Abstract
The 2011 flood on the Missouri River was one of
the largest floods since the river became regulated by a
series of high dams in the mid-20th century (greater than
150,000 cubic feet per second during the peak). The flood
persisted through most of the summer, eroding river banks,
adding sand to sandbars, and moving the thalweg of the
channel in many places. The U.S. Geological Survey monitored
and assessed the changes in two reaches of the Missouri
River: the Garrison Reach in North Dakota, bounded
by the Garrison Dam and the Lake Oahe Reservoir, and the
Recreational Reach along the boundary of South Dakota
and Nebraska bounded upstream by the Gavins Point Dam
and extending downstream from Ponca, Nebraska. Historical
cross-section data from the Garrison Dam closure until
immediately before the flood indicate that the upper reaches
of the river near the dam experienced rapid erosion, channel
incision, and island/sandbar loss following the dam closure.
The erosion, incision, and land loss lessened with time. Conversely,
the lower reach near the Lake Oahe Reservoir slackwaters
became depositional with channel in-filling and sandbar
growth through time as the flow slowed upon reaching the
reservoir. Preliminary post-flood results in the Garrison Reach
indicate that the main channel has deepened at most cross-sections
whereas sandbars and islands have grown vertically.
Sandbars and the thalweg migrated within the Recreational
Reach, however net scouring and aggradation was minimal.
Changes in the two-dimensional area of sandbars and islands
are still being assessed using high-resolution satellite imagery.
A sediment balance can be constructed for the Garrison
Reach using cross-sections, bathymetric data, sand traps for
wind-blown material, a quasi-three-dimensional numerical
model, and dating of sediment cores. Data collection and
analysis for a reach-scale sediment balance and a concurrent
analysis of the effects of riparian and island vegetation on
sediment deposition currently (2014) is ongoing.
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First posted July 8, 2014
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