Abstract
Floodwater in the Missouri River in 2011 originated in
upper-basin regions and tributaries, and then travelled through
a series of large flood-control reservoirs, setting records for
total runoff volume entering all six Missouri River main-stem
reservoirs. The flooding lasted as long as 3 months. The U.S
Geological Survey (USGS) examined sediment transport and
deposition in the lower Missouri River in 2011 to investigate
how the geography of floodwater sources, in particular the
decanting effects of the Missouri River main-stem reservoir
system, coupled with the longitudinal characteristics of civil
infrastructure and valley-bottom topography, affected sediment
transport and deposition in this large, regulated river
system.
During the flood conditions in 2011, the USGS, in cooperation
with the U.S. Army Corps of Engineers, monitored
suspended-sediment transport at six primary streamgages
along the length of the lower Missouri River. Measured
suspended-sediment concentration (SSC) in the lower Missouri
River varied from approximately 150 milligrams per liter
(mg/L) to 2,000 mg/L from January 1 to September 30, 2011.
Median SSC increased in the downstream direction from
355 mg/L at Sioux City, Iowa, to 490 mg/L at Hermann,
Missouri. The highest SSCs were measured downstream
from Omaha, Nebraska, in late February when snowmelt
runoff from tributaries, which were draining zones of high-sediment
production, was entering the lower Missouri River,
and releases of water at Gavins Point Dam were small. The
combination of dilute releases of water at Gavins Point Dam
and low streamflows in lower Missouri River tributaries
caused sustained lowering of SSC at all streamgages from
early July through late August.
Suspended-sediment ranged from 5 percent washload
(PW; percent silt and clay) to as much as 98 percent in the
lower Missouri River from January 1 to September 30, 2011.
Median PW increased in the downstream direction from
24 percent at Sioux City, Iowa, to 78 percent at Hermann, Missouri.
Measurements made in early January, when SSC was
low, indicate that suspended sediment mostly was composed
of bed material, but by mid-February, runoff from the plains
caused PW to increase at most streamgages. Total suspended-sediment
discharge (SSD) during water year 2011 at the
selected streamgages in the lower Missouri River ranged from
approximately 29 to 64 million tons. Total estimated SSD had
the lowest exceedance frequencies in the reaches between
Gavins Point Dam and Nebraska City, Nebraska, but exceedance
frequencies increased substantially downstream. In 2011,
total SSD with low exceedance frequencies were reported
at Sioux City, Iowa, Omaha, Nebraska, and Nebraska City,
Nebraska, despite moderate-to-high exceedance frequencies
for annual average SSC, indicating that the duration of high-magnitude
flooding was the primary driver of total SSD.
Comparison of median SSC for samples from water year
2011 with samples in the 20 years prior indicated that median
SSC for high-action streamflows (streamflows likely to produce
a stage exceeding the National Weather Service’s “action
stage”) in 2011 were lower than those typical for high-action streamflows. Multiple-comparison analysis indicated that
median SSC values for low-action streamflows (streamflows
likely to produce stages lower than the National Weather Service’s
“action stage”) and high-action streamflows sampled in
2011 at 4 of 6 streamgages were not significantly distinguishable
from median SSC values for low-action streamflows in
the previous 20 years. Longitudinal comparison of streamflow
and SSD exceedance frequencies for 2011 with corresponding
frequencies for 2008 and 1993 indicated the important role
of tributary contributions to total SSD in the lower Missouri
River. In 1993 and 2008, tributaries were the primary source
of floodwater in the lower Missouri River, which resulted
in a 20-fold increase in total SSD from Sioux City, Iowa, to
Hermann, Missouri. In 2011, releases at Gavins Point Dam
were the primary source of floodwater in the lower Missouri
River, and total SSD at Hermann, Missouri, was only twice
that estimated for Sioux City, Iowa.
Sand deposition was estimated using analysis of multispectral
satellite imagery collected in October and November
2011. Distributions of sand in the flood plain of the lower
Missouri River also were quantified in relation to distance
from the banks of the main channel for seven discrete river
segments bounded by Gavins Point Dam and selected downstream
tributaries. The areal extent of overbank flooding
and flood-plain sand deposits increased downstream from
Sioux City, Iowa to a broad peak near Rulo, Nebraska, and
then decreased to levels near the lower limit of quantification
downstream from Kansas City, Missouri. Most of the flood
plain inundation and sediment-deposition damage to agricultural
fields was observed between river miles 480 and 700,
where 2011 peak streamflows had low exceedance frequencies,
and the lower Missouri River channel was less incised
or had aggraded recently. As channel capacity increased
in the downstream direction, the relative magnitude of the
flood decreased downstream, and overbank flooding was less
extensive. In the constricted reaches, flood-plain sand deposits
mainly were observed in association with levee breaks.