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Lewis and Clark Lake sediment

Sediment flowing into Lewis and Clark Lake from the Missouri River and its tributaries has formed a large delta on the west end of the lake. A section in the 2016 Water Resources Development Act has authorized the development of a sediment management plan for U.S. Army Corps of Engineers reservoirs in the upper Missouri River basin.

SPRINGFIELD, S.D. -- A section in a recent federal water bill has given optimism to those seeking a solution to the increasing sediment deposits in Lewis and Clark Lake that a plan for removing sediment can be developed sooner rather than later.

The 2016 Water Resources Development Act includes a section authorizing the U.S. Army Corps of Engineers to develop a sediment management plan for reservoirs in the upper Missouri River basin. Though the act provides no financial appropriations, it sends the message that lawmakers are aware of the problem.

"I think once you have vehicles in legislation, it's on people's radars to do something," said Sandy Stockholm, executive director of the Springfield-based Missouri Sedimentation Action Coalition, formed in 2001 to educate the public and seek solutions to Missouri River reservoir sedimentation.

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Sandy Stockholm 2017

Stockholm

An estimated 5.1 million tons of sediment enter Lewis and Clark Lake annually, and the Corps of Engineers estimates that the reservoir behind Gavins Point Dam has lost approximately 30 percent of its water storage capacity since it began filling in 1955. At the current rate, the lake would be filled with sand in about 150 years or sooner.

The problem isn't sneaking up on anyone who has seen the growing sand delta on the lake's west end. The delta, continually reaching to the east, has contributed to rising ground water levels and increased flooding threats near Springfield and in Niobrara, Nebraska, and has affected water supplies and the ability of recreational boaters to navigate the area.

The corps has been studying the issue for years, using computer models to test various scenarios that adjust upstream dam releases to flush sediment through Gavins Point Dam. The federal legislation gives a push to explore other aspects required in any sediment management plan, said Paul Boyd, a Corps of Engineers hydraulic engineer.

"It allows us to leverage a lot of work that's already been done," Boyd said. "We're all for as many tools as we can get to move this discussion forward. We know a little fix here, a tiny fix there is not enough."

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Lewis and Clark Lake sediment

A sediment delta fills the west end of Lewis and Clark Lake near the confluence of the Niobrara and Missouri Rivers. The view, in 2016, is from Springfield, South Dakota, looking south toward Nebraska. The authorization of the development of a sediment management plan included in recent federal legislation is expected to lead to new studies on how to remove sediment from the reservoir.

Boyd said sediment studies thus far have focused mainly on engineering solutions. The corps and MSAC hope to partner on a cost-benefit study to analyze how increasing sedimentation at the lake and its removal would impact, both negatively and positively, not just the area bordering the lake, but locations up and down the river.

Such a study would merge economic data with engineering findings, enabling the corps to better develop a management plan and seek government appropriations to enact it.

"The cost-benefit ratio is definitely something we need to take a look at," Stockholm said. "Really what we hope it does is pull all these studies together and come up with a plan of action."

An economic study could take up to 18 months to complete, and Boyd said the earliest the corps could come forward with a plan would be 2021. Any proposal would be followed by public meetings, environmental studies and more review. It would probably be at least five years before any project could start, he said.

"We're not two years away from actively removing sediment, but at least we're on a path we weren't on 10 years ago," Boyd said.

The sooner the better, said Rollin Hotchkiss, a Brigham Young University civil and environmental engineering professor who has studied reservoir sedimentation worldwide.

Every day, more silt is dumped into Lewis and Clark Lake by the Missouri River and its tributaries. A likely participant in the upcoming economic study, Hotchkiss said the impact of moving that sediment through the lake so it can flow downstream must be determined.

"Ideally, we would put that sediment in motion and move it past the dam. It's the only sustainable solution," said Hotchkiss, who was at the University of Nebraska-Lincoln for nine years prior to going to BYU in 2005.

Hotchkiss said the challenge is to get sediment through the discharge gates at Gavins Point Dam. The gates are not at the bottom of the dam, but, like the windows of a house, are toward the middle. Some of those gates need to be lowered, an engineering feat Hotchkiss said he observed in Japan this summer at two dams and reservoirs facing the same circumstances as Lewis and Clark Lake.

If that sediment were to begin moving through the Gavins Point gates and flowing downstream, folks living down river naturally have concerns about the Missouri carrying more sediment and how that might affect them.

Study stakeholders must assess how sediment removal can both avert and cause damages and try to attach a dollar figure to each. If sediment moves past the dam and downstream, for example, how much could the corps save on building sandbars to provide habitat for endangered bird species? Or, how might increased sediment in the river affect downstream cities such as Sioux City?

Typical studies aim to look 50 years into the future. This study, Hotchkiss said, might need to look 200-300 years ahead or have no time limit, though that makes it harder to predict outcomes and costs.

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Rollin Hotchkiss mug

Hotchkiss

"As part of this study, we want to look at things a little bit differently," Hotchkiss said.

Stockholm said that, in addition to seeking grants, MSAC has met with municipal and county governing bodies and water districts bordering the lake to seek financial commitments to help fund the study. Her group hopes for major movement by the end of the year, she said. It's unknown how much the study will cost, though the corps is likely to share in the expense, especially if it partners with MSAC.

Once that study is completed, Boyd said, he's confident that a credible plan can be developed. Though undertaking yet another study may seem to some like another delay in addressing the sediment problem, it's a step forward, he said.

"It is progress. We're now asking questions about management that we weren't asking a decade ago," Boyd said.

Stockholm, too, sees the study as a necessary step in a process that's had its starts and stops.

"It's a long journey ahead, but I think we're identifying things that need to happen in order for something to big to happen," she said.

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