U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2013–3074

August 2013

Recent U.S. Geological Survey research is providing important 

insights into how best to monitor 

changes in the amount of tributaryderived sand stored on the bed of 

the Colorado River and in eddies in 

Marble Canyon, Arizona. Before the 

construction of Glen Canyon Dam and 

other dams upstream, sandbars in 

Glen, Marble, and Grand Canyons were 

replenished each year by sedimentrich floods. Sand input into the 

Colorado River is crucial to protecting 

endangered native fish, animals, and 

plants and cultural and recreational 

resources along the river in Glen 

Canyon National Recreation Area and 

Grand Canyon National Park.

There is longstanding interest in 

the condition and trend of river-related 

resources in and along the Colorado River 

in Glen Canyon National Recreation Area 

and Grand Canyon National Park, Arizona. 

These resources include endangered native 

fish, native riparian flora and fauna (plants 

and animals that live along streams), 

riverside campsites, and many sites of 

cultural significance to Native Americans. 

The physical underpinnings for many of 

these resources are river-deposited sandbars 

that occur intermittently along the banks of 

more than 300 miles of the Colorado River 

in Glen, Marble, and Grand Canyons. 

In the Colorado River, most sandbars 

form along the shoreline in eddies, which 

are areas of recirculating, relatively low 

velocity flow. The sandbars are typically 

inundated during floods and exposed 

at times of low stream flow. Before the 

completion of Glen Canyon Dam in 1963 

and the construction of other large dams 

upstream, sandbars were replenished each 

year by sediment-rich floods. Sandbars 

are now smaller because Lake Powell, 

the nearly 200-mile long reservoir formed 

by the dam, traps all upstream sediment 

before it reaches the canyons downstream. 

The sandbars that remain in Glen, Marble, 

and Grand Canyons are maintained by 

replenishment of sand that is supplied by 

tributaries and redistributed by occasional 

controlled floods released from Lake 

Powell. The two largest sources of tributary 

sand are the Paria and Little Colorado 

Rivers, which enter the Colorado River 17 

and 79 miles downstream from the dam, 

respectively. The sand delivered by these 

tributaries is temporarily stored on the 

riverbed until the water released during 

controlled floods transports it to sandbars 

along the river’s banks. 

Sandbars in Grand Canyon—

Previous Research

Decline in the size and abundance of 

sandbars since the pre-Glen Canyon Dam 

era has been documented by analysis of old 

aerial and ground-level photographs and 

by topographic surveys that began in the 

mid-1970s. Scientists have estimated that 

sandbar area in the upstream 100 miles of

Glen, Marble, and Grand Canyons was 25 

percent less in 2000 than in average pre-dam 

years (Wright and others, 2005). This decline 

occurred because releases of water from 

Lake Powell are virtually free of sediment. 

The tributaries that enter the Colorado River 

downstream from the dam supply only a 

fraction of the pre-dam sand supply (Topping 

and others, 2000), and the capacity of the 

post-dam river to transport that sand greatly 

exceeds this limited supply. Normal dam 

operations, therefore, tend to erode, rather 

than build, sandbars.

By experimentation, scientists have 

learned that controlled floods, if released 

from the reservoir immediately following 

large inputs of sand from tributaries, 

can build sandbars (Schmidt and Grams, 

2011). These sandbars are built during 

controlled floods when sand is carried 

from the riverbed and temporarily 

suspended at high concentration in the 

flow. The suspended sand is transported 

into eddies where it is then deposited 

in areas of low stream-flow velocity. 

Sandbars enlarged by this process provide 

larger camping beaches for river-rafting 

trips and create backwater habitats used 

by native fish. Newly deposited sandbars 

also provide areas for riparian vegetation 

A Sand Budget for Marble Canyon, Arizona—Implications for 

Long-Term Monitoring of Sand Storage Change

View of the Colorado River in Grand Canyon, Arizona, showing sandbars that form downstream from 

tributary debris fans, which constrict the flow of the river and cause eddies. The view is looking upstream 

at a point approximately 217 miles downstream from Glen Canyon Dam. (Photo by Sam Jansen.)
to grow and are a source of windblown 

sand. Windblown sand carried upslope 

from sandbars helps to cover and 

potentially preserve some of the culturally 

significant archeological sites in Grand 

Canyon (Draut and Rubin, 2008). 

Scientists have also learned that 

controlled floods may erode sandbars 

if the concentration of suspended sand 

during a controlled flood is too low. The 

concentration of sand during a flood is 

directly proportional to the amount of 

the riverbed covered by sand and the 

size of that sand. Higher concentrations 

of suspended sand occur when the sand 

is relatively fine and large amounts 

of the riverbed are covered by sand. 

These findings are incorporated in the 

current reservoir-release management 

strategy for Glen Canyon Dam, which 

involves releasing controlled floods— 

administratively referred to as High 

Flow Experiments (HFEs)—whenever 

the Paria River has recently delivered 

large amounts of sand to the Colorado 

River. The magnitude and duration of the 

controlled floods is adjusted to transport 

just the amount of sand that has recently 

been delivered from the Paria River. 

In support of the protocol to 

implement HFEs, the U.S. Geological 

Survey’s Grand Canyon Monitoring and 

Research Center (GCMRC) monitors the 

amount of sand supplied from the Paria 

River and other tributaries and measures 

the size and distribution of sand on the 

riverbed and in eddies. Data on long-term 

trends in the amount of sand stored on the 

riverbed and in eddies allows scientists 

to evaluate whether the cumulative effect 

of the recently adopted HFE management 

strategy is causing a net increase or 

decrease in the size and volume of sand 

deposits along the Colorado River. A 

significant and progressive decline in 

sand storage would indicate that sandbars 

created during controlled floods are 

short-lived and that the total volume of 

sandbars is declining. Such a long-term 

decline in sand storage would likely result 

in declining future effectiveness of HFEs 

in maintaining desired sandbar size.

Recent Findings—How Much 

Sand is in the River?

Although sandbars that are exposed 

above the water surface are the resources 

of management interest, the visible part of 

a sandbar represents only 10 to 20 percent 

of the total sandbar. Like icebergs, a small 

Glen Canyon Dam Lake Powell

Glen Canyon National 

Recreation Area

Lees Ferry

Little Colorado River

UTAH

ARIZONA

0 30 MILES

ARIZONA

Map

Area

112°W

37°

36°

RM 30

RM 61

RM 87

Streamflow and sediment 

transport measurement stations

Short monitoring reaches

Paria

River

Colorado River

exposed sandbar is merely the highest part 

of a much larger sandbar, most of which is 

submerged. Thus, tracking trends in sand 

storage requires tracking both the exposed and 

submerged parts of sandbars, as well as the 

total amount of sand stored elsewhere on the 

riverbed. Monitoring the submerged parts of 

sandbars in the Colorado River has required 

the development of specialized methods.

Two complementary methods are used 

to monitor changes in sand deposits for 

long segments of the Colorado River. The 

flux-based method for measuring changes 

in sand deposits is based on measurements 

of total sand transport, referred to as “flux,” 

past measurement stations spaced 30 to 60 

miles apart. The difference in flux between 

stations represents the amount of sand that 

has been deposited in or eroded from the 

intervening river segment. Because stations 

operate continuously, this method provides 

a continuous record of changes in storage 

between the measurement stations. These 

continuous measurements can be compared 

to dam releases during the same time period 

to better understand how dam operations 

affect sandbar resources. Disadvantages 

of this method are that uncertainty 

grows in proportion to the length of the 

monitoring period and that it does not 

provide information about how much each 

individual sand deposit changed within 

each long river segment. The alternative 

method to measure change in sand deposits 

is based on repeat measurements of the 

underwater topography of the sand deposits 

using an instrument called a multibeam 

echosounder. This topographic method, 

for which uncertainty does not increase 

for longer monitoring periods, provides a 

relatively precise portrayal of the locations 

and amounts of changes in sand deposits. 

Repeat topographic measurements are, 

however, difficult to implement over long 

river segments and are made infrequently.

Flux-based measurements are ideally 

suited to monitoring short-term changes in 

sand storage, which are essential for making 

decisions about dam operations. Topographic 

measurements, on the other hand, are more 

appropriate for monitoring long-term trends 

in the deposits, which are needed to evaluate 

the cumulative effect of management actions 

over several years. Grams and others (2013) 

integrated these two measurement methods 

for the 61-mile length of Marble Canyon. 

Because mapping channel topography is time 

consuming and costly, five short (less than 3 

mile) reaches were selected for monitoring as 

a practical alternative to mapping the entire 

61-mile segment. For such an approach to 

be successful, topographic changes in the 

monitoring reaches must be representative of 

changes throughout the study area. Thus, the 

primary purpose of the study was to compare 

the two measurement methods and thereby 

determine if this sampling strategy could be 

Before the construction of Glen 

Canyon Dam and other dams 

upstream, sandbars in Glen, 

Marble, and Grand Canyons, 

Arizona, were replenished each 

year by sediment-rich floods. To 

provide information crucial to 

managing these sandbars, U.S. 

Geological Survey scientists are 

monitoring changes in sandbars 

and the amount of sediment 

supplied to the river in Marble 

Canyon. This map shows the 

locations of the five short (less 

than 3 mile) monitoring reaches 

used to measure topographic 

changes in sandbars and the 

locations of stream-flow and 

sediment-transport measurement 

stations. The brown-shaded area 

is within Grand Canyon National 

Park. Glen Canyon extends from 

Glen Canyon Dam to Lees Ferry; 

upper Marble Canyon extends from 

Lees Ferry to river mile (RM) 30; 

lower Marble Canyon extends from 

RM 30 to RM 61; and Grand Canyon 

begins at RM 61.
used to reliably track long-term trends in 

sand storage. 

Each of the short topographic 

monitoring reaches was mapped at least 

four different times between 2002 and 

2009. Comparison of the maps for different 

times allowed determination of the amount 

and spatial pattern of changes in riverbed 

and eddy sand deposits. These results were 

compared with sediment-flux measurements 

spanning the same time periods.

The repeated mapping of riverbed 

topography demonstrates that very large 

changes in individual sand deposits can occur 

in the Colorado River in Marble Canyon. 

Adjacent sand deposits often change in 

different ways—one deposit may erode 

greatly, but another nearby deposit may 

grow significantly. For example, between 

December 2004 and May 2009, some parts 

of the river channel near river mile 56 

eroded, whereas other areas accumulated 

sand. Grams and others (2013) also found 

that local changes in the topography of sand 

deposits are very large when compared to the 

changes in storage calculated from flux-based 

measurements. The amount of sand that is 

either deposited or eroded from just a few 

hundred yards of river channel can equal 

the total change in sand storage for an entire 

30-mile river segment. 

Thus, changes in sand storage that are 

measured over long segments of the river 

channel by the flux-based measurement 

program are not the result of uniform 

changes evenly distributed throughout the 

channel. Instead, changes in sand storage 

that are measured over long segments result 

from the sum of topographic changes in 

(1) small areas where there has been large 

erosion and (2) other areas where there 

has been large deposition. This finding 

highlights the difficulty in identifying short 

“representative” topographic monitoring 

reaches for extending results to longer 

river segments. Changes measured at a 

few locations in a few short reaches are 

unlikely to be representative of the average 

topographic response over a long segment 

of the Colorado River.

The “Sand Budget”

Tracking sand storage in Grand Canyon 

is like managing a budget or a bank account. 

“Deposits to the account” occur when sand is 

supplied by tributaries, but these “deposits” 

are typically only made to the riverbed. Sand 

transported further downstream towards 

Lake Mead reservoir (along the ArizonaNevada border) by normal dam operations 

and controlled floods are withdrawals from 

the account. Sandbars built during controlled 

floods may be viewed as “transfers” from 

the “riverbed account” to the “eddy sandbar 

account.” Thus, it is essential that river 

managers are provided information about 

the amount of sand in the total bank account, 

the amount of sand in the eddy sandbar 

56

0 200 METERS

Elevation change

< -2 m

-1.9– -1.0

-0.9– -0.5

-0.4– -0.3

-0.2– -0.1

0

0.1

0.2–0.3

0.4–0.5

0.6–1.0

1.0–2.0

>2.0

1

2

3

4

5

This aerial image with superimposed map 

shows areas of erosion (red) and deposition 

(blue) between December 2004 and May 2009 

for a segment of the short monitoring reach 

near river mile 56 in Marble Canyon (see 

map). A multibeam echosounder was used 

to measure the changes in the underwater 

topography of sediment deposits. The 

numbered gray outlines show individual 

eddies, where changes in sediment storage are 

entirely sand. Of the three largest eddies in this 

short river segment, one (2) is almost entirely 

dominated by erosion and two (1 and 4) are 

dominated by deposition. The Colorado River 

channel outside of the eddies includes areas of 

deposition and erosion. (Changes in elevation 

shown in meters (m); 1 m=~3.28 feet. <, less

than; >, greater than.)

Glen Canyon Dam was completed in 1963, affecting stream flow and sand movement in the 

Colorado River below the dam. These photographs show the river in Grand Canyon, 150 miles 

downstream from the dam. The top image taken in 1952 shows a large sandbar; in the bottom 

image taken in 2003 little sand remains. View is downstream from the right (north) bank of the river. 

(Photos courtesy of Kent Frost, top, and Steve Tharnstrom, bottom; from Webb and others, 2002.)
Paul E. Grams

Edited by James W. Hendley II

Graphics and layout by Jeanne S. DiLeo

account, and the amount of sand that has been 

withdrawn from the account and transported to 

Lake Mead. 

Repeat topographic mapping of the 

riverbed shows where large deposits and 

withdrawals of sand from the account typically 

occur. Grams and others (2013) showed that 

the sand is stored in hundreds of separate 

accounts, some larger than others, distributed 

throughout the river channel. Understanding 

this sand budget—where the sand-storage 

locations are, the size of these accounts, 

and how much these accounts change—is 

essential to improving predictions about how 

dam operations will affect sand storage and, 

ultimately, sandbars.

The sand budget also showed that 

between 2002 and 2009 there were periods of 

depletion of sand from storage and periods of 

accumulation of sand in storage. Sand storage 

always declined between tributary sediment 

input events, whereas tributary sand inputs in 

2006, 2007, and 2008 were large and resulted 

in progressive sand accumulation. Although 

much of this sand was transported downstream 

in the March 2008 controlled flood (that is, a 

“withdrawal” from the account), some of this 

sand was transferred to newly enlarged eddy 

sandbars or remained on the riverbed. 

Management Implications

Because the absolute quantity of sand 

available to build sandbars along the Colorado 

River cannot be reliably measured, the only 

option is to measure trends in sand storage—

that is, changes in the size of eddy sandbars 

and changes in the amount of sand on the 

riverbed. Initial results from efforts to monitor 

these long-term trends in storage indicate that 

storage in Marble Canyon did not decline 

between 2002 and 2009. This period was one 

of average to above average tributary sand 

inputs and average to below average dam 

release volumes. Grams and others (2013) 

conducted their study during a period that was 

favorable to sand accumulation. Periods when 

dam release volumes are greater and tributary 

sediment inputs are less frequent will likely 

result in less sand accumulation. Grams and 

others (2013) showed that measurements of 

channel change made in short reaches can be 

used to track changes in deposits and transfers 

of sand among the storage locations, but the 

results cannot be extrapolated to long segments 

of the river, because the size and distribution of 

sand storage locations is highly variable.

Next Steps

On the basis of the above findings, 

GCMRC scientists and cooperators now 

measure topographic changes over long 

segments of the Colorado River channel. 

Instead of monitoring short reaches, which 

are unlikely to be representative of the entire 

system, scientists now map a much larger 

proportion of the sand-storage locations in long 

river segments. This monitoring strategy will 

provide a detailed accounting of the changes 

in sand deposits for entire 30-mile monitoring 

segments. Scientists will learn much more 

about the size, distribution, and behavior 

of those sand-storage locations that are of 

significant management interest, hopefully 

enabling development of a more cost-effective 

and spatially representative sandbar monitoring 

program.

References Cited

Draut, A.E., and Rubin, D.M., 2008, The role of 

eolian sediment in the preservation of archeologic sites along the Colorado River corridor in 

Grand Canyon National Park, U.S. Geological 

Survey Professional Paper 1756, 71 p., available 

at http://pubs.usgs.gov/pp/1756/.

Grams, P.E., Topping, D.J., Schmidt, J.C., 

Hazel, J.E., and Kaplinski, M., 2013, Linking morphodynamic response with sediment 

mass balance on the Colorado River in Marble 

Canyon—Issues of scale, geomorphic setting, 

and sampling design: Journal of Geophysical Research, v. 118, no. 2, p. 361–381, doi: 

10.1002/jgrf.20050, accessed July 29, 2013, 

at http://onlinelibrary.wiley.com/doi/10.1002/

jgrf.20050/abstract.

Schmidt, J.C., and Grams, P.E., 2011, The high 

flows—Physical science results, in Melis, T.S., 

ed., Effects of three high-flow experiments on 

the Colorado River ecosystem downstream from 

Glen Canyon Dam, Arizona: U.S. Geological 

Survey Circular 1366, p. 53–91, available at 

http://pubs.usgs.gov/circ/1366/.

Topping, D.J., Rubin, D.M., and Vierra, 

L.E., 2000, Colorado River sediment transport—1; natural sediment supply limitation 

and the influence of Glen Canyon Dam: Water 

Resources Research, v. 36, no. 2, p. 515–542, 

doi: 10.1029/1999WR900285, accessed 

July 29, 2013, http://onlinelibrary.wiley.com/

doi/10.1029/1999WR900285/abstract.

Webb, R.H., Melis, T.S., and Valdez, R.A., 

2002, Observations of environmental change in 

Grand Canyon: U.S. Geological Survey WaterResources Investigations Report 02–4080, 33 p., 

available at http://pubs.usgs.gov/wri/wri024080/.

Wright, S.A., Melis, T.S., Topping, D.J., and 

Rubin, D.M., 2005, Influence of Glen Canyon 

Dam operations on downstream sand resources 

of the Colorado River in Grand Canyon, in

Gloss, S.P., Lovich, J.E., and Melis, T.S., eds., 

The state of the Colorado River ecosystem in 

Grand Canyon: U.S. Geological Survey Circular 

1282, p. 17–31, available at http://pubs.usgs.gov/

circ/1282/.

For more information contact:

U.S. Geological Survey

Southwest Biological Science Center

Grand Canyon Monitoring and Research Center

2255 N. Gemini Drive

Flagstaff, AZ 86001

(928) 556-7094

http://www.gcmrc.gov/

This fact sheet and any updates to it are available

online at http://pubs.usgs.gov/fs/2013/3074/

These graphs show the “sand 

budget” (sand available to build 

sandbars) for upper and lower Marble 

Canyon, as well as Colorado River 

water discharged from Glen Canyon 

Dam, during 2002–2009. The solid 

brown lines show the sand budget 

based on sediment flux recorded at 

measurement stations (see map). 

The upper solid line is the upper limit 

of the uncertainty range, and the 

lower line is the lower limit of the 

uncertainty range. The blue dots with 

“error bars” show the sand budget 

based on repeat measurements of 

the topography of sand deposits. 

The error bars show the estimated 

measurement error. These estimates 

have additional uncertainty (not 

shown) associated with the 

extrapolation from short monitoring 

reaches (see map) to the 30-mile river 

segments between measurement 

stations. The thick blue lines are 

discharge measured at Lees Ferry. 

(1metric ton =~1.10 U.S. short ton.)

3.0

2.0

1.0

0

-1.0

50,000

40,000

30,000

20,000

10,000

0

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40,000

30,000

20,000

10,000

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3.0

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-1.0

2002 2003 2004 2005 2006

lower Marble Canyon

upper Marble Canyon

Sand budget, in million metric tons

Discharge, in cubic feet per second

2007 2008 2009 2010