ons in Sand Storage Measured at

mented Cross Sections in

lorado River Between

anyon Dam and Lava Falls Rapid,

ern Arizona, 1992–99

operation with the

ON MONITORING AND RESEARCH CENTER

ources Investigations Report 03—4104

THE INTERIOR

RVEY
U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Variations in Sand Storage Measured at 

Monumented Cross Sections in 

the Colorado River Between 

Glen Canyon Dam and Lava Falls Rapid, 

Northern Arizona, 1992–99

Prepared in cooperation with the

GRAND CANYON MONITORING AND RESEARCH CENTER

By Marilyn E. Flynn and Nancy J. Hornewer

Tucson, Arizona

2003

Water-Resources Investigations Report 03—4104
For additional information write to:

District Chief

U.S. Geological Survey

Water Resources Division

520 N. Park Avenue, Suite 221

Tucson, AZ 85719–5035

Copies of this report can be purchased from:

U.S. Geological Survey

Information Services

Box 25286

Federal Center

Denver, CO 80225–0046

The use of firm, trade, and brand names in this report is for identification purposes only and does not constitute 

endorsement by the U.S. Geological Survey.

Information about U.S. Geological Survey programs in Arizona is available online at http://az.water.usgs.gov.

U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director
Contents iii

CONTENTS

Page

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

Introduction .......................................................................................................................................................... 1

Background ................................................................................................................................................. 3

Purpose and scope....................................................................................................................................... 4

Acknowledgments....................................................................................................................................... 4

Description of the study area....................................................................................................................... 4

Physical and hydrological conditions.......................................................................................................... 4

Controlled releases and gaged tributary inputs ........................................................................................... 5

Sediment-monitoring network .................................................................................................................... 8

Methods................................................................................................................................................................ 8

Data collection ............................................................................................................................................ 8

Data processing ........................................................................................................................................... 19

Presentation of data..................................................................................................................................... 21

Cross-section variability....................................................................................................................................... 23

Spatial variability in changes in cross-sectional area.................................................................................. 23

Temporal variability in cross-sectional area ............................................................................................... 23

Changes in cross-sectional area............................................................................................................................ 31

Factors affecting sediment storage changes......................................................................................................... 31

Conclusion............................................................................................................................................................ 37

Selected references............................................................................................................................................... 38
iv Contents

FIGURES

Page

1. Map showing location of areas of monumented cross sections and streamflow-gaging stations ........ 2

2. Graphs showing summary of daily discharge statistics for the streamflow-gaging station 

Colorado River at Lees Ferry (09380000), Arizona, water years 1992–99 ......................................... 6

3. Graphs showing daily maximum discharge and daily suspended-sediment discharge on 

tributaries, water years 1992–99........................................................................................................... 9

4–8. Maps showing:

4. Location of monumented cross sections between Glen Canyon Dam and 

Lees Ferry, Arizona........................................................................................................................ 11

5. Location of monumented cross sections between the confluence of the Paria and

Colorado Rivers and Badger Creek Rapid..................................................................................... 12

6. Location of monumented cross sections between Badger Creek Rapid and the 

confluence of the Little Colorado and Colorado Rivers ................................................................ 13

7. Location of monumented cross sections between the confluence of the Little Colorado 

and Colorado Rivers and Tanner Rapid ......................................................................................... 14

8. Location of monumented cross sections between just above the confluence of

Bright Angel Creek and the Colorado River and Lava Falls Rapid............................................... 15

9. Schematic diagram showing data-collection and processing steps

A. Steps for collecting and preliminary processing of cross-section elevation data........................... 18

B. Procedure for calculating the change in cross-sectional area between two 

successive measurements............................................................................................................... 18

10–20. Graphs showing:

10. Discharge-elevation curves for three water-surface reference points

A. Sections p01–p15 .................................................................................................................... 22

B. Sections 1a1–1a7..................................................................................................................... 22

C. Sections lf1–lf5........................................................................................................................ 22

11. Normalized change-in-area data, excluding 1996 controlled-flood measurements, for 

cross sections in the primary data set

A. Sections between the confluences of the Paria and Colorado Rivers, and the 

Little Colorado and Colorado Rivers ...................................................................................... 24

B. Sections between the confluences of the Little Colorado and Colorado Rivers, and 

Bright Angel Creek and the Colorado River ........................................................................... 24

12. Non-normalized change-in-area data, excluding 1996 controlled-flood measurements, 

for cross sections in the primary data set

A. Sections between the confluences of the Paria and Colorado Rivers, and the 

Little Colorado and Colorado Rivers ...................................................................................... 25

B. Sections between the confluences of the Little Colorado and Colorado Rivers, and 

Bright Angel Creek and the Colorado River ........................................................................... 25

13. Bed-elevation measurements for selected cross sections in the primary data set below 

the confluence of the Paria and Colorado Rivers........................................................................... 26
Contents v

FIGURES—Continued Page

14. Bed-elevation measurements for selected cross sections in the primary data set below 

the confluences of the Paria and Colorado Rivers, and the Little Colorado and 

Colorado Rivers ............................................................................................................................. 27

15. Bed-elevation measurements for cross section 1a1 in the primary data set below the 

confluence of the Little Colorado and Colorado Rivers ................................................................ 28

16. Discharge and normalized change-in-area data for cross sections in the primary data set 

between the confluences of the Paria and Colorado Rivers, and Bright Angel Creek and the 

Colorado River

A. Daily maximum discharge at the streamflow-gaging station at Colorado River at Lees Ferry 29

B. Daily maximum discharge at the streamflow-gaging station at Paria River at Lees Ferry ..... 29

C. Sections p01–p32 .................................................................................................................... 29

D. Sections h1–h5 and w1–w5..................................................................................................... 29

E. Daily maximum discharge at the streamflow-gaging station at 

Colorado River above Little Colorado River........................................................................... 30

F. Daily maximum discharge at the streamflow-gaging station at 

Little Colorado River near Cameron ....................................................................................... 30

G. Sections la1–lf5. ..................................................................................................................... 30

H. Sections g0–g5 ........................................................................................................................ 30

17. Net change in area for cross sections in the matching-date data set between the confluence 

of the Paria and Colorado Rivers and Badger Creek Rapid

A. Sections p01–p08 .................................................................................................................... 32

B. Sections p09–p15 .................................................................................................................... 32

C. Sections p15a–p20................................................................................................................... 32

D. Sections p21–p26 .................................................................................................................... 33

E. Sections p27–p31 .................................................................................................................... 33

18. Net change in area for cross sections in the matching-date data set between the confluence 

of the Little Colorado and Colorado Rivers and Tanner Rapid

A. Sections la1–la7....................................................................................................................... 34

B. Sections lb1–lb4 ...................................................................................................................... 34

C. Sections lc1–lc5....................................................................................................................... 34

D. Sections ld1, ld4, and le1–le5.................................................................................................. 35

E. Sections lf1–lf4........................................................................................................................ 35

19. Associations between average changes in area and five hydrologic conditions for cross 

sections in the matching-date data set between the confluence of the Paria and Colorado 

Rivers and Badger Creek Rapid

A. Average of daily mean flows ................................................................................................... 36

B. Average of time-weighted daily mean flows........................................................................... 36

C. Average of daily mean range of flows..................................................................................... 36

D. Average of time-weighted daily mean range of flows............................................................. 36

E. Suspended-sediment discharge from the Paria River .............................................................. 36

20. Associations between average changes in area and five hydrologic conditions for cross 

sections in the matching-date data set between the confluence of the Little Colorado and 

Colorado Rivers and Tanner Rapid

A. Average of daily mean flows ................................................................................................... 37

B. Average of time-weighted daily mean flows........................................................................... 37

C. Average of daily mean range-of-flows.................................................................................... 37

D. Average of time-weighted daily mean range of flows............................................................. 37

E. Suspended-sediment discharge from the Paria and Little Colorado Rivers ............................ 37
vi Contents

TABLES

Page

1. Summary statistics for discharge at the streamflow-gaging station Colorado River at 

Lees Ferry (09380000), Arizona, water years 1992–99 ....................................................................... 7

2. Summary statistics for discharge and measured suspended-sediment discharge at the 

streamflow-gaging station Paria River at Lees Ferry (09382000), Arizona, 

water years 1992–99............................................................................................................................. 10

3. Summary statistics for discharge and measured suspended-sediment discharge at the 

streamflow-gaging station Little Colorado River near Cameron (09402000), Arizona, 

water years 1992–99............................................................................................................................. 11

4. Inventory of cross-section measurement dates..................................................................................... 16

5. Standard error estimates applied to changes in area............................................................................. 21

CONVERSION FACTORS AND DATUMS

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29)—a geodetic datum 

derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level

Datum of 1929; horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27). Altitude, as 

used in this report, refers to distance above or below NGVD 29.

Multiply By To obtain

meter (m) 3.281 foot (ft)

kilometer (km) 0.6214 mile (mi)

square meter (m2) 10.76 square foot (ft2)

square meter (m2) 1.1960 square yard (yd2)

cubic meter (m3) 35.31 cubic foot (ft3)

cubic meter (m3) 1.3079 cubic yard (yd3)

cubic meter per second (m3/s) 35.31 cubic feet per second (ft3/s)

metric ton (megagram) 1.1023 tons, short (2,000 pounds)
Abstract 1

Variations in Sand Storage Measured at Monumented Cross Sections 

in the Colorado River Between Glen Canyon Dam and Lava Falls Rapid, 

Northern Arizona, 1992–99

By Marilyn E. Flynn and Nancy J. Hornewer

Abstract

Bed elevations were measured at 131 monumented cross sections in the Colorado River between

Glen Canyon Dam and Lava Falls Rapid from June 1992 to September 1999 to provide data on

channel sand storage. This report documents the location of the 131 monumented cross sections,

dates of measurements for all cross sections, methods of data collection and processing, and

spatial and temporal variation and variability in changes in cross-sectional area for selected cross 

sections. Additionally, data were analyzed to determine if changes in sediment storage could be 

related to main channel flow conditions and tributary sediment inputs. Most of the cross sections

showed a limited capacity, both in terms of amount and residence time, to store sediment. Data for

83 of the 131 cross sections were comprehensive and complete, and were used for analyses in this

report. This data set is referred to as the primary data set. Of these 83 cross sections, 19 had a net

gain in stored sediment, 61 had a net loss of stored sediment, and 3 had no change in stored

sediment for the period of data collection, excluding data collected during the high release from

Glen Canyon Dam in 1996. A subset of the primary data set consisting of the sections downstream

from the Paria and Little Colorado Rivers with measurements made on or nearly on the same day,

referred to as the matching-date data set, was used to explore the effects of controlled flows and

tributary flows on the changes in cross-sectional area. The matching-date data set consists of data 

from 57 cross sections. Of these 57 cross sections, 1 had a net gain in stored sediment, 55 had a net

loss of stored sediment, and 1 had no change in stored sediment. Results of the analysis did not show

that changes in cross-sectional area were strongly related to main channel flow conditions or tributary 

sediment inputs.

INTRODUCTION

Between 1992 and 1999, the U.S. Geological 

Survey (USGS), initially in cooperation with Glen 

Canyon Environmental Studies (GCES) and later in 

cooperation with the Grand Canyon Monitoring and 

Research Center (GCMRC), established a network 

of monumented cross sections along the Colorado 

River between Glen Canyon Dam and Lava Falls Rapid 

(fig. 1) to measure changes in cross-sectional area. 

These changes in cross-sectional area were intended to 

be an indication of changes in sand stored on the 

channel bottom. 
2 Variations in Sand Storage Measured in the Colorado River Between Glen Canyon Dam and Lava Falls Rapid, Northern Arizona, 1992–99

113° 112°

35°

36°

37°

Between just above the

confluence of Bright Angel

Creek and the Colorado River 

 and Lava Falls Rapid (see fig. 8)

Grand Canyon

National Park

09402000

09402000

09383100

09402500

09382000

Glen Canyon

National Recreation Area

Flagstaff

Peach Springs

Cameron

09380000

Page

Glen Canyon

Dam

U.S. Geological Survey streamflow-gaging

 station— Number is station identifier

Area of monumented cross sections 

Base from U.S. Geological Survey digital data, 1:100,000, 1980

Universal Transverse Mercator Projection, Zone 12

EXPLANATION

Havas u

C reek

Kanab Cree k

L ittle Colorado

River

River

Paria River

Diamond Creek

Yuma

o C

lorado

Page

Phoenix

Flagstaff

UNITED STATES

MEXICO

A R I Z O N A

Tucson

Study area

 Between the confluence of the

Paria and Colorado Rivers

and Badger Creek Rapid

(see fig. 5)

Between the confluence 

of the Little Colorado and

Colorado Rivers and

Tanner Rapid (see fig. 7)

0 25 KILOMETERS

0 25 MILES

Colo

ra

d o

Between Badger Creek Rapid and

the confluence of the Little

Colorado and Colorado Rivers

(see fig. 6)

Between Glen Canyon

Dam and Lees Ferry

(see fig. 4)

River

64

64

89

66

67

89

180

89

ALT

40

17

40

Figure 1. Location of areas of monumented cross sections and streamflow-gaging stations.
Introduction 3

Background

In the early 1980s, agencies charged with 

management of the Colorado River in Grand Canyon, 

white-water rafters, and anglers became concerned 

that flow releases from Glen Canyon Dam were 

eroding sandbars that are critical to the riparian 

system in Grand Canyon National Park. Concern 

about sandbars focused on potential degradation by 

unsteady dam releases for power generation. In 1982, 

the Bureau of Reclamation (BOR) began coordinating 

a comprehensive program of investigations—Phase I of 

GCES—to determine the effects of dam releases on the 

downstream riverine environment. During Phase I, 

unexpected high flows prevented adequate study of 

flows typical of the dam’s powerplant operating 

criteria. After a review of Phase I by the National 

Academy of Science, the U.S. Department of the 

Interior directed that further studies be done. Phase II 

of the GCES was initiated in 1988 to gather additional 

data on specific dam operational elements and included 

flow-monitoring and model-development programs.

On July 27, 1989, the Secretary of the Interior 

directed the BOR to prepare an environmental impact 

statement (EIS) on the operation of Glen Canyon Dam. 

To protect the downstream resources until completion 

of the EIS, interim operating criteria and a monitoring 

program were implemented in August 1991. The EIS 

was completed in March 1995, and the Record of 

Decision (ROD) on the operation of Glen Canyon Dam 

was signed on October 9, 1996. The ROD stipulated 

new operating criteria for the dam based on the 

preferred modified low fluctuating flow (MLFF) 

alternative presented in the EIS. Under the MLFF 

alternative, limits were set on maximum and minimum 

daily flow releases and on the rates of increase 

(upramp) and decrease (downramp) of releases. 

Specifically, the maximum flow of 708 m3/s was 

imposed in order to conserve sediment in Grand 

Canyon, especially in the Marble Canyon area. The 

downramp limit of 42 m3/s was imposed to reduce 

beach erosion in the Marble Canyon reach that resulted 

from rapid leaching of water in the beach sand. 

Because it was assumed that limiting maximum 

releases would cause portions of sandbars above the 

normal peak stage to not be rebuilt; sediment to 

accumulate at low elevations, including backwaters and 

camping beaches; and return-current channels to 

become filled with sediment and eventually overgrown 

with vegetation, the MLFF also included habitat 

maintenance flows (high, steady releases within 

powerplant capacity) to re-form backwaters and 

maintain sandbars (U.S. Department of the Interior, 

1995). The EIS concluded that Glen Canyon Dam 

reduced the sediment-transport capacity of the river to 

a greater degree than it reduced the supply of sediment, 

and thus, sediment would accumulate over time (U.S. 

Department of the Interior, 1995). Using MLFF 

operating criteria, the EIS predicted that there would be 

a 73-percent probability of sand accumulating in the 

main channel over a 50-year period. Topping, Rubin, 

and Vierra, 2000; Topping, Rubin, Nelson, and others, 

2000; and Rubin and Topping, 2001 have concluded 

that multiyear sediment accumulation does not occur.

Beginning in 1992, as a part of the GCES Phase II 

interim-flow monitoring and model-development 

programs, the USGS established a network of 

monumented cross sections between the confluence 

of the Paria and Colorado Rivers and Lava Falls Rapid, 

with greater emphasis on the reaches downstream 

from the two largest tributaries—the Paria and Little 

Colorado Rivers. Measurements were designed around 

three hydrologic conditions in the year: (1) before the 

snowmelt runoff in tributaries in early spring, (2) after 

the snowmelt runoff in late spring or early summer, and 

(3) after summer rains in the fall. Between 1993 and 

1996, additional cross sections were added between 

Badger Creek Rapid and the confluence of the 

Colorado and Little Colorado Rivers, and between 

just upstream from Bright Angel Creek and Lava Falls 

Rapid. In 1997 the number of cross sections was 

reduced; sections in reaches that showed little to no 

change were discontinued, and an index section was 

chosen for each pool in the remaining reaches. 

The measurement frequency of these index sites also 

was reduced from three times per year to twice per 

year: (1) in April to capture information on deposition 

of sediment from winter tributary floods, and (2) in 

September to capture information on deposition of 

sediment from late summer runoff in tributaries.

One purpose of the cross-section network was to 

provide data on bed-elevation changes to compare to 

computations of bed evolution made using multidimensional flow, sediment-transport, and bedevolution models. The network proved useful for this 

purpose (Wiele and others, 1996). A second purpose of 

the cross-section network was to provide a direct, 

empirical method for monitoring the amount of sand 

stored in the channel to determine if a multiyear 

accumulation of sediment was occurring. Data indica-
4 Variations in Sand Storage Measured in the Colorado River Between Glen Canyon Dam and Lava Falls Rapid, Northern Arizona, 1992–99

ted that cross sections could be used to provide an 

assessment of the sand storage in the channel, the 

storage conditions for possible bar-building high 

releases, and the effectiveness of those releases in 

scouring stored sand and rebuilding sandbars.

 Purpose and Scope

The purpose of this report is to document locations 

of the 131 monumented cross sections, the measurement dates during the study period June 1992 to 

September 1999, and the methods of data collection 

and processing, and to present a general assessment 

of the variability in cross-sectional area. A subset of the 

data, referred to as the primary data set, was used 

to illustrate the temporal and spatial variability in 

cross-sectional area. In addition, a subset of the 

primary data set consisting of data from measurements 

made on or nearly on the same day at sections 

downstream from the Paria and Little Colorado Rivers, 

referred to as the matching-date data set, was used to 

further explore the effects of hydro-logic conditions on 

the changes in cross-sectional area. All cross-section 

measurement data are available electronically as ASCII 

files from the Arizona District Office of the USGS 

(address on back of title page).

Acknowledgments

The authors thank the many people who provided 

assistance throughout the life of this project. In particular, we acknowledge Julia B. Graf, USGS (retired), for 

her guidance and leadership in establishing the 

foundations upon which this project was developed. 

We also thank Dave Anning, Greg Fisk, and Gail 

Cordy of the USGS for their help in the preparation of 

this report.

Description of the Study Area

The study area includes the Colorado River 

between Glen Canyon Dam and Lava Falls Rapid, 

which is about 195 miles downstream from Glen 

Canyon Dam in northern Arizona. The study area 

encompasses parts of Glen Canyon National 

Recreational Area and Grand Canyon National Park 

(fig. 1). In this report, distances along the Colorado 

River are expressed in river miles, and all other 

measurements are expressed in metric units.

Physical and Hydrological Conditions

For much of its length through the study area, the 

Colorado River is confined by bedrock, by large blocks 

of talus, or by alluvial debris fans that are composed of 

sediment too coarse to be transported except during 

high flows. Flow depths in the thalweg of the river 

range from about 1.5 to 32 m at a discharge of 

708 m3/s. Depths can change abruptly, and there are 

near-vertical drops of as much as 15 m in gorges 

formed in the metamorphic and igneous rock (Wilson, 

1986). Between Lees Ferry (river mile 0) and Diamond 

Creek (river mile 226), the elevation of the river 

decreases about 542 m. Much of the decrease in 

elevation occurs in short, steep reaches, many of which 

are the well-known rapids of the Colorado River. 

Although the average gradient between Lees Ferry and 

Diamond Creek is 0.0015, the gradient of many of the 

short reaches exceeds 0.01 (Schmidt and Graf, 1990). 

Many of the reaches between rapids have gradients less 

than 0.0005 (Birdseye, 1923).

In the study area, tributaries of the Colorado River 

can be categorized as those having headwaters above 

the rim and those having headwaters below the rim. 

The Paria and Colorado Rivers are tributaries having 

headwaters above the rim; their drainage basins include 

large plateau areas, and they supply most of the 

suspended sediment to the Colorado River within the 

study area. Tributaries having headwaters below the 

rim have drainage basins largely within the inner 

canyon and can transport fine sediment as well as large 

boulders in debris flows (Cooley and others, 1977; 

Webb and others, 1989). The debris flows from these 

smaller tributaries define the longitudinal profile and 

control the geomorphic framework of the Colorado 

River by forming rapids (Webb, 1996). In addition, the 

debris fans directly control the formation and stability 

of most sandbars (Schmidt, 1990; Schmidt and Graf, 

1990; Schmidt and Rubin, 1995).

In general, the channel pattern of the Colorado 

River through the study area is dominated by an 

alternating rapid-pool sequence (Howard and Dolan, 

1981). Flow through the constrictions created by the 

debris-flow deposits at the mouths of tributaries forms 

the rapids. Flow through rapids ends in pools (Dolan 
Introduction 5

and others, 1978; Kieffer, 1985). The downstream ends 

of these pools are controlled by bedrock outcrops or 

alternating debris bars that are the outwash from the 

upstream debris fans (Howard and Dolan, 1981; Webb 

and others, 1989; Melis and Webb, 1993; Melis and 

others, 1994). Because the debris fans raise the bed 

elevation (Howard and Dolan, 1981), large pools also 

form upstream from rapids. The length of the pools is 

determined by the spacings between debris fans, which 

are controlled by bedrock geology (Howard and Dolan, 

1981). These pools are recirculation zones and may 

contain one or more eddies and low-velocity areas, thus 

providing an environment for sediment deposition 

(Schmidt and Graf, 1990).

Prior to the closure of Glen Canyon Dam in 1963, 

the Colorado River in Grand Canyon had highly 

variable discharge and suspended-sediment 

concentration annually. Between May and July, melting 

of the snowpack in the headwaters produced mean 

annual peak discharge of 2,420 m3/s. Except for brief 

but occasionally substantial summer tributary flash 

floods, flow in the Colorado River typically fell to less 

than 200 m3/s after the snowmelt flood subsided. Postdam flow regulation has removed seasonal-flow 

variations by reducing the frequency of discharges that 

are greater than powerplant capacity (940 m3/s) and 

increasing the overall range and rate of change of 

discharge during each day.

 The average sediment load for the Colorado River 

at Lees Ferry before the dam was 60 million metric 

tons per year (Webb, Wegner, and others, 1999). The 

suspended-sediment load of the post-dam Colorado 

River has been reduced by 95 percent (Webb, Wegner, 

and others, 1999) and is primarily supplied by the 

major tributaries, the Paria and Little Colorado Rivers, 

and numerous smaller, ungaged tributaries. The mean 

annual supply of fine sediment was 2.74 million metric 

tons for 1947–76 from the Paria River and 8.4 million 

metric tons for 1947–72 from the Little Colorado River 

(Webb, Wegner, and others, 1999). The smaller 

tributaries create and maintain the debris fans and 

rapids on the Colorado River and serve as important 

sources of fine-grained sediment (Powell, 1895; 

Hamblin and Rigby, 1968; Dolan and others, 1974; 

Howard and Dolan, 1981; Keiffer, 1985). Estimating 

the sediment yield from these smaller tributaries is 

complicated because sand is contributed by a 

combination of fluvial and hillslope processes (Webb 

and others, 2000). Using a sediment-yield model for 

debris flow, Webb and others (2000) estimate that 

approximately 2.0 million metric tons of total sediment 

enters the river from the ungaged tributaries between 

Glen Canyon Dam and Diamond Creek (river mile -15 

to 226).

Controlled Releases and Gaged Tributary Inputs

Changes in area measured at cross sections within 

the study area are affected by controlled flows from 

Glen Canyon Dam and sediment input from tributaries. 

Under the MLFF preferred alternative specified in the 

EIS, peak releases are limited to 708 m3/s, minimum 

releases are limited to 227 m3/s during the day and 

142 m3/s at night, and the maximum daily fluctuation 

or range in flow is limited to 227 m3/s. The preferred 

alternative also provides for short duration, high-flow 

releases to accomplish specific management objectives 

and to simulate some of the dynamics of a natural river 

(U.S. Department of the Interior, 1995). In MarchApril 1996, a controlled high-flow release experiment 

(1996 controlled flood) was conducted. A flow of 

1,275 m3/s was released from Glen Canyon Dam for 

a period of 7 days to determine the effectiveness with 

which discharges greatly in excess of the normal 

powerplant-restricted maximum could renew the 

riparian environment along the Colorado River. Several 

papers on the controlled flood are compiled in Webb, 

Schmidt, and others (1999).

For this study period, the box plots shown in 

figure 2 graphically summarize the distribution of 

maximum, minimum, and range (daily maximum - 

daily minimum) of daily discharge values for water 

years 1992–99 at the USGS streamflow-gaging station 

Colorado River at Lees Ferry (09380000; fig. 1), 

approximately 16 miles downstream from Glen 

Canyon Dam. The distribution of maximum and 

minimum daily discharge values is lower during water 

years 1992–95 than during water years 1996–99. 

Beginning in water year 1996, there is an increase in 

the annual median daily maximum and minimum 

discharge. The annual median daily discharge 

decreased in water year 1999, but was still larger than 

medians of the earlier period (1992–95). The maximum 

daily discharge at this station occurred during the 1996 

controlled flood, and the minimum daily discharge 

occurred in water year 1992. Table 1 shows annual 

summary statistics for discharge at Lees Ferry. 

Suspended-sediment discharge from the Glen Canyon 

reach, measured at the Lees Ferry gaging station, is 

negligible and therefore is not included in table 1 or 

other analyses in this report. 
6 Variations in Sand Storage Measured in the Colorado River Between Glen Canyon Dam and Lava Falls Rapid, Northern Arizona, 1992–99

Daily maximum discharge

700

800

700

600

500

400

300

200

200

250

300

100

150

100

1992 1993 1994 1995 1996

95th Percentile

90th Percentile

75th Percentile

Mean

Median

25th Percentile

10th Percentile

5th Percentile

1997 1998 1999

50

0

600

500

400

300

200

800

Daily minimum discharge

Daily range in discharge NOTE: Daily range is difference between daily maximum

 discharge and daily minimum discharge

DISCHARGE, IN CUBIC METERS PER SECOND

WATER YEAR

EXPLANATION

Figure 2. Summary of daily discharge statistics for the streamflow-gaging station Colorado River at Lees Ferry 

(09380000), Arizona, water years 1992-99
Introduction 7

Table 1. Summary statistics for discharge at the streamflow-gaging station Colorado River at Lees Ferry (09380000), Arizona, 

water years 1992–99

Water year

1992 1993 1994 1995 1996 1997 1998 1999

Minimum daily discharge summary (values in cubic meters per second)

Minimum 148 151 160 158 230 187 201 249

Median 232 230 240 259 345 575 470 320

Mean 234 238 250 310 376 504 442 345

Maximum 323 408 345 541 1,290 759 855 702

Standard deviation 42 44 44 113 138 165 76 62

Mean daily discharge summary (values in cubic meters per second)

Minimum 179 205 206 194 231 220 249 292

Median 303 306 314 328 436 595 547 430

Mean 312 317 324 361 449 542 525 446

Maximum 428 490 447 558 1,290 762 869 708

Standard deviation 54 61 59 107 135 143 78 72

Maximum daily discharge summary (values in cubic meters per second)

Minimum 228 226 232 221 232 232 281 348

Median 367 368 377 388 518 603 612 504

Mean 379 385 389 410 518 576 592 518

Maximum 530 564 629 569 1,300 767 872 711

Standard deviation 66 76 72 102 133 124 77 78

Daily range (daily maximum - daily minimum) discharge summary (values in cubic meters per second)

Minimum 39 2.0 2.0 3.1 2.0 2.8 5.7 0.0

Median 148 142 140 102 139 34 147 178

Mean 145 146 139 101 142 72 150 173

Maximum 227 286 331 193 1,050 383 368 261

Standard deviation 36 41 41 37 75 67 50 33
8 Variations in Sand Storage Measured in the Colorado River Between Glen Canyon Dam and Lava Falls Rapid, Northern Arizona, 1992–99

Tributary streamflow and suspended-sediment 

discharge measured at the USGS streamflow-gaging 

stations Paria River at Lees Ferry (09382000) and the 

Little Colorado River near Cameron (09402000; fig. 1) 

are shown in figure 3, and annual statistical summaries 

are shown in tables 2 and 3. Because sediment input 

from the Paria and Little Colorado Rivers is only 

significant during storms, suspended-sediment samples 

were collected only during storms; consequently, 

annual daily maximums and minimums for suspendedsediment discharge are not available. These samples 

were collected using automated pump samples or 

single-stage samplers, or were collected as equalwidth-increment or dip samples. On the basis of 

sediment-discharge rating curves, discharges greater 

than 30 m3/s in the Paria River and 60 m3/s in the Little 

Colorado River generally can contribute large 

quantities of sediment to the mainstem of the Colorado 

River, (G.G. Fisk, U.S. Geological Survey, oral 

commun., 2000). During the study period, the largest 

annual suspended-sediment contribution from the Paria 

River as the result of storms, 4.4 million metric tons, 

occurred in water year 1997 (table 2). The largest 

annual suspended-sediment contribution from the Little 

Colorado River as the result of storms, 18.8 million 

metric tons, occurred in water year 1993 (table 3). 

Most of this sediment was input from a storm in 

January 1993 that has a 15-year recurrence interval 

(Smith and others, 1998).

Sediment-Monitoring Network

A total of 131 monumented cross sections were 

established to measure bed elevation on the Colorado 

River between Glen Canyon Dam and Lava Falls Rapid 

(fig. 1) between August 1992 and April 1998. Cross 

sections were established at locations judged to be 

favorable for sand storage on the basis of channel 

geometry, character of bank and visible bed materials, 

and presence of sand waves on depth-sounder charts 

(Graf and others, 1995). An emphasis was placed on 

locating sections immediately downstream from major 

tributaries, the Paria and Little Colorado Rivers, 

because of the large sediment inputs from these tributaries and for flow- and sediment-model verification 

purposes. The four cross sections between Glen 

Canyon Dam and Lees Ferry (fig. 4) were established 

and first measured in April 1998. Monitoring downstream from the Paria River began in August 1992. 

Thirty-four cross sections were established between the 

confluence of the Paria and Colorado Rivers and 

Badger Creek Rapid at river mile 8 (fig. 5). Twenty-six 

cross sections were established between Badger Creek 

Rapid and the mouth of the Little Colorado River at 

river mile 61.3 (fig. 6). Most of these cross sections 

were established and first measured in January 1994. 

Monitoring downstream from the Little Colorado River 

(fig. 7) began in June and July 1992 when the 15 cross 

sections at the upstream end were established and first 

measured. The remaining 17 cross sections at the 

downstream end were established and first measured in 

January and February 1993. Thirty-five cross sections 

were established between river mile 87 (just above 

Bright Angel Creek) and Lava Falls Rapid at river 

mile 179.5 (fig. 8). A list of all measurement dates by 

cross-section name in downstream order and the 

subsets of the data, primary and matching-date data 

sets, used in this report is given in table 4. Location 

information for the monumented cross sections is 

available from the Arizona District Office of the 

USGS.

Beginning with the fall 1997 measurement trip, 

bed-material samples were collected at the cross 

sections being measured at the time (table 4). Because 

of the small number of samples collected, these data 

are not considered in this report.

METHODS

A sonic-depth sounder was used to measure the 

depth of the river bed below the water surface, and the 

graphical record produced by the sounder was digitized 

to create the digital record used to calculate changes in 

cross-sectional area. The data-collection and 

processing steps, and the steps used to calculate the 

change in area between two measurements for a cross 

section, are shown in figure 9 and described below.

Data Collection

For each measurement, a kevlar line with flags at 

6.08-m (20-ft.) intervals was strung across the river 

between the cross-section end points. Where feasible, 

the zero point on the line was positioned on the leftbank side. The positions of each monument and the 

edge of water on each bank were noted as distances 

along the line from the zero point. A boat equipped 

with a sonic-depth sounder was driven back and forth 

under the line. 
Methods 9

DISCHARGE, IN CUBIC FEET PER SECOND

SUSPENDED-SEDIMENT DISCHARGE, IN METRIC TONS PER DAY

200

180

160

140

120

100

80

60

600

40

20

500

400

300

200

500,000

1,000,000

1,500,000

2,000,000

200,000

400,000

600,000

800,000

1,000,000

1,200,000

1,400,000

1,600,000

0

2,000,000

0

100

0

0

Paria River at Lees Ferry, Arizona

Little Colorado River near Cameron, Arizona

1993

WATER YEAR

1992 1994 1995 1996 1997 1998 1999

EXPLANATION

Discharge Sediment

Figure 3. Daily maximum discharge and daily suspended-sediment discharge on tributaries, 

water years 1992–99.
10 Variations in Sand Storage Measured in the Colorado River Between Glen Canyon Dam and Lava Falls Rapid, Northern Arizona, 1992–99

Table 2. Summary statistics for discharge and measured suspended-sediment discharge at the streamflow-gaging station Paria River at 

Lees Ferry (09382000), Arizona, water years 1992–99

Water year

1992 1993 1994 1995 1996 1997 1998 1999

Minimum daily discharge summary (values in cubic meters per second)

Minimum 0.0 0.0 0.1 0.1 0.1 0.0 0.1 0.0

Median .3 .4 .3 .4 .3 .4 .4 .4

Mean .4 .7 .4 .5 .4 .5 .5 .6

Maximum 5.9 9.1 6.8 3.5 3.1 10.5 8.2 25.5

Standard deviation .7 1.0 .7 .4 .4 .7 .7 1.6

Mean daily discharge summary (values in cubic meters per second)

Minimum .1 .1 .1 .1 .1 .1 .1 .1

Median .4 .4 .4 .5 .4 .5 .5 .5

Mean .8 1.4 .6 .8 .4 1.1 .8 .9

Maximum 16.0 19.3 17.7 17.6 6.2 56.6 40.8 25.1

Standard deviation 1.8 2.4 1.4 1.4 .4 3.8 2.6 1.8

Maximum daily discharge summary (values in cubic meters per second)

Minimum .1 .1 .1 .1 .1 .1 .1 .1

Median .6 .7 .5 .7 .5 .8 .6 .6

Mean 1.9 2.6 1.6 1.4 0.7 2.9 2.1 1.9

Maximum 50.7 43.3 94.3 51.3 14.4 116 191 62.6

Standard deviation 6.1 4.8 6.8 3.9 1.3 12.1 12.4 5.6

Daily range (daily maximum - daily minimum) discharge summary (values in cubic meters per second)

Minimum .0 .0 .0 .0 .0 .0 .0 .0

Median .3 .3 .1 .2 .1 .2 .1 .2

Mean 1.5 1.8 1.1 .9 .3 2.4 1.6 1.2

Maximum 49.9 42.7 94.2 49.9 13.1 116 190 47.2

Standard deviation 5.7 4.2 6.7 3.8 1.2 11.7 12.2 4.7

Measured suspended sediment discharge (values in millions of metric tons)

Total from storms during water year 2.5 2.1 .78 .94 .24 4.4 2.3 3.3
Methods 11

 

Table 3. Summary statistics for discharge and measured suspended-sediment discharge at the streamflow-gaging station Little Colorado 

River near Cameron (09402000), Arizona, water years 1992–99

Water year

1992 1993 1994 1995 1996 1997 1998 1999

Mean daily discharge summary (values in cubic meters per second)

Minimum 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Median 2.2 .8 0 .3 0 .3 .3 0

Mean 8.4 23.3 3.1 7.1 .7 2.6 5.1 4.2

Maximum 73.6 464 68.2 186 40.8 42.5 51.0 70.8

Standard deviation 12.8 59.7 8.4 21.4 3.1 5.8 10.6 10.9

Measured suspended sediment discharge (values in millions of metric tons)

Total from storms during water year 8.5 18.8 2.8 4.1 1.4 3.8 4.0 8.6

Ferry Swale

gl1

gl2

gl3

gl4

Honey Dr aw

Wa ter Holes

Canyon

Colorado

River

0 1 KILOMETER

0 1 MILE

36°50'

52'30''

36°55'

Lees

Ferry

09380000

09380000

Cre ll aF

ek

gl2 Monumented cross section and identification

U.S. Geological Survey streamflow-gaging

 station— Number is station identifier 

EXPLANATION

Base from U.S. Geological Survey digital data, 1:100,000, 1980

Alber Conic Equal-Area projection,

Standard parallels 29°30'00'' and 45°30'00'', central meridian 111°30'00''

32'30'' 111°30'

111°35'

Figure 4. Location of monumented cross sections between Glen Canyon Dam and Lees Ferry, Arizona.
12 Variations in Sand Storage Measured in the Colorado River Between Glen Canyon Dam and Lava Falls Rapid, Northern Arizona, 1992–99

Cathedra l

Wash

Badger Creek

Jackass

Creek

p01–p08

p09

p10

p11

p12

p13 p14 p15

p15a

p16 p15b p18 p17

p19

p20

p21–p26

p27–p32

Colorado

River

0 1 KILOMETER

0 1 MILE

111°40' 37'30'' 111°35'

36°45'

47'30''

50'

36°52'30''

Paria River

89

ALT

09380000

09380000

p13

Navajo Bridge

Base from U.S. Geological Survey digital data, 1:100,000, 1980

Alber Conic Equal-Area projection,

Standard parallels 29°30'00'' and 45°30'00'', central meridian 111°30'00''

Lees Ferry

Monumented cross section and identification

U.S. Geological Survey streamflow-gaging

 station— Number is station identifier 

EXPLANATION

Figure 5. Location of monumented cross sections between the confluence of the Paria and Colorado Rivers and 

Badger Creek Rapid.
Methods 13

Soap Creek

Salt Wate

Wa r

Wa

sh

Wash

Wash

Wash

Wash

Wash

Tanner

House Rock

North Canyon

Tiger

Shinumo

Canyon

S

Canyon addle

South

Tatahatso

Point Hansbrough

Nankoweap

Creek

Creek

Kwagunt

s1

e1– e4

t1–t4

m31

h1–h5

m45

m47

w1–w5

lg1–lg4

0 5 KILOMETERS

0 5 MILES

36°15'

20'

25'

30'

35'

40'

36°45'

C

lo

o

rado

Ri ev r

m31

River Colorado

Little

45'

111°40'

50'

111°55'

sh

Base from U.S. Geological Survey

digital data, 1:100,000, 1980

Alber Conic Equal-Area projection,

Standard parallels 29°30'00'' and 45°30'00'',

central meridian 111°30'00''

Monumented cross section

 and identification 

EXPLANATION

Figure 6. Location of monumented cross sections between Badger Creek Rapid and the confluence of 

the Little Colorado and Colorado Rivers.
14 Variations in Sand Storage Measured in the Colorado River Between Glen Canyon Dam and Lava Falls Rapid, Northern Arizona, 1992–99

Tanner

Canyon

Cree k

Little Colorado River

Cape

Solitude

Ca rbon Creek

Chuar

Creek

Lava

Palisades

Creek

Espejo Creek

Comanche Creek

Basalt Canyon

Creek

la1–la5

la7 la6

lb1

lb1b

lc1–lc5

lb2 lb3

lb4

ld1–ld5

le1–le5

lf1–lf5

0 1 KILOMETER

0 1 MILE

111°50' 111°47'30'

36°7'30''

36°10'

36°12'30''

36°5'

Creek

Colorado

River

Monumented cross section and identification

U.S. Geological Survey streamflow-gaging

 station— Number is station identifier 

EXPLANATION

09383100

lb3

Base from U.S. Geological Survey digital data, 1:100,000, 1980

Alber Conic Equal-Area projection,

Standard parallels 29°30'00'' and 45°30'00'', central meridian 111°30'00''

09383100

Figure 7. Location of monumented cross sections between the confluence of the Little Colorado and Colorado Rivers 

and Tanner Rapid.
Methods 15

Mohawk

Crystal

Creek

Powell Plateau

Havasu

g0–g6

c1–c6

n0–n2800

n0–n2800

v1–v8

k1–k5

0 5 KILOMETERS

0 5 MILES

112°10'

113°

50'

40'

30'

20'

36°10'

36°20'

Canyon

National

Canyon

Bri ht g

Angel Cr

Gr a nite

Gorge

M iddle Grani t e

Gorge

12 mileCr

Kanab

Creek

Creek

7-

09402500

09402500

Base from U.S. Geological Survey digital data, 1:100,000, 1980

Alber Conic Equal-Area projection,

Standard parallels 29°30'00'' and 45°30'00'',

central meridian 111°30'00''

Monumented cross sections and identification

U.S. Geological Survey streamflow-gaging

 station— Number is station identifier 

EXPLANATION

Colorado

River

Figure 8. Location of monumented cross sections between just above the confluence of Bright Angel Creek and the 

Colorado River and Lava Falls Rapid.
16 Variations in Sand Storage Measured in the Colorado River Between Glen Canyon Dam and Lava Falls Rapid, Northern Arizona, 1992–99

Between Glen Canyon

Dam and Lees Ferry

Between Paria River and

Badger Creek Rapid

Upstream from

Soap Creek Rapids

Upstream from EighteenMile Wash 

Upstream from

North Canyon Rapids

Upstream from

South Canyon

Upstream from President

Harding Rapids

Near Eminence Break

Downstream from

Saddle Canyon

Upstream from

Nankoweap Rapids

Upstream from the

Little Colorado River

gl1

gl2

gl3

gl4

p01

p02

p03

p04

p05

p06

p07

p08

p09

p10

p11

p12

p13

p14

p15

p15a

p15b

p16

p17

p18

p19

p20

p21

p22

p23

p24

p25

p26

p27

p28

p29

p30

p31

p32

s1

e1

e2

e3

e4

t1

t2

t3

t4

m31

h1

h2

h3

h4

h5

m45

m47

w1

w2

w3

w4

w5

lg1

lg2

lg3

lg4

SECTION

1992 1993 1994 1995 1996 1997 1998 1999

Table 4. Inventory of cross-section measurement dates

[Data collected for measurement dates in the nonshaded and gray areas, hereafter referred to as the primary data set, are presented in this report. A subset 

of the primary data set, shown in the gray areas, will be referred to as the matching-date data set. Data collected for measurement dates in the blue areas 

are not shown in this report.]
Methods 17

Between the Little

Colorado River

and Tanner Rapid

Upstream from

streamflow-gaging

station 09402500,

Colorado River near

Grand Canyon

Upstream from

Crystal Rapids

Upstream from

Lava Falls Rapid

la1

la2

la3

la4

la5

la6

la7

lb1

lb1b

lb2

lb3

lb4

lc1

lc2

lc3

lc4

lc5

ld1

ld2

ld3

ld4

ld5

le1

le2

le3

le4

le5

lf1

lf2

lf3

lf4

lf5

g0

g1

g2

g3

g4

g5

g6

c1

c2

c3

c4

c5

c6

k1

k2

k3

k4

k5

n0

n200

n400

n800

n1200

n1600

n2000

n2400

n2800

v1

v2

v3

v4

v5

v6

v7

v8

SECTION

Upstream from

streamflow-gaging

station 09404120,

Colorado River above

National Canyon

near Supai

Upstream from

127-mile Rapids

1992 1993 1994 1995 1996 1997 1998 1999

Table 4. Continued.
18 Variations in Sand Storage Measured in the Colorado River Between Glen Canyon Dam and Lava Falls Rapid, Northern Arizona, 1992–99

B. Calculating change in area at cross-section x, between measurements 1 and 2

A. Collecting and preliminary processing

Measurement 1 at cross-section x

Start

Measure depths along a 

cross section using a sonic-depth

sounder, marking horizontal 

position every 6 meters on chart.

Total of 10 transects completed

Measurement 2 at cross-section x

Digitize sonic-depth sounder charts Digitize sonic-depth sounder charts

Calculate a mean bed elevation (data set

B2) for each 0.25-meter increment using

data from the 10 transects

End

Start

Measure depths along a 

cross section using a sonic-depth

sounder, marking horizontal 

position every 6 meters on chart.

Total of 10 transects completed

Start

Convert depths to bed elevations (data set

A2) and interpolate elevation data at each

0.25-meter increment, for all 10 transects

Interpolate horizontal positions

to 0.25-meter increments

Apply

Wilcoxon ranksum test to determine if

bed-elevation data (A1(i) and A2(i)), each

using 10 transects, are significantly

different from each other

 p-value<=0.05

Yes No

Use A1 and A2 data for increment i where i is

the 0.25-meter increment along the cross section 

Interpolate horizontal positions

to 0.25-meter increments

Calculate change in area for increment i

Area(i) = Diff(i) * 0.25

Accumulate total change in area

Areasubtotal = Areasubtotal + Area(i)

Convert depths to bed elevations (data set

 A1) and interpolate elevation data at each

0.25-meter increment, for all 10 transects

Use B1 and B2 data

for increment i

i = i + 1 until each 0.25-meter

increment has been evaluated

Calculate a mean bed elevation (data set

B1) for each 0.25-meter increment using

data from the 10 transects

Subtract mean elevations between

measurements 1 and 2 at increment i

Diff(i) = (B2(i) – B1(i))

Total change in area between

measurements 1 and 2 for cross section x

Areatotal = Areatotal

Calculate change in area for increment i

Area(i) = Diff(i) * 0.25 = 0

Mean elevation between

measurements 1 and 2 at increment i

Diff(i) = 0

End

End

Step 5

Step 4

Step 1

Step 2

Step 3

Figure 9. Data-collection and processing steps. A, Steps for collecting and preliminary processing of cross-section 

elevation data. B, Procedure for calculating the change in cross-sectional area between two successive measurements.
Methods 19

One pass across the river is referred to as a transect, 

and 10 transects define a measurement. The pole, 

mounted directly over the depth-sounder transducer, 

was used as a reference guide along the kevlar line. 

One person in the boat watched the line and pole and 

used a switch attached to the depth sounder to activate 

a fixed mark on the graphical depth-sounder record 

when the pole passed under a flag (fig. 9A, step 1). 

A second person in the boat made notes on the 

graphical record. Date, time, distance of end points, 

edges of water, and distance along the line of each 

fixed mark were noted on the graphical charts.

Variability in the recorded depth at a given location 

is caused by measurement error of the instrument, 

changes in boat position, and actual changes in the 

water-surface elevation during a measurement. 

Of these, the changes in boat position are by far the 

largest contributor to depth variability between 

transects. The depth sounder reported data to 0.03 m 

and had an accuracy of 0.5 percent of the measured 

depth according to manufacturer specifications 

(0.075 m at a depth of 15 m). Changes in boat position 

occurred because a fixed position under the flagged 

line could not be maintained in the strong and variable 

current of the river. Because the river bed is irregular, 

small changes in boat position could yield large 

differences in the recorded depth. The method devised 

to minimize the uncertainty in depth caused by boat 

position was to measure the cross section 10 times in 

rapid succession and compute the mean of the 

10 transects. The number of transects was selected to 

provide enough data for statistical analysis and to keep 

the measurement time short so that changes in watersurface elevation during the measurement would be 

insignificant. This method resulted in a measurement 

duration of 10 to 50 minutes, and changes in watersurface elevations during the measurements were 

typically small.

Changes in river stage were documented by 

measuring the vertical distance from a water-surface 

reference point, typically an “x” chiselled into a large 

boulder, to the water surface before and after a crosssection measurement (10 transects) was completed. 

The water-surface data were used in processing the 

cross-sectional measurement data and enabled 

comparisons of cross-sectional change in area between 

measurement dates. A water-surface reference point 

was established for individual reaches, and that 

reference point was typically used for all cross sections 

in that reach. In most cases, the reference point was 

used throughout the data-collection period; however, 

because of changing flow conditions, some reaches had 

several water-surface reference points. Relations were 

established between the original and subsequent watersurface reference points.

Data Processing

The graphical record of each cross-section transect 

was digitized by recording a point at a preset distance 

of horizontal cursor movement across the paper 

(fig. 9A, step 2). The interval between digitized points 

per unit of paper was kept constant and was selected to 

give about two digitized points for each foot of distance 

along the river bed. Once the data were digitized, the 

distance of each point from the left-bank end point (the 

left-bank monument) in inches of graph paper was 

converted to ground distance in meters using the known 

locations of the fixed marks on the graphical record and 

assuming constant boat speed between marks. To 

provide depths at equal distances from the end point for 

the statistical analysis of the data, points were selected 

or interpolated at 0.25-m intervals across the cross 

section (fig. 9A, step 3).

The sonic-depth sounder recorded depths of the 

river bed below the water surface. Data were converted 

to bed elevation by subtracting the measured depth 

from a water-surface elevation measured at watersurface reference points (fig. 9A, step 4). Those watersurface reference points that were not surveyed were 

assigned an arbitrary elevation of 500 m. Water-surface 

elevation typically was measured before and after the 

10 transects had been completed. If there was a 

difference between the before and after water-surface 

elevation measurements, a time-weighted average was 

used to represent the water-surface elevation. After the 

depths were converted to bed elevations at each 0.25-m 

increment for a transect, a mean bed elevation was 

computed at each increment using data from all 

10 transects (fig. 9A, step 5). This is referred to as the 

cross-section measurement. Each cross-section 

measurement was processed as described above.

To calculate the cross-sectional change in area 

between two measurements at a cross section, the 

difference in mean bed elevation was determined for 

each 0.25-m increment (fig. 9B). If the difference was 

deemed statistically significant, the actual difference 

value was used in the change-in-area calculation. If the 

difference was not statistically significant, zero was 
20 Variations in Sand Storage Measured in the Colorado River Between Glen Canyon Dam and Lava Falls Rapid, Northern Arizona, 1992–99

used in the change-in-area calculation. The Wilcoxon 

rank-sum test (Devore, 1991; Helsel and Hirsch, 2000) 

was used to detect statistically significant differences in 

mean bed elevations. This test was used because it does 

not require knowledge of the underlying statistical 

distribution of the bed-elevation recordings. In this 

application the test has the null hypothesis that, at each 

increment, the mean bed elevation in one measurement 

is equal to the mean bed elevation in the next 

measurement. To evaluate the hypothesis, the test 

statistic W is computed on the basis of bed-elevation 

data for the two measurements, and then a two-sided 

p-value associated with rejecting the null hypothesis is 

determined on the basis of the value of W and the 

number of depth recordings for each measurement. 

For this investigation the null hypothesis was rejected 

for p-values less than or equal to 0.05, which means 

that the null hypothesis is rejected with a 5-percent 

chance of it being true.

Because of differences in water level or in crosssection geometry, or because of missing records as a 

result of interference of the depth-sounder signal, the 

depth at some parts of a cross-sectional transect may 

not have been recorded; therefore, fewer than 10 depth 

recordings exist for some 0.25-m increments in some 

cross-section measurements. For increments at which 

the number of depth recordings equaled or exceeded 

eight for both measurements, the p-value was 

computed from the normal approximation for the 

distribution of W. For increments at which the number 

of depth recordings for either or both measurements 

was less than eight but greater than two, the p-value 

was determined from the exact distribution of W. 

The test was not applied for increments at which the 

number of depth recordings for either or both 

measurements was equal to or less than two. 

The fraction of the cross section to which the rank-sum 

test was applied and the fraction of the total cross 

section for which bed-elevation differences between 

measurements were significant were also calculated.

Changes in cross-sectional area between 

measurements were calculated for each incremental 

distance (0.25 m), using the selected or interpolated 

point as the center of the increment, by first computing 

the incremental area as the difference in mean bed 

elevations multiplied by 0.25 m. The change in area for 

the entire cross section then was computed by summing 

those incremental areas for which the p-value was less 

than or equal to 0.05. Although the original data from 

the 10 transects recorded for cross section p06 on 

August 24, 1992, were unrecoverable from the 

database, the processed mean bed elevations at the 

0.25-m increments had been calculated from the 

original data and stored in the database. Calculations of 

change in area using these data from this date, 

therefore, did not consider the p-values. The change in 

area of the bed elevation represents the change in 

sediment storage in the cross section. As a consequence 

of determining changes in cross-sectional area using 

this method, increases in area indicate sediment 

deposition and decreases indicate sediment removal.

 A standard error that can be applied to the 

computed cross-sectional change in area was calculated 

for an averaged cross section. The standard error is a 

measure of the accuracy with which the computed 

(mean) change in area estimates the true mean change 

in area. A range, in which there is a 

68.3-percent level of confidence that the true mean 

change in area lies within this range, can be calculated 

by subtracting the standard error from the computed 

mean change in area for the lower end of the range and 

by adding the standard error to the computed mean 

change in area for the upper end of the range. For a 

95-percent level of confidence, the range is calculated 

as previously described, but the standard error is first 

multiplied by 2 (Barford, 1985). 

The standard error uses the standard deviation 

calculated when computing mean bed elevations from 

the 10 transects collected during a measurement 

(fig. 9A, step 5), and the number of 0.25-m increments 

for each transect, which is a function of stream width. 

The standard error must be propagated through the 

change-in-area calculations (fig. 9B) and accumulated 

change-in-area calculations (summing changes in area 

for a cross section across many measurements). 

A representative standard error was estimated and 

propagated through the calculations. This representative standard error was calculated using a standard 

deviation of 0.15 m for the elevations and a stream 

width of 131 m. These values were calculated from the 

transect data from all cross sections in the primary data 

set (table 4) and represent an average standard 

deviation and stream width. Table 5 shows the 

estimated standard error that can be applied to changes 

in area documented in this report. The table shows the 

standard error for a single change in area calculated 

between two measurements (1 period) and accumulated changes in area over several measurements 

(periods), for 68.3-percent and 95-percent levels of 

confidence. For example, if the change in area between 
Methods 21

two measurements (1 period) is calculated to be 22 m2, 

the standard error is 0.39 m2, and the range is 

22 ± 0.39 m2 with a 68.3-percent level of confidence 

that the true mean change in area lies within this range. 

For a 95-percent level of confidence, the standard error 

is 0.78 m2, and the range is 22 ± 0.78 m2. If, for 

example, changes in area are accumulated (summed) 

between 13 measurements (12 periods) and calculated 

to be -519 m2, the standard error is 1.4 m2, and the 

range is -519 ± 1.4 m2 with a 68.3-percent level of 

confidence. For a 95-percent level of confidence, the 

standard error is 2.8 m2, and the range is -519 ± 2.8 m2. 

Bed-elevation corrections from the water-surface 

reference points enable comparisons between 

measurements for a given cross section, but do not 

enable relative comparisons between cross sections 

because differences in channel geometry are not taken 

into account. For example, an absolute change in area 

of 5 m2 is more significant in a narrower cross section 

with limited capacity for sediment storage than in a 

wider cross section with great capacity for sediment 

storage. To compare relative changes between cross 

sections without regard to differences in channel 

geometry, the changes in area were normalized 

(normalized change in area) for cross sections in the 

primary data set (table 4).

Changes in area from measurement to 

measurement were normalized by dividing the change 

in area by the cross-sectional area. The cross-sectional 

area used for normalization was calculated by 

estimating the elevation of the water surface at the 

water-surface reference points at a discharge of 

850 m3/s. A discharge of 850 m3/s was chosen to 

maximize the cross-sectional area and because many of 

the cross sections were measured at discharges near 

this value. Using the date and time of the measurement 

and data from the nearest upstream streamflow-gaging 

station, a discharge value was estimated for each watersurface elevation measurement made at the watersurface reference points. Discharge reference-point 

elevation curves were plotted for each water-surface 

reference point and regression equations were fitted to 

each curve. Using the regression equations, a watersurface elevation was calculated for a discharge of 

850 m3/s for each water-surface reference point. 

Selected discharge-elevation curves are shown in 

figure 10. Figure 10A,C illustrate discharge-elevation 

relations having high R2 values. Figure 10B shows a 

discharge-elevation relation having a low R2 value. 

Possible reasons for the outliers in the dischargeelevation curves include actual changes in the channel 

that affected the water-surface elevation or inaccurate 

water-surface measurements. To determine the crosssectional area for a discharge of 850 m3/s, the area for 

each incremental distance then was computed as the 

difference between the water-surface elevation at a 

discharge of 850 m3/s and the lowest measured bed 

elevation at that distance multiplied by 0.25 m. The 

normalized area for the cross section was computed by 

summing the incremental areas. 

Presentation of Data

Changes in sediment storage in the Colorado River 

downstream from the Paria and Little Colorado Rivers 

are documented from June 1992 to February 1994 

(Graf and others, 1995) and April 1994 to August 1995 

(Graf and others, 1997). Konieczki and others (1997) 

documented changes in sediment storage before and 

after the 1996 controlled flood. All cross-section data 

collected from June 1992 through September 1999 are 

available from the USGS as three types of tabdelimited ASCII electronic files. The first file type 

contains the interpolated data for each measurement. 

These files contain the transect number; the distance 

from the left-bank end point, in meters; and the bed 

Table 5. Standard error estimates applied to changes in area

Number of

measurements

Number of

period(s)

Standard error

(68.3-percent 

level of 

confidence;

square meters)

Standard error

(95-percent level 

of confidence;

square meters)

2 1 0.39 .78

3 2 .55 1.1

4 3 .67 1.4

5 4 .77 1.6

6 5 .86 1.8

7 6 .95 1.9

8 7 1.1 2.2

9 8 1.1 2.2

10 9 1.2 2.4

11 10 1.2 2.6

12 11 1.3 2.6

13 12 1.4 2.8
22 Variations in Sand Storage Measured in the Colorado River Between Glen Canyon Dam and Lava Falls Rapid, Northern Arizona, 1992–99

elevation for that distance, in meters. The second file 

type contains the averaged data for each measurement. 

These files contain the distance from the left-bank end 

point, in meters; the number of bed-elevation 

measurements for that distance; the mean, median, 

minimum, and maximum bed elevations for that 

distance, in meters; and the standard deviation from the 

mean bed elevation, in meters. The third file type 

contains changes in area between measurements for all 

cross sections. These files contain cross-section name, 

measurement date 1 and measurement date 2, change in 

area between the two measurement dates, fraction of 

tested cross section having differences significant at a 

p-value less than or equal to 0.05, and fraction of cross 

section for which the rank-sum test was applied. Bedmaterial sample analyses also are available. Readers 

who would like to obtain the electronic data should 

contact the Arizona District Office of the USGS.

A.

826

827

825

824

823

947

946

945

944

943

948

B.

ELEVATION, IN METERS

DISCHARGE, IN CUBIC METERS PER SECOND

y = –0.000002x2 + 0.0027x + 823.77

R2 = 0.5955

y = –0.000002x2 + 0.006 x + 924.37

R2 = 0.9769

812

813

811

810

0 200 300 400 500 700 800 100 600 900 1,000 1,100 1,200 1,400 1,300

809

C.

y = –0.000001x2 + 0.0052x + 808.34

R2 = 0.9652

Figure 10. Discharge-elevation curves for three water-surface reference points. A, Sections p01–p15. 

B, Sections 1a1–1a7. C, Sections lf1–lf5.
Cross-Section Variability 23

CROSS-SECTION VARIABILITY

Because of channel geometry and hydraulics, some 

cross sections have a larger capacity to store and 

subsequently lose sediment. This capacity is reflected 

in the range of change-in-area values.

Spatial Variability in Changes in Cross-Sectional 

Area

Normalized and non-normalized change in area 

calculated between successive measurement dates for 

cross sections in the primary data set (table 4) between 

the Paria River and Bright Angel Creek (fig. 1) are 

shown in figures 11 and 12, respectively. All measurements were used in the change-in-area calculations, 

except for those made during the 1996 controlled flood, 

and some cross sections were measured more 

frequently than others. Measurements made during the 

1996 controlled flood were not included because flows 

during the flood were extreme and not representative of 

normal hydrologic conditions. Some cross sections 

were more dynamic than others, showing a wider range 

of sediment deposition and scour. Cross sections p05 

and p06 are examples of more dynamic cross sections 

(fig. 13). Both cross sections are immediately below 

the mouth of the Paria River and are able to store 

sediment in a large eddy, as well as in the deep main 

channel, for a limited period of time (fig. 13). 

For example, the largest increase in sediment was 

measured for p06 on September 14, 1998 (fig. 13), 

after significant sediment input from the Paria River 

(fig. 3); however, the next measurement at p06, 

April 13, 1999 (fig. 13), indicated the largest decrease 

in sediment. These sections, farthest upstream, also are 

the first to be exposed to clear water flowing from the 

dam and thus are more susceptible to winnowing and 

erosion. Cross sections p12 and ld1 are examples of 

less dynamic cross sections; unlike sections p05 and 

p06, they do not have large eddies and thus have a 

limited capacity to store sediment in the main channel 

(fig. 14). 

Although cross section la1 is immediately downstream from the confluence of the Little Colorado and 

Colorado Rivers and has a channel geometry (fig. 15) 

similar to that of cross sections p05 and p06 (fig. 13), it 

does not show the same range of deposition and scour 

(figs. 11 and 12). The largest gain for cross section la1 

was measured on January 29, 1993 (fig. 15), 

immediately following significant inputs of water and 

sediment from the Little Colorado River (fig. 3); 

however, only a small loss was measured when cross 

section la1 was measured on April 22, 1993 (fig. 15). 

A negative change of comparable magnitude was not 

measured until the April measurement following the 

1996 controlled flood. A possible explanation is that 

the 1993 flood on the Little Colorado River deposited 

large-sized sediment immediately downstream from 

the confluence, armoring the channel at cross section 

la1 (Anima and others, 1998; Wiele and others, 1999). 

Releases from Glen Canyon Dam were such that the 

large-sized sediment was not moved downstream until 

the controlled flood occurred in 1996.

Temporal Variability in Cross-Sectional Area

The effects of large suspended-sediment inputs 

from tributaries on changes in the normalized cross 

sections are evident in the cross-section measurements 

when viewed chronologically (fig. 16). The cross 

sections downstream from the confluence of the Paria 

and Colorado Rivers measured in September 1998 

show a significant increase in area in the component of 

the cross section occupied by sediment the day after the 

storm that produced an input of 1.7 million metric tons 

of sediment from the Paria River (fig. 3). The subsequent measuring trip in April 1999, which occurred 

after 7 months of almost no suspended-sediment 

inputs, reported decreases in cross-sectional area, 

indicating decreases in sediment, from these same 

cross sections. Large increases in cross-sectional area 

also were measured during September 1999 following 

a storm. The results from this sequence of measurements demonstrate the rapid response of these cross 

sections to tributary inputs and indicate that because 

sediment storage in these cross sections is only temporary, the timing of the measurements is critical in terms 

of capturing the full effect of a tributary input.

Positive changes in cross-sectional area were also 

measured in the absence of known tributary inputs 

(fig. 16). For example, around September 1996, there 

was little sediment input from the Paria River, yet 

some cross sections in the h1–h5 and the w1–w5 

groups (fig. 6) showed net positive changes in area 

(fig. 16D). Cross sections in the la1–lf5 group (fig. 7) 

also showed net positive changes in area around 

March 1999, when there was little sediment input 

from either the Paria River or the Little Colorado River 

(fig. 16G). The increase in sediment may be the result 

of a sediment pulse from a previous tributary input. 
24 Variations in Sand Storage Measured in the Colorado River Between Glen Canyon Dam and Lava Falls Rapid, Northern Arizona, 1992–99

–0.6

B.

NORMALIZED CHANGE IN AREA

CROSS SECTION NAME

Bright

Angel Creek

confluence

Little Colorado

River

confluence la1 la2

la3la4la5la6la7

lb1

lb1b

lb2

lb3

lb4

lc2lc3lc4lc5

ld1ld2ld3ld4ld5

le1le2le3le4

lf1lf2lf3lf4lf5

g0g1g2g3g4g5g6

–0.4

–0.2

0

0.2

0.4

0.6

lc1le5

–0.6

A.

NORMALIZED CHANGE IN AREA

CROSS SECTION NAME

Paria

River

confluence

Little Colorado

River

confluence

–0.4

–0.2

0

0.2

0.4

0.6

p01p02p03p04p05p06p07p08p09p10p11p12p13p14p15

p15a

p15b

p16p17p18p19p20p21p22p23p24p25p26p27p28p29p30p31p32

h1h2h3h4h5

w1w2w3w4w5

Figure 11. Normalized change-in-area data, excluding 1996 controlled-flood measurements, for cross sections in the primary 

data set. A, Sections between the confluences of the Paria and Colorado Rivers, and the Little Colorado and Colorado Rivers. 

B, Sections between the confluences of the Little Colorado and Colorado Rivers, and Bright Angel Creek and the Colorado River.
Cross-Section Variability 25

B.

CROSS SECTION NAME

Bright

Angel Creek

confluence

Little Colorado

River

confluence la1 la2

la3la4la5la6la7

lb1

lb1b

lb2

lb3

lb4

lc2lc3lc4lc5

ld1ld2ld3ld4ld5

le1le2le3le4

lf1lf2lf3lf4lf5

g0g1g2g3g4g5g6

0

lc1le5

–1,500

A.

CHANGE IN AREA, IN SQUARE METERS

CROSS SECTION NAME

Paria

River

confluence

Little Colorado

River

confluence

–1,000

–500

0

500

1,000

1,500

–1,500

CHANGE IN AREA, IN SQUARE METERS

–1,000

–500

500

1,000

1,500

p01p02p03p04p05p06p07p08p09p10p11p12p13p14p15

p15a

p15b

p16p17p18p19p20p21p22p23p24p25p26p27p28p29p30p31p32

h1h2h3h4h5

w1w2w3w4w5

Figure 12. Non-normalized change-in-area data, excluding 1996 controlled-flood measurements, for cross sections in the primary 

data set. A, Sections between the confluences of the Paria and Colorado Rivers, and the Little Colorado and Colorado Rivers.

B, Sections between the confluences of the Little Colorado and Colorado Rivers, and Bright Angel Creek and the Colorado River.
26 Variations in Sand Storage Measured in the Colorado River Between Glen Canyon Dam and Lava Falls Rapid, Northern Arizona, 1992–99 ELEVATION, IN METERS

948

948

930

Section p05

DISTANCE FROM LEFT-BANK REFERENCE POINT, IN METERS

946

944

942

940

938

936

934

932

930

Section p06

September 14, 1998

April 13, 1999

946

944

942

940

938

936

934

932

0 20 40 60 80 100 120 140 160 180 200

Figure 13. Bed-elevation measurements for selected cross sections in the primary data set below the confluence of the Paria 

and Colorado Rivers.
Cross-Section Variability 27

ELEVATION, IN METERS

946

820

934

Section p12

DISTANCE FROM LEFT-BANK REFERENCE POINT, IN METERS

944

942

940

938

936

818

816

814

812

810

808

806

Section ld1

0 20 40 60 80 100 120 140 160 180 200

Figure 14. Bed-elevation measurements for selected cross sections in the primary data set below the confluences of the 

Paria and Colorado Rivers, and the Little Colorado and Colorado Rivers.
28 Variations in Sand Storage Measured in the Colorado River Between Glen Canyon Dam and Lava Falls Rapid, Northern Arizona, 1992–99 ELEVATION, IN METERS

830

805

Section la1

January 29, 1993

April 22, 1993

DISTANCE FROM LEFT-BANK REFERENCE POINT, IN METERS

825

820

815

810

0 20 40 60 80 100 120 140 160 180 200

Figure 15. Bed-elevation measurements for cross section 1a1 in the primary data set below the confluence of the Little Colorado 

and Colorado Rivers.
Cross-Section Variability 29

NORMALIZED CHANGE IN AREA

A.

1992 1994 1995 1996 1997 1998 1999 1993

DISCHARGE, IN CUBIC METERS PER SECOND

1,400

1,200

1,000

800

600

400

200

150

100

50

0

200

C.

B.

0.6

0.4

0.2

0

–0.2

–0.4

–0.6

0.6

0.4

0.2

0

–0.2

–0.4

–0.6

D.

Figure 16. Discharge and normalized change-in-area data for cross sections in the primary data set between the confluences 

of the Paria and Colorado Rivers, and Bright Angel Creek and the Colorado River. A, Daily maximum discharge at the streamflowgaging station at Colorado River at Lees Ferry. B, Daily maximum discharge at the streamflow-gaging station at Paria River at Lees 

Ferry. C, Sections p01–p32. D, Sections h1–h5 and w1–w5. E, Daily maximum discharge at the streamflow-gaging station at 

Colorado River above Little Colorado River. F, Daily maximum discharge at the streamflow-gaging station at Little Colorado River 

near Cameron. G, Sections la1–lf5. H, Sections g0–g5.
30 Variations in Sand Storage Measured in the Colorado River Between Glen Canyon Dam and Lava Falls Rapid, Northern Arizona, 1992–99 NORMALIZED CHANGE IN AREA

E.

1992 1994 1995 1996 1997 1998 1999 1993

DISCHARGE, IN CUBIC METERS PER SECOND

1,400

1,200

1,000

800

600

400

0

200

G.

F.

0.6

0.4

0.2

0

–0.2

–0.4

–0.6

0.6

0.4

0.2

0

–0.2

–0.4

–0.6

H.

600

500

400

300

200

100

0

Figure 16. Continued.
Changes in Cross-Sectional Area 31

Alternatively, because of the changing flow regime 

from Glen Canyon Dam, sediment in the system is 

undergoing constant change so that the measurement 

also may have included existing sediment within the 

pool or upstream of the pool that was reworked. 

Sediment also can be supplied by mass failure of 

sandbars, as documented by Cluer (1991), or from 

ungaged tributaries.

CHANGES IN CROSS-SECTIONAL AREA

To determine whether cross sections were gaining 

or losing sediment over time, the net change in area 

(running total or accumulation) was calculated by 

summing the non-normalized change in area between 

measurements for each cross section in the matchingdate data set (table 4). The first date, considered the 

initial measurement for these analyses, is assigned to 

be the baseline with a net area equal to zero. Changes 

in area then are calculated between subsequent 

measurements and summed over time to form the net 

change in area or the running total. Using data from a 

sequence of dates for which the same cross sections 

were measured provides a way to observe how 

individual cross sections respond to similar hydrologic 

conditions over time. The two study reaches with the 

longest and most complete time-series data are the 

reaches below the mouths of the Paria and Little 

Colorado Rivers. 

For the 5 groups of cross sections downstream 

from the Paria River, there were 13 matching dates or 

12 periods (matching-date data set, Paria group) 

between August 24, 1992, and August 19, 1996, for 

which the running totals were calculated for all cross 

sections except p02 and p32 (fig. 17). All the cross 

sections in the first group (p01–p08) had a negative 

running total, ranging from -22 to -519 m2, for the first 

period. This first-period value represents the change in 

area between the August 24, 1992, and the January 21, 

1993, measurement dates. The initial measurement on 

August 24, 1992, was shortly after large sediment 

inputs from the Paria River, and it is likely that the 

cross sections just below the confluence still contained 

sediment from this tributary input during this 

measurement. Because the sediment deposited before 

the August 24, 1992, measurement had scoured from 

the cross sections before the January 21, 1993, 

measurement, there is a large decrease in the running 

total for the first period for these cross sections.

All but two of the cross sections downstream from 

the Paria showed net losses between August 24, 1992, 

and August 19, 1996. Cross section p21 had a minimal 

gain of 70 m2, and loss or gain could not be determined 

at p19 because the standard error, ±2.8 m2 (95-percent 

level of confidence, table 5), added to the net change in 

area, 2 m2, is too close to zero. Cross section p06 

showed the largest net loss, 562 m2. Overall there was a 

wide range of responses among cross sections; cross 

section p06 showed a large capacity to store and lose 

sediment, whereas cross sections p27–p31 did not 

show large net changes.

For the 6 groups of cross sections downstream 

from the Little Colorado River, there were 

11 matching-measurement dates, or 10 periods 

(matching-date data set, Little Colorado River group), 

for which all cross sections, except la4, la5, lb1b, ld5, 

and lf5, were measured between January 29, 1993, and 

April 16, 1997 (fig. 18). Cross sections ld2 and ld3 

were excluded because data sets were incomplete. 

The first measurement in January 1993 was made 

approximately 2.5 weeks after a large flood (recurrence 

interval of 15 years) occurred on the Little Colorado 

River. Unlike cross sections just downstream from the 

confluence of the Paria River, the cross sections just 

downstream from the confluence of the Little Colorado 

River changed little until after the 1996 controlled 

flood (fig. 18).

All the cross sections just downstream from the 

Little Colorado River showed net losses for the time 

between January 31, 1993, and April 16, 1997. Cross 

section lc2 showed the largest net loss, 1,400 m2, and 

cross section 1d1 showed the smallest net loss, 5 m2. 

Similar to the cross sections in the Paria reach, the 

cross sections in the lower portion of the Little 

Colorado River reach consistently showed less net 

change than upstream cross sections. 

FACTORS AFFECTING SEDIMENT STORAGE 

CHANGES

An analysis was done to determine if there was a 

correlation between changes in cross-sectional area and 

the corresponding hydrologic conditions (factors) that 

occurred between cross-section measurements. The 

hydrologic conditions considered included high flow, 

low flow, mean flow, range of flow, and sediment input 

from tributaries. 
32 Variations in Sand Storage Measured in the Colorado River Between Glen Canyon Dam and Lava Falls Rapid, Northern Arizona, 1992–99

1992 1994 1995 1996 1997 1993

CHANGES IN AREA, IN SQUARE METERS

100

0

–100

–200

–300

–400

–500

–600

200

1 23 4 56 7

PERIOD

8 9 10 11 12

A.

100

0

–100

–200

–300

–400

–500

–600

200

B.

100

0

–100

–200

–300

–400

–500

–600

200

C.

Aug. 24, 1992Jan. 21, 1993Apr. 5, 1993Aug. 30, 1993Jan. 10, 1994Apr. 4, 1994Aug. 29, 1994May 9, 1995July 3, 1995 Aug. 28, 1995Feb. 12, 1996May 6, 1996Aug. 19, 1996

p01

p03

p04

p05

p06

p07

p08

p09

p10

p11

p12

p13

p14

p15

p15a

p15b

p16

p17

p18

p19

p20

Figure 17. Net change in area for cross sections in the matching-date data set between the confluence of the Paria and 

Colorado Rivers and Badger Creek Rapid. A, Sections p01–p08. B, Sections p09–p15. C, Sections p15a–p20. D, Sections p21–p26. 

E, Sections p27–p31.
Factors Affecting Sediment Storage Changes 33

CHANGES IN AREA, IN SQUARE METERS

100

0

–100

–200

–300

–400

–500

–600

200

1 23 4 56 7

PERIOD

8 9 10 11 12

D.

100

0

–100

–200

–300

–400

–500

–600

200

E.

Aug. 24, 1992Jan 21, 1993Apr. 5, 1993Aug. 30, 1993Jan. 10, 1994Apr. 4, 1994Aug. 29, 1994May 9, 1995July 3, 1995 Aug. 28, 1995Feb. 12, 1996May 6, 1996Aug. 19, 1996

1992 1994 1995 1996 1997 1993

p27

p28

p29

p30

p31

p21

p22

p23

p24

p25

p26

Figure 17. Continued.
34 Variations in Sand Storage Measured in the Colorado River Between Glen Canyon Dam and Lava Falls Rapid, Northern Arizona, 1992–99

1992 1994 1995 1996 1997 1993

CHANGES IN AREA, IN SQUARE METERS

1 2 34 5 6 7

PERIOD

8 9 10

A.

B.

0

–200

–400

–600

–800

–1,000

–1,200

–1,400

200

–1,600

C.

Jan. 31, 1993Apr. 22, 1993Sept. 15, 1993Jan. 30, 1994Apr. 30, 1994Sept. 14, 1994Apr. 22, 1995Feb. 25, 1996Apr. 17, 1996Sept 13, 1996Apr. 16, 1997

–200

0

–400

–600

–800

–1,000

–1,200

–1,400

200

–1,600

–200

0

–400

–600

–800

–1,000

–1,200

–1,400

200

–1,600

lc1

lc2

lc3

lc4 

lc5

lb1

lb2

lb3

lb4

la1

la2

la3

la6

la7

Figure 18. Net change in area for cross sections in the matching-date data set between the confluence of the Little Colorado and 

Colorado Rivers and Tanner Rapid. A, Sections la1–la7. B, Sections lb1–lb4. C, Sections lc1–lc5. D, Sections ld1, ld4, and le1–le5. 

E, Sections lf1–lf4.
Factors Affecting Sediment Storage Changes 35

Changes in cross-sectional area between 

measurements were calculated for sections in the 

matching-date data set (table 4) described in the section 

entitled, “Changes in Cross-Sectional Area.” The 

matching-date data set was chosen because all the cross 

sections in the same group typically were measured 

within the same week for a given measurement and 

thus were exposed to similar hydrologic conditions 

between measurements. 

To generate the values for high flow, low flow, 

mean flow, and range of flow, daily discharge data from 

the Colorado River at Lees Ferry were used for the 

Paria group of cross sections and data from the 

Colorado River near Grand Canyon were used for 

the Little Colorado River group of cross sections. 

The discharge data from the Lees Ferry gaging station, 

which is upstream from the Paria River, can be used to 

represent flow conditions for the Paria group of cross 

sections because the Paria River does not contribute 

substantial volumes of water to the Colorado River. 

Because flows from the Little Colorado River can be 

significant, however, discharge for the Little Colorado 

River group of cross sections is represented using data 

from the Grand Canyon gaging station, which is 

downstream from the Little Colorado River (fig. 1). 

1992 1993

lf1

lf2

lf3

lf4

1994 1995 1996 1997

CHANGES IN AREA, IN SQUARE METERS

1 2 34 5 6 7

PERIOD

8 9 10

D.

E.

Jan. 31, 1993Apr. 22, 1993Sept. 15, 1993Jan. 30, 1994Apr. 30, 1994Sept. 14, 1994Apr. 22, 1995Feb. 25, 1996Apr. 17, 1996Sept. 13, 1996Apr. 16, 1997

–200

0

–400

–600

–800

–1,000

–1,200

–1,400

200

–1,600

–200

0

–400

–600

–800

–1,000

–1,200

–1,400

200

–1,600

ld1

ld4

le1

le2

le3

le4

le5

Figure 18. Continued.
36 Variations in Sand Storage Measured in the Colorado River Between Glen Canyon Dam and Lava Falls Rapid, Northern Arizona, 1992–99

Additionally, time-weighted values for high 

flow, low flow, mean flow, and daily range of flow 

were generated. The weight was obtained by 

sequentially ordering days between each crosssection measurement and then multiplying the 

daily flow by the ordered number and dividing by 

the total number of days between cross-section 

measurements. This placed greater emphasis on 

flows that occurred closest to the date that the 

cross section was measured. 

Suspended-sediment discharge data from the 

Paria River at Lees Ferry gaging station were used 

to determine tributary inputs to compare to the 

changes in the Paria group of cross sections. 

Suspended-sediment discharge data from the Paria 

River at Lees Ferry and Little Colorado River near 

Cameron gaging stations were used to determine 

tributary inputs to compare to the changes in the 

Little Colorado River group of cross sections. 

Suspended-sediment input from the Glen Canyon 

reach of the Colorado River measured at the Lees 

Ferry gaging station is negligible and, therefore, 

was assumed to be zero for this analysis. The total 

suspended-sediment load between cross-section 

measurements was calculated by summing the 

loads for those days when flow and suspendedsediment discharge increased in the tributaries 

because of storms.

Because of the uncertainty in the underlying 

distributions and relatively small sample sizes, 

several nonparametric statistical tests using ranked 

and unranked data were used to determine if any of 

the factors were more influential than others in 

explaining the changes in cross-sectional area. 

Test results showed only a weak relation between 

changes in cross-sectional area and some of the 

hydrologic conditions considered—daily mean, 

daily range of flow, time-weighted daily mean, 

time-weighted daily range of flow, and sediment 

inputs from the Paria and Little Colorado Rivers. 

Scatter plots using the unranked data in figures 19 

and 20 illustrate the associations between changes 

in area and these weakly related hydrologic 

conditions for the Paria and Little Colorado River 

groups of sections, respectively. The p-values 

shown on the scatter plots were based on results 

from Kendall’s tau test using unranked data. 

AVERAGE CHANGE

IN AREA, IN

SQUARE METERS

TOTAL SUSPENDED-SEDIMENT DISCHARGE FROM

THE PARIA RIVER, IN MILLIONS OF METRIC TONS

E.

–0.5 0 0.5 1.0 1.5

60

AVERAGE CHANGE

IN AREA, IN

SQUARE METERS

AVERAGE TIME-WEIGHTED FLOW, IN

CUBIC METERS PER SECOND

D.

20 40 60 80 100

AVERAGE CHANGE

IN AREA, IN

SQUARE METERS

AVERAGE FLOW, IN CUBIC METERS PER SECOND

C.

100 150 200

300 350 400 450 500

50

AVERAGE CHANGE

IN AREA, IN

SQUARE METERS

AVERAGE TIME-WEIGHTED FLOW, IN

CUBIC METERS PER SECOND

B.

100 150 200 250 300

AVERAGE CHANGE

IN AREA, IN

SQUARE METERS

AVERAGE FLOW, IN CUBIC METERS PER SECOND

A.

250 550

30

0

–30

–60

–90

–120

60

30

0

–30

–60

–90

–120

60

30

0

–30

–60

–90

–120

60

30

–30

–60

–90

0

–120

60

30

–30

–60

–90

0

–120

Kendall's tau = 0.121

p –value = 0.638

Kendall's tau = –0.303

p –value = 0.196

Kendall's tau = –0.242

p –value = 0.310

Kendall's tau = –0.394

p –value = 0.086

Kendall's tau = –0.273

p –value = 0.250

Figure 19. Associations between average changes in area and 

five hydrologic conditions for cross sections in the matching-date data 

set between the confluence of the Paria and Colorado Rivers and 

Badger Creek Rapid. A, Average of daily mean flows. B, Average of 

time-weighted daily mean flows. C, Average of daily mean range of 

flows. D, Average of time-weighted daily mean range of flows. 

E, Suspended-sediment discharge from the Paria River. All flow data 

are from the streamflow-gaging station Colorado River at Lees Ferry 

(09380000).
Conclusion 37

AVERAGE CHANGE

IN AREA, IN

SQUARE METERS

TOTAL SUSPENDED-SEDIMENT DISCHARGE FROM

THE PARIA RIVER, IN MILLIONS OF METRIC TONS

E.

–1 0 1 2 3 4 5 6 7 8

60

AVERAGE CHANGE

IN AREA, IN

SQUARE METERS

AVERAGE TIME-WEIGHTED FLOW, IN

CUBIC METERS PER SECOND

D.

20 50 70

AVERAGE CHANGE

IN AREA, IN

SQUARE METERS

AVERAGE FLOW, IN CUBIC METERS PER SECOND

C.

50 200

AVERAGE CHANGE

IN AREA, IN

SQUARE METERS

AVERAGE TIME-WEIGHTED FLOW, IN

CUBIC METERS PER SECOND

B.

150 300

150 200 250

AVERAGE CHANGE

IN AREA, IN

SQUARE METERS

AVERAGE FLOW, IN CUBIC METERS PER SECOND

2000 250

A.

300 550

30

0

–30

–60

–90

–120

30 40 60

60

30

0

–30

–60

–90

–120

60

30

0

–30

–60

–90

–120

60

30

–30

–60

–90

0

–120

60

30

–30

–60

–90

0

–120

Kendall's tau = 0.333

p–value = 0.216

Kendall's tau = –0.733

p –value = 0.002

Kendall's tau = –0.644

p –value = 0.009

Kendall's tau = 0.333

p –value = 0.216

350 400 450 500

Kendall's tau = –0.289

p –value = 0.292

Figure 20. Associations between average changes in area and 

five hydrologic conditions for cross sections in the matching-date data 

set between the confluence of the Little Colorado and Colorado Rivers 

and Tanner Rapid. A, Average of daily mean flows. B, Average of timeweighted daily mean flows. C, Average of daily mean range of flows. 

D, Average of time-weighted daily mean range of flows. E, Suspendedsediment discharge from the Paria and Little Colorado Rivers. All flow 

data are from the streamflow-gaging station Colorado River at Grand 

Canyon (09402500).

For the Paria group of sections, unranked daily 

mean range of flow, time weighted (fig. 19D), 

shows the strongest association with change in area 

and has a p-value of 0.086. For the Little Colorado 

River group of sections, unranked daily mean flow, 

both unweighted (fig. 20A) and time-weighted (fig. 

20B), shows the strongest associations with change 

in area. The corresponding p-values are 0.009 for 

daily mean flows and 0.002 for time-weighted 

daily mean flows. 

The analyses did not strongly indicate that any 

one of the hydrologic factors considered was more 

influential in causing changes in area than any 

other hydrologic factor considered, nor could any 

causation between factors and changes in area be 

established. This could indicate that an important 

governing process was not considered in the 

analyses, or that the spatial and temporal 

resolutions of the collected data were inadequate to 

explain the changes in area.

CONCLUSION

This data-collection effort was successful in 

providing data to support the development of 

multidimensional sediment-transport models 

(Wiele and others, 1996). Because of the highly 

variable nature of sediment in this system, both 

temporally and spatially, the data cannot adequately be used to answer questions about the 

sediment-storage condition of the channel or for 

tracking the movement of sediment pulses through 

the system. At the measurement frequency that 

these data were collected, the data indicate a short 

residence time for sediment storage in the cross 

sections. This result is consistent with the conclusion of Topping, Rubin, and Vierra (2000) that 

multiyear accumulation of sediment has not occurred under the MLFF operating criteria, as was 

anticipated in the EIS.

The nonparametric statistical tests using 

ranked and unranked data showed only a weak 

relation between changes in cross-sectional area 

and some of the hydrologic conditions considered: 

daily mean flow, daily range of flow, timeweighted daily mean flow, time-weighted daily 

range of flow, and sediment inputs from the Paria 

and Little Colorado Rivers. This lack of correlation 

indicates that an important governing process was 

not considered in the analyses, or that the spatial 

and temporal resolutions of the collected data were 

inadequate to explain the changes in area.
38 Variations in Sand Storage Measured in the Colorado River Between Glen Canyon Dam and Lava Falls Rapid, Northern Arizona, 1992–99

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