GCMR-700

September 18, 2000

MEMORANDUM

To: Kathleen Wheeler

From: Barry D. Gold

Subject: Summary of Recent Sediment related Research Findings

A few months ago I sent you five published scientific papers that presented the results of recent 

sediment-related research sponsored or conducted by GCMRC. At that time you asked that we 

prepare a summary of those papers for your information. I asked Ted Melis, the GCMRC Program 

Manager for Physical Resources and the sediment researchers whose work we were citing to develop a 

summary memo. While the attached memo was reviewed by myself and Randy Peterson it represents 

the researcher's own perspectives on their recent findings.

Their conclusions could have important implications for the Glen Canyon Dam Adaptive Management 

Program. They challenge the two hypotheses on which the EIS and ROD were based. First they challenge 

the notion that sand can be stored in the channel bed over a number of years and then once sufficient 

accumulation has occurred it can be redistributed through a Beach/Habitat- Building Flow. Rather, they 

argue that the sand which enters the system from tributary events is transported downstream relatively 

rapidly. Second, they postulate that the fraction of sand that is remaining will not be sufficient to build bars 

and provide a positive sand balance.

The implications for their findings for sediment resources only are: (1) that releases above peak-power-plant 

discharge may need to be conducted immediately after substantial inputs of sand from tributaries; and (2) 

flows following sand inputs from tributary events should be maintained at 8,000 to 10,000 cfs to maximize 

sediment storage until peak power-plant discharges can be implemented. 

Their findings also suggest two hypotheses that will need to be evaluated based on the data collected from 

the GCMRC long-term monitoring program. One is that the system will exist in the post-dam era in some 

United States Department of the Interior

GRAND CANYON MONITORING AND RESEARCH CENTER

2255 N. Gemini Dr., Room 341

Flagstaff, AZ 86001

520 556-7094 Telephone

520 556-7092 Fax

www.gcmrc.gov
sort of degraded equilibrium as compared to the pre-dam sediment balance. The other is that the system will 

continue to experience a long-term loss of sediment.
Pacific Science Center

University of California at Santa Cruz

1156 High Street

Santa Cruz, CA 95064

MEMORANDUM

August 29. 2000

To: Barry D. Gold, Chief, Grand Canyon Monitoring and Research Center

From: David M. Rubin and David J. Topping, U.S. Geological Survey

John C. Schmidt, Utah State University

Joe Hazel, Northern Arizona University

Re: Summary and Discussion of Recent Research Findings Related to Dam Operations and Sand Bar 

Resources of the Colorado River Ecosystem

Background

Sand bars are an essential component of the Colorado River ecosystem downstream from Glen Canyon Dam. 

They create habitat utilized by endangered fish; they contain and protect an array of Native American cultural 

resources; they provide campsites used by recreational boaters; and they are a distinctive attribute of the pre- and postdam river landscape. Improving and maintaining sand bars below the dam is a fundamental long-term management 

objective of the Grand Canyon Protection Act, the Operation of Glen Canyon Dam Final Environmental Impact 

Statement, and the Glen Canyon Dam Adaptive Management Program (See Attachment 1).

Sand bars and sandy banks of the Colorado River in Grand Canyon are maintained by the sand that is transported 

through the canyon. The high-elevation parts of these sand bars (those parts at elevations above peak power-plant 

discharge) can be constructed only by flows that exceed peak power-plant discharge (i.e. flows greater than 31,000 cfs); 

in the absence of such high flows, these high-elevation areas are eroded by lower flows or canyon winds or are rapidly 

colonized by both native and exotic vegetation. Flows above peak power-plant discharge are necessary to maintain 

these high-elevation sand bars, but are effective only when the river contains sufficient sand resources.

Evaluating restoration and sustainability of sand resources is a complicated problem that involves sand storage on 

the Colorado River’s bed, tributary resupply of sand, sand deposition induced by flows above peak power-plant 

discharge, erosion and transport of sand during normal power-plant operations, and recolonization by vegetation. 

Improving or sustaining sand resources is a difficult challenge because Glen Canyon Dam traps all of the sediment from 

the upper Colorado River, resulting in an approximate 94% reduction (relative to pre-dam inputs) in the amount of sand 

supplied to the Colorado River at the upstream boundary of Grand Canyon National Park.

With respect to restoration and sustainability of sand bars, the Secretary of the Interior’s 1996 Record-of-Decision 

(ROD) for operations of Glen Canyon Dam is based primarily on two hypotheses: 

(1) that much of the sand introduced to the Colorado River by tributaries downstream from Glen Canyon Dam can 

accumulate in the channel over multiple years if dam releases do not exceed average volume, and

(2) that flows above peak power-plant release (such as the 45,000 cfs flow in 1996) can effectively move that 

accumulated sand from the channel bed to bars, thereby rebuilding sand bars that are eroded by typical dam releases. 

Recent Findings

Work conducted since the 45,000 cfs release in 1996 has shown that the first hypothesis on which the 1996 

ROD was based is false and that the second hypothesis is only partially true. The 45,000 cfs release in1996 
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increased the amount of sand at high elevations (Figure 1), but the sand that was deposited at high elevations came 

largely from the lower portions of the sand bars (Schmidt, 1999) and not from the channel bed as originally 

hypothesized. 

Under the dam operations imposed by the 1996 ROD, most newly input sand is not stored on the channel bed 

for long periods of time (Topping et al., 2000a; Topping et al., 2000b). Flows above peak power-plant release 

cannot take advantage of multiple years of sand accumulation, because substantial multi-year accumulation of 

sand does not occur. Instead, this sand is transported downstream relatively rapidly. The time required to export 

(transport downstream past the Grand Canyon gage) one-half of a 500,000 metric ton input of tributary sand (the 

contribution of a typical, moderate, Paria flood) varies from less than one week (for dam discharges of 25,000-

30,000 cfs) to roughly one year (for discharges of 10,000 cfs), as illustrated in Figure 2. 

The time required to export the second half of a tributary input is greater than for the first half (for a constant 

water discharge), because the second half is coarser, as a result of winnowing of the bed (Topping et al., 2000b; 

Rubin and Topping, in press). The remaining half, however, is not necessarily sufficient to enable both barbuilding and a positive sand balance. For example, the 45,000 cfs release in 1996 exported 700,000 metric tons of 

sand from Marble Canyon in one week. Thus, a release above peak power-plant discharge is a double-edged 

sword: high discharges are indispensable for rebuilding high-elevation parts of bars, but high discharges deplete 

sand resources rapidly (Figure 2). Conducting a release above peak power-plant discharge when recent tributary 

sand inputs are greatest will tend to minimize the negative impact on the sand resources.

Since the 45,000 cfs release in 1996, six kinds of sediment and topographic data have been examined: sediment 

input and output, changes in grain size of sand on the river bed, changes in sand-bar size, geomorphic mapping, and 

changes in channel cross-sections. Some of these studies document rapid export of tributary sand (transport past the 

Grand Canyon gage), whereas others demonstrate a lack of substantial multi-year accumulation of sand, especially in 

upper Marble Canyon:

• Both measurements and calculations of sediment input and output have shown that most fine sediment (sand, silt, 

and clay) introduced by tributaries is exported within a few months (Topping et al., 2000a; Topping et al., 2000b). For 

example, field measurements show that most sediment introduced by floods on the Paria River in September, 1999, was 

exported within 6 weeks. On a longer time scale (August 11, 1999 to May 14, 2000), the Paria supplied approximately 

0.8 million metric tons of sand to the Colorado River, while roughly twice this amount of sand (1.5-2 million metric 

tons) was exported past the Lower Marble Canyon gage.

• Changes in grain size of sand on the river bed also demonstrate rapid export of tributary sand. The bed was 

measurably enriched in finer sand as a result of Paria floods in September, 1998 (median grain size of Paria River sand 

is 0.11 – 0.13 mm). When sampled next (May, 1999), most of the new fine-grained sand on the bed had been

winnowed (Topping et al., 2000b). The remaining sand in the channel was generally too coarse to be transported onto 

the high-elevation areas of sand bars.

• Topographic surveys of 11 sand bars in the first 76 miles downstream from the dam document a continuing 

depletion of sand-bar area from 1991 to 1999 (Figure 1A). High flows in 1996 and 1997 temporarily reversed this trend 

but did not halt the continuing decrease in sand-bar area. The sand bars (above 20,000 cfs) were 22% smaller in surface

area in 1999 (Figure 1A), although they contained 2-3% more sand than in 1991 (Figure 1B). 

• Topographic surveys of 35 sand-bar sites documented scour of sand during the 45,000 cfs release in 1996, 

followed by net accumulation (J. Hazel, personal communication). Comparison with tributary-input data for the same 

time, however, indicates that most of the observed accumulation occurred when there was no substantial tributary sand 

input. 

• Repeated surveys of channel cross-sections from 1991 to 1999 have shown relatively large and rapid fluctuations 

in the amount of sediment present (M. Flynn and N. Hornewer, personal communications). These fluctuations are 

interpreted to represent temporary storage and subsequent down-river transport of sediment. These studies have not 

detected multi-year accumulation of sediment.
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• Analysis of bed-elevation data at the historical Marble Canyon dam sites suggests considerable loss of sediment 

from the 1950’s to the present. Not only does the post-dam river contain less sand than the pre-dam river, but the 

remaining sand is generally coarser (Rubin and Topping, in press).

• Geomorphic mapping indicates that deposition of the 45,000 cfs release in 1996 was least near Lees Ferry and 

was greatest downstream from the Little Colorado River (Schmidt et al., 1999; H. Sondossi, personal communication). 

The magnitude of “improvement” is greatest further downstream where more tributaries have delivered fine sediment to 

the channel. Thus, the “improvement” caused by any specific release above peak power-plant discharge differs both 

temporally and spatially, depending on how enriched or depleted a particular reach is at the time.

Implications for Current Management Actions

The features listed above characterize a system where increases in sand abundance result not from incremental 

multi-year accumulation but rather from temporary storage of individual tributary inputs. In such a system, where 

increases in sand abundance are temporary, the goal for building sand bars should be to exploit tributary inputs as soon 

as possible, because the volume of sand available for bar-building is greatest immediately after large tributary inflows. 

To be effective in rebuilding sand bars, releases above peak power-plant discharge should occur soon after these 

tributary inflows, before the new sand is lost downstream (Figure 2).

Large Paria tributary inflows typically occur during late summer and early fall. Under the rules of the 1996 ROD, 

however, releases above peak power-plant discharge cannot be implemented on a schedule that takes advantage of such 

inputs. If a release above peak power-plant discharge cannot be scheduled immediately following a tributary input, 

another option might be to maintain low flows until a release above peak power-plant discharge could be implemented; 

the low flows would reduce the amount of sand lost downstream. The magnitude of an acceptable low flow that limits 

the rate of sand export depends on the volume of sand introduced by tributary flooding, the length of time following the 

tributary input, and what loss of sand downstream is considered acceptable. At dam releases that are typical of recent 

years, half of the sand introduced by a tributary flood can be exported within days or weeks (Figure 2). Retention of 

sand for more than a few months requires sustained dam releases at the lower discharges currently permitted under the 

ROD (8,000 -10,000 cfs).

Recommendations for Future Management Actions

Even if rules for releases above peak power-plant discharge are revised to allow scheduling during or shortly after 

periods of sand inputs, the objectives of improving or sustaining the desired abundance, form, and function of sand bars 

may still not be possible because the long-term sand supply from tributaries in critical reaches may be too small. The 

76-mile reach downstream from Glen Canyon Dam has but one large sand source: the Paria River. The supply of sand 

from the Paria River is only about 6% of the sand that was supplied to this reach prior to the construction of Glen 

Canyon Dam. Natural floods from the Paria River may be too infrequent and too small to restore sand resources in this 

critical upstream reach, which includes the 60-mile length of Marble Canyon within Grand Canyon National Park.

Altering the timing of releases above peak power-plant discharge (or drastically reducing the dam’s discharge until 

such flows can be released) may be insufficient to rebuild sand resources above existing levels or to achieve 

sustainability at present levels; additional monitoring will be required to see if these options are successful. If 

alternative timing of releases above peak power-plant discharge proves to be insufficient for sand-bar management 

goals, then other more effective alternatives should be evaluated.

One approach would be to selectively add sand downstream of the dam. This alternative (“sediment 

augmentation”) was considered and eliminated during the Operations of Glen Canyon Dam EIS process. We are 

unaware of engineering feasibility studies of such a program, but sediment by-pass is an attribute of some recently built 

dams, as well as harbors and estuaries. A review of sediment pipeline technology is included on the EPA web site, 

http://www.epa.gov/glnpo/arcs/EPA-905-B94-003/B94-003.ch5.html. Addition of enough sediment (continuously, 

seasonally, or perhaps only during releases above peak power-plant discharge) would offer greater flexibility in dam 

operations, and it is conceivable that such an approach might cost less than imposing new constraints on dam 

operations. It is possible that sediment augmentation, substantial seasonal modification of flows released from Glen 
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Canyon Dam, or both, might be able to restore the sand resources in the Colorado River ecosystem in Grand Canyon 

National Park without more extreme actions.

Conclusions

The post-dam Colorado River is depleted in sand resources relative to the pre-dam river. The existing 

management strategy permitted under the ROD is failing to restore sand resources in the ecosystem in Grand Canyon 

National Park. The bars are continuing to decrease in surface area, and no long-term retention of tributary sand has 

been detected.

Our opinion, based on the information presented in this summary, is that any of the following approaches will 

have a significantly greater likelihood of success in restoring or retaining sand resources in the Grand Canyon 

ecosystem: 

 (1) Implement releases above peak power-plant discharge immediately after substantial inputs of sand from 

tributaries. 

 (2) Maintain low flows following sand inputs until releases above peak power-plant discharge can be implemented. 

 (3) Add sediment downstream from the dam. 

Dam operations of the last decade must have caused one of the following possible effects on sediment resources in 

the Colorado River ecosystem: sediment resources were enhanced or replenished relative to conditions in the early-tomid 1990’s, sediment resources were maintained in a degraded (post-dam) condition, or long-term export and loss of 

sediment resources is continuing. Distinguishing between such possibilities has been—and should continue to be—an 

important function of the GCMRC Adaptive Monitoring Program. The research reviewed above demonstrates that 

current operations are failing to increase sediment resources. At least one significant measure of sediment resources, 

surface area of sand bars above 20,000 cfs, documents continuing depletion of sand resources.
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A

B

Figure 1. Changes in sand-bar surface area and volume at all 11 long-term Northern Arizona University study 

sites in Marble Canyon. A. Surface area of sand bars decreased by 22% from 1991 to 1999 despite temporary 

increases caused by high releases in1996 and 1997. B. Volume of sand bars in 1999 was 2-3% greater than in 

1991.
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Figure 2. Calculated time to export one-half of a 500,000 metric ton input of tributary sand past the Grand Canyon 

gage. Calculations are based on sediment-transport data collected at the Grand Canyon and above LCR gages 

during the 1990’s. The upper limit of the shaded area (slowest export) is calculated using the average suspendedsediment concentration for each specified discharge; the lower limit of the shaded area (most rapid export) is 

calculated for high concentrations of suspended sediment at each discharge; the solid line in the center of the 

shaded band is calculated using concentrations that decrease through time from high values (during and 

immediately following tributary inputs) to mean concentrations (after half of the tributary sand has been exported). 

At the upper range of dam operations, half of the sediment is exported within a few days; multi-year accumulation 

is only likely to occur if discharge is restricted to less than 8,000-10,000 cfs. To maximize the benefit of sand 

supplied by tributaries, releases above peak power-plant discharge should be implemented as soon as possible after 

tributary input events.
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References

Rubin, D.M., Anima., R.A., and Nelson. J.M., 1997, Sedimentologic and hydraulic observations of depositional 

processes during the 1996 experimental high flow: Proceedings of the Grand Canyon Monitoring and 

Research Center's 1997 Symposium on the Glen Canyon Dam Beach / Habitat-building flow, p. 9.

Rubin, D.M., Nelson, J.M., and Topping, D.J., 1998, Relation of inversely graded deposits to suspended-sediment 

grain-size evolution during the 1996 flood experiment in Grand Canyon: Geology, v. 26, p. 99-102.

Rubin, D.M. and Topping, D.J., 1999, Seasonal and Multi-Year Sediment Balance in the Colorado River

Downstream from Glen Canyon Dam: Colorado River Ecosystem Science Symposium, Grand Canyon, AZ.

Rubin, D.M., and Topping, D.J., in press, Quantifying the relative importance of flow regulation and grain-size 

regulation of suspended-sediment transport (α), and tracking changes in bed-sediment grain size (β): Water 

Resources Research.

Schmidt, J.C., 1999, Summary and synthesis of geomorphic studies conducted during the 1996 controlled flood in 

Grand Canyon, in Webb, R.H., Schmidt, J.C., Marzolf, G.R., and Valdez, R.A., eds., The 1996 controlled 

flood in Grand Canyon: Washington, D.C., American Geophysical Union, Geophysical Monograph 110, p. 

329-342.

Schmidt, J.C., Grams, P.E., and Leschin, M.F., 1999, Variation in the magnitude and style of deposition and 

erosion in three long (8-12 km) reaches as determined by photographic analysis, in Webb, R.H., Schmidt, 

J.C., Marzolf, G.R., and Valdez, R.A., eds., The 1996 controlled flood in Grand Canyon: Washington, D.C., 

American Geophysical Union, Geophysical Monograph 110, p. 185-203.

Topping, D.J., 1997, Flow, sediment transport, and channel geometric adjustment in the Grand Canyon, Arizona 

gage reach of the Colorado River during the 1996 Grand Canyon flood experiment: Proceedings of the Grand 

Canyon Monitoring and Research Center's 1997 Symposium on the Glen Canyon Dam Beach / Habitatbuilding flow, p. 20-22.

Topping, D.J., Rubin, D.M., Nelson, J.M., Kinzel, P.J., III, and Bennett, J.P., 1999, Linkage between grain-size 

evolution and sediment depletion during Colorado River floods, in Webb, R.H., Schmidt, J.C., Marzolf, 

G.R., and Valdez, R.A., eds., The 1996 controlled flood in Grand Canyon: Washington, D.C., American

Geophysical Union, Geophysical Monograph 110, p. 71-98.

Topping, D.J., Rubin, D.M., and Schmidt, J.C., 1999, Why knowing real-time sediment inputs from tributaries is 

important: Colorado River Ecosystem Science Symposium, Grand Canyon, AZ.

Topping, D.J., Rubin, D.M., and Vierra, Jr., L.E., 2000a, Colorado River sediment transport 1. Natural sediment 

supply limitation and the influence of Glen Canyon Dam: Water Resources Research, v. 36, p.515-542.

Topping, D.J., Rubin, D.M., Nelson, J.M., Kinzel, III, P.J., and Corson, I.C., 2000b, Colorado River sediment 

transport 2. Systematic bed-elevation and grain-size effects of sand supply limitation: Water Resources 

Research, v. 36, p. 543-570. 

Attachment 1. Management objectives for sediment resources within the Glen Canyon Dam Adaptive 

Management Program are stated as follows in the June 10, 1998 Management Objectives document adopted by the 

Adaptive Management Work Group.

SEDIMENT RESOURCES

Goal: to maintain a range of sediment deposits over the long-term, including an annually flooded baresediment (unvegetated) active zone, a less frequently flooded vegetated zone, terraces (within the 45,000 cfs river 
8

stage), and backwater channels. Managing sediment resources will be on a reach-scale basis. Should significant 

and localized adverse impacts occur, site-specific mitigation would be considered.

Definition: Sediment resources include a broad array of material, ranging from suspended fines to coarse 

gravels. Primary interest relates to both material in suspension, which affects benthic capability, as well as stored 

sediment in beaches and channel margins, which affects recreation.

MANAGEMENT OBJECTIVES

MO 1: Maintain a long-term balance of river-stored sand to support maintenance flow (in years of low reservoir 

storage), beach/habitat-building flow (in years of high reservoir storage), and unscheduled flood flows. Maintain 

system dynamics and disturbance by annually (in years which Lake Po well water storage is low) redistributing 

sand stored in the river channel and eddies to areas inundated by river flows between 20,000 cfs and maximum 

power plant capacity.

MO 2: As a minimum for each reach, maintain the number and average size (area and thickness) of sandbars and 

backwaters between the stages associated with flows of 8,000 and 45,000 cfs that existed during the 1990/91 

research flows.

MO 3: Periodically increase the average size of sandbars above the 20,000 cfs river stage and number and average 

size of backwaters to the amounts measured during the high period of 1990/91 or the 1996 test of the 

beach/habitat-building flow in as many years as reservoir and downstream conditions allow.

M) 4: Maintain system dynamics and disturbance by redistributing sand stored in the river channel and eddies to 

areas inundated by river flows up to 45,000 cfs in as many years as possible when BHBF hydro logic and resource 

criteria are met.

RECREATION

MANAGEMENT OBJECTIVES

MO 2: Maintain flows (under approved operating criteria) and sediment processes that create an adequate 

quantity, distribution and variety of beaches for camping, as long as such flows are consistent with management of 

natural recreation and cultural resource values (other natural resource values).