Prepared in cooperation with the U.S. Army Corps of Engineers

Sediment Transport and Deposition in the Lower 

Missouri River During the 2011 Flood

Chapter F of

2011 Floods of the Central United States

Professional Paper 1798–F

U.S. Department of the Interior

U.S. Geological Survey
Front cover. Top photograph: View of flooding from Nebraska City, Nebraska, looking east across the Missouri River, 

August 2, 2011. Photograph by Robert Swanson, U.S. Geological Survey. 

Right photograph: USGS scientist collecting a sediment sample from the Missouri River at Sioux City, Iowa, 

September 16, 2011. Photograph by Ryan Tompkins, U.S. Geological Survey.

Back cover. Sand from the 2011 flood on the flood plain in northwestern Missouri, September 22, 2012. Photograph by 

Robert Jacobson, U.S. Geological Survey.
Sediment Transport and Deposition in the 

Lower Missouri River During the 2011 Flood

By Jason S. Alexander, Robert B. Jacobson, and David L. Rus

Chapter F of

2011 Floods of the Central United States

In cooperation with the U.S. Army Corps of Engineers

Professional Paper 1798–F

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013

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as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Alexander, J.S., Jacobson, R.B., and Rus, D.L., 2013, Sediment transport and deposition in the lower Missouri River 

during the 2011 flood: U.S. Geological Survey Professional Paper 1798–F, 27 p., http://dx.doi.org/10.3133/pp1798F.

2330-7102 (online)
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................2

Importance of Understanding Sediment Transport in the Lower Missouri River......................3

General Morphologic and Hydrologic Characteristics of the Lower Missouri River ...............5

Measurement and Monitoring of Sediment Transport in the Lower Missouri River ................5

Sediment Transport and Deposition in the Lower Missouri River in 2011............................................6

Temporal and Spatial Characteristics of Sediment Transport in the Lower Missouri River 

in 2011........................................................................................................................................6

Suspended-Sediment Concentration ......................................................................................7

Washload ......................................................................................................................................7

Suspended Load ..........................................................................................................................9

Comparison of Sediment Transport in 2011 with Other Recent Floods......................................11

Patterns of Sediment Deposition Along the Lower Missouri River during the Flood of 

2011..........................................................................................................................................17

Summary........................................................................................................................................................23

References Cited..........................................................................................................................................25

Figures

1. Map showing the Missouri River Basin, associated physiographic provinces, 

primary tributaries, main-stem dams, and suspended-sediment monitoring stations 

along lower Missouri River.........................................................................................................3

2. Graphs showing frequency distribution of selected measurements of suspendedsediment concentration at streamgages along the lower Missouri River, and daily 

streamflow of major tributaries upstream from each measurement location, 

January 1 to October 1, 2011.......................................................................................................8

3. Graphs showing frequency distribution of measured percentage of suspendedsediment as washload (percentage of suspended-sediment mixture finer than 

0.0625 millimeter in diameter) at selected streamgages along the lower Missouri 

River and daily streamflows of major tributaries upstream from each measurement 

location, January 1 to October 1, 2011 ....................................................................................10

4. Graphs showing frequency distributions of sampled suspended-sediment concentrations for selected streamgages along the lower Missouri River for low- and highaction streamflows in water year 2011 and in the preceding 20 water years..................15

5. Pie charts showing relative contributions of tributary streamflow to annual average 

streamflow of the Missouri River at Hermann, Missouri (U.S. Geological 

Survey streamgage number 06934500), during water years 1993, 2008, and 2011, 

and average for the 20-year period-of-record preceding water year 2011, as 

percentage of total .....................................................................................................................16

6. Diagrams showing annual average streamflow for the lower Missouri River and 

major tributaries and annual total suspended-sediment discharge for selected mainstem streamgages, water years 1993, 2008, and 2011..........................................................18

7. Areal images showing method of mapping sand deposition and deposited sand on 

the flood plain of the lower Missouri River , upstream of Sioux City, Iowa, in Union 

County, Iowa after the flood conditions of 2011.....................................................................19
iv

8. Graphs showing longitudinal distribution of sand deposition and overbank flooding 

along the lower Missouri River in 2011 and potential covariates.......................................20

9. Conceptual diagrams showing progression of levee-break ...............................................21

10. Multispectral satellite image taken in Novermber 2011 showing a typical levee-break 

complex fromed in the valley bottom of the lower Missouri River valley during overbank flooding in 2011..................................................................................................................22

11. Graph showing cumulative sand depositional area by distance away from channel 

boundary in the valley bottom of seven hydrologic segments of the lower Missouri 

River after overbank flooding in 2011 ......................................................................................22

Table

1. Magnitude and excedance frequency of annual average and peak streamflows, annual 

average suspended-sediment concentration, and annual total suspended-sediment 

discharge for selected main-stem and tributary streamgages, lower Missouri River, 

water year 2011...........................................................................................................................12

Rock jetty in Plattsmouth backwater on the Missouri 

River near Plattsmouth, Nebraska, October 24, 2011.

Photograph by Justin Krahulik, U.S. Geological Survey.
v

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 25.4 millimeter (mm)

foot (ft) 30.48 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 0.4047 hectare (ha)

acre 0.004047 square kilometer (km2

)

Volume

ounce, fluid (fl. oz) 0.02957 liter (L) 

cubic foot (ft3

) 28.32 cubic decimeter (dm3) 

cubic foot (ft3

) 0.02832 cubic meter (m3) 

cubic yard (yd3

) 0.7645 cubic meter (m3) 

acre-foot (acre-ft) 1,233 cubic meter (m3

)

Flow rate

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

Mass

ounce, avoirdupois (oz) 28350 milligram (mg) 

ounce, avoirdupois (oz) 28.35 gram (g) 

ton per day (ton/d) 0.9072 metric ton per day

ton per day (ton/d) 0.9072 metric ton per day

ton per day (ton/d) 0.9072 megagram per day (Mg/d)

ton per year (ton/yr) 0.9072 megagram per year (Mg/yr)

ton per year (ton/yr) 0.9072 metric ton per year

Concentration

Milligram per liter (mg/L) 1 part per million 

Milligram per liter (mg/L) 0.058 grain per gallon 

Horizontal coordinate information is referenced to the North American Datum of 1983 

(NAD 83).

Sediment Transport and Deposition in the Lower Missouri 

River During the 2011 Flood

By Jason S. Alexander, Robert B. Jacobson, and David L. Rus

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 highsediment 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 suspendedsediment 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 highmagnitude 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 

Sand from the 2011 flood on the flood plain in northwestern 

Missouri, September 22, 2012. Photograph by Robert Jacobson, 

U.S. Geological Survey.
2 Sediment Transport and Deposition in the Lower Missouri River During the 2011 Flood

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.

Introduction

Sediment transported with floodwater can cause substantial damage to agricultural fields, homes, and infrastructure 

along the valley bottom adjacent to the river. Sediment trapped 

behind dams also decreases storage capacity and reduces the 

viable lifespan of flood-control reservoirs (Graf and others, 

2010); however, the transport and deposition of sediment can 

be an important driver of ecosystem processes, particularly 

during large, landscape-scale disturbances such as floods, 

which often rejuvenate existing channel and flood-plain 

habitats, and construct new physical templates for riverine 

and riparian habitats (Junk and others, 1989; Scott and others, 

1996). Thus, understanding processes of sediment transport 

and deposition in large rivers is crucial to striking a balance 

between protecting infrastructure, agriculture, and urban areas 

adjacent to the river while also sustaining natural physical processes that are important to ecosystems of value to the Nation. 

Flooding along rivers in the central United States was 

widespread in 2011 (Holmes and others, 2013). Flooding in 

the Missouri River Basin in 2011 was caused by a combination of above-normal snowpack in headwater regions in the 

Rocky Mountains of Montana and Wyoming, near record 

snowfall and wet soil conditions on the plains, and record rainfall in May across the upper Missouri River Basin (Vining and 

others, 2013; Grigg and others, 2012). Total volume of runoff 

from these conditions into all six Missouri River main-stem 

reservoirs in 2011 was the most since record keeping began, 

exceeding previous peaks set in 1881 (50 million acre-feet) 

and 1997 (49 million acre-feet) by more than 10 million acrefeet, and forcing releases of record high-magnitude streamflows from several of the main-stem dams operated by the 

U.S. Army Corps of Engineers (USACE). Releases at Gavins 

Point Dam near Yankton, South Dakota, the most downstream 

dam in the Missouri River main stem, equaled or exceeded 

150,000 cubic-feet per second (ft3

/s) from mid-June through 

late August, and were the largest measured in magnitude and 

volume since operations began in 1955 (Grigg and others, 

2012). 

Flooding in the Missouri River in 2011 transported and 

deposited large quantities of sediment and the U.S. Geological Survey (USGS) monitored the characteristics of sediment 

transport in the Missouri River at several streamgages along 

the main stem from Montana to Missouri (Galloway and 

others, 2013; U.S. Geological Survey, 2012a); however, the 

effects of floodwater geography (that is, where floodwaters 

originate within a large river basin) on sediment transport 

were most apparent in the floodwater of the Missouri River 

downstream from Gavins Point Dam (hereafter referred to as 

“lower Missouri River”), where substantial sediment deposition and damage to adjacent properties was observed (Grigg 

and others, 2012). 

The USGS, in cooperation with the U.S. Army Corps 

of Engineers, examined sediment transport and deposition 

in the lower Missouri River in 2011 to investigate how the 

geography of floodwater sources, coupled with the alongstream characteristics of civil infrastructure and valley-bottom 

topography, affected characteristics of sediment transport and 

deposition in this large, regulated river system. Investigation 

of sediment transport and deposition focused on three key 

questions related to the unique combination of floodwater 

geography, magnitude, and volume in the lower Missouri 

River during 2011: (1) What were the temporal and spatial 

characteristics of sediment transport in the lower Missouri 

River in 2011? (2) How did floodwater geography differ from 

other high streamflow years in the lower Missouri River and 

did the difference cause detectable differences in sediment 

transport relative to previous years? (3) How did the longitudinal variation in channel geomorphology and civil infrastructure interact with floodwater geography to affect the spatial 

distribution and style of sediment deposition in the lower 

Missouri River valley?
Figure 1. The Missouri River Basin, associated physiographic provinces, primary tributaries, main-stem dams, and suspendedsediment monitoring stations along the lower Missouri River.

115°

Base from U.S. Geological Survey National Elevation Dataset, 2006 

Albers Equal-Area Conic projection

Horizonatal coordinate information is referenced to the 

North American Datum of 1983 (NAD 83)

Physiographic provinces from Fennemen (1928)

45°

40°

110° 105° 100° 95° 90°

0 100 200 300 400 KILOMETERS

0 100 200 300 400 MILES

EXPLANATION

Central Lowland

Great Plains

Middle Rocky Mountains

Northern Rocky Mountains

Ozark Plateaus

Southern Rocky Mountains

Wyoming Basin

Main-stem dam

Daily suspended-sediment monitoring station

Continuous suspended-sediment monitoring 

station

Periodic suspended-sediment monitoring 

station

Physiographic provinces within Missouri River Basin

Fort Peck Dam

Garrison Dam

Missouri River Basin boundary

Part of Missouri River Basin 

upstream from Gavins Point Dam

Oahe Dam

Big Bend Dam

Fort Randall Dam

Gavins Point Dam

Fort Benton

Yankton

Ponca

Alberta Saskatchewan

Manitoba

CANADA

UNITED STATES

Ontario

Minnesota

North Dakota

South Dakota

Nebraska

Kansas

Colorado Utah

Wyoming

Idaho

Montana

Arizona New 

Mexico Oklahoma

Wisconsin

Iowa

Illinois

Missouri

Sioux 

City

Fort Calhoun

Rulo

Nebraska City

Omaha

St. Joseph

Kansas CityHermann

St. Louis

Missouri River

Yellowstone River

Chey enne River

Niobrara River

Platte River

Kan sas River

Osage River

Gasconade River

Grand River

Platte River

Nodaway River

Nishnabotna River

Little Sioux River

Big Sioux River

James River

Introduction 3

Importance of Understanding Sediment 

Transport in the Lower Missouri River

The Missouri River Basin encompasses all or portions 

of 10 U.S. States and two Canadian Provinces, and composes 

roughly 43 percent of the total Mississippi River Basin land 

area. The Missouri River Basin encompasses several physiographic provinces (fig. 1), each with differing topographic 

relief and surficial materials. Most of the Missouri River Basin 

lies within the Great Plains and Central Lowland physiographic provinces (Fennemen, 1928), which broadly are characterized by gently sloping, unconsolidated, easily erodible 

fine sediments (sand, silt, and clay). Despite its vast expanse, 

the Missouri River contributes only about 12 percent of the 

total average annual streamflow of the Mississippi River Basin 

because much of its watershed is semi-arid (Meade, 1995); 
4 Sediment Transport and Deposition in the Lower Missouri River During the 2011 Flood

however, the landscapes of the Great Plains and Central Lowland physiographic provinces produce an abundance of sediment relative to annual water runoff (Langbein and Schumm, 

1958) and, consequently, the Missouri River is the largest 

source of sediment to the Mississippi River system (Heimann 

and others, 2011) despite the large number of dams and reservoirs. The sediment-laden, turbid waters of the Missouri River 

were the source of the original nickname “Big Muddy” given 

to the Missouri River by early explorers, trappers, boatmen, 

and settlers in the region (Blevins, 2006). 

The Missouri River main-stem reservoir system 

(MRMRS) was constructed between 1933 and 1964, and 

consists of six large earthen dams blocking lengths of the 

main channel of the Missouri River between Fort Benton, 

Mont, and Yankton, South Dakota (fig. 1) (National Research 

Council, 2002). Approximately one-half of the Missouri River 

Basin lies upstream from the most downstream impoundment, 

Gavins Point Dam (fig. 1). Sediment trapping by reservoirs 

within the MRMRS has disrupted the supply of sediment to 

the segments between dams in the middle Missouri River, and 

along the lower Missouri River. Primary sediment sources to 

the lower Missouri River are now limited to tributary contributions from the eastern regions of South Dakota and Nebraska, 

northeastern Kansas, western Iowa, and northwestern and 

central Missouri (fig. 1). The sediment-supply disruption created by MRMRS has caused channel incision in the reaches 

below the dams, and reduced sediment delivery to the Mississippi River by more than one-half of the estimated pre-settlement rate (Jacobson and others, 2009; Heimann and others, 

2010). Since the pronounced disruption in sediment transport 

coinciding with completion of the MRMRS, streamflowweighted suspended-sediment concentrations in the lower 

Missouri River have continued to decline at rates between 2 

and 3 percent per year (Heimann and others, 2011). Although 

the primary mechanisms are still poorly quantified, continued 

declines in sediment loads have been attributed to improved 

agricultural tillage and soil conservation practices (Meade and 

Moody, 2010; U.S. Department of Agriculture, 2012). 

The reductions in sediment transport and water turbidity, 

in combination with alterations of river-channel morphology, have reduced the quality and extent of natural riverine 

habitats in the lower Missouri River, and have been implicated 

as causes for the decline of some fish and bird species native 

to the Missouri River ecosystem (National Research Council, 

2002; Blevins, 2011). Riverbed incision, particularly in the 

reaches of the Missouri River just downstream from Gavins 

Point Dam, has affected recreation facilities, municipal and 

power-generation water intakes and well fields, and tributary 

channel stability, and has disconnected the river from its flood 

plain, preventing natural rejuvenation of flood-plain forest and 

wetland habitat and preventing deposition of nutrients in the 

valley bottom (U.S. Army Corps of Engineers, 2006; Dixon 

and others, 2010). Channel straightening, narrowing, and bank 

stabilization created a deeper, self-scouring navigation channel, and have reduced the area of emergent sand, a primary 

nesting habitat feature for migratory birds (Funk and Robinson, 1974; Hallberg and others, 1979). The reduction in sediment delivery to the Mississippi River also has been cited as 

one of several stressors contributing to the decline of coastal 

wetlands in the Mississippi River Delta system (not shown 

in fig. 1) (National Research Council, 2011). Habitat recovery and restoration efforts are ongoing in the lower Missouri 

River, as well as coastal Louisiana (National Research Council, 2011; U.S. Army Corps of Engineers, 2009). Proposed 

Langdon backwater on the Missouri River near Nemaha, 

Nebraska, looking upstream from the mouth, October 24, 

2011. Photograph by Justin Krahulik, U.S. Geological Survey.
Introduction 5

restoration options have included sediment nourishment projects to replenish diminished supplies, including an engineered 

sediment routing to the lower Missouri River downstream 

from Gavins Point Dam (Coker and others, 2009). 

General Morphologic and Hydrologic 

Characteristics of the Lower Missouri River 

Downstream from the MRMRS to St. Louis, Missouri, 

the lower Missouri River can be divided broadly into three 

distinct sub-segments based on channel engineering and levee 

confinement: (1) an unchannelized and unconfined segment 

from Gavins Point Dam to approximately Ponca, Nebraska; 

(2) a channelized and partially-confined segment from approximately Ponca to Fort Calhoun, Nebr.; and (3) a channelized 

and confined segment from Fort Calhoun to St. Louis, Mo. 

From Gavins Point Dam at river mile 811.1 (RM 811.1, miles 

upstream from the confluence with the Mississippi River along 

the Missouri River channel, based on unpublished data from 

the U.S. Army Corps of Engineers) to approximately Ponca, 

Nebr. (RM 753), the lower Missouri River is unchannelized, 

has numerous midchannel sandbars and islands, and has high 

bank-erosion rates relative to segments downstream (Elliot 

and Jacobson, 2006). River-channel width in the unchannelized and unconfined segment varies from approximately 

600 to 6,000 feet (ft). Downstream from RM 753, the lower 

Missouri River has been engineered into a roughly uniform, 

narrow, self-scouring channel to support barge navigation 

(referred to as “the navigation channel”), is deeper than segments upstream, and channel width varies from only about 

600 to 1,000 ft (National Research Council, 2002). Between 

RM 753 and approximately Fort Calhoun, Nebr. (RM 634), 

there are no Federal levees on the flood plain, except near 

Sioux City, Iowa, and the valley bottom is wide, so the lower 

Missouri River is less confined during overbank streamflows 

than segments downstream, but overbank streamflows are less 

likely because the valley bottom is high relative to the channel 

bottom (Jacobson and others, 2007). Downstream from 

RM 634, the valley bottom narrows, and the system of Federal 

and private levees confines the lower Missouri River from 

Omaha, Nebr, to St. Louis, Mo. 

The width and elevation of the lower Missouri River 

channel have changed in response to shifts in sediment supply 

and hydrology associated with completion of the MRMRS, 

channelization, and within-channel sand extraction (Jacobson 

and others, 2009). These changes in channel morphology have 

caused spatially variable changes to the conveyance capacity 

of the lower Missouri River channel. Between Gavins Point 

Dam and Sioux City, Iowa, the river channel has incised by as 

much as 10 ft since completion of the MRMRS (U.S. Army 

Corps of Engineers, 2006). Between Ponca, Nebr., and Sioux 

City, Iowa, channel degradation has offset decreases in channel capacity associated with flood-plain encroachment into the 

channel (narrowing) (U.S. Army Corps of Engineers, 2006), 

and streamflows of as much as 70,000 ft3

/s cause only minor 

flood damage (Grigg and others, 2012). Downstream from 

Sioux City, Iowa, the magnitude of incision of the lower Missouri River channel since completion of the MRMRS gradually decreases and the lower Missouri River is channelized 

completely. Between Sioux City, Iowa, and Omaha, Nebr., the 

combination of progressively decreasing incision and floodplain encroachment has decreased the conveyance capacity 

of the lower Missouri River relative to upstream segments, 

and minor flooding here begins at streamflows exceeding 

approximately 65,000 ft3/s (U.S. Army Corps of Engineers, 

2006). Between Omaha, Nebr., and St. Joseph, Mo., the lower 

Missouri River channel has lost conveyance capacity in recent 

decades because of flood-plain encroachment and channel 

aggradation, and flooding begins at streamflows exceeding 

approximately 90,000 ft3/s. Despite the system of levees in 

place downstream from Omaha, Nebr., flooding of agricultural land in the segment between Omaha and St. Joseph, 

Mo., begins at river stages 2 ft below the official flood stage 

because the height of the adjacent valley bottom is closer to 

the channel-bottom elevation relative to upstream reaches, and 

the valley bottom is poorly drained (U.S. Army Corps of Engineers, 2006; Chojnacki and others, 2012). Downstream from 

St. Joseph, Mo., patterns of channel incision and aggradation 

are variable, but flooding does not begin until streamflows 

exceed approximately 150,000 ft3/s (U.S. Army Corps of Engineers, 2006).

Measurement and Monitoring of Sediment 

Transport in the Lower Missouri River 

Sediment transport in rivers is measured in two primary modes: (1) suspended load, and (2) bedload. Numerous 

sediment-sampling devices have been tested, approved, and 

are associated with measurement methods standardized by a 

multi-agency U.S. government entity referred to as the Federal 

Interagency Sedimentation Program (FISP). The reader is 

referred to Edwards and Glysson (1999) for detailed descriptions of standard sediment-sampling devices, associated 

theory, and sampling methods. 

Most of the sediment load of the lower Missouri River is 

transported in suspension (National Research Council, 2011; 

Galloway and others, 2013). The suspended-sediment concentration (SSC) was obtained from water samples that represent 

the vertical and horizontal variability of suspended-sediment 

in the river channel. Samples typically were collected across 

the entire vertical water column at multiple locations along 

a river transect by lowering a standard, approved sampler 

(Davis, 2005) from the water surface down to a sampler-specific point just above the streambed (from 0.5 to 2 ft) before 

raising the sampler back up again, with the sampler transited 

vertically at a constant rate (Edwards and Glysson, 1999). 

In large turbid rivers, the sampler often is not lowered to the 

point of contact with the riverbed to prevent the sampler nozzle from sampling the riverbed. As a consequence, there often 

is an enlarged “unsampled zone,” which may result in underestimation of SSC. The subsamples collected at each location 

along the transect were either composited into one sample or 
6 Sediment Transport and Deposition in the Lower Missouri River During the 2011 Flood

analyzed separately for SSC (Guy, 1969; Edwards and Glysson, 1999). Suspended-sediment samples were analyzed at the 

USGS laboratories in Iowa City, Iowa, and Rolla, Mo. 

In the lower Missouri River, suspended sediment was 

monitored at selected USGS streamgages where the streamflow of the river is measured continually or easily computed 

from a statistical relation with river stage. The USGS used 

three types of sediment monitoring in the lower Missouri 

River: (1) daily, (2) continuous, and (3) periodic (fig. 1). Daily 

monitoring happens at streamgages where a daily value of 

SSC and suspended-sediment load (SSL) are computed using 

methods described in Porterfield (1972), which generally 

employ a combination of frequent (typically 2 to 3 times per 

week) sampling of SSC, as well as statistical relations between 

streamflow and SSC (Horowitz, 2003). Continuous monitoring is a type of daily monitoring where the daily record of 

SSC and SSL were computed using continuous measurements (at most one measurement recorded every 15 minutes) 

of a surrogate(s), usually turbidity (Rasmussen and others, 

2009), for which a statistical model of the relation to SSC 

subsequently is developed using frequent samples of SSC. 

Periodic monitoring happens at streamgages where the river 

was sampled for SSC at varying frequencies, and no daily 

record was computed. Periodic SSC data were not considered 

adequate for computation of daily SSL, but were considered 

sufficient to compute annual SSL using multivariate statistical 

approaches (Runkel and others, 2004). 

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: (1) Sioux City, 

Iowa (USGS streamgage 06486000); (2) Omaha, Nebr. (USGS 

streamgage 06610000); (3) Nebraska City, Nebr. (USGS 

streamgage 06807000); (4) St. Joseph, Mo. (USGS streamgage 

06818000); (5) Kansas City, Mo. (USGS streamgage 

06893000); and (6) Missouri River at Hermann, Mo. (USGS 

streamgage 06934500). These stations are hereinafter referred 

to simply as “Sioux City,” “Omaha,” “Nebraska City,” “St. 

Joseph,” “Kansas City,” and “Hermann,” respectively, or 

collectively as “the selected streamgages.” Five of the six 

selected streamgages in the lower Missouri River have daily or 

continuous monitoring for sediment, and Kansas City has periodic monitoring (fig. 1). Bedload sediment transport also was 

measured periodically at several streamgages along the lower 

Missouri River in 2011 (Galloway and others, 2013).

Sediment Transport and Deposition in 

the Lower Missouri River in 2011

Investigation of sediment transport and deposition 

focused on three key questions related to the unique combination of floodwater geography, streamflow magnitude, and 

volume in the lower Missouri River in 2011: (1) What were 

the temporal and spatial characteristics of sediment transport 

in the lower Missouri River in 2011? (2) How did floodwater 

geography in 2011 differ from other high streamflow years in 

the lower Missouri River, and did the difference in geography 

cause detectable differences in sediment transport relative 

to previous years? (3) How did the longitudinal variation 

in channel geomorphology and civil infrastructure interact 

with floodwater geography to affect the spatial distribution 

and style of sediment deposition in the lower Missouri River 

valley?

Temporal and Spatial Characteristics of 

Sediment Transport in the Lower Missouri River 

in 2011

For the purpose of describing the characteristics of suspended-sediment transport in the lower Missouri River during 

the high-streamflow conditions of 2011, three measures were 

used: (1) SSC; (2) washload, as a percentage of SSC (PW); 

and (3) suspended-sediment discharge (SSD), sometimes 

called the “suspended load.” For the purposes of the analysis 

described in this report, the term “washload” refers to the fraction of particles in a suspended-sediment mixture that are finer 

than 0.0625 millimeters (mm), also referred to as “silt and 

clay,” or collectively as “mud.” Washload particles typically 

are considered to be derived from upland and bank erosion, 

and may travel in perpetual suspension in the water column, 

whereas sand and other coarser particles are considered to 

be derived mostly from the riverbed and travel intermittently 

in suspension or in frequent contact with the riverbed (bedmaterial load) (Church, 2006). The actual diameter of the the 

physical separation between washload and bed-material load 

may be different depending on streamflow conditions. 

For the purposes of this report, the term “major tributaries” is assigned to streams that are tributary to the lower 

Missouri River, have streamgages with continuous periods of 

record spanning several decades, and are known to contribute substantial amounts of water and sediment to the lower 

Missouri River main stem. Under these criteria, the following 

tributaries were selected as “major tributaries” for the analyses 

described in this report: James River, Big Sioux River, Little 

Sioux River, Platte River (of Nebraska), Nishnabotna River, 

Nodaway River, Platte River (of Missouri), Kansas River, 

Grand River, Chariton River, Osage River, and Gasconade 

River (fig. 1).

Examination of spatial and temporal characteristics of 

SSC and PW in water year 2011 was focused on the period 

starting immediately before snowmelt and ice break up on the 

Great Plains, through the high-streamflow period and end of 

the water year (January 1 to September 30, 2011). Measurements of SSC made at the selected streamgages along the 

lower Missouri River were retrieved from the USGS National 

Water Information System (NWIS) (U.S. Geological Survey, 

2012b). 

Measurements taken on the same day, at the same 

streamgage, and using the same method were averaged to 
Sediment Transport and Deposition in the Lower Missouri River in 2011 7

minimize bias. Stacked time-series plots and summary statistics were used to interpret and describe temporal and spatial 

characteristics of SSC. Measurements of PW were derived 

from the filtered set of SSC data by excluding samples that did 

not include measurement of the percentage of the suspendedsediment finer than 0.0625 mm. USGS laboratory methods 

(Guy, 1969) provide two methods to measure the fraction of 

the suspended sediment that is finer than 0.0625 mm: (1) by 

physical sieving of the sediments, or (2) by visual-accumulation tube methods, which measure fractions according to the 

fall diameter of the sediments. In most cases, the available 

data for each day involved only one of these methods, but if 

both methods were used on a particular set of data for a single 

day, the result from the visual-accumulation tube method was 

chosen because it is a more precise measure (Hubbell and 

Stevens, 1986). Estimations of total annual SSD and average annual SSC for water year 2011 were characterized at the 

selected streamgages by longitudinal comparison of magnitudes and annual exceedance frequency during the period 

since completion of the MRMRS (1967 to 2011). Exceedance frequency was used instead of the observation rankings 

because the number of observations available at each mainstem streamgage within the period of record since closure of 

the MRMRS was not uniform. Annual exceedance frequency 

was estimated using the Cunnane (1978) plotting position 

formula as presented in Helsel and Hirsch (2002):

F(i) = (i – 0.4)/(n + 0.2) (1)

where

F(i) is the exceedance frequency of observation i,

i is an observation, and 

n is the total number of observations.

Estimations of total SSD and average annual SSC for 

water year 2011 were available in NWIS for all streamgages 

except Kansas City. Total SSD for water year 2011 was 

estimated for the Kansas City streamgage using the methods 

outlined in Heimann and others (2010). Records of annual 

total SSD for water years before 2011 were available for the 

selected streamgages from NWIS, Heimann and others (2010), 

and Heimann and others (2011). 

Suspended-Sediment Concentration 

Measured SSC in the lower Missouri River varied from 

approximately 150 milligrams per liter (mg/L) to 2,000 mg/L 

from January 1 through September 30, 2011 (fig. 2). Median 

SSC increased in the downstream direction from 355 mg/L at 

Sioux City to 490 mg/L at Hermann. The highest concentrations were measured downstream from Omaha in late February when snowmelt runoff from tributaries draining zones of 

high sediment production was entering the lower Missouri 

River, and releases of water from Gavins Point Dam were 

small. Variability in SSC, shown by the interquartile ranges of 

measured SSC (fig. 2), also increased in the downstream direction, except at Hermann.

From early March to late May, 2011, sediment concentrations were variable, and frequently exceeded the study-period 

median, as spikes in tributary runoff intermittently discharged 

along the length of the lower Missouri River. Measurements 

of SSC were most variable at Kansas City, where the effects 

of several upstream tributaries, notably the Platte (Missouri) 

and Kansas Rivers, caused SSC to vary from approximately 

200 mg/L to 1,000 mg/L throughout the spring. Near the end 

of May, despite a spike in inflows from tributaries, measured 

SSC at all selected streamgages was lower than the intermittently high SSC observed during the winter and early spring. 

The lower magnitude of spikes in SSC likely was a result of 

a combination of progressive increases in the magnitude of 

clear-water releases at Gavins Point Dam as well as natural 

sediment-supply limitations on the landscape caused by vegetation and crop re-growth in late spring. 

In late May, releases at Gavins Point Dam began to 

increase more rapidly and continued to increase until June 

5, when releases sharply increased, reaching approximately 

150,000 ft3

/s by June 16, and 160,000 ft3/s by June 26 (fig. 

2). From early June through the end of September, releases 

at Gavins Point Dam constituted most of the streamflow 

observed at Hermann (fig. 2). The large magnitude increase 

in releases from Gavins Point Dam in early June caused 

small rises in SSC along the lower Missouri River, ranging from 300 to 600 mg/L above the station median (fig. 2). 

From mid-June through August 20, while releases at Gavins 

Point Dam were sustained at or above 150,000 ft3/s, measured 

SSC steadily decreased at all selected streamgages to levels 

between approximately 115 to 250 mg/L below observed 

station-median values. The combination of dilute releases at 

Gavins Point Dam, and stable, low-flows in tributaries caused 

sustained lowering of SSC at all streamgages from early July 

through late August, despite the high-magnitude streamflows 

along the main stem. SSC observations at most streamgages 

increased slightly and temporarily when releases at Gavins 

Point Dam began to decrease in late August, and again in midSeptember. The cause for the increase in SSC magnitude likely 

is either the reduced dilution of a base-level sediment concentration coming from tributaries, or enrichment with sediments 

from drainage and gullying of flooded areas along the valley 

bottom (Jacobson and Oberg, 1997). 

Washload

Measured PW of suspended sediment ranged from 

5 percent to as much as 98 percent in the lower Missouri 

River from January 1 to September 30, 2011 (fig. 3). Median 

PW increased in the downstream direction from 24 percent 

at Sioux City to 78 percent at Hermann. The total range of 

observed PW generally increased from upstream to downstream from 40 to 65 percent, but a large range of 59 percent also was observed at Nebraska City, where PW values 

likely were affected by sediment inputs from the Platte River 

of Nebraska. Measurements made in early January, when 

SSC was low, indicate that suspended sediment mostly was 
Figure 2. Frequency distribution of selected measurements of suspended-sediment concentration at 

streamgages along the lower Missouri River, and daily streamflow of major tributaries upstream from each 

measurement location, January 1 to October 1, 2011. Data for releases from Gavins Point Dam from U.S. Army 

Corps of Engineers.

40,000

30,000

20,000

10,000

0

40,000

30,000

20,000

10,000

0

40,000

30,000

20,000

10,000

0

40,000

30,000

20,000

Streamflow, in cubic feet per second

Suspended-sediment concentration, in milligrams per liter

10,000

0

80,000

60,000

40,000

20,000

0

80,000

2,000

1,500

1,000

500

0

2,000

1,500

1,000

500

0

2,000

1,500

1,000

500

0

2,000

1,500

1,000

500

0

2,000

1,500

1,000

500

0

2,000

1,500

1,000

500

0

2,000

1,500

1,000

500

0

60,000

40,000

20,000

0

300,000

200,000

250,000

150,000

100,000

50,000

0

Jan. 1 Feb. 1 Mar. 1 Apr. 1 May. 1 June 1

January 1 to October 1, 2011

July 1 Aug. 1 Sept. 1 Oct. 1

Missouri River at Sioux City (06486000)

Missouri River at Omaha (06610000)

Missouri River at Nebraska City (06807000)

Missouri River at St. Joseph (06818000)

Missouri River at Kansas City (06893000)

Missouri River at Hermann (06934500)

Maximum

EXPLANATION

Percent washload measurements

Minimum

Median

75th percentile

25th percentile

Interquartile

range

Big Sioux River

(06486000)

Little Sioux River

(06607500)

Platte River (0605500)

Nishnabotna River

(06810000)

Kansas River (06892350) Platte River (06821190)

Osage River

(06926510)

Gavins Point Dam releases 

Grand River

(06902000)

Missouri River 

at Hermann, 

Missouri

(069345000)

James River

(06478513)

A

B

C

D

E

F

G

8 Sediment Transport and Deposition in the Lower Missouri River During the 2011 Flood
Sediment Transport and Deposition in the Lower Missouri River in 2011 9

composed of bed material (PW ranged from 5 to 33 percent), 

although observations supporting this interpretation are 

limited. In mid-February, snowmelt runoff from the plains 

began to contribute streamflow to the lower Missouri River, 

and PW increased at most streamgages, ranging from 20 to 

85 percent, indicating that early spring runoff was carrying 

mainly washload into the lower Missouri River. At Sioux City, 

PW increased only slightly through March, but spiked to a 

maximum observed value of 45 percent when runoff from the 

James River also spiked. Downstream from Nebraska City, 

increases in PW during snowmelt runoff were only observed at 

Hermann, but PW increased from a low of 33 percent in early 

January, to 80 percent by the end of February. Although no 

observations were made before snowmelt runoff at St. Joseph 

and Kansas City, both stations had high PW in late February, 

78 and 85 percent respectively, and these values coincided 

with spikes in runoff from the Nishnabotna River and Platte 

River (of Missouri).

After spring runoff began, PW was variable at most 

stations, fluctuating in response to spikes in tributary streamflows as well as clear-water releases from Gavins Point Dam 

(fig. 3). Measured PW decreased by 16 to 55 percentage 

points as releases of water from Gavins Point Dam abruptly 

increased in early to mid-June, which coincided with more 

subtle decreases in SSC; however, after this initial decrease, 

observed levels of PW rebounded. This rebound of PW was 

most apparent in the observations at Omaha, Nebraska City, 

St. Joseph, and Kansas City, and likely was the result of finesediment inputs from the Big and Little Sioux Rivers and the 

Platte River (Nebraska). From late July through mid-August, 

decreases in PW ranging from 26 to 50 percentage points 

were observed at Omaha, Nebraska City, Kansas City, and 

Hermann, reflecting a reduction of tributary contributions, and 

the sustained high releases at Gavins Point Dam. After August 

20, 2011, releases at Gavins Point Dam began to decrease, and 

observed levels of PW increased by 8 to 26 percentage points. 

The increase in PW likely was caused by a combination of 

decreased streamflow intensity reducing suspension of coarser 

bed material, as well as contributions of mud from drainage of 

adjacent flood plains. 

Suspended Load

Total SSD at the selected streamgages in the lower Missouri River during water year 2011 ranged from approximately 

29 million tons at Sioux City to 64 million tons at Kansas City 

(table 1). At Sioux City, total SSD for 2011 was the largest 

magnitude in the 33 years of published estimates. More than 

630 miles downstream at Hermann, Mo., the total SSD was 

estimated at approximately 57 million tons, or nearly double 

the load estimated at Sioux City. Relative to the respective 

periods of record, total estimated SSD had the lowest exceedance frequencies (low exceedance frequency indicates an 

uncommon occurrence; a high exceedance frequency indicates 

a more common occurence) in the reaches between Gavins 

Point Dam and Nebraska City (2 to 21 percent), but increased 

substantially downstream (25 to 65 percent). Annual total 

SSDs with low exceedance frequencies were reported at Sioux 

City, Omaha, and Nebraska City in 2011 despite the moderateto-high exceedance frequencies for 2011 average SSC (38 to 

84 percent), indicating that the duration of high-magnitude 

flooding, as opposed to SSC, was the primary driver of total 

SSD in 2011. 

The dilute nature of the 2011 floodwater similarly is 

reflected in a longitudinal comparison of the exceedance frequencies of annual average streamflow and annual total SSD. 

Exceedance frequencies of annual average streamflow in 2011 

in the lower Missouri River from Sioux City to Hermann were 

all approximately within the highest percentile of streamflow 

for the period of record (table 1). Despite the small exceedance frequencies of the 2011 average streamflow throughout 

the system, exceedance frequencies of total SSD increased 

from 2 percent at Sioux City to 65 percent at Hermann. Likewise, exceedance frequencies of annual average and annual 

peak streamflows in tributaries generally increased in the 

downstream direction, and those downstream from Nebraska 

City generally were high or very high, indicating that many 

tributaries were not contributing substantial amounts of additional water (or sediment) relative to a typical high-streamflow 

year on the lower Missouri River. The median exceedance 

frequency of 2011 annual average streamflow in tributaries 

between Gavins Point Dam and Nebraska City was 5 percent, 

and ranged from 2 to 11 percent; median annual peak streamflow exceedance frequency was 34 percent, and ranged from 2 

to 55 percent. Downstream from Nebraska City, the 2011 

annual average streamflow in tributaries had a median exceedance frequency of 34 percent, and ranged from 15 to 64 percent; annual peak streamflow of these tributaries had a median 

exceedance frequency of 39 percent, and ranged from 5 to 

59 percent.

Although annual total SSD approximately doubled from 

Sioux City to Hermann, substantial along-stream variation 

indicates incongruities in the net sediment balance. Between 

Nebraska City and St. Joseph, total SSD for 2011 decreased 

by approximately 7.5 million tons, indicating net sediment 

deposition between streamgages. Such deposition is feasible between these streamgages because, as indicated in a 

subsequent section of this report (see “Patterns of Sediment 

Deposition Along the Lower Missouri River during the Flood 

of 2011”), the segment of the lower Missouri River between 

Omaha and St. Joseph was a zone of substantial overbank 

flooding and sand deposition. Although the area of the lower 

Missouri River upstream from Omaha also was a zone of 

substantial overbank streamflow and sediment deposition, 

the influx of sediment from the Little Sioux and Platte Rivers 

likely contributed to the net increase in sediment discharge 

between the Sioux City and Nebraska City streamgages. An 

increase in total SSD of nearly 30 million tons was estimated between St. Joseph and Kansas City. Although such 

an increase is substantial, data from Heimann and others 

(2010) indicate that the Kansas River may contribute more 

than 20 million tons of suspended sediment to the lower 
Figure 3. Frequency distribution of measured percentage of suspended-sediment as washload (percentage 

of suspended-sediment mixture finer than 0.0625 millimeter in diameter) at selected streamgages along the 

lower Missouri River and daily streamflows of major tributaries upstream from each measurement location, 

January 1 to October 1, 2011. Data for releases from Gavins Point Dam from U.S. Army Corps of Engineers.

Missouri River at Sioux City (06486000)

Missouri River at Omaha (06610000)

Missouri River at Nebraska City (06807000)

Missouri River at St. Joseph (06818000)

Missouri River at Kansas City (06893000)

Missouri River at Hermann (06934500)

Percent washload measurements

40,000

30,000

20,000

10,000

0

40,000

30,000

20,000

10,000

0

40,000

30,000

20,000

10,000

0

40,000

30,000

20,000

Streamflow, in cubic feet per second

Percent of suspended-sediment finer than 0.0625 millimeters

10,000

0

80,000

60,000

40,000

20,000

0

80,000

100

80

60

40

20

0

100

80

60

40

20

0

100

80

60

40

20

0

100

80

60

40

20

0

100

80

60

40

20

0

100

80

60

40

20

0

100

80

60

40

20

0

60,000

40,000

20,000

0

300,000

200,000

250,000

150,000

100,000

50,000

0

Jan. 1 Feb. 1 Mar. 1 Apr. 1 May. 1 June 1

January 1 to October 1, 2011

July 1 Aug. 1 Sept. 1 Oct. 1

Maximum

EXPLANATION

Minimum

Median

75th percentile

25th percentile

Interquartile

range

Big Sioux River

(06486000)

Little Sioux River

(06607500)

Platte River (0605500)

Nishnabotna River

(06810000)

Kansas River (06892350) Platte River (06821190)

Osage River

(06926510)

Gavins Point Dam releases 

Grand River

(06902000)

Missouri River at Hermann, 

Missouri (069345000)

James River

(06478513)

A

B

C

D

E

F

G

10 Sediment Transport and Deposition in the Lower Missouri River During the 2011 Flood
Sediment Transport and Deposition in the Lower Missouri River in 2011 11

Missouri River when its annual instantaneous peak streamflow 

approaches 80,000 ft3/s. Thus, such an increase in total SSD is 

feasible for the St. Joseph-to-Kansas City reach and within the 

confidence bounds of previous annual estimates of total SSD 

for the Kansas River (Heimann and others, 2010). Between 

Kansas City and Hermann, total SSD in the lower Missouri 

River in 2011 decreased by approximately 6.5 million tons, 

and this decrease likely was due to a combination of deposition between streamgages (either on low-lying regions of the 

flood plain or aggradation on the riverbed), uncertainties in the 

total SSD estimation at Kansas City, or both. 

Comparison of Sediment Transport in 2011 with 

Other Recent Floods

The geography of tributaries contributing to flooding in 

a large river system is important to the understanding of sediment transport and deposition, because spatial variations in 

geology and climate affect the texture and erodibility of soils, 

and infrastructure, such as dams, channel engineering, and 

levees, may affect sediment routing through a river network 

(Schmidt and Wilcock, 2008; Meade and Moody, 2010). One 

of the primary goals of the study described in this report was 

to place the characteristics of sediment transport during the 

flood of 2011 in the context of previous floods, and of particular interest were floods with differing floodwater geography. 

Sediment transport in the lower Missouri River in 2011 was 

contextualized using two primary approaches: (1) comparison 

of SSC at selected locations with the recent period of record, 

and (2) comparison of floodwater geography and total SSD 

with two other recent floods in the lower Missouri River, 1993 

and 2008. 

Comparison of sediment transport in the lower Missouri 

River during 2011 with other periods in the available period 

of record requires precaution because streamflow-adjusted 

sediment loads decreased abruptly after the construction of 

the MRMRS (Meade and Moody, 2010; Heimann and others, 

2011), and have been progressively decreasing during the past 

three decades (Heimann and others, 2010). Thus, to reduce the 

trend effect of declines in magnitude of streamflow-weighted 

SSC, only the 20-year period of record preceding water year 

2011 was selected for comparison (hereinafter referred to as 

“the selected period of record”). Suspended-sediment concentrations in 2011 and for the selected period of record were 

divided into two primary groups for comparison within ranges 

of streamflow magnitude concurrent with SSC sampling. 

High-action streamflows (group 1) were defined as flows that 

would be likely to generate a river stage that exceeded the 

National Weather Service’s (NWS) “action stage.” In 2011, 

the NWS action stage was between 2 and 4 ft below the NWS 

“flood stage” at the selected streamgages, and represents a 

trigger point at which flood mitigation and preparations begin 

(http://w1.weather.gov/glossary/index, accessed December 

12, 2012). Low-action streamflows (group 2) were defined 

as streamflows less than those producing the NWS action 

stage. Suspended-sediment concentration measurements and 

SSD records for the Sioux City, Omaha, and Nebraska City 

streamgages were retrieved from NWIS (http://waterdata.

usgs.gov/nwis, accessed December 12, 2012). Suspendedsediment concentration measurements and SSD records for the 

St. Joseph, Kansas City, and Hermann streamgages were taken 

from Heimann and others (2011) and Heimann and others 

(2010) for the period of record before water year 2009, and 

retrieved from NWIS for water years 2010 and 2011. Examination of streamflow measurements from the 1991 to 2011 

water years indicated that, within an approximate 10-percent 

margin, a streamflow of approximately 70,000 ft3

/s would 

be likely to trigger the NWS action stage at the Sioux City, 

Omaha, Nebraska City, and St. Joseph streamgages, and a 

streamflow of approximately 150,000 ft3/s would potentially 

trigger the NWS action stage at the Kansas City and Hermann 

streamgages. These estimates were conservative by design 

because the streambed elevation at several streamgages has 

changed during the past several decades, precluding the identification of a single, temporally stable streamflow value corresponding to an action stage. After SSC data were categorized, 

a nonparametric multiple-comparison test (Tukey distribution, 

alpha=0.05; Helsel and Hirsh, 2002; Lorenz and others, 2011) 

was conducted on the data ranks to test for statistical 

significance of differences in median SSC among the four 

data groups: (1) low-action streamflows, 1991–2010; 

(2) high-action streamflows, 1991–2010; (3) low-action 

streamflows, 2011; and (4) high-action streamflows, 2011. To 

reduce the magnitude of imbalance in sample sizes between 

groups, any group with a sample size greater than 3 times 

larger than the largest sample size from its corresponding 

2011 group was reduced using a subsampling procedure that 

reduced that sample size to 30 based on the quantiles of the 

distribution (see Helsel and Hirsh, 2002, p. 44). 

Total suspended sediment discharge in the lower Missouri River during water years 1993 and 2008 were chosen for 

direct comparison with SSD for 2011. These two years were 

chosen because the geography of floodwaters were well documented (Parrett and others, 1993; Holmes, 1996; Holmes and 

others, 2010), the geographies of floodwater differ from those 

of 2011, and 1993 and 2008 annual average and annual peak 

streamflows at the selected streamgages differed from those 

in 2011. In contrast to 2011, floodwater in the lower Missouri 

River in 1993 and 2008 originated primarily downstream from 

the MRMRS (Wahl and others, 1993; Holmes and others, 

2010). Floodwater geography was characterized for each water 

year by examining the percent contribution of tributaries to the 

annual average streamflow of the lower Missouri River at Hermann. Longitudinal differences in total SSD in the lower Missouri River were examined by comparison of the exceedance 

frequencies of annual average and annual peak streamflows 

at the selected main-stem streamgages and streamgages 

in major tributaries. Exceedance frequencies for annual average and annual peak streamflows were calculated using equation 1. Annual average and annual peak streamflow records 

were retrieved from NWIS for the selected main-stem and 
Table 1. Magnitude and exceedance frequency of annual average and peak streamflows, annual average suspended-sediment concentration, and 

sediment discharge for selected main-stem and tributary streamgages, lower Missouri River, water year 2011.[USGS, U.S. Geological Survey; No., number; n, number of observations in record up to and including water year 2011; ft3/s, cubic feet per second; Exc. Freq., estimated exce

Army Corps of Engineers; %, percentage of years within indicated period of record that a magnitude was equaled or exceeded. A larger percentage indicates a more common m

indicates less common magnitude; conc., concentration; mg/L, milligrams per liter; --, data not available; total suspended-sediment discharge, total mass of sediment passing a shown]

USGS 

station 

numbera

Station name Locationb

Annual average streamflow Annual peak streamflow

Annual average 

suspended-sediment 

concentrationd

n

Flow 

(ft3/s)

Exc. freq.c 

(%) n

Flow 

(ft3/s)

Exc. freq.c

(%) n

Conc. 

(mg/L)

Exc. freq.c

(%) 

USACEe Gavins Point Dam 811.1 45 65,600 1 45 160,700 1 -- -- -- 

06478513f James River near Scotland, South Dakota 800.6 83 5,800 2 83 28,400 2 -- -- -- 06485500 Big Sioux at Akron, Iowa 734.5 83 5,800 2 83 19,700 28 -- -- -- 06486000 Missouri River at Sioux City, Iowa 732.4 45 79,100 1 45 192,000 1 33 330 38 

06607500 Little Sioux River near Turin, Iowa 668.9 53 3,300 9 72 13,400 40 --

06610000 Missouri River at Omaha, Nebraska 616.0 45 86,400 1 45 217,000 1 45 440 84 

06805500 Platte River at Louisville, Nebraska 595.5 58 11,500 11 59 45,400 55 --

06807000 Missouri River at Nebraska City, 

Nebraska

562.6 45 96,700 1 45 229,000 1 40 440 84 

06810000 Nishnabotna River at Hamburg, Iowa 543.3 84 2,000 15 85 16,800 49 -- -- -- 

06817000 Nodaway River at Clarinda, Iowa 462.7 78 560 24 84 30,000 5 -- -- -- 06818000 Missouri River at St. Joseph, Missouri 448.2 45 104,500 1 45 277,000 4 45 380 96 

06821190 Platte River at Sharps Station, Missouri 391.0 32 1,300 64 33 14,900 59 -- -- -- 

06892350 Kansas River at DeSoto, Kansas 367.2 94 5,600 58 38 80,200 23 -- -- -- 06893000 Missouri River at Kansas City, Missouri 366.1 45 109,300 1 45 245,000 12 38 590 83 

06902000 Grand River near Sumner, Missouri 250.2 87 3,800 52 91 53,000 59 -- -- -- 

06905500 Chariton River near Prarie Hill, Missouri 239.0 82 1,800 25 83 22,900 28 -- -- -- 

06926510g Osage River below St. Thomas, Missouri 130.4 80 10,400 51 80 58,600 54 -- -- -- 06933500 Gasconade River at Jerome, Missouri 104.9 89 3,700 19 93 85,100 8 -- -- -- 06934500 Missouri River at Hermann, Missouri 97.9 45 139,200 8 45 279,000 63 45 340 92 

aFor most stations this is the streamgage nearest the confluence with the Missouri River main channel (most downstream streamgage).bDistance in miles upstream from the confluence with the Mississippi River, based on unpublished data from U.S. Army Corps of Engineers.cCalculated using Cunane (1978) plotting position formula.dData shown for, at most, 1967 to 2011, the period after completion of the Missouri River Mainstem Reservoir System.eData provided by Dan Pridal (U.S. Army Corps of Engineers, written commun., 2012).fStation used has longest period of record but is not station nearest confluence with Missouri River.gValues from station combined with nearby discontinued station (06926500) to create longer period of record.hValue calculated using methods of Heimann and others (2010) and Dave Heimann (U.S. Geological Survey, written commun., 2012).
Sediment Transport and Deposition in the Lower Missouri River in 2011 13

tributary streamgages. For tributaries, with the exception of 

the James and Osage Rivers, the annual average and annual 

peak streamflow records for the entire period of record were 

selected and data for the most downstream streamgage were 

used. In the case of the James River, a streamgage near the 

mouth of the watershed (the James River at Scotland, S. Dak., 

USGS station no. 06478500, not shown on fig. 1) was used 

instead of the most downstream gage (the James River near 

Yankton, S. Dak., USGS station no. 06478513) because it had 

a much longer and continuous period of record. In the case of 

the Osage River, the annual average and annual peak streamflow records from two streamgages (the Osage River near St. 

Thomas, Mo., USGS Station no. 06926500, not shown on 

fig. 1, and the Osage River below St. Thomas, USGS Station 

no. 06926510) were combined because the two gages were 

near each other, and their combined periods of record did not 

overlap, but created an unbroken period of record from 1931 

to 2011. For Missouri River main-stem gages, only exceedance frequencies for the period from 1967 to 2011 were 

examined because of the pronounced change in hydrology 

associated with completion of the MRMRS in 1967. 

Median values of SSC from samples at selected 

streamgages along the lower Missouri River from 1991 to 

2010 indicate that SSC generally increased from upstream to 

downstream during low-action and high-action streamflow 

conditions. Median values of SSC for samples taken during 

low-action flows ranged from a low of approximately 190 

mg/L at Sioux City to a high of 490 mg/L at Kansas City, 

but decreased to 290 mg/L at Hermann (fig. 4). Similarly, 

median SSC for samples taken during high-action streamflows 

ranged from a low of approximately 530 mg/L at Sioux City 

to 1,340 mg/L at Kansas City but decreased to approximately 

1,120 mg/L at Hermann. Decreases in median sample SSC 

values at Hermann likely are associated with contributions 

of large volumes of more dilute water from the Ozark Plateaus by the Gasconade River and highly impounded Osage 

River, which flow into the lower Missouri River just upstream 

from Hermann. Differences between median values of SSC 

for samples taken during high-action streamflows and those 

taken during low-action streamflows for the selected period of 

record ranged from approximately 350 to 850 mg/L among the 

selected sites, and the interquartile ranges of the two sample 

groups show little (2 sites) to no distributional overlap (4 sites; 

fig. 4), indicating that differences in sample SSC between 

the two streamflow conditions likely are distinct. A multiplecomparison analysis indicated that the differences in sample 

median SSC between high-action and low-action streamflows 

for the selected period of record are significant statistically 

(p-values less than 0.001) for all streamgages where such 

analysis was considered valid (fig. 4).

Median values of SSC for samples during high-action 

streamflows in 2011 were greater than the median values for 

samples during low-action streamflows in 2011 for a few of 

the selected streamgages, and statistically significant differences in median values for both streamflow conditions were 

only detected at Hermann (p-value <0.01) (fig. 4), but median 

SSC for both streamflow conditions consistently were lower 

than the median values for high-action streamflows during 

the 20 years before. Similar to the 20 years before, median 

sampled SSC in 2011 during low-action streamflows generally increased from Sioux City to Kansas City, then decreased 

between Kansas City and Hermann; the spatial pattern for 

median sampled SSC in 2011 during high-action streamflows 

was not consistent with the 20 years before, showing decreases 

between Nebraska City and Kansas City, and increases 

between Kansas City and Hermann. Median SSC during 

low-action streamflows in the lower Missouri River in 2011 

ranged from a low of approximately 270 mg/L at Sioux City 

to a high of 460 mg/L at Kansas City. Median SSC during 

high-action streamflows ranged from a low of 380 mg/L at 

Sioux City to a high of 620 mg/L at Hermann (fig. 4). Differences between median values of SSC for low- and high-action 

streamflows at the selected streamgages in 2011 were small 

relative to the corresponding differences between the same 

streamflow groups for the 20 years before, ranging only from 

130 to 350 mg/L among the selected streamgages in 2011. 

The signature of reservoir decanting on SSC in the lower 

Missouri River in 2011 was evident in the contrasting frequency distributions of SSC for low- and high-action streamflows at the selected monitoring stations. Simple differences 

between median SSC for low- and high-action streamflows 

in 2011, when compared with the corresponding differences 

for the selected period of record, indicate that median SSC 

values for 2011 are, in most cases, closer to the median values 

for low-action streamflows for the 20 years before. Differences between median SSC for samples during high-action 

streamflows in 2011 and those for the selected period of record 

ranged from approximately 170 mg/L to 1,010 mg/L, whereas 

differences between high-action streamflow samples from 

2011 and samples during low-action streamflows for 

the selected period of record ranged from approximately 

0 mg/L to 340 mg/L. Multiple-comparison analysis indicated 

that median SSC values for low- and high-action streamflows 

sampled in 2011 at 4 of 6 monitoring stations were not significantly distinguishable from median SSC values for low-action 

streamflows during the select period of record (fig. 4). Similarly, multiple-comparison analysis indicated that median SSC 

for high-action streamflows sampled in 2011 were not significantly distinguishable from those of low-action streamflows 

sampled in 2011 at 4 of 5 streamgages where such analysis 

was considered valid. 

During the 20-year period of record preceding water year 

2011, the annual average streamflow of the lower Missouri 

River at Hermann, Mo., was 94,200 ft3

/s. On average, the 

largest contributions to the annual streamflow at Hermann 

are from major tributaries originating in the high sedimentproduction areas of the Great Plains and Central Lowland 

(37 percent), releases of water through Gavins Point Dam 

(28 percent), and small tributaries plus groundwater inflows 

(hereinafter referred to collectively as “small tributaries”) 

(19 percent) (fig. 5). In 2011, the annual average streamflow 

of the lower Missouri River at Hermann was 139,200 ft3

/s; the 
Langdon backwater on the Missouri River near Nemaha, 

Nebraska, looking upstream from the mouth, October 24, 2011. 

Photograph by Justin Krahulik, U.S. Geological Survey.

14 Sediment Transport and Deposition in the Lower Missouri River During the 2011 Flood

sustained, very large releases of water through Gavins Point 

Dam constituted approximately 47 percent of this streamflow, 

but no tributary downstream from the dam contributed more 

than 10 percent, and small tributary contributions accounted 

for only 13 percent. By contrast, in 1993 and 2008, years 

when heavy rains were concentrated in the downstream 

regions of the Missouri River Basin (Wahl and others, 1993; 

Holmes and others, 2010), releases of water from Gavins 

Point Dam accounted for only 7 and 12 percent, respectively, 

of the annual average streamflow at Hermann, reflecting the 

operation of the MRMRS for downstream flood control. Small 

tributary contributions accounted for approximately 25 percent 

of the average annual streamflow at Hermann in 2008 and 

1993, and major tributaries flowing through regions with high 

sediment production accounted for approximately 50 percent 

of the annual average streamflow at Hermann in 1993, and 

40 percent in 2008 (fig. 5). In 2011, major tributaries in high 

sediment-production regions accounted for a total of only 28 

percent of the average annual streamflow in the lower Missouri River at Hermann. 

Longitudinal comparison of streamflow and annual total 

sediment-discharge exceedance frequencies for the lower Missouri River in 2011 with corresponding frequencies for 2008 

and 1993 showed the important role of tributary contributions 

to total SSD of the lower Missouri River and the effect of 

clear-water releases on total SSD in 2011. Annual streamflow 

exceedance frequencies at the selected main-stem streamgages 

decreased from 74 percent at Sioux City to 1 percent at 

Hermann in 1993, and from 99 percent to 26 percent in 2008. 

Likewise, annual peak-streamflow exceedance frequencies at 

the selected main-stem streamgages in 1993 decreased from 

19 percent at Sioux City to 1 percent at Hermann, and from 

77 percent to 26 percent in 2008. Much of the downstream 

increase in streamflow in 1993 and 2008 was a consequence 

of heavy rains falling on saturated soils in watersheds of the 

Great Plains and Central Lowlands (Wahl and others, 1993; 

Holmes and others, 2010), producing streamflows of large 

magnitude and long duration in major tributaries to the lower 

Missouri River. These contributions are reflected in low 

exceedance frequencies of annual average and peak streamflows for major tributaries. In 1993, exceedance frequencies of 

annual average and peak streamflows in major tributaries were 

almost uniformly within the upper decile of streamflows for 

their respective periods of record (fig. 6), and annual average 

streamflows in 10 of 12 major tributaries were within the top 

3 percent. In 2008, tributary contributions were also important 

to the pattern of streamwise-decreasing exceedance frequencies of annual average streamflow along the lower Missouri 

River. Annual average and annual peak streamflows for 2008 

in the Platte (Nebraska), Nishnabotna, Nodaway, Platte (Missouri), Grand, Chariton, and Gasconade Rivers all had exceedance frequencies approximately within the upper quartile of 

streamflows for their respective periods of record ending in 

2011; annual peak streamflows in the Nodaway and Chariton 

Rivers were the highest on record.

In contrast with 1993 and 2008, exceedance frequencies of annual average and peak streamflows at the selected 

streamgages in 2011 increased from upstream to downstream. 

Exceedance frequencies of annual average streamflow in 2011 

were in the upper 1 percent during the post-dam period of 

record from Sioux City to Kansas City, and was 8 percent at 

Hermann. However, exceedance frequencies of instantaneous 

peak streamflows were in the highest-streamflow percentile 

during the post-dam period of record at Sioux City, Omaha, 

and Nebraska City, but exceedance frequency then increased 

to 12 percent by Kansas City and 63 percent at Hermann. The 

pattern of downstream increasing exceedance frequencies 

for streamflow highlights the effect of the lack of synchrony 

between reservoir-release flows and variable inflows from 

downstream tributaries in 2011. Exceedance frequencies of 
Figure 4. Frequency distributions of sampled suspended-sediment concentrations for selected streamgages 

along the lower Missouri River for low- and high-action streamflows in water year 2011, and in the preceding 

20 water years. High-action streamflows are defined as streamflows likely to trigger the National Weather 

Service action stage. Low-action streamflows are defined as streamflows likely to produce stages less than 

the National Weather Service action stage.

*

**

**

*

*

*

*

*

*

***

*

*

*

*

*

*

*

***

**

**

*

***

*

***

*

***

Missouri River at Sioux City, Iowa [U.S. Geological 

Survey (USGS) station number 06486000]

Missouri River at Omaha, Nebraska 

(USGS station number 06610000)

Missouri River at Nebraska City, Nebraska 

(USGS station number 0607000)

Missouri River at St. Joseph, Missouri 

(USGS station number 06818000)

Missouri River at Kansas City, Missouri 

(USGS station number 06893000)

Missouri River at Hermann, Missouri 

(USGS station number 06934500)

(39)

(19)

(34)

(30)

(51)

(37)

(23)

(58)

(30)

(30)

(27)

(12)

(28)

(4)

(30)

(14)

(30)

(30)

(30)

(37)

(30)

(3) N/A

N/A

(25)

(30)

0 500 1,000 1,500

Concentration of suspended sediment, in milligrams per liter

2,000 2,500 3,000

EXPLANATION

Median

75th percentile90th percentile

Sample size

10th percentile25th percentile

Interquartile

range

Water year 2011—Samples taken at high-action streamflows

Water year 2011—Samples taken at low-action streamflows

Water years 1991 to 2010—Samples taken at high-action streamflows

Water years 1991 to 2010—Samples taken at low-action streamflows

Median of distribution is significantly different than distribution of 

category with indicated color (based on nonparametric 

multiple-comparison analysis at 95-percent confidence level) 

Indicates statistical test not considered valid because of small 

sample size

N/A

Sediment Transport and Deposition in the Lower Missouri River in 2011 15
Figure 5. Relative contributions of tributary streamflow to annual average streamflow of the Missouri River at Hermann, Missouri 

(U.S. Geological Survey streamgage number 06934500), during water years 1993, 2008, and 2011, and average for the 20-year 

period-of-record preceding water year 2011, as percentage of total. Data shown for individual tributary basins are represented by 

most downstream streamgage on each tributary. Actual contributions may be different than shown.

A. 1993 annual average flow = 182,000 ft3/s B. 2008 annual average flow = 115,000 ft3/s

C. 2011 annual average flow = 139,200 ft3

/s D. 1991–2010 annual average flow = 94,200 ft3/s

Nodaway River (1%)

Platte River (3%) Nishnabotna River (3%)

Little Sioux River (3%)

Big Sioux River (3%)

James River (1%)

Gavins Point 

Dam (7%)

Gavins Point Dam (47%)

Small tributaries or

groundwater

accretion (25%)

Small tributaries or

 groundwater

 accretion (13%)

Gasconade River (2%)

Osage River 

(14%)

Chariton 

River (2%)

Grand River ( 10%)

Kansas River (17%)

Nodaway River (1%)

Platte River (2%) Nishnabotna River (3%)

Little Sioux River (2%)

Big Sioux River (2%)

James River (1%)

Gavins Point 

Dam (12%)

Small tributaries or

groundwater

accretion (25%)

Gasconade River (4%)

Osage River 

(18%)

Chariton 

River (3%)

Grand River (9%)

Kansas River 

(8%)

James River

(4%)

Big Sioux 

River (4%)

Little Sioux 

River (2%)

Platte River (8%)

Nishnabotna River

(1%) Platte River (1%)

Kansas River (4%)

Grand River (3%)

Chariton River (1%)

Osage River (7%)

Gasconade

River (3%)

Little Sioux River (2%)

Big Sioux River (2%)

Nishnabotna River 

(2%)

Platte 

River

(9%)

Platte River (2%)

Nodaway River 

(1%)

Kansas River (9%)

Grand River (6%)

Chariton River (2%)

Osage River 

(14%)

Gasconade River (3%)

Small tributaries or

groundwater

accretion (19%)

Gavins Point Dam 

(28%)

James River (2%)

Water originating upstream from and passing through 

Gavins Point Dam

Small tributary basins and/or groundwater accretions

Tributaries along South Dakota border

Tributaries along Nebraska border

Tributaries along Iowa border

Tributaries along Kansas border

 Tributaries along Missouri border

Cubic feet per second

Percent

ft3

/s

%

Ozark Plateaus

Great Plains/Central Lowlands

Great Plains/Central Lowlands/

Ozark Plateau Undifferentiated

Rocky Mountain Belt and 

Northern Great Plains

Landscape origin Confluence along Missouri River by State

EXPLANATION

Abbreviations

16 Sediment Transport and Deposition in the Lower Missouri River During the 2011 Flood
Sediment Transport and Deposition in the Lower Missouri River in 2011 17

annual average streamflows in tributaries draining snowmelt 

from mountains or northern Great Plains regions such as the 

James, Big Sioux, Little Sioux, and Platte Rivers ranged from 

2 percent to 11 percent for the respective periods of record, but 

increased substantially in major tributaries downstream from 

the confluence of the Platte River (Nebraska), ranging from 15 

to 64 percent (fig. 6). Potentially more important to sediment 

transport, exceedance frequencies of annual peak streamflows 

in major tributaries to the lower Missouri River were generally 

low in 2011, but 6 of 12 tributaries had exceedance frequencies of 40 percent or greater, and 4 had frequencies greater 

than 50 percent. 

Differences between relative streamflow contributions 

from Gavins Point Dam releases and downstream tributaries in 1993, 2008, and 2011 translated into real differences in 

the magnitude of total SSD along the lower Missouri River. 

In 1993 and 2008, the combination of smaller dam releases 

and greater downstream tributary contributions resulted in 

a 20-fold increase in annual total SSD from Sioux City to 

Hermann. In 2011, record releases at Gavins Point Dam 

were paired with tributary contributions that, although large 

between Gavins Point Dam and the Platte River (Nebraska), 

were less consequential downstream from there, and annual 

total SSD at Hermann was only twice that estimated for Sioux 

City. In 1993 and 2008, total SSD at Hermann was approximately 177 million tons and 89 million tons, respectively, and 

annual average streamflow was 182,800 ft3

/s and 114,600 ft3/s, 

respectively. In 2011, annual total SSD was approximately 

57 million tons, and annual average streamflow at Hermann 

was 139,200 ft3/s. Thus, in 1993, the lower Missouri River discharged 300 percent more suspended sediment with an average streamflow that was only 30 percent larger than in 2011, 

and in 2008 transported 50 percent more suspended-sediment 

with an annual average streamflow that was 20 percent lower 

than in 2011. Despite the much larger overall total sediment 

load of the 1993 flood relative to the 2008 and 2011 floods, 

Holmes (1996) indicated that the 1993 flood transported less 

sediment on a per-day basis than did other large floods in 1973 

and 1951. Thus, the comparisions reported herein further indicate declines in suspended-sediment transport of the Missouri 

River relative to its pre-development condition.

Patterns of Sediment Deposition Along the 

Lower Missouri River during the Flood of 2011

The purpose of estimating sand deposition along the 

flood plain of the lower Missouri River was to assess sediment-related geomorphic effects and damages. Sand deposition was estimated using analysis of multispectral satellite 

imagery collected in October and November 2011. Twenty-six 

10- or 20-meter resolution multispectral images obtained by 

the Système Pour l’Observation de la Terre (SPOT)-4 or -5 

satellites (data available from the U.S. Geological Survey—

Earth Resources Observation and Science Center) provided 

coverage of the entire 811 miles of the lower Missouri River 

when streamflows were entirely within the riverbanks. Sand 

was mapped using a supervised classification of image pixels 

(fig. 7), and confirmed by field visits. Exposed sand within the 

banks (sandbars) was excluded from the analysis. Computation of areal extent of sand deposits provides a conservative 

estimate of sand deposition because sand overlain by mud 

is excluded, and sand deposits are undermapped in wooded 

areas. Neither volume nor mass of sand was estimated; 

however, a similar analysis of deposition on the Mississippi 

River flood plain after the 1993 flood indicated that minimum 

detectable thickness of sand deposits was approximately 2 ft 

(Jacobson and Oberg, 1997). 

The distributions of sand deposits along the flood plain 

of the lower Missouri River also were analyzed in relation 

to distance from the banks of the main channel by summing 

the area of sand within progressively wider buffer-distance 

swaths calculated using digital map analysis software (ESRI, 

2010). Sand deposition accounting was subtotaled for seven 

discrete hydrologic segments, bounded by Gavins Point Dam 

and subsequent downstream tributaries: (1) Gavins Point Dam 

to near Ponca, Nebr. (hereinafter named “Gavins”); (2) Ponca 

to the confluence with the Big Sioux River (hereinafter named 

“Ponca”); (3) Big Sioux River to the confluence with the 

Platte River (hereinafter named “Big Sioux”); (4) the Platte 

River to the confluence with the Kansas River (hereinafter 

named “Platte”); (5) the Kansas River to the confluence with 

the Grand River (hereinafter named “Kansas”); (6) the Grand 

River to the confluence with the Osage River (hereinafter 

named “Grand”); and (7) the Osage River to the confluence 

with the Mississippi River (hereinafter named “Osage”).

The longitudinal distribution of sand deposition on the 

flood plain along the lower Missouri River in 2011 (fig. 8) 

indicates depositional patterns were affected by annual peak 

streamflow magnitude, channel capacity, flood-plain confinement, sediment supply, and connectivity between channel 

and flood plain (fig. 8). The spatial distributions of overbank 

flooding and sand deposition on the lower Missouri River 

flood plain in 2011 are consistent with transport by floodwater 

that originated mostly from Gavins Point Dam, but did not 

substantially overflow the banks of the Missouri River until 

downstream from Sioux City, Iowa. Overbank flooding and 

sand deposition increased downstream from Sioux City to 

a broad peak near Rulo, Nebr. (fig. 1) (approximately 

RM 498), and then decreased to be intermittently near 

unmeasurable levels (lower limit of quantification using stated 

method) downstream from Kansas City, Mo. This broadly 

defined distribution parallels the pattern of incision and aggradation downstream from Gavins Point Dam (fig. 8E), a pattern 

that is related strongly to flood-plain connectivity (Jacobson 

and others, 2009). 

The longitudinal variation in flood volume relative to 

channel capacity and magnitude of recent channel incision 

is evident from maximum flooded area and flood plain sand 

depositional area (fig. 8A, B). In the Gavins segment, despite 

the relatively low exceedance frequencies of 2011 annual peak 

streamflow magnitude, the incised, wide channel accommodated much of the floodwater, and overbank flooding was not 
Gavins

Point Dam

34

83

14

83 1

83

2

83

1

53

8

72

1

84

4

85

1

78

9

84

1

87

2

91

2

32

5

33

2

82

9

83

1

45

1

45

4

45

1

45

9

89

5

93

1

80

28

80

1

94

2

38

3

58

1

59 

James River

Big Sioux River

Little Sioux River

Nishnabotna River

Nodaway River

Platte River

Platte River

Osage River

Gasconade River

Kansas River

Grand River

Chariton River

53

33

17

45

14

40

1

45 4

45

2

38

12

45

1

45

32

45

4

45

43

45

8

45

74

45

19

45

12

83

9

83 20

83

33

83

12

53

41

72

3

84

8

85

3

78

1

84

3

87

13

91

17

32

26

33

3

82

1

83

1

45

1

45

4

45

1

45

5

89

3

93

8

80

42

80

29

94

80

38

16

58

15

59 

James River

Big Sioux River

Little Sioux River

Nishnabotna River

Nodaway River

Platte River

Platte River

Osage River

Gasconade River

Kansas River

Grand River

Chariton River

80

33

77

45

14

40

1

45

4

45

2

38

12

45

1

45

32

45

4

45

94

45

9

45

99

45

77

45

2

83

2

83 2

83

28

83

9

53

40

72

15

84

49

85

1

78

9

84

52

87

59

91

64

32

59

33

25

82

28

83

8

45

63

45

8

45

63

45

1

45

12

45

19

89

8

93

19

89

8

93

51

80

54

80

58

94

23

38

11

58

55

59 

James River

Big Sioux River

Little Sioux River

Nishnabotna River

Nodaway River

Platte River

Platte River

Osage River

Gasconade River

Kansas River

Grand River

Chariton River

2

33

8

45

21

40

63

45

65

45

65

45

25

38

1

45

4

45

1

45

1

45

1

45

1

45

1

45

1

45

Gavins

Point Dam Gavins

Point Dam

Total estimated suspended-sediment discharge,

 in millions of tons per year

EXPLANATION

0 20 50 100

Estimated average annual streamflow, 

 in thousands of cubic feet per second

Lower Missouri River Basin streamgages

Tributary gaging station

200

0 200

Annual average

streamflow percentile*

Number of years of

streamflow record

Annual average

streamflow percentile*

Number of years of

streamflow record

Annual average

streamflow percentile*

Number of years of 

peak-flow record

Annual average

streamflow percentile*

Number of years of 

peak-flow record

* Percentiles based on Cunnane (1978) plotting position formula

Annual sediment discharge 

percentile*

Number of years of record

A. 1993 B. 2008 

C. 2011

Yankton

Omaha

Nebraska City

St. Joseph

Kansas City

Hermann St. Louis

Yankton

Omaha

Nebraska City

St. Joseph

Kansas City

Hermann St. Louis

Yankton Sioux City

Omaha

Nebraska City

St. Joseph

Kansas City

Hermann St. Louis

Sioux City Sioux City

Figure 6. Annual average streamflow for the lower Missouri River and major tributaries and annual total suspended-sediment 

discharge for selected main-stem streamgages, water years 1993, 2008, and 2011. Estimates of average annual streamflow for 

tributaries are represented by most downstream streamgage, and may differ from actual magnitudes shown. Estimates are compiled 

from USGS National Water Information System; Heimann and others, 2010; Dave Heimann (U.S. Geological Survey, written commun., 

2012). Estimates of sediment discharge near St. Louis were assumed to be approximately equivalent to those at Hermann for 

illustrative purposes in this figure. 

18 Sediment Transport and Deposition in the Lower Missouri River During the 2011 Flood
Figure 7. Images showing method of mapping sand deposition and deposited sand on the flood plain of the lower Missouri River, 

upstream of Sioux City, Iowa, in Union County, Iowa after the flood conditions of 2011. A, 2006 natural-color aerial photograph. B, Postflood multispectral image. C, Same image showing areas mapped as new sand deposits in yellow from supervised classification.

A B C

0 0.5 1.0 MILES

0 1.0 KILOMETERS

Base image from the United States Department of 

Agriculture−National Agriculture Imagery Program, 2006

Base image is copyrighted from Centre National 

d’Etudes Spatiales, France, 2011

Base image is copyrighted from Centre National 

d’Etudes Spatiales, France, 2011

North North North

Sediment Transport and Deposition in the Lower Missouri River in 2011 19

extensive. Overbank flooded area and sand depositional area 

increased in the Ponca segment, where channel capacity is 

reduced relative to the upstream reach, because the riverbed 

has undergone less degradation. Most of the flood-plain inundation and sediment-deposition damage to agricultural fields 

occurred between RM 480 and 700, where 2011 annual peak 

streamflows had low exceedance frequencies, and where the 

lower Missouri River channel is less incised or has aggraded 

in recent decades (fig. 8E), allowing floodwater easier access 

to the flood plain. As channel capacity increased in the downstream direction, the relative magnitude of the flood decreased 

downstream, overbank flooding was less extensive, and sand 

depositional area consequently decreased. 

The spatial distribution of flood plain sand deposits also 

relates to the distribution of floodwalls and levees that mediate 

connection of the channel to the flood plain, and the broad, 

wave-like pattern of sand deposition is punctuated with local 

highs related to levee breaks (fig. 8D). Gaps in flood plain 

sand deposits exist between RM 720 and 735, coincident with 

flood walls protecting much of Sioux City, Iowa, and between 

RM 610 and 620, coincident with flood protection around 

Omaha, Nebr. Flood plain sand deposition also was affected 

by levee constrictions and levee breaks. Floodway constrictions by agricultural levees are highly variable. The distances 

between levees (or levees on one side and valley wall on the 

other) are relatively wide upstream from RM 620, whereas 

downstream from RM 620, overbank constricted widths 

decrease as a result of levee alignments close to the channel 

(fig. 8D). 

The typical morphologies of small and large levee-break 

complexes have been interpreted by previous investigators (Jacobson, 2003) to indicate that levee breaks involved 

exceedance of at least one of three geomorphic thresholds, 

and those breaks that most effectively transported sediment 

onto the flood plain exceeded all three. The first threshold to 

be overcome is the energy threshold, inherent in exceedance 

or failure of the levee, which can happen by various processes, 

including overtopping (design exceedance), geotechnical slope 

failure, lateral erosion, or liquefaction through sand boils 

(fig. 9A, B). A second geomorphic threshold is the erosion 

threshold. The erosion threshold is exceeded when streamflow is concentrated through the levee-break opening, often 

at locally steepened slopes, and streamflow velocities are 

sufficient to erode through the cohesive top stratum of alluvial 

deposits (fig. 9C). Typical Missouri River flood plain deposits have a silt-clay top stratum of variable thickness that also 

serves as productive agricultural soil. When the top stratum is 

eroded by hydrodynamic forces concentrated through a leveebreak, turbulent floodwater gains access to the underlying noncohesive sand that is eroded easily, creating extensive floodplain scours. The flood-plain scours can constitute substantial 

damage to flood plain cropland, soils, and infrastructure, and 

also deliver additional sediment for downstream transport. The 

third threshold, a connectivity threshold, is exceeded when 
Figure 8. Longitudinal distribution of sand deposition and overbank flooding along the lower Missouri River in 2011 and potential 

covariates: A, area of sand deposits on valley bottom; B, peak flooded area along valley bottom; C, width of valley bottom; D, 

confinement of main-stem channel between levees or between a levee and valley wall, locations of levee breaks, and locations of

levee-break complexes with ramp structures connecting the channel to the flood plain; E, change in Construction Reference Plane (a 

conceptual, water-surface-elevation plane for the streamflow having 75-percent exceedance frequency at all locations, based on U.S. 

Army Corps of Engineers, unpublished data) 1990–2005, showing areas of channel incision and aggradation (negative values indicate 

net degradation or incision), and cumulative downstream increase in drainage area, and cumulative total drainage area. River miles are 

based on unpublished data from the U.S. Army Corps of Engineers. 

Distance upstream from confluence with the Mississippi River, in river miles

Big Sioux Platte Kansas Grand Osage

* *** *** * * * Levee break locations

Gavins Ponca

800 700 600 500 400 300 200 100 0

Sand area

Flooded area

Valley width

Levee/valley confining width

Change in construction 

reference plane Cumulative drainage area

Levee-break complex with 

ramp structure

Gavins Big Sioux Platte Kansas Grand Osage

A Ponca

B

C

D

E

Distance upstream from confluence with the Mississippi River, in river kilometers

1,200 1,000 800 600 400 200 0

-20 200

400

600

-15

-10

-5

0

5

10

0

20,000

40,000

60,000

80,000

100,000

0

3

6

9

12

15

18

0

2,000

4,000

6,000

8,000

10,000

0

300

600

900

Sand area, in acres Flooded area, in acres Valley width, in miles

Levee confinement,

in feet

Distance in construction

reference plane 1990 to

2005, in feet

Discharge area, in thousands

of square miles

1,200

1,500

1,800

20 Sediment Transport and Deposition in the Lower Missouri River During the 2011 Flood

the scour hole downstream from the levee break has had sufficient time to erode upstream to connect to the main channel 

(fig. 9D). In this situation, the levee-break complex has created 

a ramp-like structure, allowing a direct path for suspended 

and bedload sediment to be transported from the main channel 

onto the flood plain. Approximately 10 levee-break complexes 

created by the 2011 flood on the lower Missouri River were 

interpreted to have progressed to this third stage (fig. 9). 

In the constricted reaches, flood plain sand deposits are 

mainly located near levee breaks. As many as 33 separate 

levee breaks were observed on the lower Missouri River in 

2011 (fig. 8D). Levee breaks concentrate streamflow, causing 

high current velocities across the flood plain, and creating 

distinctive sand splay deposits (fig. 10). The concentrated 

streamflow resulted in enhanced transport of sand onto the 

flood plain in these areas. Similar levee-break complexes 

were documented to have resulted from a large Missouri 

River/Mississippi River flood in 1993 (Jacobson and Oberg, 

1997; Schalk and Jacobson, 1997; Jacobson, 2003), where 

some levee breaks were shown to be particularly effective 

in transporting sediment from the main channel onto flood 

plains. For example, a large levee-break complex on the Missouri River near Hermann, Mo., deposited 4.4 million cubic 

yards (yd3

) of sediment, equivalent to 10–16 percent of the 
Overtopping

Slumping

Sand boils - undermining

Enhanced bed-material transport

Ramp

A

B

C

D Connection threshold: Ramp forms allowing 

sand to be transported from main channel to 

splay on flood plain

Erosion threshold: Cohesion of top 

stratum and undermining in scour 

Energy threshold: Potential 

energy from levee elevation

Figure 9. Conceptual diagrams showing progression of levee break: A, pre-levee-break situation, indicating variety of possible 

exceedance or failure mechanisms; B, levee height creates local potential energy; the levee creates a local threshold that, when 

exceeded, releases energy locally; C, an overtopped or failed levee concentrates streamflow, creating locally high velocities, resulting 

in flood-plain scour. Scour through a cohesive top stratum into underlying non-cohesive sand deposits is a second, erosional threshold 

that, once overcome, results in rapid upstream and downstream expansion of the flood-plain scour area; D, a third threshold, a 

connection threshold, happens when flood-plain scour extends upstream and intersects the main channel, resulting in direct, ramp-like 

access of suspended and traction sediment load onto the flood plain. 

Sediment Transport and Deposition in the Lower Missouri River in 2011 21

suspended-sediment load of the 1993 flood (Holmes, 1996; 

Schalk and Jacobson, 1997), and a complex on the Mississippi 

River near Miller City, Illinois, had a deposit of 10.7 million 

yd3

, or 22–36 percent of the Mississippi River suspended-sediment load (Jacobson and Oberg, 1997). These volumes indicate that levee-break complexes can create substantial sinks 

for sediment during large floods, and are a likely explanation 

for downstream decreases in total SSD in 2011, such as those 

between Nebraska City and St. Joseph, and between Kansas 

City and Hermann (table 1). During the 1993 flood, Holmes 

(1996) reported substantial decreases in the suspended-sediment load between the Missouri River at Hermann and the 

Mississippi River at St. Louis, and attributed the decreases to 

a combination of sediment deposition on the Missouri River 

flood plain at levee breaks and the lower overall sedimenttransport capacity of the Mississippi River at St. Louis.
Figure 10. Multispectral satellite image taken in November 2011 showing a typical leveebreak complex formed in the valley bottom of the lower Missouri River during overbank 

flooding in 2011. Labels in yellow indicate river miles upstream from the confluence with the 

Mississippi River at St. Louis, Missouri, and are based on unpublished data from the U.S. Army 

Corps of Engineers. 

Figure 11. Cumulative sand depositional area by distance away from channel boundary in the valley 

bottom of seven hydrologic segments of the lower Missouri River after overbank flooding in 2011. 

Hydrologic segments are named for the upstream-bounding tributary or, in the case of Gavins, a large, 

main-stem dam bounding the segment at the upstream end.

5 4 9

5 5 0

5 5 1

5 5 2

0 0.5 1.0 MILE

0 North 0.5 1.0 KILOMETER

Base image is copyrighted from Centre National 

d’Etudes Spatiales, France, 2011

Sand splay

Indicate river miles upstream from St. Louis, 

Missouri and are based on unpublished data 

from the U.S. Army Corps of Engineers

EXPLANATION 549

Distance from channel, in feet

0 2,000 4,000 6,000 8,000

Cumulative sand area, in proportion of total sand area

0

0.2

0.4

0.6

0.8

1.0

Hydrologic segment

EXPLANATION

Gavins

Ponca 

Big Sioux 

Platte 

Kansas 

Grand

Osage

22 Sediment Transport and Deposition in the Lower Missouri River During the 2011 Flood
Summary 23

Analysis of the calculated distance of sand deposits from 

the main channel also indicates the effect of levee breaks on 

flood-plain sedimentation in the lower Missouri River in 2011. 

Sand deposition extended furthest from the river channel in the 

Big Sioux, Platte, and Kansas hydrologic segments, and was 

nearest to the channel in the Gavins, Ponca, Grand, and Osage 

segments. This pattern may be the result of a combination of 

the concentration of streamflow and current velocities in levee 

breaks, which enabled advection of sand further across the 

flood plain, and the generally wider valley width of these three 

reaches (fig. 8C). Sand deposition in the Platte segment, which 

had the greatest number of levee breaks, extended farther 

away from the channel than in upstream reaches with nearly 

equivalent, or greater, overall depositional areas per mile. For 

example, in the Platte segment, only 50 percent of the total 

sand-depositional area was accounted for within 4,000 ft from 

the channel; however, at the same distance, approximately 70 

and 80 percent of total sand-depositional area was accounted 

for in the Ponca and Big Sioux segments, respectively (fig. 

11). The greater dispersion of sediment across the flood plain 

in the Platte segment relative to the Sioux and Ponca segments 

could also be the result of finer grain sizes in the suspendedsediment load being transported farther away from the channel; however, in the absence of grain-size data on the flood 

plain, this hypothesis could not be tested. 

Summary

Flooding in the Missouri River Basin in 2011 was caused 

by a combination of above-normal snowpack in headwater 

regions in the Rocky Mountains of Montana and Wyoming, 

near record snowfall and wet soil conditions on the plains, and 

record rainfall in May across the upper Missouri River Basin. 

Floodwater originating in these upper-basin regions then 

travelled through the series of large flood-control reservoirs, 

setting records for total runoff volume entering all six Missouri River main-stem reservoirs in 2011, forcing releases 

from the dams. The flooding lasted as long as 3 months, and 

releases at Gavins Point Dam near Yankton, S. Dakota, the 

most downstream of the series of six large dams on the Missouri River main stem, were the largest measured in magnitude and volume since operations began in 1955. Sediment 

caused substantial damage to agricultural fields, homes, and 

infrastructure along the valley bottom adjacent to the river. 

The U.S. Geological Survey (USGS), in cooperation with the 

U.S. Army Corps of Engineers, examined sediment transport 

and deposition in the lower Missouri River in 2011 to investigate how the geography of floodwater sources, coupled with 

the along-stream characteristics of civil infrastructure and 

valley-bottom topography, affected characteristics of 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 suspendedsediment transport at six streamgages along the length of the 

lower Missouri River: (1) Sioux City, Iowa; (2) Omaha, Nebr.; 

(3) Nebraska City, Nebr.; (4) St. Joseph, Mo.; (5) Kansas City, 

Mo.; and (6) Hermann, Mo. 

Investigation of sediment transport and deposition 

focused on three key questions related to the unique combination of floodwater geography, magnitude, and volume in the 

lower Missouri River in 2011: (1) What were the temporal 

and spatial characteristics of sediment transport in the lower 

Missouri River in 2011? (2) How did floodwater geography 

differ from other high streamflow years in the lower Missouri 

River and did the difference cause detectable differences in 

sediment transport relative to previous years? (3) How did the 

longitudinal variation in channel geomorphology and civil 

infrastructure interact with floodwater geography to affect 

the spatial distribution and style of sediment deposition in the 

lower Missouri River valley?

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 

through September 30, 2011. Median SSC increased in the 

downstream direction from 355 mg/L at Sioux City to 490 

mg/L at Hermann. The highest concentrations were measured 

downstream from Omaha in late February, when snowmelt 

runoff from tributaries draining zones of high sediment production was entering the lower Missouri River, and releases 

at Gavins Point Dam (Gavins Point Dam) were small. Variability in SSC also increased in the downstream direction, 

except at Hermann. From mid-June through August 20, while 

releases at Gavins Point Dam were sustained at or above 

150,000 cubic feet per second, measured SSC decreased at all 

selected streamgages to levels between approximately 115 to 

250 mg/L below observed station-median values. The combination of dilute releases at Gavins Point Dam and stable, low 

flows in tributaries caused sustained lowering of SSC at all 

streamgages from early July through late August, despite the 

high-magnitude streamflows along the main stem. SSC observations at most streamgages increased slightly and temporarily 

when releases at Gavins Point Dam began to decrease in late 

August, and again in mid-September. 

Measured percent washload (PW) of suspended sediment 

ranged from 5 percent 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 to 78 percent at Hermann. Measurements made in 

early January, when SSC was low, indicate that suspended sediment mostly was composed of bed material (PW ranged from 

5 to 33 percent). In mid-February, snowmelt runoff from the 

plains began to contribute streamflow to the lower Missouri 

River, and PW increased at most stations, ranging from 20 to 

85 percent, indicating that early spring runoff was carrying 

mainly washload into the lower Missouri River. After spring 

runoff began, PW was variable at most stations, fluctuating 

in response to spikes in tributary flows as well as clear-water 

releases at Gavins Point Dam.

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 
24 Sediment Transport and Deposition in the Lower Missouri River During the 2011 Flood

estimated SSD had the lowest exceedance frequencies in the 

reaches between Gavins Point Dam and Nebraska City, but 

exceedance frequencies increased substantially downstream. 

Annual total SSDs with low exceedance frequencies were 

reported at Sioux City, Omaha, and Nebraska City in 2011 

despite moderate-to-high exceedance frequencies for average 

SSC, indicating that the duration of high-magnitude flooding 

was the primary driver of total SSD. Exceedance frequencies 

of annual average streamflow in 2011 in the lower Missouri 

River from Sioux City to Hermann were all approximately 

within the highest percentile of streamflow for the period of 

record. Despite small exceedance frequencies for 2011 total 

streamflow throughout the system, exceedance frequencies of 

total SSD increased from 2 percent at Sioux City to 65 percent 

at Hermann.

The signature of reservoir decanting on SSC in the lower 

Missouri River in water year 2011 was evident in the frequency distributions of SSC for low- and high-action streamflows (respectively, streamflows producing stages lower and 

higher than the National Weather Service “action stages”) at 

the selected streamgages, relative to those for similar streamflows in the previous 20 years. Differences between median 

SSC for samples during high-action streamflows in 2011 and 

those for the selected period of record ranged from approximately 170 mg/L to 1,010 mg/L, whereas differences between 

high-action streamflow samples from 2011 and samples during 

low-action streamflows in the previous 20 years ranged from 

approximately 0 mg/L to 340 mg/L. Multiple-comparison 

analysis indicated that median SSC values for low- and highaction streamflows sampled in 2011 at 4 of 6 monitoring stations were not significantly distinguishable from median SSC 

values for low-action streamflows in the previous 20 years. 

Longitudinal comparison of streamflow and annual total 

sediment-discharge exceedance frequencies for 2011 with 

corresponding frequencies for 2008 and 1993 showed the 

important role of tributary contributions to annual total SSD 

and the effect of clear-water releases on total SSD in 2011. 

In 1993 and 2008, exceedance frequencies for annual average and annual peak streamflow decreased substantially from 

upstream to downstream. In contrast, exceedance frequencies 

for annual average and annual peak streamflow at the selected 

streamgages in 2011 increased from upstream to downstream. 

The pattern of downstream increasing exceedance frequencies 

for streamflow highlights the downstream diminishing effect 

of tributaries on 2011 flood flows in the lower Missouri River 

relative to releases at Gavins Point Dam. Exceedance frequencies of annual peak streamflows in major tributaries to the 

lower Missouri River were generally low in 2011, but 6 of 12 

major tributaries had exceedance frequencies of 40 percent or 

greater, and 4 had frequencies greater than 50 percent. Differences between relative streamflow contributions from Gavins 

Point Dam releases and downstream tributaries in 1993, 2008, 

and 2011 translated into real differences in total SSD along 

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 to Hermann. In 2011, releases at Gavins Point Dam were 

the primary source of floodwater, and annual total SSD at 

Hermann was only twice that estimated for Sioux City.

Sand deposition along the flood plain of the lower Missouri River was estimated to assess sediment-related geomorphic effects and damages. Sand deposition was estimated 

using analysis of multispectral satellite imagery collected in 

October and November 2011 along the entire 811 miles of the 

lower Missouri River. The distributions of sand deposits along 

the flood plain of the lower Missouri River also were analyzed 

in relation to distance from the banks of the main channel by 

summing the area of sand within progressively wider bufferdistance swaths, and subtotaled for seven discrete hydrologic 

segments. The seven hydrologic segments, bounded by Gavins 

Point Dam and selected downstream tributaries, were named 

for their upstream-bounding dam or tributary: (1) Gavins, 

(2) Ponca, (3) Big Sioux, (4) Platte, (5) Kansas, (6) Grand, and 

(7) Osage.

The spatial distributions of overbank flooding and sand 

deposition on the lower Missouri River flood plain in 2011 are 

consistent with transport by floodwater that originated mostly 

from Gavins Point Dam, but did not substantially overflow 

the banks of the Missouri River until downstream from Sioux 

City, Iowa. Overbank flooding and sand deposition increased 

downstream from Sioux City to a broad peak near Rulo, Nebr., 

and then decreased to levels near the lower limit of quantification downstream from Kansas City. Most of the flood-plain 

inundation and sediment-deposition damage to agricultural 

fields was observed between river miles 480 and 700, where 

2011 annual peak streamflows had low exceedance frequencies, and where the lower Missouri River channel is less 

incised or has aggraded in recent decades, allowing floodwater 

easier access to the flood plain. As channel capacity increased 

in the downstream direction, the relative magnitude of the 

flood decreased downstream, and overbank flooding was less 

extensive. Gaps in flood plain sand deposits exist between 

river miles 720 and 735, coincident with flood walls protecting 

much of Sioux City, Iowa, and between river miles 610 and 

620, coincident with flood protection around Omaha, Nebr. 

 Flood plain sand deposition also was affected by levee 

constrictions and levee breaks. In the constricted reaches, 

flood plain sand deposits were mainly located near levee 

breaks. The levee breaks concentrated streamflow, causing 

high local current velocities and enhanced transport of sand 

onto the flood plain. Sand deposition extended furthest from 

the river channel in the Big Sioux, Platte, and Kansas segments, and was nearest to the channel in the Gavins, Ponca, 

Grand, and Osage segments. Sand deposition in the Platte 

segment, which had the greatest number of levee breaks, 

extended further away from the channel than upstream reaches 

with nearly equivalent, or greater, overall depositional areas 

per mile. Approximately 10 levee-break complexes created by 

the 2011 flood on the lower Missouri River were interpreted to 

have developed when conditions exceeded three geomorphic 

thresholds, and water flowing through a levee break advected 

large volumes of sand onto the flood plain.
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Chief, USGS Office of Surface Water 

415 National Center 

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