Muddy Mo: Reservoir Siltation and the Flood of 2011

The Army closed Gavin’s Point Dam near Yankton, South Dakota in 1955.  The reservoir behind the dam stretched upstream to the town of Niobrara, Nebraska, situated at the mouth of the Niobrara River.  By the early 1970s, silt from the Niobrara River had created a shallow delta at the head of Lewis and Clark Lake.  This accumulated silt did two things.  First, it raised the water table at Niobrara.  As a result, buildings in town suffered structural damage from water seepage.  Second, it caused an increase in flooding in Niobrara when high flows came down either the Missouri or Niobrara rivers, struck the shallow delta, and then backed-up into residential and commercial areas.  Because of the higher water table and the increasing likelihood of floods, the residents of Niobrara moved their town to higher ground in 1972.

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 presently exiting Fort Randall Dam are hitting the delta near Niobrara, piling up, and flowing outward onto town property.  The Army has repeatedly acknowledged that Niobrara’s flood problems are a result of sedimentation at the head of Lewis and Clark Lake.  Niobrara 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 in 1963.  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 6 miles above Pierre), rush downstream, strike the delta formed by inflows from the Bad River, slow down, and then back-up into neighborhoods.  Fort Yates, North Dakota, stands at the head of Lake Oahe which filled after the closure of Oahe Dam in 1958.  The citizens of that reservation community are today constructing berms around town buildings 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 70 miles north of Bismarck) have no where else to go but into Fort Yates.  North Dakota officials stated as recently as September 2010 that siltation in the upper Oahe reservoir had exacerbated the flooding situation next to Oahe Lake.  And then there is Williston, North Dakota, at the head of giant Lake Sakakawea.  The Army formed this reservoir after closure of Garrison Dam in 1953.  Williston too 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 the muddy inflows from the Yellowstone River – which comes into the Missouri 21 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 silt-laden deltas located at their headwaters, are contributing to the flooding.

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