Abstract
An analysis of recent and historical U.S. Geological Survey
streamgage information was used to assess geomorphic
changes caused by the 2011 flood, in comparison to selected
historical floods, at three streamgage sites along the lower
Missouri River—Sioux City, Iowa; Omaha, Nebraska; and
Kansas City, Missouri. Channel-width change was not evident
at the three streamgage sites following the 2011 flood and
likely was inhibited by bank stabilization. Pronounced changes
in channel-bed elevation were indicated.
At Sioux City and Omaha, the geomorphic effects of the
2011 flood were similar in terms of the magnitude of channelbed
scour and recovery. At both sites, the 2011 flood caused
pronounced scour (about 3 feet) of the channel bed; however,
at Omaha, most of the channel-bed scour occurred after the
flood had receded. More than 1 year after the flood, the channel
bed had only partially recovered (about 1 foot) at both
sites. Pronounced scour (about 3 feet at Sioux City and about
1.5 feet at Omaha) also was caused by the 1952 flood, which
had a substantially larger peak discharge but was much shorter
in duration at both sites. Again, at Omaha, most of the channel-
bed scour occurred after the flood had receded. At Sioux
City, substantial recovery of the channel bed (about 2.5 feet)
was documented 1 year after the 1952 flood. Recovery to
the pre-flood elevation was complete by April 1954. The
greater recovery following the 1952 flood, compared to the
2011 flood, likely was related to a more abundant sediment
supply because the flood predated the completion of most of
the main-stem dam, channelization, and bank stabilization
projects. At Omaha, following the 1952 flood, the channel bed
never fully recovered to its pre-flood elevation.
The geomorphic effect of the 2011 flood at Kansas City
was fill (about 1 foot) on the channel bed followed by relative
stability. The 1952 flood, which had a substantially larger peak
discharge but was much shorter in duration, caused modest
fill (about 0.5 foot) on the channel bed. The 1993 flood, which
also had a substantially larger peak discharge but was much
shorter in duration, caused pronounced scour of the channel
bed (possibly as much as 4 feet). Similar to the floods
at Omaha, much of the channel-bed scour at Kansas City
occurred after the 1993 flood had receded. More than 1 year
after the 1993 flood, following partial recovery (about 1 foot), the channel bed had stabilized, at least temporarily. Following
the 1993 flood, the channel bed never fully recovered to its
pre-flood elevation.
For each flood in the post-dam era that resulted in
substantial channel-bed scour (Sioux City in 2011, Omaha in
2011, Kansas City in 1993), recovery of the channel bed to its
pre-flood elevation had not occurred more than 1 year after the
flood (20 years after the 1993 flood at Kansas City). Thus, the
possibility exists that channel-bed scour caused by large floods
may have a cumulative effect along the lower Missouri River.
The persistence of the flood-related decreases in channel-bed
elevation may be indicative of the constrained ability of the
channel to recover given a limited sediment supply caused by
one or more of the following factors: upstream storage of sediment
in reservoirs, bank stabilization, commercial sand dredging,
depletion of readily available sediment by the flood, and a
lack of post-flood sediment contributions from tributaries.