The buildup of sediment greatly increased in areas along the Missouri River following the record 2011 flood. This area, visible from the Double Ditch Village historic site north of Bismarck, shows a vast sandbar in the aftermath of the flood. Water leaving Garrison Dam, upstream of Bismarck, is clearer than in the pre-dam era, allowing capacity to pick up and later deposit sediment. Patrick Springer / Forum Communications Co.
Riverbank erosion along Missouri River can affect water quality in region
Patrick Springer - Reporter - The Forum
MANDAN, N.D. – Chuck Mork’s farm on the high bottomland along the Missouri River saw almost nine quarters of land submerged by the record flood in the summer of 2011.
Altogether, almost a third of his crop acres were flooded – all planted before the flood warnings came in late May.
After the floodwaters receded in the fall, he was pleasantly surprised to find most of his land survived the ordeal in pretty good shape, despite the massive crop loss.
The worst damage involved the loss of land along a half mile of riverbank, where the surging river nibbled away between 2 feet and 10 feet of pasture, also taking a dozen trees, mostly old cottonwoods.
“We did have some erosion,” Mork says. “Not super bad. Our situation is not near as bad as some.”
Riverbank erosion along the Missouri River can be a significant problem, an issue that has a surprising connection to water quality.
Garrison Dam, 75 miles upstream of Bismarck-Mandan, regulates the river’s flows. But the water coming out of the dam is clearer than the “Muddy Missouri” that preceded Garrison, giving it the capacity to pick up and redeposit sediment.
The riverbank along the area near the Mork farm suffered severe erosion in the late 1950s and early 1960s after the dam was closed in 1953.
The late Andy Mork, Chuck Mork’s father, was an outspoken advocate of riverbank stabilization downstream of the dam.
The stabilization projects, involving placement of a protective shield of rip-rap rocks, were the first on the Missouri River in North Dakota.
A neighbor north of the Mork farm, eight miles north of Mandan, was also among landowners spared from significant riverbank erosion – thanks in large part to the efforts years ago of Mork’s late father, The upstream neighbor’s flood-prone fields, protected by more than a mile of rip-rap laid in the 1960s, survived without noticeable erosion, Mork says.
“Without that bank protection, the flows we had last summer, it would have eaten away the ever-living heck out of them,” he says.
Actually, Mork adds, hundreds of acres of his neighbor’s fields already would have been gobbled up by the river if the bank hadn’t been protected by rip-rap.
Officials of the Army Corps of Engineers, which built and operates Garrison Dam and the other Missouri River dams, maintain that riverbank erosion has been reduced as a result of the regulated river flows.
Still, the erosion of the riverbed and banks and deposition of sediment downstream remains a significant problem in areas.
Sedimentation, as the process is called, is an aggravating factor in areas as diverse as flood control and hydropower generation.
The exacerbation of flooding is most evident a few miles downstream from the Mork farm, around Bismarck and Mandan and nearby housing developments.
One culprit is a delta formed by silt deposited by the Missouri River as it encounters the still headwaters of Lake Oahe, the reservoir of Oahe Dam near Pierre, S.D., and extending almost to Bismarck-Mandan.
The delta has created a bottleneck, narrowing the river channel to a width of 30 or 40 feet, resulting in ice jams that can exacerbate flooding, as happened in a brief 2009 flood.
State and local officials have complained about the flood risk posed by the delta for years. It caused the federal government to raise the elevation of the 100-year flood by one foot in 1999 for an area of south Bismarck and south of the city.
The flood risk from the continued buildup of sediment was again acknowledged in a 2009 study for the corps, but state and local officials complain that so far no action has been taken to alleviate the problem.
“We’re just building up bigger and bigger headaches,” says Todd Sando, North Dakota’s state engineer and top staff official of the State Water Commission. “Having a delta in a shallow area is just inviting ice jams.”
A study by the U.S. Geological Survey in 1999 concluded that more than 90 percent of the sediment in the Missouri River running through North Dakota comes from the river’s banks and bed.
Less than 10 percent of the siltation originates from tributaries, including the Heart River and Knife River.
Ironically, the sedimentation problem partly stems from the clearer water that flows out of Garrison and the other dams along the Missouri River.
It’s the downside of the Missouri’s relatively clear water, among the highest-quality water in North Dakota, according to the State Water Commission.
Here’s a simplified explanation of how the dams have altered the Missouri River’s ability to carry and deposit sediment.
Almost all of the sediment suspended in the river settles in the dam reservoirs before moving farther downstream – leaving the water with the capacity to pick up more silt and sediment as it flows.
“It’s a hungry river,” Sando says of the Missouri downstream from Garrison Dam. “It’s looking to pick up its bed load and sediment load.”
Pick up, then deposit, as eroded riverbed and riverbank soils are transformed into sandbars and deltas.
The heavier sediment settles to form sandbars. The lighter silt remains in the river, however, until reaching a slow zone, notably the still, upper reaches of Lake Sakakawea or Lake Oahe, and deposits, forming deltas.
Williston, upstream along the headwaters of Lake Sakakawea, has sedimentation headaches similar to those plaguing Bismarck and Mandan.
Siltation forced the abandonment of a nearby agricultural irrigation project in the early 1970s. Buildup of the Williston delta raised the river bottom more than 17 feet by the early 2000s, resulting in a large swampy section of the river rife with mosquitos.
Andy Mork saw it coming in the summer of 1953, when he took the family on a drive to see Lake Sakakawea as it was forming from the closure of Garrison Dam.
After driving over the divide, he peered down into the Missouri River Valley and was surprised by what he saw – beautiful blue water, not the tan-brown “Old Muddy” river he had known since boyhood.
It was just starting to cultivate the appetite Sando would later describe as a hungry river.
Filling with sediment
Garrison Dam’s Lake Sakakawea, a colossus that ranks as the nation’s third-largest artificial reservoir, is slowly filling with sediment.
Every year, the lake collects silt and sediment that reduces its storage capacity by a volume estimated to equal a lake one-foot deep covering 25,900 acres.
That amounts to 0.11 percent of its storage capacity, needed for uses including flood protection, water supply and recreation, every year.
Engineers project that Lake Sakakawea will be completely filled with sediment in 900 years.
Most of that sediment comes from upstream on the Missouri River and Yellowstone River.
Most of the sediment is deposited on the lake’s upper reaches, but some of it accumulates at the base of the dam.
As of 2009, Sakakawea’s permanent pool had been reduced by 3.5 percent as a result of sedimentation.
The problem is much more severe – and pressing – for the smaller Missouri River reservoirs downstream in South Dakota.
For instance, Gavins Point Dam’s Lewis and Clark Lake, near Yankton, S.D., has lost 30 percent of its storage from sedimentation. The reservoir is projected to lose half its capacity by 2045, according to a 2009 study by the corps.
The Missouri River system of six reservoirs, with total storage equaling 73.1 million acre-feet, has lost 5 million acre-feet, or 6.8 percent.
The loss adds up at a yearly rate that equals storage equaling a flood 100 miles long, 10 miles wide, with an average depth of more than 7 feet, according to the Missouri Sedimentation Coalition, an advocacy group based in South Dakota.
Put another way, the yearly loss to the Missouri River system’s water supply from sedimentation would provide more than 800,000 people with more than 100 gallons of water a day for an entire year, according to the coalition’s figures.
The advocacy coalition was formed in 2001, spearheaded by officials from the city of Pierre, S.D., who were forced to buy out homes near the river that were threatened by water tables driven up by sediment buildup.
Bismarck’s water table also has increased because of sedimentation, but faces a more immediate problem from the threat of ice-jam floods exacerbated by the delta.
So far, no long-term plans have been devised to deal with the growing problems caused by sediment accumulation along the Missouri River.
But the corps is studying near-term steps to better protect areas around Bismarck-Mandan.
Severe erosion at a bend in the river by Hoge Island north of Bismarck caused a bank collapse that damaged several homes, including one washed away in the 2011 flood.
The corps is considering rip-rapping the area, a half-mile stretch of riverbank flanked by areas that have protection that worked well in the flood but must be repaired.
Officials also are looking at dredging an area of delta to prevent a large drainage canal in south Bismarck from backing up from an ice jam.
A sandbar at the mouth of the Heart River in Mandan poses another problem. The corps will work with state and local officials to monitor and mitigate ice buildup at the sediment sites.
As for a permanent solution to the sandbar problems bedeviling Bismarck-Mandan and nearby subdivisions, the corps maintains it lacks authority to tackle the issue.
“That is an issue for the locals to address if they feel they need to,” says John Remus, the corps’ Omaha District chief of hydraulic engineering.
A federal program to provide bank stabilization projects might help reduce riverbank erosion, Remus said. A task force will meet early in 2012 to explore the possibility.
Any project funding would require state and local matching funds, and would have to obtain environmental permits.
“Nothing is in the works at this time,” Remus says.
More than five decades after Garrison Dam began operations, its effects on downstream river dynamics have not been studied in depth.
Prompted by the record flood of 2011, the U.S. Geological Survey is proposing a major three-year study to explore issues including riverbank erosion and sediment accumulation.
Has the Missouri River reached a steady state, in terms of the sediment load it can carry? Or is the river, following the massive flood, still seeking to reach that balance?
“It’s something that’s been needed for a long time,” Joel Galloway, a research hydrologist for the USGS in Bismarck, says of the study.
“Streams are always trying to reach some kind of equilibrium,” he says. Garrison Dam altered the river’s natural equilibrium, including Lake Sakakawea’s capture of sediment.
“If we choke off that sediment, it will find other places,” Galloway says. “It still may be adjusting itself.”
The study will focus on the 70-mile, free-flowing reach from Garrison Dam to the headwaters of Lake Oahe.
The study’s findings, expected in 2014, could help guide steps, such as bank stabilization or delta dredging.
Otherwise, Galloway says, “They’re kind of going in blind, not knowing what to do. We hope to answer some good questions.”
Chuck Mork is among those who complain that it has become too difficult to obtain a permit for riverbank stabilization projects.
North Dakota officials believe that river management to create sandbars to provide habitat for endangered or threatened bird species, such as the least tern and piping plover, impede flood control.
Flushing water to create sandbars, Sando and others maintain, increases riverbank erosion, and therefore contributes to delta formation, which can exacerbate ice-jam flooding.
But officials with the corps and U.S. Fish and Wildlife Service argue that managing the river to benefit birds does not sacrifice riverbanks or aggravate flooding.
“Flows out of the dam have very little if anything to do with endangered species protection,” says Carol Aron, a biologist with the U.S. Fish and Wildlife Service in Bismarck.
Nonetheless, Sando and Mork say, riverbank stabilization projects have stalled in the past decade as permits have become hard to obtain.
As of the late 1990s, less than a third of the 170 miles of riverbank from Garrison Dam to Lake Oahe were rip-rapped. The estimated cost of protecting the 10 percent of shoreline vulnerable to erosion was $13.8 million in 1997, or
$19.45 million in 2011 dollars.
“We’ve got plenty of sand and Fish and Wildlife wants more sand,” Sando says. “That’s why people love the river. It’s great for recreation.”
As for Andy Mork, who died in 2010, his son believes he would have been pleased to see how the rip-rap withstood the test of the record flood of 2011.
“He would have been very happy, with the results of the areas where the stabilization was done,” Chuck Mork says.
Patrick Springer reports for The Forum of Fargo-Moorhead.