Flooding fallout: Sedimentation on the Mo. River accelerated during ’11 flood

The historic 2011 Missouri River flooding accelerated sedimentation and will affect the river for years in ways still not known, according to a river expert.

This aerial photos show the Missouri River near Vermillion before the 2011 flood. The high waters dumped tons of sand and other sediment downstream. Vermillion is located at the top-center of both images. (Photo: Missouri Sedimentation Action Coalition)

This aerial photo show the Missouri River near Vermillion after the 2011 flood. The high waters dumped tons of sand and other sediment downstream. Vermillion is located at the top-center of both images. (Photo: Missouri Sedimentation Action Coalition)

Tim Cowman, director of the Missouri River Institute (MRI) in Vermillion, spoke on the flooding during Monday's annual meeting of the Missouri Sedimentation Action Coalition (MSAC) in Wagner.

Cowman told the Press & Dakotan that the flooding's aftermath continues to exert a profound impact on the river itself.

"Moving this much water down the free-flowing river for three months at a time has had a significant impact. It's starting to impact the channel itself," he said.

"There has been a lot of sediment moved around on the channel. There has also been significant degradation and the significant deposition of sand in the channel. Now, we have a lot more sandbars. They are up to 10 feet out of the water, which is something unusual."

The flood has impacted backwaters adjacent to the Discovery Bridge at Yankton and at Ponca State Park, Cowman said. In addition, the three-month inundation of the cottonwood forests weakened the trees' root system, with many cottonwoods that could normally withstand high winds now blowing over.

"The river will pull itself back to equilibrium over the next few years," he said. "It's a much different river, and it will be several years like this."

This year's snowpack and precipitation in the Missouri River basin are back to normal and below-normal levels, Cowman said. However, he warned that a similar scenario existed last year before historic rainfalls in the upper basin during late spring overburdened the system.

"The Missouri River still has a lot of power. We had gotten complacent and felt that we had tamed it when we put in the dams," he said. "But 2011 showed us that a lot can still happen to overwhelm what engineers put on the river. We need to keep that in mind as we go forward."

The lasting impact of the flood and the movement of sediment were on the minds of people who spoke to the Press & Dakotan prior to Monday's meeting.

To capture the flood's ongoing impact, MSAC commissioned more than 700 aerial images last November after the floodwaters receded, according to MSAC executive director Sandy Korkow of Springfield.

An estimated 89,700 acre-feet of sediment accumulates in the six Missouri River reservoirs annually, Korkow said.

However, she wasn't prepared for what she saw in the aerial photos.

"It has always been shocking to see how much sediment is in the river. But to see its movement from the air, it just puts things in perspective. You know it's there and it's not going to run away," she said.

"We're not sure how much more sediment entered the reservoirs as a result of the flooding. We know the energy involved gave the sediment more fuel to move, but it wasn't going past the dams as some might feel. People thought the sediment would get flushed down the system, but we know it didn't and is still in the reservoirs."

MSAC is finalizing a 16-minute video using the November shots, showing sediment's threat to all of the river's authorized purposes, Korkow said.

"I did hear a presentation at a river conference in Pierre on Friday, and the Corps is looking at different modelings and methods of forecasting (the river)," she said. "I think everyone is trying to gather more information and move forward in a positive way to better predict the future."

Mary Hurd and her husband, Rick, are still dealing with the flooding's aftermath for their riverfront property. The Hurds live about 15 miles west of Springfield.

Mary Hurd said several friends last weekend brought back the couple's dock that had floated downstream to Niobrara, Neb., even with a 1,000-pound weight attached to the structure.

 "We have reclaimed 250 acres of our bottom ground. We thought we would try planting, but it just didn't look very good, so we may hold off a year," she said. "We still have a lot of sand that blew out of our family park and picnic shelter. Yesterday, the wind blew so hard from the south that we had such a sandstorm that we didn't see any fields south of the road."

The remaining sediment after the massive force of floodwater at 160,000 cubic feet per second – more than twice the old record – shows that flushing isn't a likely option for moving sediment, said MSAC technical coordinator Howard Paul.

"We could look at Dr. (Howard) Coker's proposed pipeline. We are also trying to find ways to reduce the amount of sediment getting into the river," he said. "This sediment issue is a major problem. It has to be addressed because it's not something that's quietly going away."

Despite the remaining sediment problems, MSAC has raised the issue's visibility since the organization was founded in 2000, according to MSAC chairman Larry Weiss.

"The flood last summer raised the visibility of sediment and how it impacts us," he said. "We are continuing to push what we can with the resources available to make improvements."

Weiss saw the flooding and sediment impact as a Pierre resident, but he was still shocked at what he saw downstream.

"The amount of sediment on the banks right below Sioux City was amazing," he said. "In Pierre, there was a lot of sediment moved, which brings to light the fact that there is bed erosion and bank erosion."

Despite the challenges of sedimentation, Weiss remains hopeful for the future.

"This (problem) didn't start yesterday, and it's not going to end tomorrow," he said. "People shouldn't become discouraged. We have made a lot of progress since we started this organization."

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