This is the first in a three-part series examining sedimentation issues along the Missouri River, including the impact on the reservoirs, the changing river environment and how to best manage the system.
The Missouri River is the longest river in North America, flowing for more than 2,300 miles and passing through portions of seven states, and providing water, electricity and recreation for those living and working along its banks.
But under the surface, the river carries something that makes it harder for the Missouri River reservoirs to deliver those benefits that people and wildlife depend on: mud.
The river is rich with sediment – silt, clay, sand and gravel – coming in from its own bed and numerous tributaries. And there are more than a few area residents who are concerned about the side effects of that sediment building up in the river and its six reservoirs now that the river is no longer free to carry that sediment downstream, as it did once.
Instead of helping to replenish coastal areas on the Gulf of Mexico, as in the days when the Big Muddy tumbled its load of sediment into the Mississippi and so on south, much of the sediment from the upper Missouri is now stranded behind six Missouri River dams that were built in the mid-20th century. Four of those six dams – Oahe, Big Bend, Fort Randall and Gavins Point – are in South Dakota.
The largest problem sedimentation creates, especially in areas such as Pierre and Fort Pierre, is flooding.
When the sediment-rich, free-flowing parts of the river hit slower waters, such as the reservoirs, the water loses velocity and drops sediment, creating deltas. The same thing occurs where tributaries such as the Bad River, White River and Niobrara River enter the Missouri.
John Cooper, a member of the state Game, Fish and Parks Commission, said the dropped clay-based soil can set much like concrete and form speed bumps in the river. During high flows, the river must rise in elevation to get over those speed bumps, and that high water will flood areas around it, he said.
The added sediment also displaces water, raising surface elevations. This is especially problematic in southeast Pierre, where sandy soil makes it easier for that displaced water to percolate through and flood houses.
Larry Weiss, president of the Missouri Sedimentation Action Coalition, knows first-hand the problems with flooding. He owned a house in southeast Pierre that was purchased by the U.S. Army Corps of Engineers in 2001.
“The reason for that buyout was groundwater problems and flooded basements. We had our basement flooded three times,” Weiss said. “Sedimentation raised the bottom of the river, or the flow line.”
Weiss was not alone in facing that issue. Since 1999 the Corps of Engineers has spent more than $25 million on buying out 110 similarly-flooded homes in Pierre and Fort Pierre and flood-proofing an additional 18.
John Remus, chief of the Hydrological Engineering Branch for the Corps’ Omaha District, said the Corps has noticed more flooding in Pierre and Fort Pierre associated with sedimentation around the reservoir and Missouri River tributaries, such as the Bad River.
“In those areas we have increased groundwater elevations and increased water surface elevations,” Remus said.
Howard Paul, former executive director for the Missouri Sediment Action Coalition, said the plant-dense deltas at the head of the reservoirs can create a damming effect, backing up the river and cause flooding.
This could be seen during the last major drought, when the water level behind the delta formed by the Niobrara River upstream of Lewis and Clark Lake was seven feet higher than below it, he said.
The entrapment of sediment in the reservoirs has also led to concerns over lost capacity as mud takes the place of water, especially for the three smaller lakes.
The larger reservoirs – Oahe, Sakakawea and Fort Peck – are in no immediate danger, as each has an extremely long sediment life of 900 to 1,100 years, according to Jody Farhat, chief of the Missouri River Basin Water Management for the Corp.
In contrast, Remus said Lewis and Clark Lake is already 25 percent full of sediment, and will take about 150 years to fill completely. Lake Sharpe will take an estimated 240 years and Lake Francis Case will be full in about 400.
Lewis and Clark Lake has become the focal point for many concerned about sediment, as a multi-lobed delta fed by sediments delivered by the Niobrara River has filled in the reservoir’s eastern edge.
Cooper said that diminished capacity means in the future the Corps of Engineers will have to scramble to release enough water to meet navigation and electrical needs.
“Lewis and Clark Lake reservoir cannot handle the sediment from the Niobrara,” he said.
Weiss said Lewis and Clark Lake sees a lot of recreational use during the summer, and if the reservoir fills with sediment, it will be a major economic loss for that part of the state.
More immediately, Remus said the town of Springfield, on the eastern edge of the reservoir has seen issues of sediment blocking water intake valves for several years, a problem other lakeside towns may experience in the future.
In the Pierre area this filling in could also affect recreation, perhaps even eliminating access behind La Framboise or Farm Island in time, he said.
Cooper said that for anglers, the reservoirs’ filling means less habitat for fish spawning and greater likelihood of boat ramps becoming unusable, he said.
“That’s a minor inconvenience, but a reminder of that fact that these reservoirs every year are filling up,” Cooper said.