Prepared in cooperation with the Lower Platte South Natural Resources District

Emergent Sandbar Dynamics in the Lower Platte 

River in Eastern Nebraska—Methods and Results 

of Pilot Study, 2011

Scientific Investigations Report 2013–5031

U.S. Department of the Interior

U.S. Geological Survey
Cover photograph. Sandbars in the lower Platte River near South Bend, Nebraska.
Emergent Sandbar Dynamics in the Lower 

Platte River in Eastern Nebraska—

Methods and Results of Pilot Study, 2011

By Jason S. Alexander, Devin M. Schultze, and Ronald B. Zelt

Prepared in cooperation with the Lower Platte South Natural Resources District

Scientific Investigations Report 2013–5031

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

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

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Suggested citation:

Alexander, J.S., Schultze, D.M., and Zelt, R.B., 2013, Emergent sandbar dynamics in the lower Platte River in

eastern Nebraska—Methods and results of pilot study, 2011: U.S. Geological Survey Scientific Investigations 

Report 2013–5031, 42 p. with appendixes.
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................4

Physical and Hydrologic Setting ........................................................................................................4

Definitions and Descriptions of Sandbars in the Lower Platte River...........................................5

Previous Studies ...................................................................................................................................5

Approach and Methods................................................................................................................................6

Hydrologic Context ...............................................................................................................................6

Field Data Collection and Analysis ....................................................................................................7

Sandbar Number and Area ........................................................................................................8

Field Data Collection ..........................................................................................................8

Data Analysis.......................................................................................................................8

Sandbar Height ..........................................................................................................................10

Field Data Collection ........................................................................................................10

Data Analysis.....................................................................................................................11

Sandbar Location.......................................................................................................................13

Field Data Collection ........................................................................................................13

Data Analysis.....................................................................................................................13

Quality Control.....................................................................................................................................13

Replicate Surveys of Sandbar Geometry...............................................................................13

Data Analysis.....................................................................................................................14

Detailed Surveys of Sandbar Topography .............................................................................14

Data Analysis.....................................................................................................................14

Emergent Sandbar Dynamics in the Lower Platte River.......................................................................15

Seasonal Differences in Number and Areas of Emergent Sandbars ........................................15

Seasonal and Longitudinal Changes in Emergent Sandbar Height............................................18

Seasonal Changes in Longitudinal Distribution of Emergent Sandbars....................................22

Quality Control.....................................................................................................................................24

Sandbar Area..............................................................................................................................24

Sandbar Height ..........................................................................................................................25

Limitations of the Study .....................................................................................................................26

Implications for Ecology and Further Geomorphic Study ............................................................27

Summary and Conclusions.........................................................................................................................28

References Cited..........................................................................................................................................31

Appendixes ...................................................................................................................................................35

Appendix 1. Estimates of Error in Measured Sandbar Height Associated with Errors in 

Estimation of Water Travel Time .........................................................................................36

Appendix 2. Validation of Predicted Bar Heights Using Observations of Inundation from 

Remote Time-Lapse Photographs ......................................................................................41
iv

Figures

1. Map showing location of lower Platte River study area, U.S. Geological Survey 

gaging stations, and reconnaissance time-lapse cameras, Nebraska...............................3

2. Hydrograph showing daily average discharge in calendar year 2011 and selected 

daily discharge statistics for Platte River at Louisville, Nebraska.......................................7

3. Schematic diagrams showing typical levels and shapes of emergent sandbars in 

lower Platte River, Nebraska, and associated point spacing methods for surveying 

sandbar height: (A) streamwise tapering, (B) rectangular, and (C) dissected and 

tapering ..........................................................................................................................................9

4. Graphs showing example of stage-correlation method used to estimate travel time 

of water between gaging stations by (A) comparison of stage hydrographs with 

various offset hydrographs; and (B) results from correlation analyses comparing 

downstream hydrograph with various offset hydrographs of upstream stage 

hydrograph...................................................................................................................................12

5. Aerial photograph map showing examples of point-and-transect spacing of detailed 

topographic measurements collected with a total-station system, summer 2011, lower 

Platte River, Nebraska ...............................................................................................................15

6. Box-and-whisker plots showing frequency distribution of emergent sandbar area 

by survey period and sandbar type, lower Platte River study segment, Nebraska, 

2011: (A) all bar types, (B) mid-channel sandbars, (C) island-attached sandbars, 

and (D) bank-attached sandbars .............................................................................................16

7. Box-and-whisker plots showing frequency distributions of differences in emergent 

sandbar area among collocated pairs of sandbars between survey seasons by 

sandbar type, lower Platte River, Nebraska, 2011: (A) spring–summer difference, 

(B) summer–fall difference, and (C) spring–fall difference.................................................17

8. Graph showing cumulative frequency distribution of measured sandbar heights 

from three survey periods, lower Platte River, Nebraska, 2011..........................................18

9. Graphs showing longitudinal distributions of measured sandbar heights from three 

survey periods in 2011 and river-channel morphometric characteristics, lower Platte 

River, Nebraska: (A) sandbar heights measured during the spring survey period; 

(B) sandbar heights measured during the summer survey period; (C) sandbar 

heights measured during the fall survey period; and (D) bank heights measured 

during the spring and summer survey periods, channel sinuosity, and unvegetated 

channel width..............................................................................................................................19

10. Box-and-whisker plots showing frequency distributions of measured sandbar 

heights above river stage at 4,000 cubic feet per second for three survey periods 

and different bar types, lower Platte River, Nebraska, 2011: (A–C) median measured 

sandbar heights, and (D–F) maximum measured sandbar heights....................................20

11. Bar charts showing longitudinal distribution of emergent sandbar area for three 

survey periods and net change in emergent sandbar area between river miles 3 

and 26, lower Platte River, Nebraska, 2011: (A) spring survey period,(B) summer

survey period, (C) fall survey period, and (D) difference in percent of total sandbar 

area by river mile between spring and fall survey periods .................................................22

12. Box-and-whisker plots showing frequency distributions of shifts in sandbar areal 

centroids among collocated sandbar pairs between three survey periods, for 

different sandbar types, lower Platte River, Nebraska, 2011: (A) between spring 

and summer survey periods, (B) between summer and fall survey periods, and 

(C) between spring and fall survey periods............................................................................23
v

13. Graph showing differences between original and replicate differential-grade 

global-positioning-system surveys of sandbar area, as percentage of original 

sandbar area, relative to sandbar size, lower Platte River, Nebraska, 2011 ....................25

14. Graphs showing quality-control data associated with spot measurements of 

sandbar height, lower Platte River, Nebraska, 2011: (A) original versus replicate 

ranked spot measurements of sandbar height, and (B) differences between ranked 

original and replicate spot measurements of sandbar height relative to sandbar 

area ...............................................................................................................................................25

15. Graphs showing cumulative frequency distribution of sandbar topography, medians, 

and ranges of spot measurements of sandbar height using the rapid-assessment 

survey method, and medians and ranges of heights of previously used tern nests 

on four sandbars of varying size, lower Platte River, Nebraska, 2011: (A) sandbar 

number 1, 3.4 acres; (B) sandbar number 2, 8.7 acres; (C) sandbar number 3, 

19 acres; and (D) sandbar number 4, 29.9 acres ...................................................................26

Tables

1. Ranges of daily discharge and stage range of Platte River at Louisville, Nebraska 

(USGS 06805500), during three survey periods, 2011..............................................................9

2. Number and total measured and estimated areas of emergent sandbars in the 

study segment of the lower Platte River, Nebraska, by bar type and survey period, 

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

3. Summary statistics of differences between measured median and maximum 

sandbar heights and results of Wilcoxon signed-rank comparison tests between 

survey periods at collocated sandbar pairs, lower Platte River, Nebraska, 2011 ...........21

4. Summary of replicate surveys of sandbar area measured using differential-grade 

global-positioning system, lower Platte River, Nebraska, 2011 ..........................................24

5. Differences between sandbar areas measured with differential-grade globalpositioning-system and using a total-station laser-surveying instrument, 

lower Platte River, Nebraska, 2011 ..........................................................................................24

6. Summary statistics of differences in measured sandbar height and results of 

Wilcoxon signed-rank comparison tests between replicate surveys using laserleveling method, lower Platte River, Nebraska, 2011............................................................27
vi

Conversion Factors and Datums

Inch/Pound to SI

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2)

acre 0.4047 hectare (ha)

acre 0.4047 square hectometer (hm2)

acre 0.00405 square kilometer (km2)

square mile (mi2) 2.590 square kilometer (km2)

Velocity

mile per hour (mi/hr) 0.44704 meter per second (m/s)

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

Pressure

pound per cubic foot (lb/ft3) 16.02 kilogram per cubic meter (kg/m3)

pound per foot-second (lb/ft-s) 14.59 Newton per meter-second 

(N/m-s)

Altitude or vertical coordinate information is referenced to the National Geodetic Vertical 

Datum of 1929 (NGVD 29). 

Altitude refers to distance above or below NGVD 29. 

NGVD 29 can be converted to North American Vertical Datum of 1988 by using the National 

Geodetic Survey Conversion utility available at http://www.ngs.noaa.gov/TOOLS/Vertcon/

vertcon.html.

Horizontal coordinate information is referenced to the Nebraska State Plane-Feet, North 

American Datum of 1983 (NAD 83).

Acknowledgments

The authors thank Meghan Sittler, Lower Platte River Corridor Alliance (LPRCA) Coordinator, and all members of the LPRCA for their continued support of this project. Thanks also 

goes to the Nebraska Game and Parks Commission for allowing river reconnaissance camera 

deployment on their observation tower at Platte River State Park, and to the Lower Platte South 

Natural Resources District for allowing a temporary stage gage to be deployed on the Lied 

Bridge.
Abstract

The lower Platte River corridor provides important 

habitats for two State- and federally listed bird species: the 

interior least tern (terns; Sternula antillarum athallassos) and 

the piping plover (plovers; Charadrius melodus). However, 

many of the natural morphological and hydrological characteristics of the Platte River have been altered substantially by 

water development, channelization, hydropower operations, 

and invasive vegetation encroachment, which have decreased 

the abundance of high-quality nesting and foraging habitat for 

terns and plovers. The lower Platte River (LPR), defined as 

103 miles (mi) of the Platte River between its confluence with 

the Loup River and its confluence with the Missouri River, 

has narrowed since the late-19th and early-20th centuries, 

yet it partially retains many geomorphologic and hydrologic 

characteristics important to terns and plovers. These birds nest 

on the sandbars in the river and along shorelines at sand- and 

gravel-pit lakes in the adjacent valley.

The need to balance continued economic, infrastructure, 

and resource development with the conservation of important physical and aquatic habitat resources requires increased 

understanding of the physical and biological dynamics of the 

lower Platte River. Spatially and temporally rich datasets for 

emergent sandbar habitats are necessary to quantify emergent sandbar dynamics relative to hypothesized controls and 

stressors. In cooperation with the Lower Platte South Natural 

Resources District, the U.S. Geological Survey initiated a pilot 

study of emergent sandbar dynamics along a 22-mi segment 

of the LPR downstream from its confluence with Salt Creek, 

near Ashland, Nebraska. The purposes of the study were to: 

(1) develop methods to rapidly assess sandbar geometries and 

locations in a wide, sand-bed river, and (2) apply and validate 

the method to assess emergent sandbar dynamics over three 

seasons in 2011. An examination of the height of sandbars 

relative to the local stage of the formative discharge event, and 

how subsequent river discharges, of both high and low magnitude, alter sandbar geometries and abundance within the LPR 

was of particular interest.

A “rapid-assessment” method was developed with the 

goal of characterizing the spatial distribution and habitatrelevant geometries of the complete population of sandbars 

along the study segment. Three primary measures were used 

to assess emergent sandbar dynamics in the study segment: 

sandbar area, sandbar height, and sandbar location. Data to 

derive these measures were collected during three, week-long 

survey periods in 2011, herein named “spring survey period,” 

“summer survey period,” and “fall survey period.” Emergent 

sandbars were grouped into one of three generalized types: 

(1) bank-attached, (2) island-attached, and (3) mid-channel. 

The area of 68 sandbars, with a total estimated emergent 

area of 360 acres, was surveyed or estimated during the spring 

survey period. A total of 105 sandbars, with total estimated 

emergent area of 315 acres, were surveyed or estimated during 

the summer survey period. Seventy-three sandbars, with a total 

estimated emergent area of 402 acres, were measured during 

the fall survey period. There were 17 collocated sandbars 

common to the spring and summer survey periods, 27 collocated sandbars common to summer and fall survey periods, 

and 21 collocated sandbars common to spring and fall survey 

periods.

Changes in sandbar area between seasonal survey periods, when measured for collocated sandbar pairs, were similar 

to the seasonal differences observed for the larger population 

of sandbars. Mid-channel sandbar pairs had a median decrease 

in emergent area of 4.3 acres between spring and summer 

survey periods. Bank-attached sandbar pairs had the widest 

range of areal change between spring and summer, with most 

bars eroding, but some bars undergoing net expansion of up to 

1.9 acres.

A comparison among survey periods of cumulative 

frequency distributions of measured sandbar heights indicated that the high platform of sandbars was higher during the 

spring survey period than during the subsequent summer and 

fall survey periods. Median sandbar height, indexed to the 

stage of 4,000 cubic feet per second, decreased from about 4.5 

to 3.0 feet (ft) between the spring and summer survey periods. 

Sandbar heights tended to follow a wavy, longitudinal pattern 

whereby certain reaches of the study segment consistently had 

more abundant and higher sandbars, and others had fewer, 

Emergent Sandbar Dynamics in the Lower Platte River 

in Eastern Nebraska—Methods and Results of Pilot 

Study, 2011

By Jason S. Alexander, Devin M. Schultze, and Ronald B. Zelt
2 Emergent Sandbar Dynamics in the Lower Platte River in Eastern Nebraska—Methods and Results of Pilot Study, 2011

lower sandbars. Large sandbars tended to be more abundant 

and higher between State Highway 50 and near Cedar Creek, 

and lower and less abundant in the first 3 mi upstream from 

State Highway 50.

The high platform on sandbars was between 0.5 and 2.0 ft 

higher during the spring survey period than for the subsequent 

summer and fall survey periods. Bank-attached and mid-channel sandbars generally had the highest group-median height 

and group-median of maximum height across all seasons. 

Group-median heights for all sandbar types measured during 

the spring survey period indicated that the high platform of 

most sandbars was about 2–3 ft lower than the estimated stage 

of the 2010 peak discharge of 136,000 cubic feet per second. 

Maximum measured sandbar heights from the spring survey 

generally were higher than the estimated stages of the 2011 

peak discharge, indicating that at least small parts of many 

sandbars were not inundated.

Comparisons among seasonal survey periods of ranked 

differences between collocated sandbar pairs further confirm 

that a lowering of sandbar heights followed the spring survey 

period. Median differences in sandbar median and maximum 

heights between the spring and summer survey periods indicated decreases of 0.83 and 0.81 ft, respectively, for collocated 

sandbar pairs. Median differences among collocated pairs in 

sandbar median and maximum heights between the summer 

and fall survey periods indicated a decrease of 0.09 ft for each 

statistic.

One of the primary motivations of the study was to 

characterize the height of sandbars relative to the stage of 

formative flows. Sandbar heights surveyed during the spring 

survey period indicated that the high platform of sandbars can 

be close to the maximum stage of formative flow events, but 

this height is rare, and more often the highest part of sandbars 

is 1–2 ft less than the formative stage. Although the methods 

used for the study differ from previous studies in the LPR, the 

estimates of bar heights are consistent with previous studies, 

which, based on estimations made for this report, indicated 

sandbars formed between 0.9 and 4.2 ft lower than their 

respective formative stages.

The streamwise distribution of the emergent sandbar area 

did not change substantially among the three survey periods. 

Between spring and fall survey periods, the median change 

in the fraction of total emergent sandbar area, tabulated by 

river mile, was a decrease of only 0.21 percentage point, and 

the interquartile range of such net changes was from -1.32 to 

1.06 percentage points.

Replicate differential-grade global-positioning system 

(DGPS) surveys of the sandbar area were performed on a total 

of 13 sandbars across much of the range of sandbar sizes present during the survey periods, and the mean relative difference 

in sandbar area between original and replicate DGPS measurements was 0.62 percent with all replicate measurements 

within 5 percent of the original survey. Results from signedrank comparisons of replicate pairs indicated that differences 

in area were not significant. Paired measurements of sandbar 

area made with a total-station system and the DGPS method 

indicated that sandbar areas obtained using the DGPS method 

likely are within 5 percent of those obtained using higherprecision methods.

Replicate surveys of sandbar heights using the rapidassessment method indicated that differences in measured 

sandbar heights were not significant, averaged -0.002 ft, and 

ranged from -1.15 to 0.48 ft for spot heights. Measured sandbar heights and tern and plover nest heights were compared 

with the cumulative frequency distribution of bar heights from 

detailed measurements using total-station surveys. Results 

indicated that bar-height values greater than the median height 

from the rapid-assessment survey method adequately represented the range of nest heights observed in 2011. With continued monitoring of sandbar dynamics, the hydrogeomorphic 

characteristics of areas of the LPR that consistently produce 

habitat favored by nesting colonies of terns and plovers 

potentially can be quantified and identified. Such analyses 

would be valuable for planning and permitting of future river 

corridor development, infrastructure, and construction, as well 

as for constructing or restoring persistent and successful tern 

and plover habitats in sand-bed rivers elsewhere on the Great 

Plains.

Introduction

The lower Platte River (LPR) corridor provides important 

habitats for two State- and federally listed bird species: the 

interior least tern (terns; Sternula antillarum athallassos) and 

the piping plover (plovers; Charadrius melodus). However, 

many of the natural morphological and hydrological characteristics of the Platte River have been altered substantially by 

water development, channelization, hydropower operations, 

and encroachment into the channel by vegetation (Williams, 

1978; Eschner and others, 1983; Johnson, 1994; National 

Research Council, 2005; Ginting and Zelt, 2008). These 

alterations have resulted in a decrease of nesting and foraging 

habitat suitable for terns and plovers through channel narrowing and decreased area of barren, emergent (above the water 

surface) mid-channel sandbars (National Research Council, 

2005). Terns and plovers nest annually on the sandbars in the 

river and along shorelines of the active and abandoned openpit sand- and gravel-mine lakes on the adjacent valley floor.

The LPR, defined here as the 103-mile (mi) segment of 

the Platte River between its confluence with the Loup River 

and its confluence with the Missouri River (fig. 1), has narrowed since the late-19th and early-20th centuries (Joeckel 

and Henebry, 2008), yet it partially retains many of the geomorphologic and hydrologic characteristics important to terns 

and plovers. The LPR corridor, the land region encompassing 

the LPR channel, floodplain, valley floor, and valley walls, 

is situated within the most populous counties of Nebraska. 

The valley floor of the LPR also is home to numerous farms, 

aggregate extraction operations, waterfront developments, and 

state parks, and is crossed by several railroads, state and interstate highways, and water courses. The LPR alluvial aquifer 
Introduction 3

is a primary source of drinking water for nearby cities and 

towns, including the two most populous cities of Nebraska: 

Omaha and Lincoln.

Balancing continued economic, infrastructure, and 

resource development with the conservation of important 

physical and aquatic habitat resources requires increased 

understanding of the physical and biological dynamics of the 

LPR. An understanding of the interactions of hypothesized 

natural controls (streamflow, sediment supply, river morphology, and ice effects) with anthropogenic stressors (bridges, 

hydropower operations, bank protection, and levees), and how 

these interactions may affect the locations, abundance, quality, and persistence of emergent sandbar habitats important 

to State- and federally listed species, is of particular concern 

to local stakeholders. To approach this complex problem, 

spatially and temporally rich datasets for emergent sandbar 

habitats are necessary to begin to quantify emergent sandbar 

dynamics relative to hypothesized controls and stressors. Terns 

and plovers use emergent sandbar habitats in other river systems across the Great Plains, and substantial efforts are being 

made nationally to restore habitats in rivers where dams and 

other river engineering have degraded the quality of sandbar 

habitat (U.S. Army Corps of Engineers, 2011; Lott and Willey, 

2012). Thus, an understanding of emergent sandbar dynamics in the LPR also helps inform restoration and preservation 

of emergent sandbar habitats in the region. The term “sandbar dynamics” is used in this report to refer to the temporal 

changes in the number, geometry, and spatial distribution of 

emergent sandbars within a river channel and as related to 

controlling factors.

In cooperation with the Lower Platte South Natural 

Resources District (LPSNRD), the U.S. Geological Survey 

(USGS) initiated a pilot study (hereinafter referred to as “the 

study”) of emergent sandbar dynamics along a 22-mi segment 

of the LPR. The purposes of the study were to (1) develop 

methods to rapidly assess sandbar geometries and locations 

in a wide, sand-bed river, and (2) apply and validate the 

method to assess emergent sandbar dynamics over three seasons within a calendar year. An examination of the height of 

0

0

5

5

10 MILES

10 KILOMETERS

North Platte River

Sout h Platte River Loup River

Elkhorn River Missouri River

Platte River

96°25’

41°05’

41°

Base from U.S. Geological Survey National Hydrography Dataset, 1999, 1:100,000

Universal Transverse Mercator projection, Zone 15

North American Datum of 1983 (NAD 83)

96°20’ 96°15’ 96°10’ 96°05’ 96° 95°55’

SAUNDERS SARPY COUNTY

COUNTY

Gretna

SOUTH DAKOTA

Platte River Basin

Enlarged area

NEBRASKA IOWA

KANSAS

COLORADO

WYOMING

MINNESOTA

CASS COUNTY

Ashland

South

Bend

North Platte

Columbus

Omaha

Louisville

La Platte

Lied Bridge

06805020 06805500

06805500

06805570

06801000

Platte River State Park

Missouri River

Cedar Creek

Salt

Creek

U.S. Geological Survey gaging 

station and number

Time-lapse camera location

EXPLANATION

50

80

75

Figure 1. Location of lower Platte River study area, U.S. Geological Survey gaging stations, and reconnaissance timelapse cameras, Nebraska.
4 Emergent Sandbar Dynamics in the Lower Platte River in Eastern Nebraska—Methods and Results of Pilot Study, 2011

sandbars relative to the local stage of the formative discharge 

event, and how subsequent river discharges, of both large and 

small magnitude, alter sandbar geometries and abundance 

within the LPR, was of particular interest.

Purpose and Scope

The purpose of this report is to describe the methods 

developed and used to collect data associated with emergent 

sandbars (hereinafter called “sandbars”), as well as the findings of the initial analyses of these data, for the 22-mi segment 

of the LPR downstream from its confluence with Salt Creek, 

near Ashland, Nebraska (hereinafter referred to as the “study 

segment”) (fig. 1). These data include surveyed geometries 

and spatial distribution of sandbars within the study segment 

during three data-collection periods, each associated with a 

typical open-water (no ice cover) season. Analyses include 

seasonal comparison of metrics of sandbar geometry, statistical analysis of seasonal paired geometries, calculation and 

presentation of metrics of sandbar spatial distributions, and 

statistical analysis and presentation of quality-control measures. Additional assessments of data quality are described 

in appendixes 1 and 2. The term “sandbar geometry” in this 

report refers to the planimetric area and characteristic height 

(height indexed to some datum) of individual sandbars. The 

term “reach” in this report refers to lengths of river, greater 

than 1 average channel width in length, but shorter than the 

study segment itself. The lowest 4 mi of the Platte River were 

not included in the study because the hydraulics in that segment are controlled intermittently by backwater effects from 

the Missouri River.

Physical and Hydrologic Setting

The Platte River Basin has its headwaters on the eastern 

slope of the Rocky Mountain region of Colorado and Wyoming, and traverses the Great Plains and Dissected Till Plains 

of Nebraska, draining about 86,000 square miles (mi2) along 

its course (Fenneman, 1928; Eschner and others, 1983) 

(fig. 1). The Platte River in eastern Nebraska flows through a 

broad valley that progressively narrows as it extends downstream, funneling into the Missouri River near La Platte, 

Nebraska (Elliott and others, 2009). The valley bottom of 

the Platte River in the 22-mi study area is bounded by the 

sedimentary bedrock bluffs of the eastern Platte River Gorge 

(Joeckel and Henebry, 2008), and the river here has less variation in channel width, greater sinuosity, and deeper flow than 

in its upstream segments bounded by its confluences with the 

Loup and Elkhorn Rivers and Salt Creek (Ginting and others, 

2008; Elliott and others, 2009). The slope of the Platte River 

in the study area averages 0.075 percent, the lowest grade of 

any segment of the Platte River in Nebraska (Bentall, 1991).

The flow of the LPR is dependent on the flows in several 

primary tributaries: the Loup River, which has a confluence 

with the LPR near Columbus; the Elkhorn River, which has a 

confluence with the LPR near Gretna; and, to a lesser extent, 

on flows in Salt Creek, a smaller tributary, which has a confluence with the LPR near Ashland (Bentall and Shaffer, 1979; 

Ginting and others, 2008) (fig. 1). Streamflows in the Loup 

and Elkhorn Rivers are substantially affected by groundwater seepage, which provides a steady base flow to the LPR 

even during dry periods (Bentall and Shaffer, 1979). Because 

neither of these tributaries has a large, main-channel floodcontrol dam or reservoir, general hydrologic measures—particularly the mean annual discharge and instantaneous annual 

maximum discharge—have been less affected in the LPR 

than those in the Platte River upstream from its confluence 

with the Loup River (Eschner and others, 1983; Ginting and 

Zelt, 2008; Dietsch and others, 2009). Like other major trunk 

streams flowing over the Great Plains and with headwaters in 

the Rocky Mountains, the LPR typically has two annual rises: 

a rise associated with Great Plains snowmelt and the breakup 

of river ice cover, and a rise associated with snowmelt in the 

mountains and spring rainfall on the plains. Streamflows in 

the LPR also tend to increase because of runoff associated 

with regional outbreaks of summer thunderstorms, and these 

increases often inundate sandbars and associated resident tern 

and plover nests (Parham, 2007; Brown and Jorgensen, 2008; 

Brown and others, 2011a). 

Although generalized flow measures in the LPR have 

been less affected by water development than in upstream 

segments (Ginting and Zelt, 2008), the intra-daily flows 

throughout the LPR are characterized by a ubiquitous diel 

wave that typically cycles over a 24-hour period. The wave 

cycle is the result of hydrocycling from power generation 

at a hydroelectric facility near Columbus, Nebraska (Loup 

River Public Power District, 2008). Power generation occurs 

generally during peak-demand periods, and the facility can 

release a maximum flow of 3,500 cubic feet per second (ft3/s); 

however, in combination with naturally occurring changes 

in flow magnitude, maximum diel flow range may be larger. 

The amplitude of the wave gradually attenuates downstream, 

but commonly results in stage changes of at least 0.4 feet (ft) 

over a 24-hour period in the study segment. The confluence 

of the tailrace (released-water return flow) of the hydroelectric facility and the LPR is about 2 mi downstream from the 

confluence of the Loup River and the LPR, and the hydrocycle 

wave is apparent at all streamflow-gaging stations along the 

LPR downstream from the tailrace. Although some natural 

diel fluctuation in river levels probably occurred prior to the 

beginning of operations at the hydroelectric facility in 1937, 

large fluctuations in river stage over the course of a single day, 

in the absence of intense rainfall or snowmelt, are not apparent 

from the typical daily hydrograph of streamflow-gaging stations on any of the primary tributaries to the LPR. Therefore, 

the magnitude of the fluctuations in daily discharge and stage 

measured in the LPR during the study was likely greater than 

the magnitudes that would have been observed prior to the 

hydroelectric facility.
Introduction 5

Definitions and Descriptions of Sandbars in the 

Lower Platte River

The Platte River in Nebraska is a wide, sand-bedded, 

braided river. The term “braided” in this report is used as 

a general term to describe the division of water flow in a 

river channel into two or more primary threads by islands 

or emergent sandbars. In the case of large (loosely defined 

here as being about one average channel width or more in 

length), densely vegetated islands, braiding is commonly not 

discharge-dependent because these features generally cause 

flow division for discharge magnitudes up to or near bankfull. 

This type of braiding is usually referred to “anabranching” if 

the island is very large and flow division extends for several 

tens of channel widths or more (Bridge, 2003); anabranches 

presently occur in the LPR only upstream from the study 

area. For more transient features, such as sandbars, braiding 

is discharge-dependent because discharges less than bankfull 

often cause the submergence of these features in the LPR 

(Parham, 2007). 

One of the primary characteristics of braided rivers is the 

presence of macroforms, which are large, transient, sedimentary deposits, often referred to as “bars” or, with inferred grain 

size, “sandbars.” The term “sandbar” is used in this report 

to refer to the macroforms of the LPR described in detail by 

Smith (1970, 1971), Blodgett and Stanley (1980), and Crowley (1983), and similar types of deposits described in rivers 

elsewhere (see Bridge [2003] for general review of macroforms). More specifically, the sandbars described in this report 

were all emergent above the water surface at the time of observation. Sandbars differ from other bedforms such as dunes and 

ripples because their length is proportional to channel width, 

and their height is proportional to the depth of the river during 

their formation. In the Platte River system in Nebraska, sandbars typically are “transverse”—that is, the downstream slip 

face of the bar is aligned obliquely or nearly perpendicular to 

the primary flow direction during bankfull discharges (Crowley, 1983), although their shape may change substantially after 

flow recession. Smith (1971) described transverse sandbars as 

the “most characteristic and important bedforms in the lower 

Platte River. They comprise a varied group of tabular, essentially flat-topped, periodic or solitary sand bodies which grow 

by down current additions to slip faces.” However, Smith 

(1971) also concluded that most exposed sandbars, especially 

large sandbars that cause braiding in the LPR during low discharges, are not simply exposed transverse bars, but depositional features that initiate as transverse bars during high flows 

and evolve (through changes in texture, shape, and size) after 

flow recession during periods of moderate-to-low discharges.

Because of the difficulty of directly observing their 

formation, quantitative observations of the hydraulics and 

sediment-transport fields around Platte River-type macroforms have not been well-described (Leeder, 1999; Best and 

Rhoads, 2008), and tools to predict the location, conditions of 

formation, and geometry of these macroforms still are limited 

to field, flume, and numerical modeling descriptions. Smith 

(1971) attributed the formation of sandbars in the LPR at “low 

and intermediate discharges” to be the result of variations in 

depth and velocity over short distances, whereby sediment was 

deposited on the riverbed in locations where the tractive forces 

of the bed were locally reduced enough to stall sediment transport, similar to the mechanism of riffle formation proposed 

by Leopold and Wolman (1957). At high discharges, Smith 

(1971) attributed bar formation in the LPR to the coalescing of 

diffuse dune fields during the recession of a flood, and showed 

that, once formed, water-surface slope over the bars was substantially steeper than in other areas of the river channel, often 

forming “sand riffles.” Similar anecdotal observations of large 

sandbars emerging after flow recession in areas of the LPR, 

where sand riffles were present during high flows, have been 

made by the authors.

Previous Studies

The surfaces of mid-channel sandbars are the primary onriver nesting habitat feature for terns and plovers in the LPR. 

Previous authors have described and classified macroforms in 

the LPR (Crowley, 1983), have investigated their internal stratigraphy and sedimentology (Ore, 1964; Smith, 1970; Blodgett 

and Stanley, 1980; Crowley, 1983), and have described 

observations of their formation and evolution (Smith, 1971; 

Blodgett and Stanley, 1980; Crowley, 1983). However, 

because of sandbar importance as nesting habitat for bird species, recent work has focused on the physical characteristics 

of sandbars and surrounding channel morphology associated 

with the nesting of species of concern. This work has focused 

particularly on measuring the area of sandbars and the height 

above the stage of index discharges because these metrics provide insight into potential diagnostic differences between sites 

selected by terns and plovers and sandbars without observed 

nesting. Some recent nesting-related work also has described 

associated nearby channel morphologic characteristics. For 

example, Kirsch (1996) indicated that, although gross availability of sandbar habitat was not limiting on the LPR, terns 

nested on mid-channel sandbars where channel width tended 

to be greater than at sites without nests. More recent investigations also have indicated positive relations between nearby 

non-vegetated river channel width and the presence of tern 

and plover nesting sites (Elliott, 2011; Jorgensen and others, 

2012).

Using a combination of remote sensing and groundlevel surveys, Ziewitz and others (1992) characterized 

sandbar geometry and nest presence or absence at potential 

nesting sites along a 3-mi systematic sampling interval in 

the LPR in 1988. Sandbar area was calculated from aerial 

photographs taken during aerial surveys of birds. Mean and 

maximum sandbar height relative to the water surface were 

estimated from on-the-ground transect surveys, and indexed 

to a discharge of 4,000 ft3/s by estimating water travel time 

between bounding streamflow-gaging stations. Along the most 

downstream 54 mi of the LPR, Brown and Jorgensen (2008) 
6 Emergent Sandbar Dynamics in the Lower Platte River in Eastern Nebraska—Methods and Results of Pilot Study, 2011

surveyed sandbar heights for bars larger than 0.5 acres along a 

1/3-mi systematic interval, and referenced heights to the daily 

maximum and minimum stages at nearby gaging stations. 

They reported that bar height progressively increased in the 

downstream direction. Brown and Jorgensen (2009) used similar methods to measure the height and area of sandbars along 

most of the 103-mi segment of the LPR. They measured the 

height and area of 36 sandbars: 26 with nests and 10 without 

nests. Sandbar height was indexed to the long-term mean 

stage for June 15 at the nearest representative streamflowgaging station. The index stages used by Brown and Jorgensen 

(2009) represented a discharge ranging from about 6,200 to 

12,000 ft3/s on the latest stage-discharge rating tables for the 

gaging stations along the length of the LPR (M.J. Andersen, 

U.S. Geological Survey, written commun., 2011). For the 

Platte River streamflow-gaging stations referenced in this 

study, near Ashland (USGS 06801000) and at Louisville 

(USGS 06805500), the stages used by Brown and Jorgensen 

(2009) to index bar heights were equivalent to flows of about 

11,100 and 12,000 ft3/s, respectively. Brown and Jorgensen 

(2009) again indicated that sandbar heights progressively 

increased in the downstream direction, but the rate of downstream increase was substantially higher in the segment of the 

LPR downstream from the Elkhorn River.

Approach and Methods

Previous studies of sandbar geometry have indicated 

substantial spatial variation including spatial trends in sandbar 

geometry in the LPR (Brown and Jorgensen, 2008, 2009). To 

capture both the seasonal and spatial variation in sandbar locations and geometries in the LPR, a “rapid-assessment” method 

was developed and used for the study, with the goal of characterizing the spatial distribution and habitat-relevant geometries 

of the complete population of sandbars along the 22-mi study 

segment. These geometries and distributions are represented 

by data collected during week-long survey periods within 

the three open-river seasons during calendar year 2011. The 

rapid-assessment method was used instead of selecting shorter, 

fixed study reaches or systematic intervals because a primary 

goal of the study was to understand if the spatial distribution 

and geometries of bars changed substantially from season to 

season, and this required a more spatially enriched dataset.

Hydrologic Context

Previous studies have indicated that less frequent, highmagnitude discharges are a fundamental process for the formation of large, higher sandbars that are used most frequently 

for nesting by terns and plovers in the LPR (Parham, 2007). 

Models presented in previous work have assumed that sandbar 

geometries persist until at least the peak flow in the year 

following initial formation (Parham, 2007). Moreover, more 

recent work indicates that nesting has occurred on sandbars 

that are in about the same location during as much as a 3-year 

period (Brown and Jorgensen, 2010). Unless otherwise noted, 

descriptions of streamflow discharges on the Platte River are 

referenced to the hydrologic record at the streamflow-gaging 

station on the Platte River at Louisville, Nebraska (hereinafter, 

the “Louisville gage”), which is the only gaging station on the 

Platte River downstream from Salt Creek that has a currently 

applicable rating of the stage-discharge relation (U.S. Geological Survey, 2012).

The data-collection activities described in this report 

spanned three survey periods of 7 days or less, during three 

open-water (no ice) seasons in the LPR in calendar year 2011 

(fig. 2). Sandbar geometries and locations during the first 

survey period, April 6–12, 2011, likely were a reflection of 

the collective processes of flooding, low flows, and winter 

ice effects and ice melt in the year that preceded the survey. 

In particular, the instantaneous peak discharge during water 

year 2010 of 136,000 ft3/s (U.S. Geological Survey, 2011), 

the third highest instantaneous discharge since the Louisville 

gage began recording in 1953, likely had a great effect on the 

arrangement and geometries of sandbars in the study area. 

Streamflows in the LPR during the three 2011 survey periods 

were commonly greater than the daily averages for the period 

of record (fig. 2). During late June, July, and August 2011, 

streamflow often ranked above the 90th percentile of daily 

flow values for the date (streamflows that, for each calendar 

date, were exceeded less than 10 percent of the time during the 

1953–2011 period of record). The higher summer base flows 

in the LPR during 2011 resulted from the very wet conditions in the upper Platte River Basin (Grigg and others, 2012), 

which required sustained releases from water-supply reservoirs to accommodate runoff into the late summer. Although 

base flows were higher than average during water year 2011, 

the annual maximum discharge of about 45,400 ft3/s ranks 

near the median (33 out of 59) of annual maximum discharges 

within the period of record for the Louisville gage. 

River stages in the LPR within the study segment 

sometimes are controlled by factors other than the streamflow of the Platte River. River stages in the most downstream 

segments of the LPR also are controlled by the stage of the 

Missouri River, which acts as a downstream altitude control 

on the water surface. The hydraulic control of the stage of a 

river by a higher stage in a downstream water body is referred 

to as a “backwater effect” because the body with the highest 

stage will “block” flow in a tributary river, causing a rise in its 

stage (Meade and others, 1991); consequently, the tributary 

river may have an unusually high stage at a low-magnitude 

discharge. 

A complete analysis of the hydraulic extent and effects 

of backwater from the Missouri River was beyond the scope 

of the study; however, an attempt was made to estimate the 

upstream extent of backwater altitude on the LPR. For the 

purposes of the study, the altitude of backwater extent on the 

LPR was defined as the interpolated altitude of the National 

Weather Service (NWS) flood stage of the Missouri River. 

This altitude was considered a conservative estimate of the 
Approach and Methods 7

potential extent of backwater effects from the Missouri River 

that may have a considerable effect on sandbar depositional 

and erosional processes and associated geometries. The 

altitude of the flood stage of the Missouri River at its confluence with the Platte River was estimated by first digitizing 

the centerline distance of the Missouri River channel between 

the two streamflow-gaging stations that bound the Missouri 

River-Platte River confluence; the Missouri River at Omaha, 

Nebraska (USGS station 06610000); and the Missouri River at 

Nebraska City, Nebraska (USGS station 06807000). Second, the slope of the Missouri River channel centerline at 

flood stage was calculated using the difference between the 

flood stage at each gage (referenced to the National Geodetic 

Survey 1929 datum), divided by the river distance from step 

1. Third, the altitude of the flood stage of the Missouri River 

at the Platte River confluence was linearly interpolated along 

the slope of the centerline of the Missouri River. Finally, the 

estimated altitude of the flood stage of the Missouri River was 

interpolated upstream on the Platte River and compared with 

the altitude contours of the USGS 7.5-minute quadrangle (U.S. 

Geological Survey, 2010).

 The estimated plane of the Missouri River flood stage 

(conservative estimate of backwater-effects limit) intersects 

the LPR at a little more than 3.5 mi upstream from the Missouri River, but was rounded up to 4 mi to accommodate 

estimation uncertainty and the expected effect of the backwater on deposition immediately upstream. The estimated 

upstream extent of Missouri River backwater effects was the 

basis for fixing the downstream end of the study area because 

it was assumed that sandbar geometries within the backwateraffected zone were governed primarily by backwater-affected 

river hydraulics, rather than by hydrologic and geomorphic 

processes driven by the LPR watershed. 

Field Data Collection and Analysis

Three primary measures were used to assess emergent 

sandbar dynamics in the study segment: sandbar area, sandbar height, and sandbar location. Data to derive measures of 

sandbar dynamics were collected during three, week-long 

survey periods in calendar year 2011. The first survey was 

conducted during April 6–12, hereinafter referred to as the 

“spring survey.” The second survey was conducted during July 

26–August 2, hereinafter referred to as the “summer survey.” 

The third survey was conducted during October 31–November 

4, hereinafter referred to as the “fall survey.” The measures of 

sandbar dynamics presented in this report are not interpreted 

as having sharply defined, singular values. Rather, these 

measures are approximations of each metric’s magnitude as 

they existed in the LPR during the three survey periods. Field 

data collection focused only on sandbars that were mostly barren. However, some sandbars, particularly during the summer 

survey, had dispersed bunchgrasses and forbes, or patches of 

more dense, but immature vegetation (defined here as growth 

that was judged to have occurred since recession of spring 

high flows). In all cases, total vegetative cover was less than 

50 percent, and in most cases much less than 50 percent.

Emergent sandbars were grouped into one of three 

generalized types: (1) bank-attached, (2) island-attached, 

and (3) mid-channel. At the discharge and stage observed, 

bank-attached and island-attached sandbars had a direct, dry 

connection to a river bank or island, respectively, whereas 

mid-channel sandbars were separated from the river banks and 

islands by a wetted channel. Sandbars attached to the river 

banks or islands are used less frequently by terns and plovers, 

possibly because mid-channel nesting habitat is less accessible to predators (Joel Jorgensen, Non-Game Bird Manager, 

Nebraska Game and Parks Commission, oral commun., 2011; 

90th percentile of

daily flows over

period of record

Spring survey

Summer survey

Fall survey

Calendar year 2011 

daily average flows

Mean daily flows over

period of record

10th percentile of 

daily flows over

period of record

0

10,000

20,000

30,000

40,000

50,000

60,000

Discharge, in cubic feet per second

January 1 March 1 May 1 July 1

2011 2012

September 1 November 1 January 1

Figure 2. Daily average discharge in calendar year 2011 and selected daily discharge statistics for Platte River at 

Louisville, Nebraska. Daily statistics selected were the 90th percentile, the mean, and the 10th percentile of daily mean 

discharges, by calendar date, during the 1954–2011 period of record. 
8 Emergent Sandbar Dynamics in the Lower Platte River in Eastern Nebraska—Methods and Results of Pilot Study, 2011

U.S. Army Corps of Engineers, 2011; Lott and Wiley, 2012). 

Although mid-channel sandbars could be considered attached 

to river banks or islands at lower discharges, there is no practical way to quantitatively assess this attachment over a range 

of river stages in the absence of detailed topographic and 

bathymetric surveys.

Sandbar Number and Area

The area of emergent sandbars was surveyed during the 

study to characterize the size distribution and to provide summary measures for the relative sizes of sandbars in the LPR 

during the study period. Changes in the discharge of the river 

and associated changes in river stage may have a substantial 

effect on the area of a sandbar that is emergent above the 

water surface, particularly if a large part of the sandbar is lowlying. Furthermore, in the absence of comprehensive surveys 

of the emergent and submerged topography of the sandbars, 

there is no practical method to normalize the area of emergent 

sandbars to an index stage or discharge. Therefore, survey 

periods representative of each season of interest were selected; 

however, some scheduling flexibility was needed to maximize 

surveying during periods of discharge with relative stability 

and similarity, so that sandbar areas were most comparable 

within and across the three survey periods. 

Field Data Collection

Sandbar areas were surveyed in two different ways, 

depending on the field-estimated size of the sandbar. A coarser 

method to survey sandbar area and location was used for 

sandbars with an estimated area of less than 2 acres, and a 

finer method was used for sandbars with estimated area of 

2 acres or more. Two acres was selected as the threshold value 

between survey methods because previous field surveys of 

sandbars with colonies of terns and plovers indicated that most 

nests generally were observed on sandbars larger than 2 acres 

(Joel Jorgensen, Nebraska Game and Parks Commission, oral 

commun., 2011). Sandbar areas were estimated in the field 

using a laser rangefinder capable of measuring the length and 

width of each sandbar from a distance. If the length-width 

product was less than 2 acres, the sandbar was grouped into 

one of four size bins: area <0.5 acres, 0.5–<1.0 acres, 1.0–<1.5 

acres, and 1.5–<2.0 acres. 

Sandbars with an estimated area of more than 2 acres 

were surveyed using a handheld, 14-channel, differentialgrade global-positioning system (DGPS) receiver. When 

operating in DGPS mode, the receiver was capable of achieving real-time sub-meter accuracy using any available satellitebased augmentation system (Ashtech LLC, 2010). The Federal 

Aviation Administration’s Wide-Area Augmentation System 

(WAAS) was the only non-commercial system available 

within the study area, and the receiver was capable of operating in DGPS mode at all times because of the wide horizons 

of the LPR. All survey waypoints were georeferenced in the 

Nebraska State-Plane coordinate system, based on the North 

American Datum of 1983 (NAD 83). Sandbar areas were surveyed by an operator holding the DGPS receiver, and walking 

the rim of the “dry crest” of each sandbar, logging positional 

waypoints every 10 ft. The “dry crest” was defined as the part 

of the sandbar emergent above the diel high-water mark from 

daily stage fluctuations (fig. 3). The dry crest was used to normalize measurements among sandbars to the daily high stage, 

which was relatively constant within each survey period, but 

differed somewhat between survey periods (table 1). The dry 

crest is identifiable in the field by a scalloped face along the 

margin of the sandbar that separates drier sand from saturated 

or wave-sculpted sand, and the rim of the dry crest was the 

area defined as “emergent” for the study.

Data Analysis 

Points logged along the margin of each sandbar were 

post-processed in a geographic information system (GIS) to 

remove redundant points from clusters or points recorded 

unintentionally by the operator. After post-processing, the 

points were used to create digital polygons and to make calculations of sandbar dry-crest areas using the GIS. In several 

cases, sandbars were composed of complexes of smaller 

sandbars separated from a larger “core” sandbar by shallow 

cross-bar channels (fig. 3C). In such cases, the dry crests of all 

individual sandbars were logged and attributed with codes to 

associate them with the larger core sandbar. Thus, the sandbar 

area for sandbar complexes was considered the total dry-crest 

area of the core sandbar and smaller sandbars surrounding the 

core.

Assessment of seasonal changes in emergent sandbar 

quantity and area was done in three primary ways. First, an 

assessment of the total number of bars and the total estimated 

emergent sandbar area for each survey period was performed. 

The total emergent sandbar area was calculated for each 

survey period by summing the total area of emergent sandbars 

measured using DGPS and the estimated area of individual 

sandbars smaller than 2 acres. For sandbars smaller than 

2 acres, areas were estimated by assigning the midpoint of the 

0.5-acre bin size to all bars within each size bin. For example, 

for all bars within the size bin “area less than 0.5 acres,” each 

bar was assumed to be 0.25 acres in area. Next, an assessment 

of changes in sandbar area by sandbar type was done for all 

sandbars where DGPS measurements were made. Because this 

assessment was focused on those sandbars generally larger 

than 2 acres, changes were quantified by comparison of basic 

summary statistics (mean, median, interquartile range) for all 

sandbar types by survey season. Finally, to assess processbased changes by survey season, an assessment of changes 

among individual, collocated sandbar pairs and by sandbar 

type was performed. Collocated sandbar pairs were identified 

by intersecting sandbar polygons from each survey period in 

the GIS. If two polygons intersected, regardless of how much 

area overlapped between the two, the sandbars were considered a non-independent pair and included in the test data. 

Changes were assessed as the difference between measured 
Approach and Methods 9

High platform (sandbar height measurements)

Area above dry crest but lower than high 

platform (sandbar height measurements)

Area below dry crest (within daily stage range)

Rotating laser level

Bar-topographic measurement point

Water-surface measurement point

EXPLANATION

Highest

Lowest

A B C

Cross-bar

channel

Figure 3. Typical levels and shapes of emergent sandbars in lower Platte River, Nebraska, and 

associated point spacing methods for surveying sandbar height: (A) streamwise tapering, (B) rectangular, 

and (C) dissected and tapering.

Table 1. Ranges of daily discharge and stage range of Platte River at Louisville, Nebraska (USGS 06805500), during three survey 

periods, 2011.

[Data are approved values retrieved from the U.S. Geological Survey National Water Information System on February 13, 2012.]

Survey period 

and date

River discharge, in cubic feet per second Stage range

Maximum Minimum Average Range (feet)

Spring survey period

04/06/2011 12,900 9,810 11,400 3,090 0.45

04/07/2011 13,600 9,930 11,700 3,670 0.53

04/08/2011 13,500 9,810 11,400 3,690 0.54

04/12/2011 12,300 8,940 10,500 3,360 0.41

Average 13,075 9,623 11,250 3,453 0.48

Summer survey period

07/26/2011 13,100 8,120 10,500 4,980 0.76

07/27/2011 12,300 8,410 10,200 3,890 0.6

07/28/2011 13,200 8,700 11,300 4,500 0.68

07/29/2011 14,200 9,870 12,000 4,330 0.62

08/02/2011 12,100 7,560 9,930 4,540 0.72

Average 12,980 8,532 10,786 4,448 0.68

Fall survey period

10/31/2011 9,740 6,650 8,190 3,090 0.54

11/01/2011 9,550 7,070 8,270 2,480 0.43

11/03/2011 10,600 7,180 8,960 3,420 0.57

11/04/2011 10,100 7,340 8,710 2,760 0.47

Average 9,998 7,060 8,533 2,938 0.50
10 Emergent Sandbar Dynamics in the Lower Platte River in Eastern Nebraska—Methods and Results of Pilot Study, 2011

sandbar areas between survey periods (summer-spring, fallsummer, and fall-spring). Changes were quantified using basic 

summary statistics of differences in sandbar areas between 

survey seasons by sandbar type. Because there was no practical way to normalize sandbar area to an index discharge, and 

because characteristics of the entire population of sandbars 

were recorded, no inferential statistical analyses of differences 

in means or variances of emergent sandbar area were performed when comparing sandbar types or seasons.

Sandbar Height 

The height of sandbars is a direct measure of the potential (probability) for inundation of a sandbar by a discharge 

having a particular stage, and is, therefore, one metric for the 

relative quality of tern and plover nesting habitat because it 

is directly related to the potential for inundation (causing nest 

failure) in any given year (Parham, 2007; Brown and Jorgensen, 2008; Lott and Willey, 2012). Sandbars in the study 

segment generally had an easily identifiable, tabular high 

surface (hereinafter, the “high platform”; fig. 3), which gently 

sloped downstream. In some cases, the high platform had 

low-relief undulations, or the sandbar was dissected into series 

of individual tabular areas separated by shallow, cross-bar 

eroded threads. Undulations and mud drapes were particularly 

noticeable during the spring survey, and these were assumed 

to be the result of backwater flooding caused by ice jams. For 

example, the peak stage recorded in 2011 at the Louisville 

gage was recorded on February 17, and was related to backwater flooding from ice, as opposed to the annual instantaneous 

maximum discharge, which occurred on May 27, 2011 (U.S. 

Geological Survey, 2012). During the subsequent summer and 

fall surveys, the high platforms of sandbars were less undulating, and had less mud, indicating they had been reworked (or 

resurfaced) either by wind or water.

Field Data Collection 

Sandbar height was surveyed for all sandbars with a fieldestimated size of more than 2 acres. Measurements of sandbar 

height were focused on characterizing the field-perceived high 

platform (fig. 3A) because it could be compared to the stage 

of the formative flow and was the part of the sandbar that was 

assumed to be the most-probable and highest-quality nesting habitat for terns and plovers. In some cases, one or two 

extensive flat surfaces were inset and lower relative to the 

highest surface, but the height of these lower surfaces was not 

surveyed (figs. 3A and B). Sandbar height was surveyed using 

a rotating laser level mounted on a tripod and a laser detection receiver mounted on a graduated stadia rod. The rotating 

laser level had a radial range of 750 ft and a nominal accuracy 

of about ±0.0033 ft per 100 ft along the radius. In most cases, 

sandbar height was surveyed by taking a series of 5–9 point 

measurements on the high platform (figs. 3A–C); however, in 

rare cases fewer or more points were taken. To negate bias in 

each height measurement caused by the slope of the sandbar 

surface, measurements of sandbar height were referenced 

to adjacent point measurements of the water surface. In rare 

cases, water-surface height was linearly interpolated upstream 

or downstream to a point adjacent to a sandbar height measurement where no water-surface height had been surveyed. 

The location of point measurements of sandbar height and 

water-surface height was recorded using either a WAASenabled recreational GPS unit, or logged into a handheld 

DGPS receiver similar to that used for sandbar area. The exact 

collection time of all point measurements of water-surface 

height was either recorded in a field notebook, or logged 

directly into the GPS unit.

Spacing of point measurements of sandbar height was 

determined in three different ways, depending on the shape of 

the sandbar. For sandbars that were prismatic in shape, point 

samples of sandbar heights were collected using a quasi-grid 

approach that weighted the number of point measurements to 

larger (wider) parts of the sandbar (more points were taken in 

wider areas than in narrower areas) (fig. 3A). For sandbars that 

had a simplistic streamwise elongated shape and a singular, 

unbroken high platform, a rectangular grid system was used to 

locate point samples of sandbar height along equally spaced 

transects (fig. 3B). The rectangular grid was established by: 

(1) using a laser rangefinder to estimate the streamwise center 

of the high platform; (2) setting the rotating laser level at 

the estimated streamwise center of the high platform; and 

(3) surveying equally spaced point measurements of height 

along transects aligned perpendicular to the high banks of the 

channel, and equally spaced at the approximate 25th, 50th, 

and 75th percentiles of the length of the high platform. For 

sandbars that had a dissected topography, or those that were 

split by a series of shallow channels, point measurements of 

sandbar heights were taken on all individual bar units that 

had a maximum height approximately equivalent to the high 

platform of the core bar, and the number of point measurements were weighted approximately proportional to the 

field-estimated relative size of each bar’s high platform area 

(fig. 3C). Sandbars that were nearby, but separated from the 

core channel by a deep, non-wadeable, channel thread, were 

considered independent bar units.

The natural high banks of the river also were measured 

relative to the water surface to provide an additional reference datum where possible. The term “natural high banks” 

is used to refer to banks of the river that are not constructed 

or otherwise perceptibly directly altered by humans, and that 

have a mature riparian forest community, typically indicated 

by nearly vertical banks without bank protection and the 

presence of riparian trees of large diameter. Measurements of 

the heights of the natural high banks only occurred during the 

spring and summer survey periods. The locations of sampling 

areas for the heights of the high banks were dictated mainly 

by availability because much of the study segment has bank 

protection; however, nearness to a surveyed sandbar also was 

a consideration. Heights of high banks were measured by 

placing a graduated stadia rod at water’s edge near the bank, 

and holding the stadia rod plumb while a second operator used 
Approach and Methods 11

a hand level to align the brink of the top of the bank with the 

scale on the rod. The height of the bank, the location of the 

measurement, and the exact time of the measurement were 

recorded. Because of the undulating and diffuse (grassy, illdefined brink) nature of the bank tops, three height measurements, spaced about 50–150 ft apart, were taken and averaged 

for every bank sampling area.

Data Analysis 

Because all sandbar heights were paired with an adjacent 

measurement of water-surface height, and the water-surface 

height in the LPR has a diel fluctuation, adjustments to each 

measured water-surface height were necessary to standardize 

sandbar heights to a common datum. Height standardization 

was done using simplifying assumptions of the hydrology and 

hydraulics of the study segment, and a spatial-temporal interpolation technique. First, it was assumed that stage changes 

at two streamflow-gaging stations, the Ashland and Louisville 

gages, were broadly representative of the magnitudes of stage 

changes throughout the study segment, and that stage changes 

throughout the study segment could be reasonably characterized using the rating tables from these gages. Second, it was 

assumed that the instantaneous stage at a location in the river 

away from a streamflow-gaging station could be represented 

by an interpolated stage if the streamwise distance and the 

travel time of water between the streamflow-gaging station 

and survey point were known. The first assumption is recognized to be a potential oversimplification of the system; however, such an assumption is necessary in the absence of many 

additional streamflow-gaging stations throughout the study 

area. Furthermore, the study area has less variation in channel 

width than segments upstream (Elliott and others, 2009), and, 

therefore, likely has less overall streamwise variation in stagedischarge relations.

Each surveyed water-surface height was indexed to two 

primary reference heights: (1) the stage of the water-surface 

at 4,000 ft3/s (Ziewitz and others, 1992) at the nearest gaging 

station, and (2) the height of the nearest bank-height measurement of the high banks of the river. To index individual 

water-surface heights, which were taken over variable times 

of the day and at various locations throughout the river, each 

measurement was first referenced to an interpolated stage at 

the nearest streamflow-gaging station using the following 

equations:

(1)

where

TX is the time offset between location X and 

the nearest streamflow-gaging station, in 

hours;

XG is the streamwise location of the nearest 

streamflow-gaging station, in miles;

Xi is the streamwise location of water-surface 

height point measurement i, in miles; and

V is the average velocity of a parcel of water 

between the streamflow-gaging station 

and the water-surface height point 

measurement, in miles per hour.

The interpolated stage at location X for measurement i

was obtained from the instantaneous (15-minute intervals) 

stage record of the stream gage at XG:

SXi = S (t

i

–TX) (2)

where

SXi is the interpolated stage from the nearest 

gaging station of the individual watersurface height measurement i, in feet;

t

i is the time of the individual water-surface 

height measurement i, in feet; 

TX is as before.

Once the interpolated stage was calculated, the height of 

individual sandbar measurements relative to the interpolated 

stage of the river at 4,000 ft3/s was calculated as:

Z4kj = ZXj

i

– [SXi – S4k ] (3)

where

Z4kj is the height of an individual point 

measurement of sandbar height j above the 

interpolated local stage of 4,000 ft3/s, in 

feet;

ZXj

i

is the height of an individual point 

measurement j on the sandbar high 

platform adjacent to water surface height 

measurement i, in feet;

SXiis as before;

S4k is the stage of 4,000 ft3/s from the rating table 

of the streamflow-gaging station at XG, in 

units of length.

The streamwise distances between the streamflow-gaging 

stations and each water-surface height point measurement 

were determined using a linear referencing tool in the GIS. 

The average velocity of a water parcel between the watersurface height measurement point and the nearest streamflowgaging station was determined using a stage-correlation 

technique. The stage-correlation technique progressively 

lagged the 15-minute stage record of an upstream gaging 

station, in 15-minute time steps (fig. 4A), and calculated 

the Pearson’s correlation coefficient between the lagged 

record and the downstream stage record at each time step. 

The average water-travel time was assumed to be the time 

offset for which the two records reached maximum correlation (fig. 4B). Other studies have used this type of correlation 

analysis method to estimate bedform migration velocities 

(Dinehart, 2002; Holmes, 2010), and this method inspired the 

technique used in this study.

Stage correlation entailed using a 72-hour window of 

instantaneous (15-minute) stage records, centered on the survey day of interest, and included additional, stage-only gaging 

−

V

X X

TX = G i
12 Emergent Sandbar Dynamics in the Lower Platte River in Eastern Nebraska—Methods and Results of Pilot Study, 2011

stations. To increase the spatial resolution of the records for 

the diel wave in river stage, and, therefore, to allow spatial differences in average travel time to be assessed, two additional 

stage-only gaging stations were installed for this study: one at 

South Bend, Nebraska (USGS station 06805020, hereinafter 

referred to as “the South Bend gage”), and one at La Platte, 

Nebraska (USGS station 06805570, hereinafter referred to 

as “the La Platte gage”) (fig. 1). Travel time was estimated 

between each gaging-station pair for each day on which a 

sandbar survey occurred to account for day-to-day variations 

in travel time owing to weather and minor discharge changes 

(appendix 1, tables 1-1 through 1-3). In some cases, a full 

72-hour stage record was not available for all streamgages 

because gaging-station equipment malfunctioned. In such 

cases, a shorter time period centered on a day other than the 

survey date was used to make the correlations. The height 

of the high banks relative to the 4,000 ft3/s reference stage 

was calculated using the same techniques as described in 

equations 1–3 for indexing measurements of sandbar height.

Assessment of changes in sandbar height between seasons and changes in the longitudinal pattern of sandbar heights 

only focused on those sandbars that were larger than 2 acres 

because few measurements of height were made on sandbars smaller than 2 acres. Four types of statistical summaries 

were assessed. First, cumulative frequency distributions of 

measured sandbar heights were constructed for each survey 

period, and simple comparisons of these curves and summary 

statistics were made. This simple assessment was considered rigorous because measurements were made on the high 

platform of the entire population of emergent sandbars larger 

than 2 acres. Second, scatter plots of measurements of sandbar 

heights by longitudinal location were constructed, and locally 

weighted least-squares regression (LOESS) lines were fit to 

the data using a span of 0.4 (fraction of sample size), and a 

linear, symmetric fit (Helsel and Hirsch, 2002). The LOESS 

lines were used to visually evaluate patterns in the alongstream distribution of sandbar heights, and to compare any 

noted patterns with coincident along-stream patterns in channel sinuosity (Elliott and others, 2009), channel width (Elliott, 

2011), and height of the high banks (this study). Third, the 

frequency distributions of median and maximum measured 

values of sandbar height were compared among survey periods 

and sandbar types. These particular summary statistics were 

selected because quality-control data (described in the “Quality Control” section of this report) indicated that median and 

maximum values provided the strongest evidence of overlap 

with tern nesting heights. Finally, the two-tailed Wilcoxon 

signed-rank test (Helsel and Hirsch, 2002) was used to test 

Stage at gaging station 

at Louisville (~river mile 16)

Time = 120 minutes

Time = 240 minutes

Time = 360 minutes

Offset time of Ashland

EXPLANTION

Stage at gaging station 

near Ashland (~river mile 28)

00:00

16.0

16.2

1.0

0.8

0.6

0.4

0.2

0

16.4

16.6

Water surface elevation,

in feet above arbitrary datum Pearson’s correlation coefficient

06:00

A

B

0 60 120 180 240 300 360 420 480 540 600

Local time on November 1, 2011

Time offset applied to stage at upstream gaging station, in minutes

Time offset with maximum correlation 

between stage records at Ashland 

and Louisville = 360 minutes

12:00 18:00 24:00

Figure 4. Example of stage-correlation method used to estimate travel time of water between gaging stations

 by (A) comparison of stage hydrographs with various offset hydrographs; and (B) results from correlation 

analyses comparing downstream hydrograph with various offset hydrographs of upstream stage hydrograph.
Approach and Methods 13

for significant differences of sandbar height between survey 

periods among sandbar pairs. A non-parametric test was used 

because the data indicated the potential for non-normality in 

the sandbar height dataset for the summer survey.

Sandbar Location

Data describing the location and spatial distribution of 

sandbars, and particularly the spatial distribution of sandbar 

area, are important for understanding if sandbars are uniformly 

or randomly distributed along the river, or if certain locations 

in a river tend to be more conducive to deposition and formation of emergent sandbars of high quality as nesting habitat. 

Such information could be used in a variety of ways. For 

example, an understanding of flow conditions in areas where 

high-quality emergent sandbars tend to form, persist, or both 

may be critical to develop simulation models for predicting 

the expected availability and quality of habitat on the basis of 

flow conditions. Additionally, such information is valuable for 

planning sustainable habitat creation or restoration projects if 

it enhances the likelihood of sandbar persistence, and, thus, 

the cost-effectiveness of constructed habitats. 

Field Data Collection

The location of each sandbar was approximated in one of 

two ways, depending on the size of the sandbar. For sandbars of less than 2 acres, the bar centroid was estimated by 

recording a GPS location of the boat and an offset distance 

and azimuth (uncorrected for declination) to the approximate 

center of the sandbar, which were estimated using a laser 

rangefinder with an internal digital compass. The GPS points 

then were manually post-processed in a GIS, using the offset 

distance and azimuth (uncorrected for declination) to create 

point features at the estimated center of each sandbar smaller 

than 2 acres.

For sandbars larger than 2 acres, the centroid of each was 

calculated from the GIS polygon representation of the surveyed perimeter of the dry crest, and that centroid was used as 

the sandbar’s location. The centroid of the largest sandbar was 

used as the location for complexes of multiple sandbars. The 

point location of each sandbar was referenced linearly in the 

GIS to river distance upstream from the Missouri River-Platte 

River confluence along the channel centerline of the Platte 

River (Elliott and others, 2009).

Data Analysis

Sandbar locations and spatial distributions were assessed 

in two primary ways. First, the sandbar area was summed by 

river-mile increments for each survey period, and expressed 

as a percentage of the total emergent sandbar area for the 

same period, to assess the streamwise distribution of emergent 

sandbar area by river mile. Expressing area as a percentage of period total reduces the influence of between-period 

differences in streamflow discharge on the interpretation of 

spatial distributions of sandbar area. Second, shifts in sandbar 

location between survey periods were assessed using repeated 

survey results for individual sandbars (data pairs for presumed 

same sandbar). Data pairs for location assessments were the 

same as those identified for the paired-test statistical comparison of sandbar height (described in the “Sandbar Height: Data 

Analysis” section of this report). Shifts in sandbar location 

within a data pair were characterized as the difference in the 

streamwise location of the sandbar areal centroids for spring–

summer, summer–fall, and spring–fall collocated sandbar 

pairs. Summary statistics for shift in location (including mean, 

median, interquartile range, and range) were used to quantify 

the magnitude and direction (upstream or downstream) of 

shifts.

Quality Control

Two primary procedures were implemented as checks 

on the quality of field measurements of sandbar geometry. 

First, surveys of sandbar area and height were replicated at 

10 percent or more of sandbars larger than 2 acres. Second, a 

quality-control survey was used to compare sandbar area and 

height results measured using the rapid-assessment approach 

with those from a more detailed survey measured with a laser 

total-station system. Additional quality-control procedures 

were used to approximate the uncertainty of estimations of 

sandbar height associated with errors in estimation of water 

travel time and to validate predictions of sandbar inundation. 

These additional procedures are described in appendixes 1 and 

2, respectively.

Replicate Surveys of Sandbar Geometry

Surveys of sandbar geometry were replicated for at least 

10 percent of all sandbars surveyed during each survey period. 

Under ideal conditions, a sandbar was randomly selected for 

survey replication as one of the first seven sandbars surveyed 

on each day of each survey period, and the set of replicates 

would cover the range of sizes in the population of sandbars 

being surveyed. However, when the sandbar chosen at random 

either was (1) too similar in size to a sandbar from the previous survey day, or (2) was too large for survey replication 

while maintaining overall efficiency, a different sandbar was 

chosen for survey replication. In a couple of cases, replicates 

of sandbar area were surveyed simply because time allowed 

for the technician to make an additional pass around the sandbar before height measurements were concluded. No replicate 

surveys were performed on sandbars with resident tern or 

plover colonies to minimize disturbance to those species.

Replicate surveys of sandbar geometry were performed 

differently and served two different purposes, depending on 

the geometric component being measured. Replicate surveys 

of sandbar area were performed by walking the same route 
14 Emergent Sandbar Dynamics in the Lower Platte River in Eastern Nebraska—Methods and Results of Pilot Study, 2011

around the sandbar twice with the handheld DGPS unit held 

in the same position. Replicate surveys of sandbar height 

were performed by moving the rotating laser level upstream 

or downstream an arbitrary distance of 10–50 ft, re-approximating a second survey grid with the laser rangefinder, and 

performing a second survey, with all point measurements thus 

offset from the first. The distance the rotating laser level was 

moved was more for sandbars with large high platforms and 

less for those with smaller high platforms. Replicate surveys 

of sandbar area allowed approximations of uncertainty in the 

measured sandbar area caused by positional variations in the 

DGPS-surveyed points resulting from any or a combination of 

three sources: (1) small-magnitude deviations in technician’s 

route reflecting recognizability of the dry crest’s edge, 

(2) variations in the way the technician held the GPS 

unit (possible effects on multi-path signal reception), or 

(3) variations in precision caused by differences in GPS 

satellite coverage between the individual replicate surveys. 

Replicate surveys of sandbar height provided estimations of 

uncertainty in the measured height of a sandbar caused by 

differences in human perception of the high platform extent 

and shape, as evidenced by differences in the placement of 

the rotating laser level and the layout of the survey grid, and 

ultimately resulted in different locations of spot measurements 

of height. Because the same operator always collected both 

replicates of either sandbar height or sandbar area at a specific 

sandbar, the resulting data do not allow an estimate of uncertainty attributed to observer bias.

Data Analysis

Two primary methods were used to characterize 

uncertainty of replicated sandbar-geometry measurements: 

(1) summary statistics, including confidence intervals on 

the mean difference between data pairs, and (2) two-tailed 

Wilcoxon signed-rank tests of data pairs. For sandbar area, 

data pairs were the paired original and replicate survey result. 

For sandbar height, spot measurements of height from original 

and replicate surveys were paired based on rank, as opposed 

to position in the survey grid, because differences in sandbar height ultimately were evaluated for ranked data values 

(median and maximum), and these values are not associated 

with location within the survey grid. Summary statistics and 

confidence intervals were computed for differences between 

ranked data pairs, and statistical tests of difference were 

performed to evaluate differences in median and maximum 

sampled data. A non-parametric statistical test was used 

because at least some of the datasets potentially appeared to be 

non-normally distributed, and replicate sample sizes were too 

small to confidently determine the normality of distributions.

Detailed Surveys of Sandbar Topography

Although replicate surveys of sandbar areas and geometries addressed the variability of measurements associated with 

surveying using the same method, replicate surveys could not 

address three other areas of concern: (1) the accuracy of the 

absolute magnitude of areas being measured with the handheld 

DGPS units, (2) confidence in the field technician’s ability 

to perceive the highest part of the sandbar topography, and 

(3) the representativeness of rapid-assessment bar height 

measurements relative to the height of tern and plover nesting locations on the same bar. To address these concerns, a 

quality-control survey of sandbar topography was conducted 

during a 2-day period, independent of the three primary survey 

periods, and outside the tern and plover nesting season. 

On the first day, the rapid-assessment method was used 

to survey the height and area of four bars ranging in size from 

3.40 to 29.2 acres. Flagging was used to mark the locations of 

point measurements of sandbar height taken with the rotating 

laser level so they could be precisely relocated. Two of the 

four sandbars had confirmed nesting of interior least terns during summer 2011 (Joel Jorgensen, Nebraska Game and Parks 

Commission, written commun., 2011).

On the second day, the topography of the same four 

sandbars was surveyed using a total station. The total station 

was placed near the center of the sandbar to minimize the 

greatest measurement distance. The manufacturer’s specifications for the total station indicated a horizontal and vertical 

accuracy of about ±0.01 ft per 1,000 ft of distance when using 

a reflector prism (Sokkia Company, 2004). Sandbar topography on each sandbar was surveyed along 10 transects spaced 

evenly across the longitudinal length of each sandbar (fig. 5). 

The edges of the dry crest were encoded along each transect 

data series. Point spacing of topographic measurements along 

each transect generally was determined by equal increments; 

however, different technicians used slightly different spacing when walking, and additional points were surveyed to 

capture major topographic slope breaks. Point measurements 

of sandbar topography also were surveyed at the flagged locations where height measurements were taken with the rapidassessment method, as well as on the sandbar surface next to 

nest scratches and bowls on the two sandbars where previously used nests were present. In some cases, tern or plover 

scrapes had eggshell remnants, and, in other cases, no eggshell 

remnants were present. Scrapes were assumed to have been 

either used for nesting or initially excavated as potential nesting locations by terns or plovers. In either case, the authors 

assumed that the scrapes were roughly representative of the 

heights used by terns and plovers for nesting in 2011.

Data Analysis

To address the accuracy of sandbar area measurements 

made using the rapid-assessment method, the areas of polygons made from the DGPS survey were compared with those 

made using the total-station survey data. Polygons of the sandbar area were generated from the topographic data surveyed 

with the total station by connecting the dry-crest edge points 

of each transect along the outside of the sandbar. Although this 

type of polygon is crude compared to the detailed outline of 

the dry crest taken using the DGPS survey, the transect density 
Emergent Sandbar Dynamics in the Lower Platte River 15

was sufficient to encompass the general outline of the sandbar 

and to identify any large-magnitude differences between the 

DGPS and total-station surveys.

A cumulative frequency distribution of topography was 

generated from the topographic points surveyed with the total 

station to address the ability of the technician to identify the 

high platform of a bar and to provide some characterization of 

how well measurements of height using the rapid-assessment 

method captured the highest topography of the sandbar. 

Several steps were taken to remove bias in the topographic 

distribution associated with the downstream slope of the 

sandbar. First, the height of each water-surface point was corrected for water-stage fluctuations using a linear time-based 

interpolation and the net change in stage recorded over the 

time of the total-station survey (observed on a temporary staff 

plate). Second, survey points of the water surface from the 

total station were located along a straight line drawn through 

the center of the sandbar using an addressing tool in a GIS. 

Next, an ordinary least-squares regression line (Helsel and 

Hirsch, 2002) was fit to the stage-indexed heights of surveyed 

water-surface points along the length of the sandbar. Finally, 

the slope of the ordinary least-squares regression line was 

used to normalize the topographic points from the total-station 

survey to an interpolated flat-water surface. The cumulative 

topographic distribution did not include the points collected 

to measure water-surface height. The nest heights and highplatform heights surveyed using the rapid-assessment methods 

were then compared to the percentiles of sandbar height from 

the cumulative topographic distribution of the sandbar.

Emergent Sandbar Dynamics in the 

Lower Platte River 

Along the 22-mi study segment, the variation from reach 

to reach in percentage of total emergent sandbar area and 

range of sandbar heights generally was consistent between 

survey periods (seasons). The highest part of many sandbars 

formed by the peak discharge in 2010 remained emergent 

above the stage of the peak discharges of 2011, but these 

subsequent flows caused a net lowering of sandbars overall. 

Analysis of quality-control data indicated that the rapidassessment method developed for this study was sufficiently 

sensitive to document the changes in sandbar height and area 

measured between seasons.

Seasonal Differences in Number and Areas of 

Emergent Sandbars

Average streamflows at the Louisville gage during the 

spring, summer, and fall survey periods were 11,250, 10,786, 

and 8,533 ft3/s, respectively—a discharge differential of as 

much as 30 percent, and an average stage range of about 

0.43 ft at the Louisville gage (based on rating number 14, M.J. 

Andersen, U.S. Geological Survey, written commun., 2011). 

Nevertheless, the diel fluctuation in river discharge because of 

hydrocycling within each survey period resulted in overlap in 

the daily ranges of discharges between periods (table 1). Stage 

ranges were as much as 0.76 ft during a single day, indicating 

that the measurements of sandbar area across survey periods 

had enough overlap to be comparable, but likely had some 

relative bias, particularly for comparisons with the fall survey 

period when discharges in the LPR were much lower than during the two previous survey periods.

The area of 68 sandbars, with a total estimated emergent 

area of 360 acres, was measured or estimated during the spring 

survey period (table 2). Eighty-five percent of the total estimated emergent sandbar area during the spring survey period 

was contained in the 25 sandbars surveyed with the DGPS 

method, of which only one was smaller than 2 acres. Measured 

emergent sandbar areas ranged from about 1.90 to 39.9 acres, 

with a median sandbar area of about 10.5 acres. Mid-channel 

sandbars were the most abundant of the three measured 

sandbar classes, and ranged in emergent area from 1.90 to 

22.5 acres (fig. 6). There were few island-attached sandbars 

during the spring survey, but these sandbars were generally 

Total station topographic points

(detailed topography)

Sandbar height measurement 

points (rapid assessment)

Approximate margin 

of high platform

Previously used 

nests (field 

assessed)

Margin of dry crest of sandbar 

measured using DGPS

Total station

41°2.8’

96°07’ 96°6.7’

41°2.7’

Base from U.S. Department of Agriculture Farm 

 Service Agency, 2007

Nebraska State-Plane projection

Horizontal coordinate information is referenced to 

 North American Datum of 1983 (NAD 83)

0

0 25 50 METERS

250 500 FEET

Figure 5. Examples of point-and-transect spacing of detailed 

topographic measurements collected with a total-station system, 

summer 2011; locations of previously used tern nests interpreted 

by field technicians; and locations of spot heights measured 

using the rapid-assessment survey method, lower Platte River, 

Nebraska, 2011.
16 Emergent Sandbar Dynamics in the Lower Platte River in Eastern Nebraska—Methods and Results of Pilot Study, 2011

large, ranging in emergent area from about 6.70 to 18.9 acres. 

Bank-attached sandbars contributed about half of the total 

measured emergent sandbar area during the spring survey 

period (table 2), with a median area of 13.8 acres, the largest 

of the three sandbar classes (fig. 6).

A total of 105 sandbars, with a total estimated emergent 

area of 315 acres, were surveyed or estimated during the summer survey period (table 2). Thirty-eight of these sandbars 

were measured using DGPS, of which 9 were smaller than 

2 acres. Measured sandbars contributed about 84 percent 

of the total emergent sandbar area (table 2), and sandbars 

larger than 2 acres contributed about 80 percent of the total. 

A decrease in total emergent sandbar area apparently had 

occurred since the spring survey despite slightly lower daily 

streamflows relative to the spring survey period (table 1). The 

decrease in sandbar area primarily was the result of a decrease 

in the total area of bank-attached sandbars (table 2). Despite 

a decrease in median and maximum individual area, midchannel sandbars increased in total number as well as in total 

emergent area (table 2), indicating that the intervening high 

flows not only dissected larger mid-channel bars into smaller 

bars, but also created a number of new emergent mid-channel 

sandbars.

Seventy-three sandbars, with a total estimated emergent 

area of 402 acres, were measured during the fall survey period. 

The increase in sandbar area occurred despite a net decrease 

in total number of emergent sandbars and was a consequence 

of the lower average daily flows during the fall survey period 

relative to the summer and spring survey periods (table 1). 

More than 90 percent of the total estimated emergent sandbar 

area was contained in the 30 sandbars measured using DGPS, 

29 of which were larger than 2 acres. Much of the increase in 

emergent area occurred by an increase in the area of midchannel sandbars larger than 2 acres, despite a net decrease 

in the total number of these types of bars, indicating that a 

substantial area of shallow-water habitats had surrounded the 

larger mid-channel sandbars, and this area emerged because of 

lower flows.

Intersection of sandbar polygons from each survey season 

resulted in 17 collocated sandbars common to the spring and 

Values no farther 

from the median 

than 150 percent 

of the interquartile 

range

EXPLANATION

Spring Summer Fall Spring Summer Fall

0

10

20

30

40

Area, in acres

25

38

30

13

75th percentile

Outlier

Number of values

Median

25th percentile

9

5

5

3

7

7

7

26

18

50

0

10

20

30

40

50

A. All sandbar types B. Mid-channel sandbars

C. Island-attached sandbars D. Bank-attached sandbars

Figure 6. Frequency distribution of emergent sandbar area by survey period and sandbar type, lower Platte River study segment, 

Nebraska, 2011: (A) all bar types, (B) mid-channel sandbars, (C) island-attached sandbars, and (D) bank-attached sandbars. Data 

shown are for sandbars measured using differential-grade global-positioning system, and are generally restricted to sandbars larger 

than 2 acres.
Emergent Sandbar Dynamics in the Lower Platte River 17

summer survey periods, 27 collocated sandbars common to the 

summer and fall survey periods, and 21 collocated sandbars 

common to the spring and fall survey periods (fig. 7).

The larger number of collocated sandbars common to the 

spring and fall survey periods relative to the spring and summer survey periods is likely a result of the larger overall emergent area of sandbars during the 

fall period that increased the 

potential for overlap between 

polygons. Changes in sandbar 

area between seasonal survey periods, when measured 

from collocated sandbar pairs, 

were similar to the seasonal 

differences observed for the 

larger population of sandbars. 

Changes in sandbar area measured at the individual sandbar 

level between the spring and 

summer survey periods generally were net erosional. Median 

change in sandbar area from 

spring to summer was greatest 

for island-attached sandbars, 

which decreased in area 

(fig. 7A). Mid-channel sandbar 

pairs had a median decrease of 

4.3 acres between spring and 

summer survey periods. Bankattached sandbars had the widest range in responses between 

spring and summer, with most 

bars eroding, but some bars 

undergoing net expansion of up 

to 1.9 acres (fig. 7A). Sandbar 

area expanded between the 

summer and fall survey periods 

at most collocated sandbar 

pairs; however, this expansion 

is interpreted by the authors 

to be a consequence of the 

Figure 7. Frequency distributions of differences in emergent sandbar area among collocated 

pairs of sandbars between survey seasons by sandbar type, lower Platte River, Nebraska, 2011: 

(A) spring–summer difference, (B) summer–fall difference, and (C) spring–fall difference. Data 

shown are for sandbars measured using handheld differential-grade global-positioning system, 

and are generally restricted to sandbars larger than 2 acres.

Values no farther 

from the median 

than 150 percent 

of the interquartile 

range

EXPLANATION

75th percentile

Outlier

Number of values

Median

25th percentile

7

BankattachedIslandattachedMidchannel

BankattachedIslandattachedMidchannel

-20

-10

10

Change in area, in acres

0

30

20

-20

-30

-10

10

0

30

20

40

A. Spring–summer B. Summer–fall

C. Spring–fall

7 8

2

6 16

5

7

11

3

Table 2. Number and total measured and estimated areas of emergent sandbars in the study segment of the lower Platte River, 

Nebraska, by bar type and survey period, 2011. 

[Measured sandbar areas, Emergent sandbar areas measured using differential-grade global-positioning system (DGPS) by walking perimeter of dry crest; 

generally only includes sandbars larger than 2 acres; Estimated sandbar areas, Emergent sandbar areas estimated using laser rangefinder; generally only 

includes sandbars less than 2 acres. With few exceptions, these were low, mid-channel bars; n with area, <2.0 acres, number of sandbars measured using 

DGPS method that were eventually calculated to be less than 2 acres; Total estimated emergent sandbar area, total may differ from sum of subtotals because 

of rounding; n, sample size; <, less than] 

Survey 

period

Measured sandbar areas

Estimated sandbar areas Total estimated emergent 

n sandbar area (acres) n with area 

<2.0 acres

Mid-channel Islandattached

Bankattached Total 

Area (acres)

(acres) n

Area 

(acres) n n n

Total 

(acres)

Spring 25 1 113 13 39 3 9 308 43 52 360

Summer 38 9 132 26 35 5 7 264 67 51 315

Fall 30 1 185 18 49 5 7 369 43 32 402
18 Emergent Sandbar Dynamics in the Lower Platte River in Eastern Nebraska—Methods and Results of Pilot Study, 2011

lower flows during the fall 

measurement period (fig. 7B; 

table 1). In contrast, comparisons between the spring and fall 

survey periods still indicated 

net erosion for most bank- and 

island-attached sandbars, indicating the erosion was of large 

enough magnitude to overpower 

the effect of the lower flows on 

emergent area at these types of 

sandbars (fig. 7C). Differences 

in sandbar area among spring-tofall data pairs indicated that 11 

mid-channel sandbars expanded 

by a median value of 1.5 acres 

(fig. 7C).

Seasonal and 

Longitudinal Changes in 

Emergent Sandbar Height

A comparison of cumulative frequency distributions of 

measured sandbar height values 

between survey periods indicated 

that the high platform of surveyed sandbars was much higher 

during the spring survey period than during the subsequent 

summer and fall survey periods (fig. 8). Median measured 

sandbar height decreased from about 4.5 ft to 3.0 ft (height 

indexed to discharge of 4,000 ft3/s) between the spring and 

summer survey periods. During the spring survey period, most 

measured heights were between 3.5 and 5.5 ft, whereas, during 

the summer and fall survey periods, most measured heights 

were between 2.0 and 5.0 ft. Cumulative frequency distribution curves also indicated that measured sandbar heights were 

similar between the summer and fall survey periods, which 

was not unexpected because no high-magnitude discharges 

occurred between those surveys (maximum daily mean 

discharge was less than 15,000 ft3/s; fig. 2). Median sandbar 

height measured during the summer survey period was about 

3.0 ft, and was about 3.2 ft during the fall survey period 

(fig. 8). Although this difference would indicate an increase 

in bar height, quality-control data (described in the “Quality 

Control” section of this report) indicated that this difference 

is within the range of expected measurement uncertainty, and 

this difference in medians was one of the largest differences 

between the summer and fall surveys’ cumulative frequency 

distributions (fig. 8).

The longitudinal distribution of sandbar heights by survey period indicated that, although sandbar height decreased 

between the spring survey period and subsequent survey 

periods, the general longitudinal pattern and trend of sandbar 

heights was similar across periods (fig. 9). Sandbar heights 

tended to follow a wavy longitudinal pattern whereby certain 

reaches consistently had more abundant and higher sandbars, and others had fewer, lower sandbars. LOESS lines fit 

to sandbar heights indicated that sandbar heights generally 

trended upward, if only slightly, in the downstream direction, 

although the significance of this trend was not tested. Large 

sandbars (sandbars with emergent areas larger than 2 acres) 

tended to be more abundant and higher between State Highway 50 and the Cedar Creek area, and lower and less abundant 

in a 3-mi reach immediately upstream from State Highway 50 

and in the reaches near river miles 7–12 (fig. 9). Longitudinal 

patterns in sandbar height did not show a distinct covariation 

with other channel morphometric characteristics, although 

higher channel-sinuosity values tended to be weakly spatially 

correlated with more dense clusters of emergent sandbars, 

and unvegetated, active-channel width tended to trend slightly 

downward along the study segment as sandbar height tended 

to trend slightly upward (fig. 9D).

Median and maximum measured sandbar height values for larger sandbars indicated that the high platform on 

sandbars was between 0.5 and 2.0 ft higher during the spring 

survey period than during the subsequent summer and fall 

survey periods. Bank-attached and mid-channel sandbars 

generally had the greatest group median height value (median 

of all sandbar-median values from all measured sandbars 

within each seasonal survey) and the greatest group median 

of maximum height value (median of all sandbar-maximum 

Figure 8. Cumulative frequency distribution of measured sandbar heights from three survey 

periods, lower Platte River, Nebraska, 2011. Percentage of sampled points lower than indicated height

100

80

60

40

Fall survey

Stage height at 45,400 cubic 

feet per second at Ashland 

gaging station

Stage height at 45,400 cubic 

feet per second at Louisville 

gaging station

Average stage height at 

136,000 cubic feet per 

second within study reach

Spring survey

Median

Summer survey

20

0

1 2 3 4

Measured sandbar height, in feet above stage of 4,000 cubic feet per second

5 6 7 8
Emergent Sandbar Dynamics in the Lower Platte River 19

Figure 9. Longitudinal distributions of measured sandbar heights from three survey periods in 2011 and river-channel 

morphometric characteristics, lower Platte River, Nebraska: (A) sandbar heights measured during the spring survey 

period; (B) sandbar heights measured during the summer survey period; (C) sandbar heights measured during the fall 

survey period; and (D) bank heights measured during the spring and summer survey periods, channel sinuosity, and 

unvegetated channel width. Channel sinuosity and unvegetated channel width are from Elliot and others (2009) and Elliot 

(2011), respectively. 

25 20 15

Distance upstream from the Missouri River, in miles

10 5

8

6

4

Interstate 80 Cedar Creek Lied Bridge

2

0

8

6

4

2

0

8

6

4

2

0

8

9

6

4

2

1 500

1,000

1,500

2,000

2,500

1.10 3,000

1.00

1.05

A. Sandbar heights measured during spring survey period

B. Sandbar heights measured during summer survey period

C. Sandbar heights measured during fall survey period

D. Sandbar heights measured during spring and summer survey periods

Salt Creek State Highway 50 U.S. Highway 75

Measured height above stage at 4,000 cubic feet per second, in feet

Channel sinuosity, at 6,560 foot scale

Unvegetated channel width, in feet

EXPLANATION

Locally weighted least-square regression fitting 

line (type - Gaussian: span = 40 percent)

Unvegetated channel width (Elliot, 2011)

Measured height of natural high bank above 

stage 4,000 cubic feet per second

Sinuosity of river channel measured at 6,500 

foot interval (Elliot and others, 2009)

Location of major feature along river channel 

centerline

Measured sandbar height
20 Emergent Sandbar Dynamics in the Lower Platte River in Eastern Nebraska—Methods and Results of Pilot Study, 2011

values from all measured sandbars within each seasonal 

survey) across all seasons (fig. 10). The group-median heights 

for all sandbar types measured during the spring survey period 

indicated that the high platform of most sandbars was about 

2–3 ft lower than the estimated stage of the 2010 instantaneous 

maximum discharge of 136,000 ft3/s, and group medians of 

maximum heights were between 1.5 and 2.5 ft lower than the 

stage at this discharge (fig. 10). Group-median heights for 

mid-channel and island-attached sandbars measured in the 

spring were well within the range of estimated stages of 

the 2011 instantaneous maximum discharge of 45,400 ft3/s 

(fig. 10). The group-median height of bank-attached sandbars 

Values no farther 

from the median 

than 150 percent 

of the interquartile 

range

EXPLANATION

Height above water stage at 4,000 cubic feet per second, in feet

1

Spring Summer Fall Spring Summer Fall Spring Summer Fall

2

3

4

5

6

7

7.5

1

2

3

13

13

26

18

Average height of stage at

136,000 cubic feet per second

Height of stage at 45,400 cubic feet 

per second at Louisville, Nebraska

Average height of stage 

at 45,400 cubic feet per 

second

Height of stage at 45,000 cubic feet 

per second at Ashland, Nebraska

26

18

3

5 5

3 5

5

9 7

7

7

7

4

5

6

7

7.5

75th percentile

Outlier

Number of values

Median

25th percentile

7

9

A. Mid-channel sandbars B. Island-attached sandbars C. Bank-attached sandbars

D. Mid-channel sandbars E. Island-attached sandbars

Median values

Maximum values

F. Bank-attached sandbars

Figure 10. Frequency distributions of measured sandbar heights above river stage at 4,000 cubic feet per second for three survey 

periods and different bar types, lower Platte River, Nebraska, 2011: (A–C) median measured sandbar heights, and (D–F) maximum 

measured sandbar heights.
Emergent Sandbar Dynamics in the Lower Platte River 21

measured in the spring was higher than the estimated stages 

of the 2011 peak flow. Maximum measured heights from the 

spring survey, including the group-median of maximums for 

all sandbar types, generally were higher than the estimated 

stages of the 2011 peak flow, indicating that at least small parts 

of many sandbars were not inundated. However, the decrease 

in median and maximum measured heights following that latespring peak flow indicated that bar heights were often lowered, 

likely by lateral or downstream erosion. With the exception of 

bank-attached sandbars, group-median and group-median of 

maximum measured sandbar heights generally were similar 

between the summer and fall survey periods (fig. 10).

A comparison among seasonal survey periods of ranked 

differences between collocated sandbar pairs further confirms 

that a lowering of sandbar heights followed the spring survey 

period. Median values of differences in sandbar-median and 

-maximum height among collocated sandbar pairs between 

the spring and summer survey periods indicated decreases 

of 0.83 and 0.81 ft, respectively (table 3). Differences in the 

same statistics for sandbar pairs compared between spring and 

fall survey periods indicated decreases of 0.73 and 0.99 ft, 

respectively; and between the summer and fall survey periods 

indicated a decrease of 0.09 ft, for each statistic. The p-values 

resulting from Wilcoxon signed-rank tests of seasonal data 

pairs from collocated sandbars indicated that the differences in 

median and maximum heights between the spring and summer 

survey seasons and between the spring and fall survey seasons 

were significant at the 95-percent confidence level (table 3).

One of the primary aims of the study was to characterize 

the height of sandbars relative to the stage of formative flows. 

Because of the rarity of the 2010 instantaneous peak discharge, characterizing the height of sandbars formed by it was 

of particular importance. Sandbar height measurements for 

this study were referenced to the stage of the Platte River at 

nearby gages at a discharge rate of 4,000 ft3/s. The average 

stage height difference between 4,000 ft3/s and the stage 

at 136,000 ft3/s (the magnitude of the instantaneous peak 

discharge in 2010 measured at the Louisville gage) is 

7.1 ft (fig. 8). The frequency distribution of measured sandbar 

heights from high platforms for the spring survey indicates 

that 100 percent of the measured heights were less than 

6.8 ft, 90 percent of the measured heights were less than 

5.5 ft, and more than half of measured heights were less than 

4.5 ft (fig. 8). These data indicate that, although the high 

platform of sandbars can approach the maximum stage of 

formative flow events, this height is rare, and more often the 

highest part of sandbars is 1–2 ft less than the formative stage. 

Depending on which rating table is used (Ashland number 6 

or Louisville rating number 14, M.J. Andersen, U.S. Geological Survey, written commun., 2011), between about 10 and 

50 percent of measured heights from the spring survey 

would have been inundated by the 2011 high discharge of 

45,400 ft3/s, a discharge 67 percent less than the 2010 flood. 

Using the rating from the Louisville gage, more than 98 percent of measured heights from the spring survey would have 

been inundated by a flow of about 85,000 ft3/s, a discharge 

about 38 percent less than the 2011 flood.

The methods of data collection, data analysis, and presentation for the study were different enough from previous 

reports of sandbar heights (Ziewitz and others, 1992; Brown 

and Jorgensen, 2008; Brown and Jorgensen, 2009) that direct 

comparison of measured bar heights is difficult. Nevertheless, 

sandbar height data in Ziewitz and others (1992) and Brown 

and Jorgensen (2008) were collected shortly after low-frequency, high-magnitude discharge events, and their measurements of sandbar height indicate a consistent pattern of 1 and 

3 ft of space, respectively, between the estimated stages 

of those floods and reported sandbar heights. The peak discharge of the LPR in 1988, when Ziewitz and others (1992) 

surveyed sandbar heights in the LPR, had a magnitude of only 

24,000 ft3/s at the Louisville gage, but the peak discharge in 

1987 was 103,000 ft3/s. The heights of sandbars reported by 

Ziewitz and others (1992) ranged from a mean-of-maximums 

of 2.2 ft to a maximum-of-maximums of 5.5 ft, which spans 

nearly 90 percent of the range of heights measured during 

the spring survey for the study described in this report 

(fig. 8). Using the latest stage-discharge rating table (no. 14) 

from the Louisville gage as an approximation of the stagechange ranges in 1988, mean-of-maximum sandbar heights 

would have been about 0.66 ft lower than the stage of the 

river at 24,000 ft3/s, but maximum-of-maximum reported 

heights would have been more than 2.6 ft higher, suggesting 

Table 3. Summary statistics of differences between measured median and maximum sandbar heights and results of Wilcoxon 

signed-rank comparison tests between survey periods at collocated sandbar pairs, lower Platte River, Nebraska, 2011.

[Summary statistics of paired-ranked differences, differences between indicated statistic summarizing measured heights on collocated sandbar pairs between 

seasons; p-values, probability that central value of difference between data pairs is 0 at 95-percent confidence level]

Measurement periods 

compared

Number 

of data 

pairs

Summary statistics of paired-ranked differences

p-values

Differences in median height Differences in maximum height

Median 

(feet)

Mean 

(feet)

Interquartile 

range of differences (feet)

Median 

(feet)

Mean 

(feet)

Interquartile 

range of differences (feet)

Median 

measured 

heights

Maximum 

measured 

heights

summer minus spring 17 -0.83 -0.67 -1.07 – -0.07 -0.81 -0.93 -1.34 – -0.52 <.01 <.001

summer minus fall 27 -0.09 0.02 -0.32 – 0.17 -0.09 0 -0.36 – 0.34 0.56 0.71

spring minus fall 21 -0.73 -0.74 -1.09 – -0.14 -0.99 -0.99 -1.39 – -0.49 <.01 <.001
22 Emergent Sandbar Dynamics in the Lower Platte River in Eastern Nebraska—Methods and Results of Pilot Study, 2011

that sandbars observed by Ziewitz and others (1992) in 1988 

likely were formed by the 1987 flood. Mean-of-maximum and 

maximum-of-maximum measured heights reported by Ziewitz 

and others (1992) would have been about 4.2 and 0.9 ft lower, 

respectively, than the estimated stage of the formative discharge of 103,000 ft3

/s.

The peak discharge in 2008, just prior to when Brown 

and Jorgensen (2008) surveyed sandbar heights in the LPR, 

was 96,600 ft3/s. In that part of the LPR that coincides with 

the study area described in this report, Brown and Jorgensen 

(2008) reported maximum sandbar heights between 0.5 and 

2.8 ft higher than the stage during the late summer discharge 

of 19,500 ft3/s, which, based on the latest rating table from 

the Louisville gage, indicates that bar heights formed by the 

2008 flood were about 1.1–3.4 ft lower than the maximum 

stage of the flood. Therefore, data from the study described 

in this report and data reported by Ziewitz and others (1992) 

and Brown and Jorgensen (2008) indicate that sandbar heights 

formed by a low-frequency discharge event are subject to subsequent inundation by discharges as much as 25–80 percent of 

their respective formative event.

Seasonal Changes in Longitudinal Distribution 

of Emergent Sandbars

Although the magnitude of total emergent sandbar 

area varied between surveys, the streamwise distribution of 

emergent sandbar area did not change substantially among the 

three survey periods. A comparison of plots of the longitudinal 

distribution of sandbar area, normalized as percentage of total 

sandbar area, indicated that river reaches with abundant emergent sandbar area tended to be the same across the three 

survey periods (fig. 11). The surveyed sandbar area ranged 

Interstate 80 Lied Bridge Cedar Creek

0

5

10

15

0

5

10

15

0

5

10

15

0

-5

5

-10

-15

27 25 20 15

Distance upstream from the Missouri River

Sandbar area, river mile subtotal, as percentage of total sandbar area

Net channel change, in percent

of total sandbar area

10 5 2

10

15

Salt Creek

State Highway 50 U.S. Highway 75

A. Spring survey period

B. Summer survey period

C. Fall survey period

D. Difference between spring and summer survey periods

Figure 11. Longitudinal distribution of emergent sandbar area for three survey periods and net change in emergent 

sandbar area between river miles 3 and 26, lower Platte River, Nebraska, 2011: (A) spring survey period, (B) summer 

survey period, (C) fall survey period, and (D) difference in percent of total sandbar area by river mile between spring and 

fall survey periods.
Emergent Sandbar Dynamics in the Lower Platte River 23

from 0 to 12.7 percent of 

the total study-segment 

area within an individual 

river mile, but multimile reaches where each 

individual river mile had 

at least 5 percent of total 

emergent bar area tended 

to cluster in the same 

locations during each 

survey season. In particular, river miles 8 and 20, 

and those river miles near 

Cedar Creek tended consistently to have an abundance of emergent sandbar area. Between spring 

and fall survey periods, 

the median change in the 

fraction of total emergent 

sandbar area, tabulated by 

river mile, was a decrease 

of only 0.21 percentage 

point, and the interquartile 

range of such net changes 

was from -1.32 to 

1.06 percentage points. 

The largest shift in fraction of total emergent 

area was for river mile 

24, where the percentage 

of total emergent sandbar 

area increased by about 

10 percentage points 

between the spring and fall 

survey periods (fig. 11D). 

The large shift in percentage of total sandbar area 

resulted from a combination of additional emergent sandbar area created by deposition 

during the high flows of May and June 2011 and a downstream 

shift in the areal centroid of one large sandbar, which moved 

the accounting of its bar area into river mile 24 from river mile 

25. 

When compared among collocated sandbar pairs, shifts in 

sandbar locations, measured as shifts in sandbar areal centroid 

between survey seasons, indicated that sandbars tended to 

erode or deform from upstream to downstream, but overall 

shifts in center of area tended to be small. Median streamwise 

shift in areal centroid among sandbar data pairs, between 

the spring and summer survey seasons, ranged from -0.05 to 

-0.01 mi (negative value means downstream shift), and was 

greatest for mid-channel sandbars (fig. 12A). Despite substantial increases in emergent sandbar area, shifts in sandbar areal 

centroid among data pairs between the summer and fall survey 

seasons had median values ranging from -0.02 to -0.003 mi, 

and had narrower interquartile ranges than the corresponding 

spring-to-summer shifts. Mid-channel sandbar pairs had the 

most variability in shifts between summer and fall survey periods (fig. 12B). The small median values of net shifts in sandbar areal centroid among data pairs between the spring and 

fall surveys (range of -0.02 to 0.03 mi) were dwarfed by the 

variability of shift distances within each bar type (fig. 12C).

The difference between the spring-fall and summer-fall 

shifts in areal centroid (fig. 12) indicates that the high flows 

that occurred between the spring and summer survey periods largely shifted sandbar areal centroids downstream. The 

increase in number and magnitude of positive (upstream) 

shifts between the spring and fall survey periods indicates that 

more of the submerged part of the sandbar’s mass was near the 

upstream end of some sandbars. The downstream shift in sandbar areal centroid between spring and summer surveys was, in 

some cases, offset by an upstream shift between summer and 

Figure 12. Frequency distributions of shifts in sandbar areal centroids among collocated sandbar 

pairs between three survey periods, for different sandbar types, lower Platte River, Nebraska, 

2011: (A) between spring and summer survey periods, (B) between summer and fall survey periods, 

and (C) between spring and fall survey periods. Data shown are for sandbars measured using a 

handheld differential-grade global-positioning system, and are generally restricted to sandbars 

larger than 2 acres. Sandbar pairs were identified as sandbars that overlapped when intersected 

in a geographic information system.

Values no farther 

from the median 

than 150 percent 

of the interquartile 

range

EXPLANATION

75th percentile

Outlier

Number of values

Median

25th percentile

7

Shift in sandbar areal centroid, in miles

0.3

-0.3

0.2

-0.2

0.1

-0.1

0

0.3

-0.3

0.2

-0.2

0.1

-0.1

0

7 6

7

3

11

5

16

2

8

A. Spring–summer B. Summer–fall

C. Spring–fall

BankattachedIslandattachedMidchannel

BankattachedIslandattachedMidchannel
24 Emergent Sandbar Dynamics in the Lower Platte River in Eastern Nebraska—Methods and Results of Pilot Study, 2011

fall. The net shift upstream was most common for mid-channel 

sandbars and could indicate that the high flows of 2011 did not 

simply erode the upstream ends of these bars and transport the 

sand away; rather, for some bars, these areas were reworked, 

causing net lowering and subsequent deposition of additional 

sand mass along the upstream margins of sandbars. The 

small number of bars in each sandbar type category limits the 

strength of evidence for this process inferred from the 2011 

surveys alone.

Quality Control

Sandbar location and geometry can be quantified in many 

ways, using varied methods and many associated measures. 

Each measure is sensitive to method and instrument limitations of precision and accuracy, as well as operator error or 

bias. Because the rapid-assessment data-collection methods 

developed and used for this study are new (although they are 

influenced by methods reported elsewhere), some characterization of the nature of uncertainties and sensitivity of each 

measure is required to assess the quality and repeatability of 

the data collected using these methods. 

Sandbar Area

Replicate DGPS surveys of sandbar area were performed 

on 13 sandbars across much of the range of sandbar sizes present during the survey periods, and the bars where surveys were 

replicated constituted 14 percent of all measured sandbars 

(table 4). The mean relative difference in sandbar area 

between original and replicate DGPS measurements was 

0.62 percent of the original measurement, and all replicate 

measurements were within 5 percent of the original survey 

(fig. 13). A comparison of the percent difference between original and replicate measurements of sandbar area with sandbar 

size indicated that differences in area generally were both 

positive and negative, and that magnitudes were evenly 

distributed across the range of sandbar sizes measured 

(fig. 13). Results (p-values) from the Wilcoxon signed-rank 

test of replicate pairs indicated that the central value of differences between paired areas did not differ significantly from 

zero at the 95-percent confidence level (table 4).

Differences in emergent sandbar area between measurements made with the higher-precision laser total station and 

those made with the handheld DGPS method indicated that the 

overall size of areas obtained using the DGPS method is likely 

within 5 percent of the overall size of areas obtained using the 

laser total-station method (table 5). Although comparisons can 

be made for only four sandbars, differences between sandbar 

areas obtained using the laser total station and DGPS method 

were both positive and negative, and did not appear to be 

affected by the size of the sandbar. The areas delineated in the 

total-station survey had less detail along the sandbar boundary, 

but the close agreement between the magnitudes of sandbar 

area obtained using the two methods indicates that the DGPS 

Table 4. Summary of replicate surveys of sandbar area measured using differential-grade global-positioning system, lower Platte 

River, Nebraska, 2011.

[Relative percent difference, percent difference between original and replicate surveyed sandbar area, obtained by by subtracting the quotient of original and 

replicate measured areas from 1 and multiplying by 100; p-value, probability that central value of differences between data pairs is equal to 0, at 95-percent 

confidence level, from Wilcoxon signed-rank test; No., number; na, not applicable]

Survey period No. survey bars No. replicates Percent of sandbars replicated

Range areas of 

sandbars replicated, in acres

Range of relative 

percent differencesp-value

Spring 25 6 24 2.4–13.5 -2.60–3.2 na

Summer 38 4 11 2.7–15.8 -0.1–4.7 na

Fall 30 3 10 3.5–19.8 -4.2– -0.5 na

Total 93 13 14 2.4–19.8 -0.1–4.7 0.74

Table 5. Differences between sandbar areas measured with 

differential-grade global-positioning system and using a totalstation laser-surveying instrument, lower Platte River, Nebraska, 

2011.

[Relative percent difference, percent difference between areas measured 

using total station and DGPS survey methods, obtained by subtracting the 

quotient of total station and DGPS measured areas from 1 and multiplying 

by 100; QC, quality-control; DGPS, differential-grade-global positioning 

system]

QC bar

Area obtained 

from DGPS 

(acres)

Area obtained 

from total-station 

survey (acres)

Relative 

difference 

(percent)

1 3.4 3.6 -4.7

2 8.7 8.4 3.0

3 19.0 19.7 -3.5

4 29.2 28.9 1.1
Emergent Sandbar Dynamics in the Lower Platte River 25

method likely is adequate for obtaining estimates of sandbar 

emergent area within 5 percent of the actual size (table 5).

Sandbar Height

Original and replicate spot measurements of sandbar 

height were paired based on their rank within the sample datasets. Ranked original and replicate sandbar height measurements surveyed with the rapid-assessment method indicated 

that slight differences in the placement of the survey grid did 

not result in substantial differences in the range of heights 

surveyed. Ranked original and replicate data pairs show 

strong agreement (fig. 14A); mean difference between ranked 

original and replicate spot measurements of sandbar height 

was -0.002 ft, and ranged from -1.15 to 0.48 ft (fig. 14B). The 

95-percent confidence interval of mean difference in ranked 

spot measurements of sandbar height was -0.07 to 0.07, indicating a range of uncertainty of mean difference in replicated 

sandbar height of about 0.20 ft at high levels of statistical 

confidence. Although higher sandbars tended to also be larger 

sandbars, and, therefore, potentially subject to greater uncertainty, no relation of bias or imprecision in sandbar height 

with respect to sandbar size was apparent in scatter plots of 

differences between original and replicate heights (fig. 14B). 

Results from Wilcoxon signed-rank tests of 11 replicate pairs 

of sandbar-median and -maximum measured heights indicated 

that differences between data pairs are insignificant at the 

95-percent confidence level (table 6).

A comparison of measurements of sandbar and nest 

heights relative to cumulative topographic distributions generated from the more detailed total-station surveys indicated that 

technicians could adequately identify and measure the highest part (high platform) of a sandbar. Spot measurements of 

sandbar height generated using the rapid-assessment method 

ranged from the 51st to the 98th percentile of sandbar height 

frequency distributions (fig. 15), with sandbar-median and 

-maximum values of the high-platform height between the 

73rd and 93rd, and the 95th and 98th percentiles of sandbar heights, respectively. At the two sandbars where several 

nest heights were measured, individual nest heights ranged 

between the 62nd and 99th percentile of sandbar heights, 

EXPLANATION

Spring survey period

Summer survey period

Fall survey period

Mean percent difference

95-percent confidence interval of mean

99-percent confidence interval of mean

Relative difference between replicate

measurements of sandbar area,

in percent of original measured area

-6

0 5 10

Original measured sandbar area, in acres

15 20

-2

-4

0

2

4

6

Figure 13. Differences between original and replicate 

differential-grade global-positioning-system surveys of sandbar 

area, as percentage of original sandbar area, relative to sandbar 

size, lower Platte River, Nebraska, 2011.

Figure 14. Quality-control data associated with spot 

measurements of sandbar height, lower Platte River, Nebraska, 

2011: (A) original versus replicate ranked spot measurements 

of sandbar height, and (B) differences between ranked original 

and replicate spot measurements of sandbar height relative to 

sandbar area.

EXPLANATION

Spring survey period

Summer survey period

Fall survey period

Mean percent difference

95-percent confidence interval of mean

99-percent confidence interval of mean

Difference between original and replicate ranked

spot measurements of sandbar height, in feet

0 5 10

Measured sandbar area, in acres

1 2 3 4 5 6

Original ranked spot measurement

of sandbar height, in feet

15 20 -1.5

-1.0

-0.5

0

0.5

1.0

Replicate ranked spot measurement of

sandbar height, in feet

6

5

4

3

2

Line of perfect 

agreement

1

1.5

A

B
26 Emergent Sandbar Dynamics in the Lower Platte River in Eastern Nebraska—Methods and Results of Pilot Study, 2011

and median nest heights ranged between the 86th and 

89th percentiles (fig. 15). Although nest heights only were 

measured at two sandbars, a comparison of the range of highplatform heights obtained by the rapid-assessment method 

and the range of surveyed heights of nests indicated that at 

least the highest one-half of measured high-platform heights 

overlapped with and adequately represented the range of nest 

heights observed in 2011.

Limitations of the Study

The emergent sandbar data represent only three seasons 

within a single calendar year and sandbar configurations 

chiefly affected by, at most, two high-discharge events (in 

2010 and 2011). Without additional data collected over several 

more years and several more high-flow events, it is unclear 

whether the results of this study show long- or short-term 

sandbar dynamics; therefore, the time scale over which they 

can be applied is similarly uncertain. Likewise, the geometries 

A B

C D

EXPLANATION

Cumulative percent of topography

0

20

40

60

80

100

0

20

40

60

80

100

Sandbar number 1 Sandbar number 2

Sandbar number 3 Sandbar number 4

Bar area = 3.4 acres

Total station sample size = 77

Bar height sample size = 5

Bar area = 8.7 acres

Total station sample 

 size = 151

Bar height sample 

 size = 9

Nest height sample 

 size = 6

Bar area = 19 acres

Total station sample size = 126

Bar height sample size = 9

Nest height sample size = 26

Cumulative bar 

topography method

Bar area = 29.2 acres

Total station sample 

 size = 193

Bar height sample size = 9

Median of sampled 

heights with rapidassessment method

9.9 10.0 10.1 10.2 10.3 10.4 10.5 10.6

Height, in feet above arbitrary datum

Median and range of measured high-platform heights

Median and range of measured nest heights 

9.9 10.0 10.1 10.2 10.3 10.4 10.5 10.6

Median of sampled 

heights with rapidassessment method

Median of sampled 

heights with rapidassessment method

Median of sampled 

heights with rapidassessment method

Median of sampled 

nest heights

Median of sampled 

nest heights

Cumulative bar 

topography method

Cumulative bar 

topography method

Cumulative bar 

topography method

Figure 15. Cumulative frequency distribution of sandbar topography, medians, and ranges of spot measurements 

of sandbar height using the rapid-assessment survey method, and medians and ranges of heights of previously 

used tern nests on four sandbars of varying size, lower Platte River, Nebraska, 2011: (A) sandbar number 1, 3.4 

acres; (B) sandbar number 2, 8.7 acres; (C) sandbar number 3, 19 acres; and (D) sandbar number 4, 29.9 acres.
Emergent Sandbar Dynamics in the Lower Platte River 27

and locations of sandbars present during the spring survey 

period were measured more than 9 months after the 2010 

instantaneous maximum discharge of 136,000 ft3/s occurred. 

With the information now available, we cannot know with 

absolute certainty the extent to which the geometries and spatial distributions of the emergent sandbars measured during the 

spring survey period differed from their configuration immediately after the 2010 peak discharge. A comparison of the summer and fall survey period datasets from this study, however, 

indicates that the geometry and distribution of sandbars in fall 

2010 likely were similar to the configuration immediately after 

the June 2010 peak discharge. Flows were generally low and 

stable for much of the intervening period, although a moderate 

rise owing to ice melt occurred a little more than a month prior 

to the spring survey (fig. 2). Therefore, no hydrologic evidence 

exists to indicate that a substantial scour or depositional event 

occurred. Parts of the highest sandbars measured during the 

spring survey period were higher than the stages of the spring 

and summer rises in 2011, and, therefore, were formed by a 

flow event with a magnitude greater than 45,400 ft3/s.

Implications for Ecology and Further 

Geomorphic Study

The longitudinal patterns of emergent sandbar areas and 

heights in this study indicate that certain reaches of the LPR 

are more conducive to deposition of extensive areas of higher 

sandbars, and that these locations apparently did not change 

much after one modest and two moderately large spring river 

rises partially inundated, reworked, and redeposited the sandbars. Even after the additional emergence of sandbars because 

of the lower-magnitude flows during the fall survey, the spatial 

distribution of emergent area (at the scale of river-mile intervals) did not change substantially.

Although analysis of linkages between longitudinal characteristics of sandbar geometries and tern and plover nesting 

locations was outside of the scope of the pilot study described 

in this report, the two sandbars with confirmed tern nesting 

colonies in 2011 (within the study area) were located across 

from the Cedar Creek development, an area of the LPR where 

the emergent sandbar area was abundant (fig. 11) and sandbars 

generally were higher than elsewhere in the study segment 

(fig. 9). Colonies of nesting terns, although not necessarily 

producing fledges, have been observed on sandbars near Cedar 

Creek every year since at least 2008 (Brown and Jorgensen, 

2008; Brown and Jorgensen; 2009, Brown and Jorgensen, 

2010; Brown and others, 2011b). As described in “Seasonal 

and Longitudinal Changes in Emergent Sandbar Height,” 

2008 also was a year when a larger-than-average annual 

flood created abundant emergent sandbar habitat throughout 

the LPR (Brown and Jorgensen, 2008), and the area around 

Cedar Creek was likely a locus of deposition in that year. The 

fact that habitat created in 2008 persisted or was replenished 

during the high discharge of 2010 further indicates that the 

Cedar Creek reach may have morphological conditions that 

favor either persistent habitat (lower sandbar erosion rates) 

or consistent deposition (higher deposition rates per unit of 

discharge).

Data from this study and evidence from previous studies indicate that longitudinal patterns and trends of emergent 

sandbar geometry exist in the LPR. The reporting of such 

patterns in multiple studies indicates a potential to isolate 

combinations of morphologic, hydrologic, and hydraulic 

conditions tending to favor deposition or persistence of midchannel sandbars with characteristic geometries and surrounding environments that are frequently selected for nesting by 

tern and plovers in the LPR. For example, recent studies by 

Elliott (2011) and Jorgensen and others (2012) have indicated 

that terns and plovers tend to nest in reaches with wider active 

(non-vegetated) channels, but too much width also may be 

a limitation. Although channel width may create favorable 

nesting conditions through purely ecological mechanisms (by 

limiting predator access, for example), changes in channel 

width, particularly channel expansion zones, long have been 

shown to create favorable conditions for macroform deposition (Leopold and Wolman, 1957; Mosely, 1976), whereby 

bedload sediment passes through a channel constriction, 

and deposits when tractive forces are reduced owing to flow 

expansion (Ashmore, 1993; Ashworth, 1996; Ashworth and 

others, 2000). Within this morphological framework, bedload 

sediment transport potential can be scaled to channel width 

using a unit-stream-power equation (Bagnold, 1977):

	 Ω = (γQS)/w (4)

where

Ω is unit stream power in units of pounds per 

foot per second,

γ is the specific weight of water (62 pounds per 

cubic foot [lb/ft3]),

Q is river discharge in cubic feet per second, 

S is water surface slope (dimensionless), and

w is wetted channel width in units of ft.

Table 6. Summary statistics of differences in measured 

sandbar height and results of Wilcoxon signed-rank comparison 

tests between replicate surveys using laser-leveling method, 

lower Platte River, Nebraska, 2011.

[QC, quality-control; p-value, probability that central values of differences between data pairs is equal to 0 at 95-percent confidence level, from 

Wilcoxon signed-rank test]

Sandbar 

height statistic

Summary statistics of differences in height 

between replicates

Number 

of QC data 

pairs

Mean 

(feet)

Interquartile 

range of 

differences 

(feet)

p-value

Median 11 0.03 -0.06–0.15 0.46

Maximum 11 -0.14 -0.17–0.22 0.96
28 Emergent Sandbar Dynamics in the Lower Platte River in Eastern Nebraska—Methods and Results of Pilot Study, 2011

Costa and O’Conner (1995) suggested a time integration 

of stream power over the period of a formative flow event that 

can be used as an indicator of geomorphic effectiveness: 

	 Ωtot = ∫ Ω dt (5)

where

Ωtot is total unit stream power, and

dt is the incremental time period of the flow 

event.

Applying total unit stream power in a spatial context, 

areas of maximum total unit stream power would be the narrowest reaches (bottlenecks), while areas of minimal total unit 

stream power would be the widest reaches. Therefore, midchannel sandbars with geometries that favor both selection for 

nesting and minimal probability of nest inundation may be the 

result of a relatively constrained range of unit stream powers, 

or reach-scale changes in total unit stream power, such that 

enough tractive force is exerted to construct a high sandbar, 

but not so much that deposition is unlikely during formative 

events. Conversely, a reach that is very wide in unvegetated 

width would likely be a zone of deposition during flow recession of a formative event, but such reaches also would likely 

have relatively low total unit stream power, and consequently 

may not have the tractive force necessary to build sandbars to 

sufficient heights. Moreover, these “goldilocks” depositional 

zones of higher-quality, mid-channel sandbar habitat would 

likely shift over decade-to-century time-periods, as bottlenecks and expansion zones shift in a longitudinal manner. 

These goldilocks zones are likely not the only locations 

where quality mid-channel sandbar habitat is formed. For 

example, Elliott and others (2009) suggested that reaches of 

higher-channel sinuosity also would likely have more potential for flow separation, and, therefore, would have a higher 

depositional potential. Within the study segment, reaches with 

greater channel sinuosity, such as the reach within the first 

mile downstream from the Lied Bridge (fig. 9), also had extensive areas of higher, larger, mid-channel sandbars; however, 

nesting was not documented in this reach in 2011.

With continued monitoring of sandbar dynamics, areas 

of the LPR that consistently produce habitat favored by 

nesting colonies of terns and plovers potentially could be 

quantified and identified (mapped). Such information would 

provide valuable information for planning and permitting of 

future river corridor development, infrastructure routing, and 

construction projects, as well as for constructing or restoring 

high-quality nesting habitats for terns and plovers in sand-bed 

rivers elsewhere on the Great Plains.

Summary and Conclusions

The lower Platte River (LPR) corridor provides important 

habitats for two State- and federally listed bird species: the 

interior least tern (terns; Sternula antillarum athallassos) and 

the piping plover (plovers; Charadrius melodus). The LPR, 

defined as the 103 miles (mi) of the Platte River between its 

confluence with the Loup River and its confluence with the 

Missouri River, has narrowed since the late-19th and early20th centuries, yet it partially retains many geomorphologic 

and hydrologic characteristics important to terns and plovers. 

Terns and plovers nest on the sandbars in the river and along 

shorelines at sand-and-gravel mine lakes on the adjacent valley floor. The need to balance continued economic, infrastructure, and resource development with the conservation 

of important physical and aquatic habitat resources requires 

increased understanding of the physical and biological dynamics of the LPR. Spatially and temporally rich datasets for 

emergent sandbar habitats are necessary to quantify emergent sandbar dynamics relative to hypothesized controls and 

stressors.

In cooperation with the Lower Platte South Natural 

Resources District, the U.S. Geological Survey initiated a pilot 

study of emergent sandbar dynamics along a 22-mi segment 

of the LPR downstream from its confluence with Salt Creek, 

near Ashland, Nebraska. The purposes of the study were to: 

(1) develop methods to rapidly assess sandbar geometries and 

locations in a wide, sand-bed river, and (2) apply and validate 

the method to assess emergent sandbar dynamics over three 

seasons in 2011. Of particular interest was an examination of 

the height of sandbars relative to the local stage of the formative discharge event, and how subsequent river discharges, of 

both large and small magnitude, alter sandbar geometries and 

abundance within the LPR. This report describes the methods 

developed and used to collect data associated with emergent 

sandbars, and the findings of the initial analyses of these data.

A “rapid-assessment” method was developed and used 

for the study, with the goal of characterizing the spatial 

distribution and habitat-relevant geometries of all sandbars 

along the study segment. Three primary measures were used to 

assess emergent sandbar dynamics in the study segment: sandbar area, sandbar height, and sandbar location. Data to derive 

these measures were collected during three, week-long survey 

periods in 2011: April 6–12, called the “spring survey;” July 

26–August 2, called the “summer survey;” and October 31–

November 4, called the “fall survey.” Emergent sandbars were 

grouped into one of three generalized types: (1) bank-attached, 

(2) island-attached, and (3) mid-channel. 

The area of 68 sandbars, with a total estimated emergent 

area of 360 acres, was surveyed or estimated during the spring 

survey period. Mid-channel sandbars were the most abundant 

of the three measured sandbar classes, and ranged in emergent 

area from 1.9 to 22.5 acres. The few island-attached sandbars 

surveyed during the spring generally were large, ranging in 

emergent area from 6.7 to 18.9 acres. Bank-attached sandbars 

constituted about half of the total measured emergent sandbar 

area during the spring survey period, and had median areas of 

13.8 acres, the largest of the three sandbar classes.

One hundred five sandbars, with a total estimated emergent area of 315 acres, were surveyed or estimated during the 

summer survey period. An apparent decrease in total emergent 
Summary and Conclusions 29

sandbar area occurred despite slightly lower daily discharges 

relative to the spring survey period, and primarily was the 

result of a decrease in the total emergent area of bank-attached 

sandbars. Mid-channel sandbars increased in total number and 

total emergent area, indicating that the intervening high flows 

not only dissected larger mid-channel bars into smaller bars, 

but also created many new emergent mid-channel sandbars.

Seventy-three sandbars, with a total estimated emergent 

area of 402 acres, were measured during the fall survey period. 

The increase in sandbar area occurred despite a net decrease 

in the total number of emergent sandbars, and was an effect 

of the lower average daily discharges during the fall survey 

period relative to the summer and spring survey periods. 

Much of the increase in emergent area occurred by means of 

an increase in the area of mid-channel sandbars larger than 

2 acres, indicating that a substantial area of shallow-water 

habitats had surrounded the larger mid-channel sandbars, and 

this area emerged when discharges decreased.

There were 17 collocated sandbars common to the spring 

and summer survey periods, 27 collocated sandbars common 

to the summer and fall survey periods, and 21 collocated sandbars common to the spring and fall survey periods. Changes in 

sandbar area between seasonal survey periods, when measured 

for collocated sandbar pairs, were similar to the seasonal differences observed for the larger population of sandbars. Midchannel sandbar pairs had a median decrease in emergent area 

of 4.3 acres between spring and summer survey periods. Bank 

attached sandbar pairs had the widest range of areal change 

between spring and summer, with most bars eroding, but some 

bars undergoing net expansion of up to 1.9 acres. Comparisons between the spring and fall survey periods indicated net 

erosion for most bank- and island-attached sandbars, with sufficient erosion to overpower the effect of the lower discharges 

on emergent area at these types of sandbars.

A comparison among survey periods of cumulative 

frequency distributions of measured sandbar heights indicated 

that the high platform of sandbars was much higher during the 

spring survey period than during the subsequent summer and 

fall survey periods. Median measured sandbar height, indexed 

to the stage for a discharge of 4,000 cubic feet per second 

(ft3/s), decreased from about 4.5 to 3.0 feet (ft) between the 

spring and summer survey periods. During the spring survey 

period, most measured heights were between 3.5 and 5.5 ft, 

whereas, during the summer and fall survey periods, most 

measured heights were between 2.0 and 5.0 ft. Cumulative 

frequency distribution curves also indicated that sandbar 

heights were similar between the summer and fall survey periods, which was not unexpected because no high-magnitude 

discharges occurred between those surveys. 

The longitudinal distribution of sandbar heights by survey period indicated that, although sandbar heights decreased 

between the spring survey period and subsequent survey 

periods, the general longitudinal pattern and trend of sandbar 

heights were similar across periods. Sandbar heights tended 

to follow a wavy longitudinal pattern whereby certain reaches 

had more abundant and higher sandbars, and others had 

fewer, lower sandbars. The locations of these reaches generally were the same across survey periods. Locally weighted 

least-squares regression lines fit to sandbar heights indicated 

that sandbar heights generally trended upward, if only slightly, 

in the downstream direction. Larger sandbars tended to be 

more abundant and higher between State Highway 50 and the 

Cedar Creek area, and lower and less abundant in the first 3 mi 

upstream from State Highway 50.

Median and maximum measured sandbar heights indicated that the high platform on sandbars was between 0.5 and 

2.0 ft higher during the spring survey period than during the 

subsequent summer and fall survey periods. Bank-attached 

and mid-channel sandbars generally had the greatest groupmedian height and group-median of maximum height values 

across all seasons. Group-median heights for all sandbar types 

measured during the spring survey period indicated that the 

high-platform of most sandbars was about 2 ft lower than the 

estimated stage of the 2010 peak discharge of 136,000 ft3/s, 

and group medians of maximum height values were between 

1.5 and 2.5 ft lower than this stage. Group-median heights for 

mid-channel and island-attached sandbars measured in the 

spring were well within the range of estimated stages of the 

2011 high discharge of 45,400 ft3/s, whereas the group-median 

height of bank-attached sandbars was higher than the estimated stages of the 2011 peak discharge. Maximum measured sandbar heights from the spring survey generally were 

higher than the estimated stages of the 2011 peak discharge, 

indicating that at least small parts of many sandbars were not 

inundated.

A comparison among seasonal survey periods of ranked 

differences between collocated sandbar pairs further confirms 

that a lowering of sandbar heights followed the spring survey 

period. Median values of differences in sandbar-median and 

-maximum height between the spring and summer survey 

periods indicated decreases of 0.83 and 0.81 ft, respectively, 

for collocated sandbar pairs. Differences in the same statistics 

for sandbar pairs compared between spring and fall survey 

periods indicated decreases of 0.73 and 0.99 ft, respectively; 

and between the summer and fall survey periods indicated a 

decrease of 0.09 ft, for each statistic. Results from Wilcoxon 

signed-rank tests of seasonal data pairs from collocated 

sandbars indicated that differences in median and maximum 

heights between the spring and summer survey seasons and 

the spring and fall survey seasons were significant. 

One of the primary aims of the study was to characterize the height of sandbars relative to the stage of formative 

flows. The height of sandbars during the spring survey was 

of particular interest because these bars were assumed to 

have been formed by the 2010 instantaneous peak discharge, 

which is the third highest measured discharge on record in the 

study area. Sandbar heights surveyed during the spring survey 

period indicated that, although the high platform of sandbars 

can approach the maximum stage of formative flow events, 

this height is rare, and more often the highest part of sandbars 

is 1–2 ft less than the formative stage. Furthermore, between 

about 10 and 50 percent of measured heights from the spring 
30 Emergent Sandbar Dynamics in the Lower Platte River in Eastern Nebraska—Methods and Results of Pilot Study, 2011

survey would have been inundated by the 2011 high discharge 

of 45,400 ft3/s (a discharge 67 percent less than the 2010 

flood), and more than 98 percent of measured heights from the 

spring survey would have been inundated by a flow of about 

85,000 ft3/s (a discharge about 38 percent less than the 2011 

flood). Although the methods used for the study differ from 

previous studies in the LPR, the estimations of bar heights are 

consistent with previous studies which, have indicated sandbars formed between 0.9 and 4.2 feet lower than their respective formative stages.

The streamwise distribution of emergent sandbar area did 

not change substantially among the three survey periods. A 

comparison of plots of the longitudinal distribution of sandbar 

area, normalized as percentage of total sandbar area, indicated 

that river reaches with abundant emergent sandbar area tended 

to be the same across the three survey periods. Between spring 

and fall survey periods, the median change in the fraction of 

total emergent sandbar area, tabulated by river mile, was a 

decrease of only 0.21 percentage point, and the interquartile 

range of such net changes was from -1.32 to 1.06 percentage 

points.

When compared among collocated sandbar pairs, shifts 

in sandbar locations, measured as shifts in sandbar polygon 

centroid between survey seasons, indicated that overall shifts 

in the areal centroid tended to be small, with the largest shifts 

occurring in mid-channel sandbars between the spring and 

summer survey seasons (the median streamwise shift for collocated pairs was 0.05 mi).

Replicate differential-grade global-positioning system 

(DGPS) surveys of sandbar area were performed on 13 sandbars across much of the range of sandbar sizes present during 

the survey periods, and 14 percent of all measured sandbars 

were replicated. The mean relative difference in sandbar area 

between original and replicate DGPS measurements was 

0.62 percent of the original measurement, and all replicate 

measurements were within 5 percent of the original survey. 

The relative differences in area generally were both positive 

and negative, and magnitudes were evenly distributed across 

the range of sandbar sizes measured. Results from Wilcoxon 

signed-rank comparisons of replicate pairs indicated that 

differences in area between replicate measurements were not 

significantly different from zero.

Differences in sandbar area between measurements made 

with the higher-precision laser total station and those made 

with the handheld DGPS indicated that the overall size of 

areas obtained using the DGPS is likely within 5 percent of 

the overall size of areas obtained using the laser total station. The close agreement between the overall sandbar area 

sizes obtained using the two methods indicates that the DGPS 

method likely is adequate for obtaining estimates of emergent 

sandbar area within 5 percent of the actual size.

Original and replicate sandbar height measurements, 

paired by ranks and surveyed with the rapid-assessment 

method, indicated that measured sandbar heights did not vary 

substantially between replicate measurements having slight 

differences in the placement of the survey grid. The mean difference between ranked-paired original and replicate measurements of sandbar height was -0.002 ft, and ranged from -1.15 

to 0.48 ft for individual spot heights. Results from Wilcoxon 

signed-rank tests of 11 replicate pairs of sandbar-median 

and -maximum measured heights indicated that differences 

between data pairs are insignificant.

A comparison of measurements of sandbar and nest 

heights relative to cumulative topographic distributions generated from detailed total station surveys indicated that technicians could adequately identify and measure the highest part 

of a sandbar (high platform). Measurements of spot heights 

generated using the rapid-assessment method ranged from 

the 51st to the 98th percentiles of sandbar height frequency 

distributions. At the two sandbars where several nest heights 

were measured, individual nest heights ranged between the 

62nd and 99th percentile of sandbar heights. Although nest 

heights only were measured at two sandbars, a comparison 

of the range of high-platform heights obtained by the rapidassessment method and the range of surveyed heights of nests 

indicated that at least the highest one-half of measured highplatform heights overlapped with and adequately represented 

the range of nest heights observed in 2011.

Sandbar data presented in this report represent only three 

seasons within a single calendar year and sandbar configurations chiefly affected by, at most, two high-discharge events 

(in 2010 and 2011). Without additional data collected over 

several more years and several more high-flow events, it is 

unclear whether the results of this study show long- or shortterm sandbar dynamics; therefore, the time scale over which 

they can be applied is similarly uncertain. Data from this study 

and evidence from previous studies indicate that longitudinal 

patterns and trends of emergent sandbar geometry exist in 

the LPR. The reporting of such patterns in multiple studies 

indicates a potential to isolate combinations of morphologic, 

hydrologic, and hydraulic conditions tending to favor deposition or persistence of mid-channel sandbars with characteristic 

geometries and surrounding environments that combine to 

create on-river habitats frequently selected for nesting by terns 

and plovers in the LPR. With continued monitoring of sandbar 

dynamics, areas of the LPR that consistently produce habitat 

favored by nesting colonies of terns and plovers potentially 

could be quantified and identified. Such analyses would 

provide valuable information for planning and permitting of 

future river corridor development, infrastructure routing, and 

construction projects, as well as for constructing or restoring 

persistent and successful tern and plover habitats in sand-bed 

rivers elsewhere on the Great Plains.
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Platte River, Nebraska: Geological Society of America Bulletin, v. 82, p. 3407–3420.

Sokkia Company, 2004, Sokkia Series 130R—Reflectorless 

total stations—Specifications sheet: Knagawa, Japan, Sokkia Company Ltd., International Sales Department, 8 p.
 References Cited 33

U.S. Army Corps of Engineers, 2011, Final programmatic 

environmental impact statement for the mechanical and 

artificial creation and maintenance of emergent sandbar 

habitat in the riverine segments of the upper Missouri River: 

Omaha, Nebr., U.S. Army Corps of Engineers, Omaha District [variously paged].

U.S. Geological Survey, 2010, Topographic map of the Plattsmouth quadrangle, Nebraska-Iowa: U.S. Geological Survey 

Topographic Quadrangle Map, 7.5-Minute Series, scale 

1:24,000 (photograph revised 2008).

U.S. Geological Survey, 2011, Water-resources data for the 

United States, water year 2010: U.S. Geological Survey 

Water-Data Report WDR-US-2010, with Section on station 

06805500, 18 p., accessed January 17, 2013, at http://wdr.

water.usgs.gov/wy2010/pdfs/06805500.2010.pdf.

U.S. Geological Survey, 2012, Water-resources data for the 

United States, water year 2011: U.S. Geological Survey 

Water-Data Report WDR-US-2011, with Section on station 

06805500, 18 p., accessed January 17, 2013, at http://wdr.

water.usgs.gov/wy2011/pdfs/06805500.2011.pdf.

Williams, G.P., 1978, The case of the shrinking channels—The 

North Platte and Platte Rivers in Nebraska: U.S. Geological 

Survey Circular 781, 48 p. (Also available at http://pubs.

er.usgs.gov/publication/cir781.)

Ziewitz, J.W., Sidle, J.G., and Dinan, J.J., 1992, Habitat conservation for nesting least terns and piping plovers on the 

Platte River, Nebraska: Prairie Naturalist, v. 24, p. 1–19.

Appendixes
36 Emergent Sandbar Dynamics in the Lower Platte River in Eastern Nebraska—Methods and Results of Pilot Study, 2011

Appendix 1. Estimates of Error in 

Measured Sandbar Height Associated 

with Errors in Estimation of Water 

Travel Time

A fundamental component of the estimations of sandbar 

height made for the study was estimation of water travel time 

between the four streamflow-gaging stations around or within 

the study area (see equations 1 through 3 in “Approach and 

Methods”). Water travel time was estimated between each 

pair of streamflow-gaging stations for each day of each survey 

period using a stage-correlation method that iteratively added 

time offsets to the upstream station’s record for a 72-hour window of the instantaneous stage records (15-minute time series) 

until the correlation between the two records, as measured 

using Pearson’s correlation coefficient (Helsel and Hirsch, 

2002), was maximized (see “Approach and Methods” for an 

additional description of the stage-correlation method).

Two types of travel times were estimated: (1) the 

incremental-reach average travel time (t), and (2) the studysegment average travel time (T). The incremental-reach average travel time is the travel time of water between adjacent 

streamflow-gaging stations along the larger study segment, 

and was the travel time used to interpolate a stage value corresponding to each individual water-surface height measurement. Interpolated stage values were referenced to the nearest 

streamflow-gaging station with a stage-discharge relation. The 

study-segment average travel time is the travel time of water 

between the most upstream and most downstream streamflowgaging stations on any given day, and was used as a qualitycontrol measure. By continuity, T should be equal to the sum 

of all t’s:

(1-1)

where

i is an incremental reach bounded by two 

streamflow-gaging stations, and

n is the total number of incremental reaches 

within the study segment. 

In the case of this study along the LPR in eastern 

Nebraska, three incremental reaches, were used to estimate travel time (in order from upstream to downstream): 

(1) Ashland (USGS station 06801000) to South Bend (USGS 

station 06805020), (2) South Bend to Louisville (USGS 

station 06805500), and (3) Louisville to La Platte (USGS 

station 06805570). The term “closure” is used to describe 

the continuity-based balance between separately calculated 

travel times for the left and right sides of equation 1-1. Perfect 

closure would indicate no travel-time differential between the 

two sides of equation 1-1; in the case of this study, departure 

from perfect closure is measured in unit time differentials of 

15-minute increments because this was the recording frequency in the stage records at each streamflow-gaging station. 

Two closure estimates were made for this study: (1) an “incremental closure” estimate between Ashland and Louisville, and 

(2) a “total closure” estimate between Ashland and La Platte. 

Thus, a measure of uncertainty in any given water-surface 

measurement is the corresponding estimate of departure from 

closure (in units of 15-minute increments) multiplied by the 

maximum 15-minute stage change during the respective survey period. The associated measurements of sandbar height, 

which are related to water stage at the bar’s edge, have an 

uncertainty proportional to the uncertainty in the water-surface 

height.

Estimates of water travel time between streamflow-gaging stations indicated that incremental average water velocities 

within the study area varied from 1.1 to 2.8 miles per hour 

(mi/hr), and study-segment average water velocities varied 

from 1.4 to 2.1 mi/hr (tables 1-1 through 1-3). Incremental 

water velocities generally were greatest in the South Bend-toLouisville reach, and least in the Louisville-to-La Platte reach. 

Average water velocities in the Louisville-to-La Platte reach 

potentially were decreased by backwater effects from the 

Missouri River during the spring and summer survey periods, 

causing water velocities to be less than 1.4 mi/hr on average. 

Water velocities were most uniform across increments during 

the fall survey, when water levels in both the lower Platte and 

Missouri Rivers were the lowest of those during the three 

survey seasons. 

For incremental closure, results indicated that the absolute value of travel-time closure error between the Ashland 

and Louisville gages was no more than 30 minutes; for the 

summer and fall surveys, closure was perfect about half of 

all days (tables 1-1 through 1-3). Although only 7 estimates 

of total closure could be made for the total of 12 survey days, 

these estimates indicate that the absolute value of total closure 

error was always less than 30 minutes, and closure was perfect 

in 3 of 7 cases.

Maximum stage changes over 15-minute increments 

as recorded at streamflow-gaging stations during each 

survey period varied from 0.04 to 0.11 ft, and averaged 0.04–

0.07 ft (table 1-4). Multiplying the range of maximum 

15-minute stage changes by the maximum (30 minutes, or 

2 units) and minimum (-30 minutes, or -2 units) travel-time 

closure errors, an estimate of the maximum range of the potential error in sandbar height measurement points (for a given 

day with imperfect closure) caused by errors in water travel 

time estimates is +/-0.08 for at least one reach per survey 

period to +/-0.22 ft for the upstream reach near Ashland during 

the summer survey period.

T ≅ ∑n

i

 ti
Appendix 1. Estimates of Error in Measured Sandbar Height Associated with Errors in Estimation of Water Travel Time 37

Table 1-1. Correlation, travel time, and estimated water velocity from a comparison of instantaneous stage records between 

selected gaging stations in lower Platte River, Nebraska, for time periods centered on individual days during the spring survey 

period, 2011.

[Spring survey period defined as April 6–April 12, 2011. Unless otherwise indicated, correlation coefficients and travel times were calculated using the 

15-minute stage record, applied over a maximum 72-hour time window, centered on the day indicated. See “Approach and Methods” for additional explanation of correlation technique. Values in italics indicate travel time was calculated using data centered on a different day than indicated. —, no data; NA, 

closure was not calculated]

Day Number of overlapping 

observations used

Pearson’s correlation 

coefficient

Water travel time

(minutes)

Average water velocity 

(miles per hour)

Ashland (06801000) to South Bend (06805020)

04/06/2011 229 0.99 180 2.4

04/07/2011 273 0.99 180 2.4

04/08/2011 274 0.98 180 2.4

04/12/2011 271 0.98 195 2.2

South Bend (06805020) to Louisville (06805500)

04/06/2011 258 0.90 105 2.4

04/07/2011 262 0.99 90 2.8

04/08/2011 263 0.99 90 2.8

04/12/2011 269 0.99 105 2.4

Ashland (06801000) to Louisville (06805500)

04/06/2011 218 0.99 300 2.3

04/07/2011 256 0.99 300 2.3

04/08/2011 200 0.99 300 2.3

04/12/2011 200 0.98 330 2.1

Incremental closure, in minutes: Ashland (06801000) to Louisville (06805500)

04/06/2011 — — -15 —

04/07/2011 — — -30 —

04/08/2011 — — -30 —

04/12/2011 — — -30 —

Louisville (06805500) to La Platte (06805570)

04/06/20111 — 0.98 660 1.2

04/07/20111 — 0.98 660 1.2

04/08/20111 — 0.98 660 1.2

04/12/20112 243 0.98 660 1.2

Ashland (06801000) to La Platte (06805570)

04/06/20111 — — — —

04/07/20111 — — — —

04/08/20111 — — — —

04/12/20112 221 0.93 990 1.5

Total closure, in minutes: Ashland (06801000) to La Platte (06805570)

04/06/2011 — NA —

04/07/2011 — NA —

04/08/2011 — NA —

04/12/2011 — -30 —

1

Stage data not available for La Platte gage on this day; travel time value was calculated on different day than indicated. Pearson’s correlation coefficient 

shown does not correspond to day indicated.

2

Travel time value calculated using data from 4/12/2011 to 4/15/2011.
38 Emergent Sandbar Dynamics in the Lower Platte River in Eastern Nebraska—Methods and Results of Pilot Study, 2011

Table 1-2. Correlation, travel time, and estimated water velocity from a comparison of instantaneous stage records between 

selected gaging stations in lower Platte River, Nebraska, for time periods centered on individual days during the summer survey 

period, 2011.

[Summer survey period defined as July 26–August 2, 2011. Unless otherwise indicated, correlation coefficients and travel times were calculated using 

the 15-minute stage record, applied over a maximum 72-hour time window, centered on the day indicated. See “Approach and Methods” for additional 

explanation of correlation technique. Values in italics indicate travel time was calculated using data centered on a different day than indicated. —, no data; 

NA, closure was not calculated]

Day Number of overlapping 

observations used

Pearson’s correlation 

coefficient

Water travel time

(minutes)

Average water velocity 

(miles per hour)

Ashland (06801000) to South Bend (06805020)

07/26/2011 248 0.99 195 2.2

07/27/2011 247 0.98 195 2.2

07/28/2011 252 0.97 210 2.0

07/29/2011 177 0.97 225 1.9

08/02/2011 259 0.99 195 2.2

South Bend (06805020) to Louisville (06805500)

07/26/2011 281 1.00 105 2.4

07/27/2011 281 1.00 105 2.4

07/28/2011 281 1.00 105 2.4

07/29/2011 204 0.99 105 2.4

08/02/2011 42 0.99 135 1.8

Ashland (06801000) to Louisville (06805500)

07/26/2011 250 0.99 315 2.2

07/27/2011 239 0.98 315 2.2

07/28/2011 245 0.97 315 2.2

07/29/2011 224 0.98 315 2.2

08/02/2011 251 1.00 315 2.2

Incremental closure, in minutes: Ashland (06801000) to Louisville (06805500)

07/26/2011 — — -15 —

07/27/2011 — — -15 —

07/28/2011 — — 0 —

07/29/2011 — — 15 —

08/02/2011 — — 15 —

Louisville (06805500) to La Platte (06805570)

07/26/20111 238 0.41 750 1.1

07/27/20111 238 0.41 750 1.1

07/28/20111 238 0.41 750 1.1

07/29/20111 238 0.41 750 1.1

08/02/20111 238 0.41 750 1.1

Ashland (06801000) to La Platte (06805570)

07/26/20112 — — — —

07/27/20111 218 0.33 1,050 1.4

07/28/20111 218 0.33 1,050 1.4

07/29/20113 — — — —

08/02/20113 — — — —

Total closure, in minutes: Ashland (06801000) to La Platte (06805570)

07/26/2011 — — NA —

07/27/2011 — — 0 —

07/28/2011 — — 15 —

07/29/2011 — — NA —

08/02/2011 — — NA — 1

Travel time value calculated using data centered between 0800 hours on July 26th through 0800 hours on July 29th.

2

Travel time value not calculated because of backwater effect on stage height.

3

Travel time not calculated because of gaging-station malfunction.
Appendix 1. Estimates of Error in Measured Sandbar Height Associated with Errors in Estimation of Water Travel Time 39

Table 1-3. Correlation, travel time, and estimated water velocity from a comparison of instantaneous stage records between 

selected gaging stations in lower Platte River, Nebraska, for time periods centered on individual days during the fall survey period, 

2011.

[Fall survey period defined as October 31–November 4, 2011. Unless otherwise indicated, correlation coefficients and travel times were calculated using the 

15-minute stage record, applied over a maximum 72-hour time window, centered on the day indicated. See “Approach and Methods” section for additional 

explanation of correlation technique. —, no data]

Day Number of overlapping 

observations used

Pearson’s correlation 

coefficient

Water travel time

(minutes)

Average water velocity 

(miles per hour)

Ashland (06801000) to South Bend (06805020)

10/31/2011 272 0.96 225 1.9

11/01/2011 271 0.98 240 1.8

11/03/2011 274 0.92 195 2.2

11/04/2011 275 0.89 180 2.4

South Bend (06805020) to Louisville (06805500)

10/31/2011 280 0.97 105 2.4

11/01/2011 279 0.94 120 2.1

11/03/2011 279 0.98 120 2.1

11/04/2011 279 0.98 120 2.1

Ashland (06801000) to Louisville (06805500)

10/31/2011 265 0.99 330 2.1

11/01/2011 263 0.97 360 1.9

11/03/2011 265 0.94 330 2.1

11/04/2011 267 0.94 300 2.3

Incremental closure, in minutes: Ashland (06801000) to Louisville (06805500)

10/31/2011 — — 0 —

11/01/2011 — — 0 —

11/03/2011 — — -15 —

11/04/2011 — — 0 —

Louisville (06805500) to La Platte (06805570)

10/31/20111 260 0.79 390 2.1

11/01/20111 262 0.72 375 2.2

11/03/20111 261 0.75 390 2.1

11/04/20111 260 0.70 405 2.0

Ashland (06801000) to La Platte (06805570)

10/31/20111 239 0.77 720 2.1

11/01/20111 239 0.69 720 2.1

11/03/20111 238 0.65 735 2.0

11/04/20111 240 0.59 705 2.1

Total closure, in minutes: Ashland (06801000) to La Platte (06805570)

10/31/2011 — — 0 —

11/01/2011 — — 15 —

11/03/2011 — — -30 —

11/04/2011 — — 0 —

1

Stage record at La Platte affected by sand burial of pressure line.
40 Emergent Sandbar Dynamics in the Lower Platte River in Eastern Nebraska—Methods and Results of Pilot Study, 2011

Table 1-4. Maximum 15-minute stage changes at selected 

gaging stations in the lower Platte River, Nebraska, by survey 

period, 2011.

[Spring survey period defined as April 6–April 12, 2011. Summer survey 

period defined as July 26–August 2, 2011. Fall survey period defined as 

October 31–November 4, 2011. All values calculated using 15-minute timeseries record available from the U.S. Geological Survey National Water 

Information System. NA, stage record condsidered unsuitable for analysis.]

Survey 

period

Maximum 15-minute stage changes by gaging station, 

in feet

Platte 

River near 

Ashland 

(06801000)

Platte 

River at 

South Bend 

(06805020)

Platte 

River at 

Louisville 

(06805500)

Platte River 

at La Platte 

(06805570)

Spring 0.05 0.05 0.04 0.04

Summer 0.11 0.06 0.06 0.04

Fall 0.04 0.08 0.05 NA

Average 0.07 0.06 0.05 0.04
Appendix 2. Validation of Predicted Bar Heights Using Observations of Inundation from Remote Time-Lapse Photographs 41

Appendix 2. Validation of Predicted 

Bar Heights Using Observations of 

Inundation from Remote Time-Lapse 

Photographs

Sandbar inundation by annual maximum or other peak 

discharge events is one of the common causes of tern and plover nest failure in the lower Platte River. Therefore, efforts to 

document sandbar heights are important to enable prediction 

of such events in the future. Estimations of sandbar heights 

for this study were made across varying reach lengths using 

the stage-discharge rating relations from nearby streamflowgaging stations. Although the authors believe these estimations 

of height were adequate to predict inundation status, validation 

of predicted sandbar heights may add additional confidence 

to the estimation methods for sandbar height described in this 

report. However, the difficulty in validation lies in maintaining readiness for an annual maximum or other peak discharge 

event to occur, and collecting observations of sandbar inundation coinciding with the event. To capture observations of 

sandbar inundation extent during whatever high-discharge 

events occurred, the authors deployed two time-lapse reconnaissance digital cameras on an observation tower near South 

Bend, Nebr (fig. 2-1). The cameras, focused on reaches of the 

lower Platte River, were fixed with one facing upstream, the 

other facing obliquely downstream, and programmed to take 

photographs at 15-minute intervals during all daylight hours 

for all days during the study period.

Validation of the extent of inundation during high flows 

was assessed for four surveyed sandbars visible in the field 

of view of these two time-lapse cameras. The four sandbars 

were first surveyed during the spring survey period on April 7, 

2011, and 4–9 measurements of sandbar height were taken on 

each bar’s high platform. The validation procedure for sandbar 

height consisted of two primary steps: (1) predict the number 

of individual height measurements that would be inundated 

for a stage rise associated with a particular high-magnitude 

discharge on a particular day, and (2) visually estimate the 

extent of sandbar inundation represented in the images of that 

sandbar taken by the time-lapse cameras on that same day. 

The first step was completed using the stage-discharge rating 

table of the nearest streamflow-gaging station, which was in 

this case the Platte River at Louisville, Nebraska (USGS station 06805500). Extent of inundation was predicted for 

the high-discharge events that occurred on May 27, 2011 

(45,400 ft3/s) and June 28, 2011 (43,400 ft3/s), and was 

predicted using only measurements of height from the spring 

survey. Estimation of extent of inundation in the time-lapse 

photographs was made by simple visual comparison of photographs taken at lower discharges (that is, in the weeks prior to 

the floods), with photographs taken during the peak of 

the floods. Because the photographs were oblique, visual 

estimations were limited to bin sizes of 20 percent (0, greater 

than zero and less than 20 percent, 20 to less than 40 percent, 

40 to less than 60 percent, 60 to less than 80 percent, 80 to less 

than 100 percent, and 100 percent). Although methods exist 

to extract more precise estimates of area from oblique photographs, the authors considered this method adequate for the 

purposes of validating sandbar inundation.

Although only four sandbars with measured height of 

the high platform were visible in time-lapse photographs, 

the observations of sandbar inundation from the photographs 

indicate very good agreement with predictions of inundation 

extent based on sandbar height (table 2-1). Two of four sandbars predicted to be 100 percent inundated by both the 

May 27 and June 28 high-discharge events, were observed to 

be entirely inundated in time-lapse photographs taken during 

the associated peak discharges. Similarly, two of the four 

sandbars were predicted to have inundation of more than 

60 percent of surveyed points on the high platform (table 2-1), 

and time-lapse photographs of both of these sandbars showed 

that the May (fig. 2-1) and June high discharges inundated an 

estimated 80 percent or more of each bar.

Table 2-1. Predicted number of spot measurements of sandbar height inundated and estimated percentage of sandbar area 

inundated, as shown by time-lapse photography, for four sandbars and two high-magnitude discharges, lower Platte River, Nebraska, 

2011.

[No., number; ft3/s, cubic feet per second; >, greater than] 

Sandbar 

no.

Date 

surveyed

No. 

height 

measurements

Peak discharge of 45,400 ft3/s on May 27, 20111 Peak discharge of 43,400 ft3/s on June 28, 20112

No. of points 

predicted to be 

inundated

Estimated percent of sandbar 

inundated, from visual analysis 

time-lapse photographs

No. of points 

predicted to be 

inundated

Estimated percent of sandbar 

inundated, from visual analysis 

time-lapse photographs

1 04/07/2011 4 4 100 4 100

2 04/07/2011 9 6 >80 6 >80

4 04/07/2011 9 7 >80 6 >80

5 04/07/2011 5 5 100 5 100

1

Source: U.S. Geological Survey, 2012.

2

Discharge estimate obtained from 15-minute time-series data at USGS streamflow-gaging station 06805500.
42 Emergent Sandbar Dynamics in the Lower Platte River in Eastern Nebraska—Methods and Results of Pilot Study, 2011

Sandbar number 1

Lied Bridge

A

B

Sandbar number 2

Figure 2-1. Oblique, reconnaissance photographs showing two sandbars used for validation of predicted 

inundation of sandbar-height survey points; U.S. Geological Survey photographs from tower-mounted timelapse camera facing lower Platte River near South Bend, Nebraska: (A) May 7, 2011 river discharge was 

about 10,000 cubic feet per second, and (B) May 27, 2011 river discharge was about 44,400 cubic feet per 

second. River flow is left to right.
Publishing support provided by: 

Rolla and Tacoma Publishing Service Centers

For more information concerning this publication, contact:

Director, USGS Nebraska Water Science Center

5231 South 19th Street

Lincoln, Nebraska 68512

(402) 328–4100

Or visit the Nebraska Water Science Center Web site at:

http://ne.water.usgs.gov/


Alexander, J.S. and others—Emergent Sandbar Dynamics in the Lower Platte River in Eastern Nebraska—Methods and Results of Pilot Study, 2011—Scientific Investigations Report 2013–5031