The Muddy Mo: Reservoir Siltation and the Flood of 2011

In 1955, the Army closed Gavin’s Point Dam near Yankton, South Dakota.  Once the reservoir behind the dam filled with water, it stretched upstream to the town of Niobrara, Nebraska.  By the late 1960’s, silt from the Niobrara River had created a shallow delta at the head of Lewis and Clark Lake.  This accumulated silt did two things: 1) it raised the water table at Niobrara, which caused buildings in town to suffer water damage; and 2) it contributed to more frequent floods.  High flows descending either the Missouri or Niobrara rivers struck the shallow delta, backed-up, and then flowed into residential and commercial neighborhoods in the town of Niobrara.  In 1972, the higher water table and the likelihood of future floods persuaded the residents of Niobrara to move their town to higher ground.

Now, fast forward to the present.  The low-lying sections of the relocated Niobrara still face a flood threat from the Missouri River.  High flows exiting Fort Randall Dam are hitting the delta near Niobrara, piling up, and spilling onto town property.  The Army has admitted that the continued flooding at Niobrara is the result of sedimentation at the head of Lewis and Clark Lake.

Niobrara’s flood problem is not unique.  Fort Pierre and Pierre, South Dakota, sit at the head of Lake Sharpe.  The Army formed Lake Sharpe after the closure of Big Bend Dam.  The capital of South Dakota and its sister city now face a flood of epic proportions because of a combination of high water releases from Oahe Dam and the presence of a silty delta at the head of Lake Sharpe, which begins just southeast of Fort Pierre.  In a scenario similar to the one at Niobrara, large discharges pour forth from Oahe Dam (located six miles above Pierre), rush downstream, strike the delta formed by inflows from the Bad River, slow down, and then back-up.

Fort Yates, North Dakota, is located at the head of Lake Oahe, which filled after the closure of Oahe Dam.  The citizens of that reservation community are today constructing berms to protect them from the rising waters of the 210-mile-long reservoir.  The reservoir in front of the town has so filled with silt, that the waters descending from Garrison Dam (approximately seventy miles north of Bismarck) have no where else to go but into Fort Yates.  North Dakota officials stated as recently as last September that siltation in the upper Oahe reservoir has worsened flooding along Lake Oahe.

And then there is Williston, North Dakota, at the head of giant Lake Sakakawea.  The Army formed this reservoir after the closure of Garrison Dam.  Williston also confronts a historic flood.  Major flood stage at Williston is 26 feet.  As of today, June 5, 2011, the Missouri River stage at Williston is 27.02 feet.  As you might by now have already guessed, Williston sits adjacent to another reservoir delta, this one formed by inflows from the silt-laden Yellowstone River, which enters the Missouri twenty-one miles to the west.

This year, the Dakota dams have failed to halt flooding in both the lower and upper Missouri Valley.  As a matter of fact, the reservoirs, and the deltas located at their headwaters, are contributing to the flooding.

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