U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2007–3020

March 2007

Research Furthers Conservation of Grand Canyon Sandbars

Printed on recycled paper

Grand Canyon National Park (fig. 1) 

lies approximately 25 km (15 mi) downriver from Glen Canyon Dam, which was 

built on the Colorado River just south of 

the Arizona-Utah border in Glen Canyon 

National Recreation Area (fig. 2). Before 

the dam began to regulate the Colorado 

River in 1963, the river carried such large 

quantities of red sediment, for which the 

Southwest is famous, that the Spanish 

named the river the Rio Colorado, or “red 

river.” Today, the Colorado River usually runs clear below Glen Canyon Dam 

because the dam nearly eliminates the 

main-channel sand supply (Topping and 

others, 2000b; Wright and others, 2005). 

The daily and seasonal flows of the river 

were also altered by the dam (Topping and 

others, 2003). These changes have disrupted the sedimentary processes that create and maintain Grand Canyon sandbars. 

Throughout Grand Canyon, sandbars create habitat for native plants and animals, 

supply camping beaches for river runners 

and hikers, and provide sediment needed 

to protect archaeological resources from 

weathering and erosion (Rubin and others, 

2002; Wright and others, 2005).

Maintenance of sandbars in the Colorado River ecosystem, the river corridor 

that stretches from the dam to the western boundary of Grand Canyon National 

Park, is a goal of the Glen Canyon Dam 

Adaptive Management Program (U.S. 

Department of the Interior, 1995; Patten 

and others, 2001). The program is a federally authorized initiative to ensure that the 

mandates of the Grand Canyon Protection 

Act of 1992 are met through advances in 

information and resource management. 

The U.S. Geological Survey’s Grand Canyon Monitoring and Research Center has 

responsibility for scientific monitoring and 

research efforts for the program. Extensive 

research and monitoring during the past 

decade have resulted in the identification 

of possible alternatives for operating Glen 

Canyon Dam that hold new potential for 

the conservation of sand resources.

Sand in Grand Canyon

Tributaries to the Colorado River 

below Glen Canyon Dam, such as 

the Paria and Little Colorado Rivers, 

provide the only new inputs of sediment 

that can be used to maintain sandbars 

in the Colorado River ecosystem in the 

postdam era (fig. 3). Smaller tributaries 

are also responsible for debris-fan eddy 

complexes, the ecosystem’s dominant 

landforms (Schmidt and Graf, 1990; 

Schmidt and Rubin, 1995). A debris fan 

is a triangular landform that is created 

when a tributary deposits gravel-sized 

and larger particles at its intersection 

with the Colorado River during seasonal 

floods. Debris fans create large recirculation zones in the river, or eddies, where 

sandbars form. 

Research and monitoring have 

resulted in a better understanding of how 

sandbars and sand resources are affected 

by the operation of Glen Canyon Dam. 

For example, high-flow releases from the 

dam that simulate mild natural floods can 

modify or maintain debris fans, which 

control the location and regulate the size 

of sandbars (Webb and Griffiths, 2001). 

Perhaps the single most important 

finding in the last decade is that relations between the discharge of water 

Figure 1. Sandbars in Grand Canyon create habitat for native plants and animals, supply camping beaches for river runners and hikers, and provide sediment needed to protect archaeological 

resources from weathering and erosion. Photograph © 2005 Geoff Gourley; used with permission.
from the dam and sand transport change 

over time in the Colorado River (Rubin 

and others, 1998; Topping and others, 

1999, 2000a; Rubin and Topping, 2001). 

Sand on the riverbed becomes finer when 

tributaries add fine sediment to the river 

and coarser when higher releases from the 

dam wash finer sand downstream. These 

changes in the grain size of sand affect the 

rate at which sand is transported downstream. As a result of dam operations and 

decreased sand supply, the Colorado River 

now typically transports more sand downstream than tributaries supply on a seasonal to annual basis (Topping and others, 

2000b; Rubin and others, 2002; Wright 

and others, 2005). This sediment deficit 

has resulted in progressive erosion of 

channel and sandbar deposits from Marble 

and Grand Canyons since 1963 (Schmidt 

and others, 2004). This erosion of channel 

and sandbar deposits has continued despite 

constraints placed on releases from the 

dam by the 1996 Record of Decision 

(Rubin and others, 2002; Schmidt and others, 2004; Wright and others, 2005). 

A second important finding is 

that during the high-flow releases in 

1996 and 2000, the primary sources 

of sand for building high-elevation 

sandbars were the low-elevation parts 

of the same sandbars (Andrews and 

others, 1999; Schmidt, 1999; Hazel 

and others, 2006) and not sand that had 

accumulated on the riverbed, as had 

been hypothesized. During these two 

experiments, conducted when the Colorado River was relatively sand depleted 

(Hazel and others, 2006), the erosion of 

low-elevation sandbars actually resulted 

in a net reduction in overall sandbar size. 

Maintenance of sandbar size requires 

high-flow releases from the dam to flush 

sand from the riverbed up onto sandbars 

(Schmidt and others, 1999). Recent 

research has shown, however, that 

tributary-supplied sand does not accumulate on the riverbed over multiyear 

periods under typical dam conditions 

(Topping and others, 2000b; Rubin and 

others, 2002; Wright and others, 2005). 

Because sand does not accumulate on the 

riverbed, there is generally insufficient 

sand available to be transferred from the 

riverbed to sandbars. This finding holds 

true even when the quantity of sand 

supplied by tributaries is above average, 

which was the case during the late 1990s. 

Additionally, the lower elevation parts of 

these eroded sandbars and the adjacent 

riverbed never fully recovered their former 

sand volume following the scouring that 

occurred during the 1996 high-flow release 

(Schmidt and others, 2004). 

These results indicate that high-flow 

releases conducted under sand-depleted 

conditions, such as those that existed in 

1996, will not successfully sustain sandbar 

area and volume. As understanding 

improved following the 1996 and 2000 

experiments, scientists and managers 

focused on the need to strategically 

time high-flow releases to take advantage of episodic tributary floods that 

supply new sand to the Colorado River 

downstream from the dam. 

The Importance of Tributary Floods 

In November 2004, a high-flow 

release was timed to follow tributary 

floods for the first time on the Colorado 

River (fig. 4). This experiment resulted in 

an increase in sandbar total area and volume in the upper half of Marble Canyon 

(Topping and others, 2006). Further downstream, where sand was less abundant, a 

net transfer of sand out of eddies occurred 

that was similar to that observed during 

the 1996 and 2000 experiments (Hazel and 

others, 2006; Topping and others, 2006). 

Scientists also confirmed that substantial 

increases in total eddy-sandbar area and 

volume are only possible during high-flow 

releases that follow large tributary floods 

Figure 2. The Colorado River ecosystem encompasses the Colorado River corridor from Glen 

Canyon Dam to the western part of Grand Canyon National Park, where the river empties into Lake 

Mead. The maintenance of sand resources in the Colorado River ecosystem is a primary goal of the 

Glen Canyon Dam Adaptive Management Program.

Figure 3. The Colorado River several 

miles downstream from its confluence 

with the Paria River during a flood. Paria 

floods dramatically affect sediment and 

organic loads in the Colorado River (USGS 

photograph by Theodore Kennedy).
that enrich sand supplies in the main 

channel of the Colorado River (Rubin and 

others, 2002; Topping and others, 2006). 

However, more sand will be required than 

was available during the 2004 experiment 

(800,000 to 1,000,000 metric tons) to 

achieve increases in total sandbar area 

and volume throughout all of Marble and 

Grand Canyons in the future (Topping and 

others, 2006).

Preserving Archaeological Sites 

Many archaeological sites found 

near the Colorado River are buried by 

windborne sand that helps to preserve 

these sites over time. Sandbars created 

by the 2004 experiment increased the 

windborne transport of sand toward 

some of these archaeological sites in 

Grand Canyon (Draut and others, 2005; 

Draut and Rubin, 2006). Increased sand 

carried by the wind from restored sandbars may reduce erosion and increase 

the preservation potential at some 

archaeological sites. 

Managing Sand in the Future

Recent findings suggest that it 

may be necessary to make more sand 

available for maintaining and restoring 

sandbars than is now provided by typical 

tributary flood events. One possibility 

is augmenting the sand available from 

tributaries with sand trapped behind Glen 

Canyon Dam (Randle and others, 2007). 

Alternatively, the sand supply might be 

indirectly increased through the use of 

short-duration high flows following each 

average to large tributary input of sand; 

this approach would move new sand from 

the riverbed to sandbars before it can be 

carried downstream (Topping and others, 

2006). The effectiveness of this strategy 

rests on minimizing sand export and 

sandbar erosion during periods between 

high flows; however, export and erosion 

rates are strongly dependent on powerplant release volume and daily release 

patterns (Wright and others, 2005). 

Another possibility is constraining dam 

releases following tributary sand inputs 

for a period of time until a high-flow 

release can be carried out; however, 

constraining daily operations may not 

be possible during extended periods of 

above-average runoff into Lake Powell or 

when the reservoir is near capacity.

By Theodore S. Melis, David J. Topping, David M. Rubin, and Scott A. Wright

References

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Figure 4. Repeat photographs of a sandbar along the left shore of the Colorado River near 

river mile 30 shortly before (A) and shortly after (B) the November 2004 high-flow experiment. Such substantial increases in sandbar volume and area are only possible when high 

flows are conducted following large tributary inputs of sediment. Photographs provided by 

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More Information

Theodore S. Melis

U.S. Geological Survey

Southwest Biological Science Center

Grand Canyon Monitoring and Research 

Center

2255 N. Gemini Drive

Flagstaff, Arizona, 86001

Tel: 928-556-7094 

E-mail: tmelis@usgs.gov

Web site: www.gcmrc.gov