Garrison Dam to use spillway gates to meet release goals
High releases through summer
RIVERDALE – Spillway gates at Garrison Dam are scheduled to be raised Monday as the Corps of Engineers evacuates rising water from Lake Sakakawea. It will mark only the third time in the history of the reservoir, which reached operational level in 1955, that the release gates will be utilized.
“As for the gates themselves, we opened them in the high water year of 2011 and we used them again last year,” said Todd Lindquist, Corps of Engineers project manager at Riverdale. “Using the spillway doesn’t mean you are going to flood.”
Water being released through Garrison Dam and into the Missouri River was cut to as little as 15,000 cubic feet per second a few weeks ago. The cut was necessary to avoid aggravating flooding conditions much further downstream on the Missouri River system. It also meant that water stored in Lake Sakakawea would increase more than projected by earlier forecasts.
On Thursday Lake Sakakawea reached 1.850.30 feet. Exclusive flood control for the reservoir is 1,850 to 1,854 feet, which is overflow level. Current runoff forecasts show the reservoir topping out at 1,851.8 feet.
“The forecast looks pretty good. The reservoirs downstream are coming down ahead of schedule,” said Lindquist. “The Oahe pool is coming down. The system is functioning pretty well.”
Lake Sakakawea was higher last year when it reached 1,853.2 feet and releases through Garrison Dam totaled 60,000 cfs. This year, despite the second highest projected runoff in the history of the Missouri River system, releases will reach 46,000 cfs or 20% less than a year ago.
“This is just a different event than last year,” explained Lindquist. “It is our second highest runoff on record if it comes to fruition, but very different because most of the runoff into the system is below us, dams without much flood control.”
Snowmelt runoff annually reaches Lake Sakakawea from as far away as the Rocky Mountains with the major contributors being the Missouri and Yellowstone Rivers. It is that runoff that is responsible for the annual rise of the reservoir and it is currently underway. The amount of runoff above Lake Sakakawea is only slightly above the long-term norm and much less than last year’s runoff that was 137% of normal.
Although seldom used, the use of the spillway gates is one of three options to regulate water levels in Lake Sakakawea. Water can also be released into the Missouri through the five hydroelectric turbines at Garrison Dam or through any of three release tunnels immediately adjacent to the power plant.
Lindquist said the decision to use spillway gates this year is due to construction taking place at the intake structure. In addition, only four of the Garrison Dam power generating turbines are currently operational, meaning only about 34,000 cfs of water can flow through that source. Depending on pool elevation of Lake Sakakawea, full capacity of water pushing through the facility’s five turbines can reach 42,000 cfs.
“On Monday we’ll probably open a couple of spillway gates,” said Lindquist. “We’ll gradually ramp up from 3,000 cfs to 16,000 cfs through the spillway. In the end we’ll have 16 gates about a foot open. All five of our turbines will be back in operation in a few days.”
There are 28 spillway gates at Garrison Dam. The latest schedule issued by the Corps calls for releases from Lake Sakakawea to continue to be increased until reaching 46,000 cfs on June 24. That amount of water is considered an above normal release but not enough to cause high water troubles along the river below Garrison Dam.
Based on the current release schedule, it will be a summer of high water on the Missouri in North Dakota but well within the river’s capacity. For example, the level of the Missouri at Bismarck is expected peak slightly over 11 feet. It is when the river level exceeds 13 feet that problems begin to arise.
“We shouldn’t impact people downstream,” stated Lindquist. “Last year at 60,000 cfs we were in people’s yards and boat ramps.”
The release rate of 46,000 cfs is scheduled to last through August with decreases beginning in September. Total runoff into the Missouri River system, above and below Lake Sakakawea, is forecast to be 50.0 million acre feet of water. The historic average, covering 121 years of record keeping, is 25.3 maf.