching overflow levels. It is an effort to alleviate high water and flooding at downstream points on the Missouri.

"Rainfall runoff continues to flow into the Oahe, Big Bend, Fort Randall and Gavins Point Reservoirs at significantly higher than average rates," said John Remus, chief of the Missouri River Basin Water Management Division in Omaha, Nebraska.

The reservoir system on the Missouri River consists of three major reservoirs upstream and three much smaller reservoirs downstream. The "big three" are Fort Peck in Montana, Lake Sakakawea and Lake Oahe which spans both North and South Dakota. Smaller impoundments downstream, in order, are Big Bend, Fort Randall and Gavins Point which is located in Nebraska just above Sioux City, Iowa.

Gavins Point releases were scheduled to be increased to 75,000 cubic feet per second Saturday, June 1. That follows a 5,000 cfs increase earlier in the week. The Corps is attempting to alleviate high water situations on the lower Missouri River system by evacuating as much water as possible without aggravating flooding. At the same time, water levels of the big three reservoirs continue to climb.

Additional rainfall has the potential to exacerbate the situation, especially at Lake Oahe where the water level was 1,618.41 feet Friday with a projected level of 1,619.3 feet by Tuesday. Spillway level at Lake Oahe is 1,620 feet.

Inflow into Lake Oahe was listed by the Corps at 53,000 cfs Friday with an outflow of 29,800 cfs. That outflow is scheduled to be increased to 48,000 cfs Monday to avoid reaching overflow. Lake Oahe's exclusive flood control zone of 1,617 to 1,620 feet is nearly 90% full.

Releases through Garrison Dam at Lake Sakakawea were cut earlier this week to 15,000 cfs from 25,000 cfs a few days earlier. Lake Sakakawea, the largest reservoir on the Missouri River system, had an inflow reading of 39,000 cfs Friday and rose slightly over one-tenth of a foot in 24 hours. The water level of Sakakawea was 1,847.61 feet Friday morning and, according to the latest available projections, was expected to climb to 1,850.8 feet by June 20 with inflow still exceeding outflow.

The May 1 forecast for Lake Sakakawea called for a summertime peak elevation of 1,848.4 feet at the end of June. The June outlook is expected to revise that number upward based on present conditions in the system.

Lake Sakakawea's flood control zone begins at 1,837.5. What is considered the exclusive flood control zone for Lake Sakakawea is 1,850 to 1,854 feet, or spillway level.

Sen. John Hoeven-R, ND, met with Colonel John Hudson, commander of the USACE Omaha District, this past Wednesday to relay his concerns about rising water levels.

"While we're sympathetic to the flooding downstream, the Corps needs to ensure they prevent upstream flooding. I relayed our concerns about the reduction in water releases from the Garrison Dam and again urged the Corps to manage the Missouri River to prevent flooding in North Dakota communities," Hoeven said.

The Corps response was that releases from Lake Sakakawea would begin to be increased in June. The current schedule calls for ramping up Lake Sakakawea releases on June 11 until reaching 40,000 cfs by June 20. All projections are based on normal rainfall throughout the period and subject to change.

Total flood storage remaining in all six Missouri River reservoirs was listed at 33% by the Corps Friday with 10.9 million acre feet of 16.3 maf of designated flood control storage filled.