United States Department of the Interior

U. S. GEOLOGICAL SURVEY

Memorandum

To: John Hamill, Chief, Grand Canyon Monitoring and Research Center

From: Dave Rubin, David Topping, Scott Wright

Subject: Status of sand mass balance in the Colorado River ecosystem below Glen

Canyon Dam

Date: October 19, 2006

Summary

Recent floods on the Paria River and Little Colorado River have supplied large quantities

of sand to the mainstem Colorado River downstream from Glen Canyon Dam. Sand

deliveries of such large magnitude occur on average only every five years, and the

October 6, 2006, flood was the largest on the Paria River since 1998. Resource managers

now have the relatively rare opportunity of exploiting this large quantity of sand for both

habitat restoration and additional scientific learning.

Background

The most important science question identified by sediment scientists and managers at

the 2005 Knowledge Assessment Workshop (KAW) was:

Is there a “flow-only” operation that will restore and maintain sandbar habitats over

decadal timescales?

One significant outcome of the 2005 Knowledge Assessment conducted by the GCMRC

was that, despite the great deal of learning that has occurred regarding sediment transport,

sand bars, and dam operations in this system, the information and models are not yet

capable of answering this question. The question can only be answered through a

program that utilizes rigorous experimental floods whenever triggered by the specified

amounts of new sand. The rationale for conducting experimental floods under sedimentenriched conditions has been documented in several peer-reviewed outlets, including

Webb and others (1999), Topping and others (2000), Rubin and Topping (2001), Rubin

and others (2002), Schmidt and others (2004), Wright and others (2005), Hazel and

others (2006), Topping and others (2006), and by Randall and others (written
communication). The flow chart below was developed at the KAW to guide

experimentation toward answering the sediment science question:

Figure 1. Flow chart for experiments testing the ability of operational treatments to

restore sand resources.

Recent sand inputs to the Colorado River ecosystem from tributaries below Glen Canyon

Dam provide an ideal opportunity to replicate the 2004 experimental flood—a key step in

the flow chart—and address this strategic science question.

Sediment scientists at the U.S. Geological Survey recently produced the following

estimates made on the basis of a combination of modeling and measurements over the

last two years, with the following updated information included through October 18,

2006:
Sand Supply in the Main Channel of the Colorado River

Since July 1, 2006, the Paria has supplied between 1.32 and 1.98 million metric tons of

sand, and the LCR has supplied between 260,000 and 380,000 metric tons of sand. The

lesser tributaries in upper Marble Canyon are estimated to have supplied at least 170,000

metric tons of sand during this same period and perhaps much more.

Considering the uncertainties associated with the sand mass balance methods, the lower

bound of the amount sand that accumulated in Marble Canyon since the 2004

experimental flood is 1.39 million metric tons, and the lower bound of sand that

accumulated in upper Grand Canyon (below the Little Colorado River) is 300,000 metric

tons. Following the scheme proposed by Science Planning Group and weighting the

upper Grand Canyon sand at 50% (150,000 metric tons) and adding this reduced amount

to the Marble Canyon sand gives a lower bound of 1.54 million metric tons. This

exceeds the trigger by at least 540,000 metric tons.

Compared to sand status leading to the November 2004 flood, we now have almost 2

times that amount in upper Marble Canyon and about 3 times that amount in upper Grand

Canyon. The recent Paria flood is the largest since 1998, and it is relatively rare to get

large inputs from both the Paria River and LCR during the same year. Given the recent

history of inputs from the Paria, Little Colorado, and lesser tributaries, the current

opportunity to conduct an experimental flood with the amount of sediment now in the

system should be considered rare, even if no additional sand is supplied. This large

amount of sand recently supplied from the Paria, LCR, and lesser tributaries occurs only

once in every 5-10 years. Furthermore, continued tributary flooding could result in

additional new sand. This is perhaps the best opportunity managers will have in this

decade to build on the results of 2004.

Upper Colorado River Basin Hydrology and Annual Glen Canyon Dam Releases

The same weather patterns that contributed to supply of sand to the Colorado River have

apparently also supplied a significant volume of runoff to the Upper Colorado River

Basin, and these inputs have come very early in the 2007 Water Year. It is anticipated

that such early inflows to Lake Powell might result in Glen Canyon Dam releases later in

the year (presumably, summer months with high daily peak discharges) exceeding the

minimum release volume of 8.23 million acre feet. The known influence of any

significantly increased monthly volume releases from the dam will be faster export (loss)

of these new sand inputs. Sand-transport data collected during late-1999 and early-2000

indicate that if higher than 8.23 million acre-foot dam releases occur prior to conducting

the next experimental flood, the recent large sand inputs will be exported from the

canyon within 2007, and additional erosion of older sand in storage is likely. Such an

increase in annual releases from the dam constrains the time available for exploiting the

recent sand inputs.
Sediment Scientists' Recommendation for Restoration Experiment

Timing

Sand supplied by tributaries to the mainstem Colorado River is exported downstream

relatively quickly, which constrains the time available for the new sand to be utilized for

restoration (Rubin and others, 2002). The rate at which new sand is exported depends on

water discharge and the availability of the new sediment (which in turn depends on the

quantity and distribution of new sediment). The majority of sand in a moderate input is

predicted to be lost within days (at discharges of >35,000 cfs) weeks (at discharges of

~25,000 cfs) or months (at discharges of ~15,000 cfs); at discharges of 10,000 cfs and

lower, sand is retained for periods of months to years. As discussed previously (Rubin

and others, 2002), the new sand will only be available for restoration if a flood is

implemented promptly or if releases are constrained; as noted above, basin hydrology

may prohibit low releases.

The goal of an experimental flood is three-fold: (1) to transfer inputs of tributary-supplied

sand from the channel of the Colorado River to where it is needed (on the parts of the

bars that will be emergent at typical dam releases) and where it will persist for longer

timescales than on the channel bed, (2) to temporarily (over timescales of months) reduce

the subsequent transport/export of sand by coarsening the sand on the surface of the

channel bed and lower parts of bars, and (3) to maintain the increased bar size and sand

resources until a subsequent experimental flood can be released from the dam.

Preliminary analysis of data following the 2004 flood suggest that goals (1) and (2) were

met, and that (3) can still be met if the next experimental flood is implemented in a timely

manner.

Experimental plan

1) Use a combination of suspended-sediment measurements and the USGS's sand-routing

model (Wiele and others, in press) to observe and predict downstream redistribution of

new sand as a means of identifying optimal timing of an experimental flood (presumably

in winter or spring 2007).

2) Repeat 2004 experimental flood hydrograph (in a month to be determined by 1 above,

and by managers), thereby using the flood experiment as a monitoring tool. Differences

in bar size between successive experiments will provide information about success or

failure of a sediment-management program based purely on dam operations (see above

diagram).

3) Use experience gained from the 1996 and 2004 science programs to design a smaller,

lower-cost, surveying and change detection program. For example, instead of conducting

both pre- and post-flood surveys, researchers could survey bars only after the flood.
In addition to addressing the strategic science question identified in the flow chart, this

experimental plan has the potential to increase the ecosystem’s sand resources. There is

no guarantee that flow-only restoration is possible, but we know of no better flow-only

option than an experiment to retain these large recent inputs.

Reporting of Future Inputs and Exports

Tributaries are continuing to supply additional sand to the Colorado River. We will

provide undated reports of the mass balance by email. In the meantime, please feel free

to contact me with any additional questions that you or the stakeholder group might have

regarding this updated information on the sand mass balance and associated scientific

recommendations. I can be reached at (831) 427-4736.

References

Hazel, J., Jr., Topping, D.J., Schmidt, J.C., and Kaplinski, M., 2006, Influence of a dam

on fine-sediment storage in a canyon river: Journal of Geophysical Research, v. 111,

F01025, 16 p.

Randall, T.J., Lyons, J.K., Christensen, R.J., and Stephen, R.D., Grand Canyon Sediment

Augmentation Appraisal Engineering Report: Bureau of Reclamation, draft report, Oct

1, 2006.

Rubin, D.M. and Topping, D.J., 2001, Quantifying the relative importance of flow

regulation and grain-size regulation of suspended-sediment transport (a) and tracking

changes in grain size on the bed (b): Water Resources Research, v. 37, p, 133-146.

Rubin, D.M., Topping, D.J., Schmidt, J.C., Hazel, J., Kaplinski, M., and Melis, T.S.,

2002, Recent sediment studies refute Glen Canyon Dam hypothesis: Eos, June, 2002, p.

273, 277-278.

Schmidt, J. C., Topping, D. J., Grams, P. E., and Hazel, J. E., 2004, System-wide changes

in the distribution of fine sediment in the Colorado River corridor between Glen

Canyon Dam and Bright Angel Creek, Arizona: Final report submitted to the Grand

Canyon Monitoring and Research Center, 107 p.

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

transport: Part 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., Schmidt, J.C., Hazel, J.E., Jr., Melis, T.S., Wright, S.A.,

Kaplinski, M., Draut, A.E., and Breedlove, M.J., 2006, Comparison of sedimenttransport and bar-response results from the 1996 and 2004 controlled-flood experiments

on the Colorado River in Grand Canyon: CD-ROM Proceedings of the 8th Federal

Inter-Agency Sedimentation Conference, Reno, Nevada, April 2-6, 2006, ISBN 0-

9779007-1-1.

Webb, R.H., J.C. Schmidt, G.R. Marzolf, and R.A. Valdez, eds., 1999, The Controlled

Flood in Grand Canyon: Geophys. Monogr. Ser., vol. 110, AGU, Washington, D.C.

Wiele, S.M., P.R. Wilcock, P.E. Grams, in press, Reach-averaged sediment routing

model of a canyon river: Water Resources Research.

Wright, S.A., Melis, T.S., Topping, D.J., and Rubin, D.M., 2005, Summary of the effects

of Glen Canyon Dam operations on downstream sand transport and sandbars along 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.