Influence of a dam on fine-sediment storage in a canyon river

Joseph E. Hazel Jr.,1 David J. Topping,2 John C. Schmidt,3 and Matt Kaplinski1

Received 24 June 2004; revised 18 August 2005; accepted 14 November 2005; published 28 March 2006.

[1] Glen Canyon Dam has caused a fundamental change in the distribution of fine

sediment storage in the 99-km reach of the Colorado River in Marble Canyon, Grand

Canyon National Park, Arizona. The two major storage sites for fine sediment (i.e., sand

and finer material) in this canyon river are lateral recirculation eddies and the mainchannel bed. We use a combination of methods, including direct measurement of sediment

storage change, measurements of sediment flux, and comparison of the grain size of

sediment found in different storage sites relative to the supply and that in transport, in

order to evaluate the change in both the volume and location of sediment storage. The

analysis shows that the bed of the main channel was an important storage environment for

fine sediment in the predam era. In years of large seasonal accumulation, approximately

50% of the fine sediment supplied to the reach from upstream sources was stored on

the main-channel bed. In contrast, sediment budgets constructed for two short-duration,

high experimental releases from Glen Canyon Dam indicate that approximately 90%

of the sediment discharge from the reach during each release was derived from eddy

storage, rather than from sandy deposits on the main-channel bed. These results indicate

that the majority of the fine sediment in Marble Canyon is now stored in eddies, even

though they occupy a small percentage (17%) of the total river area. Because of a

95% reduction in the supply of fine sediment to Marble Canyon, future high releases

without significant input of tributary sediment will potentially erode sediment from

long-term eddy storage, resulting in continued degradation in Marble Canyon.

Citation: Hazel, J. E., Jr., D. J. Topping, J. C. Schmidt, and M. Kaplinski (2006), Influence of a dam on fine-sediment storage in a

canyon river, J. Geophys. Res., 111, F01025, doi:10.1029/2004JF000193.

1. Introduction

[2] In canyon rivers with debris fans, the flow field is

divided between the area of downstream flow in the main

channel and areas of lateral flow recirculation, or eddies,

that occur in the lee of debris fans (Figure 1). Temporary

and persistent storage of the sediment load occurs on the

bed of the main channel, in eddies, and on the floodplain.

Although the importance of debris fans as prominent landforms affecting channel planform, gradient, and hydraulics

in canyon rivers is well documented [e.g., Howard and

Dolan, 1981; Kieffer, 1985; Webb et al., 1989; Schmidt,

1990; Miller, 1994; Schmidt and Rubin, 1995; Grams and

Schmidt, 1999], the relative role of the main-channel bed,

eddies, and the floodplain in short- and long-term storage of

the sediment load is poorly known.

[3] Many dams in the western United States cause large

reductions in the downstream supply of fine sediment,

which can alter the volume and distribution of fine sediment

stored in the river [Petts, 1979; Galay, 1983; Williams and

Wolman, 1984; Grant et al., 2003]. Understanding the

spatial patterns of sediment distribution is a key part of

understanding postdam channel evolution and in turn, the

long-term impacts of flow regulation to other resources such

as riverine ecology and recreation.

[4] This paper describes changes in predam and postdam

patterns of sand, silt and clay (hereafter referred to as finesediment) distribution in the Colorado River in Grand

Canyon National Park (GCNP), Arizona. The study area

is the 99-km length of Marble Canyon, located between

the Paria and Little Colorado Rivers, in eastern GCNP

(Figure 2). The native Colorado River ecosystem developed

in conjunction with a large seasonal flux of suspended fine

sediment and seasonal accumulation of this sediment on the

bed and banks. Fine sediment accounts for approximately

99% of the total sediment load of the Colorado River in the

study area [Rubin et al., 2001]. Completion of Glen Canyon

Dam in 1963 caused a 95% reduction in the delivery of fine

sediment to the modern river at the upstream boundary of

GCNP [Andrews, 1990, 1991; Topping et al., 2000a]. The

result was erosion of fine sediment by winnowing of the bed

and degradation of the bars that occur in eddies [Schmidt et

al., 2004]. The goal of modern river management in GCNP

is to maintain eddy bars and floodplain deposits as large as

possible under the prevailing dam-regulated conditions

[U.S. Department of the Interior, 1996; Schmidt et al.,

1998; National Research Council, 1999].

[5] We examine sediment storage changes in Marble

Canyon with aerial photographs and direct measurements

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, F01025, doi:10.1029/2004JF000193, 2006

1

Department of Geology, Northern Arizona University, Flagstaff,

Arizona, USA. 2

U.S. Geological Survey, Flagstaff, Arizona, USA.

3

Department of Aquatic, Watershed, and Earth Resources, Utah State

University, Logan, Utah, USA.

Copyright 2006 by the American Geophysical Union.

0148-0227/06/2004JF000193$09.00

F01025 1 of 16
of bed, eddy, and bar topography. We compare these

changes and the grain size of sediment found in different

storage sites to the sizes supplied to the reach and to those in

transport. Our purpose is to evaluate the change in volume

and distribution of active fine-sediment storage to flow

regulation. We define active storage as the mass of fine

sediment that is delivered by tributaries, temporarily stored

on the bed or the floodplain, and transported downstream

within a few months.

2. Background

[6] The geomorphology and ecology of the Colorado

River in Grand Canyon was transformed by the closure

and operation of Glen Canyon Dam [Carothers and

Brown, 1991; Webb et al., 1999], located 26 km upstream

from the Paria River. The downstream riverine ecosystem

is now the subject of a large-scale rehabilitation program

called the Glen Canyon Dam Adaptive Management

Program.

[7] Despite the best efforts to provide decision-makers

with accurate estimates of fine-sediment flux and change in

storage, scientists have only recently agreed on the severe

sediment-deficit condition of the Colorado River in Marble

Canyon. Although Laursen et al. [1976] predicted finesediment deficit, Howard and Dolan [1981], Randle and

Pemberton [1987], Andrews [1991], and Smillie et al.

[1993] predicted that there was a surplus of fine sediment

in the postdam river because the capacity to transport sand

had been reduced more than the supply. The predictions of

each of these studies were based on application of a longterm average fine-sediment transport relation to estimate

transport for periods when there were no measurements. We

now understand that there is substantial hysteresis and

systematic shifts in sand-transport relations, as described

below, and use of average relations to estimate transport is

inappropriate.

[8] The view that sediment-surplus conditions exist and

that the Colorado River accumulates fine sediment over

multiyear time periods was described in the Final Environmental Impact Statement (EIS) for Glen Canyon Dam

Operations [U.S. Department of the Interior, 1995] and

was the paradigm prior to the 1996 Controlled Flood

[Webb et al., 1999]. The EIS proposed that controlled

floods, released as clear water from the dam, would

entrain fine sediment delivered by tributaries during

preceding years. Entrainment and transport during controlled floods would result in transfer of fine sediment

from the main-channel bed to eddies and the floodplain,

thereby achieving management objectives [Schmidt et al.,

1999].

2.1. Fine-Sediment Flux

[9] Sand-transport relations are not stationary and depend on the grain size and amount of sand on the bed

that is available for transport [Rubin et al., 1998; Topping

et al., 2000a]. The postdam variation in sand concentration is about 2 orders of magnitude at any given

discharge [Topping et al., 2000b]. When the Paria River,

the predominant supply to Marble Canyon [Andrews,

1990, 1991; Topping et al., 2000a], delivers sand, the

bed fines and main stem transport increases. Following

cessation of sand input, fines are winnowed, the bed

coarsens, and transport rates decrease. These supplydriven changes in sand concentration and grain size occur

over timescales of hours to days, precluding the development of stable or seasonally variable sand-transport

relations. Topping et al. [2000b] and Rubin et al.

[2002] showed that sand supplied from unregulated

tributaries remains in storage for only a few months

before most of it is transported downstream, unless flows

are below approximately 250 m3

/s; longer-term sand

accumulation does not occur. Thus the postdam river is

actually in a condition of sand deficit rather than the

sediment surplus predicted by previous studies.

2.2. Fine-Sediment Storage

[10] When a river is in fine-sediment deficit, erosion of

long-term storage sites occurs and new supplies from

tributaries do not persist. In the case of canyon rivers with

Figure 1. Panchromatic aerial photograph of Saddle

Canyon, a typical fan-eddy complex of the Colorado River

in Marble Canyon. The photograph was taken on 28 May

2002 at the base flow discharge of 227 m3

/s. The

subaqueous zone is highlighted with an overlay of greenband, multispectral imagery from the same aerial overflight.

DF, tributary debris fan; R, rapid or riffle; P, main-channel

pool; HEB, high-elevation eddy bar; LEB, low-elevation

eddy bar; CM, channel-margin deposits. Arrows indicate

flow direction. Location is shown on Figure 2 as long-term

study site 47. Photograph is by the Grand Canyon

Monitoring and Research Center, U.S. Geological Survey.

F01025 HAZEL ET AL.: DAM-INDUCED STORAGE CHANGE

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F01025
numerous eddies, predicting the locations from which

persistent erosion occurs and the locations where newly

supplied fine sediment is temporarily stored is important for

interpreting the long-term effects of flow regulation. Understanding whether these locations are on the main-channel

bed, in eddies, or on the floodplain should also improve the

design of future controlled floods and ecological management of the river corridor.

[11] There is relatively little space for fine-sediment

storage on the floodplain in Marble Canyon, because the

valley is not much wider than the channel (Figure 3). The

average width of the channel at base flow is 86 m [Schmidt

et al., 2004]. The average width of the valley, approximated

by the average flow width at the predam mean annual flood,

is only 54% greater [Schmidt et al., 2004]. Thus there is

relatively little room for alluvium to be stored on the

floodplain, and most storage occurs on the main-channel

bed or in eddies.

[12] Longitudinal variation in fine-sediment storage sites

is determined by changes in river-level bedrock, which

Figure 2. Colorado River in Marble Canyon, Arizona. The shaded area is Grand Canyon National Park.

Shown are the locations of the U.S. Geological Survey streamflow-gauging stations, the 5 GIS-based

reaches, and the 11 long-term study sites. The long-term study sites are referred to by distance in river

miles downstream relative to Lees Ferry (river mile 0).

Figure 3. Longitudinal profile of the Colorado River in Marble Canyon, Arizona. Shown are channel

and alluvial valley width. Channel and alluvial valley width are defined as the water surface width at

flows of 227 m3

/s and 2750 m3/s, respectively. The vertical exaggeration of the longitudinal profile is 123

(modified from Schmidt et al. [2004]).

F01025 HAZEL ET AL.: DAM-INDUCED STORAGE CHANGE

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F01025
controls valley width, and the location of tributaries, which

determine the location of debris fans and the size and

frequency of associated eddies [Howard and Dolan, 1981;

Schmidt et al., 2004]. The longitudinal profile of the bed

includes shallow areas at rapids and riffles and deep pools

that typically occur upstream and downstream from rapids

(Figure 1). The bed of the postdam river is primarily

composed of gravel, boulders, and bedrock. Overlying this

coarse material are discontinuous patches of sand [Wilson,

1986; Anima et al., 1998]. Many of these sand patches are

thin in Marble Canyon, because gravel can be observed

protruding through the troughs of dunes [Rubin et al.,

2001].

[13] Debris fans fix the locations of eddies that occur in

the downstream channel expansion. Thus monitoring of the

volume of eddy bars is not confounded by bar migration,

because these bars do not migrate. Eddies larger than

1000 m2 occur with a frequency of between 2 and 6 per

kilometer [Schmidt et al., 2004]. Large eddies may contain

bars that are more than 10 m thick [Rubin et al., 1994;

Barnhardt et al., 2001].

[14] Floodplain deposits are typically narrow, thin

veneers of fine sediment along steeply sloping banks. These

deposits are typically composed of predam deposits that are

rarely or never inundated, and inset deposits formed by the

postdam flow regime [Schmidt and Rubin, 1995; Hereford

et al., 1996]. We hereafter refer to these deposits as channelmargin deposits (Figure 1).

3. Sediment Storage in the Predam River

3.1. Seasonal Sediment Storage

[15] In the predam Colorado River in Marble Canyon,

seasonal fluctuations in main stem discharge caused seasonal differences in the storage of the naturally large

suspended load. Analyses of the daily sediment-transport

data collected by the U.S. Geological Survey (USGS)

between October 1947 and March 1963 at the two streamflow gauging stations bracketing the study area, the Colorado River at Lees Ferry, Arizona, station 09380000

(hereafter referred to as the Lees Ferry gauge) and the

Colorado River near Grand Canyon, Arizona, station

09402500 (hereafter referred to as the Grand Canyon gauge)

indicate that sediment accumulated and was temporarily

stored in the study area when the discharge was typically

less than 250 m3

/s. This typically occurred during the

9-month period between July and the subsequent March

[Topping et al., 2000a]. During the average predam year, the

seasonal accumulation was 7 ± 6 million Mg and was

greatest during the years with the largest tributary floods

in late summer, fall, and winter. During the spring snowmelt

flood between April and June, the amount of fine sediment

exported past the Grand Canyon gauge was approximately

equal to the amount transported past the Lees Ferry gauge

plus the amount that accumulated between the two gauges

since the previous July. Thus the total amount of fine

sediment in storage was probably least immediately upon

recession of the snowmelt flood.

3.2. Data Sources and Methods

[16] Although it is impossible to construct a partitioned

sediment budget for the seasonal accumulation of fine

sediment during each predam year for which there are

sediment-transport data, the relative magnitude of storage

on the main-channel bed and in eddies can be estimated by

extrapolating from a database of channel and eddy bar area

generated from aerial photographs taken in predam and

postdam years [Schmidt et al., 2004]. The data was measured

in five reaches that range from 2.9 to 14 km in length, totaling

approximately 44% of the study area (Figure 2).

[17] Channel maps, including fine-sediment deposits,

were based on interpretation of between seven and nine

aerial photograph series (mid-1930s, 1965, 1973, 1984,

1990, 1992, 1993, 1996, and 2000) overlain in a geographic information system (GIS). For each eddy bar, a

polygon was established that enclosed the entire area with

an eddy bar in any year (Figure 4). The number of aerial

photographs mapped in each study reach varied on the

basis of availability. This polygon, referred to as the eddy

deposition zone (EDZ), is an objectively defined surrogate for the area of each eddy. Although this polygon is

smaller than the eddy that exists in the flow field, the

EDZ has the advantage of being defined without the

necessity of field observations of flow, and indicates

the area where substantial fine sediment storage has

occurred at some time in the past.

3.3. Analysis of Sediment Storage in the Predam River

[18] There are 236 EDZs in the mapped reaches, of which

150 are larger than 1000 m2 and for which we measured

the area of fine sediment in each aerial photograph series.

The average number of EDZs larger than 1000 m2

/km in the

study area is approximately 3.5, which leads to an estimate

that there are approximately 350 EDZs larger than 1000 m2

in Marble Canyon (Table 1). The average EDZ size is about

7430 m2

, which leads to an estimate that there is approximately 2.6 106 m2 of EDZ in Marble Canyon. The total

area of main channel outside of eddies is approximately

8.5 106 m2 at 227 m3

/s (Table 1).

[19] There is no way to determine the average thickness

of predam annual scour and fill in EDZs. We estimated the

maximum potential scour and fill in nine EDZs in or near

the study area by integrating field surveys made since 1990.

We determined the lowest measured elevation of the bed at

1-m-spaced grid nodes of the surveyed area. We refer to this

artificial surface of lowest measured elevations as the

‘‘minimum surface’’ (Figure 5). Details of the field surveys

are described in section 4.1.2.1. In each EDZ, the minimum

surface was subtracted from a flat surface at the elevation of

the water surface at 100 m3

/s, the lowest stage used in the

calculation of the EDZs. The difference between these two

surfaces is a conservative estimate of the maximum potential scour and fill, and thus of the active storage potential of

fine sediment in eddies.

[20] The thickness of sediment between the two surfaces

described above varies between 1.8 and 6.2 m, with a mean

thickness of 3.2 m (Table 2). Multiplication of this thickness

by the total estimated area of EDZs yields an estimate of

8.3 106 m3 for the potential active storage volume

of eddies in Marble Canyon. This volume would contain

13.1 106 Mg, which is about the same as the upper bound

of the seasonal fine-sediment accumulation between the

Lees Ferry and Grand Canyon gauges during the average

year, estimated by Topping et al. [2000a].

F01025 HAZEL ET AL.: DAM-INDUCED STORAGE CHANGE

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[21] We estimated the magnitude of average scour and fill

of the main-channel bed and eddies under: (1) different

amounts of seasonal fine-sediment accumulation, (2) different proportions of this accumulation occurring on the mainchannel bed and in eddies, and (3) different proportions of

the main-channel bed where large amounts of fine sediment

could be stored. We compared these estimates of scour and

fill with historical observations. We made these estimates

for three scenarios, including the upper bound of the seasonal

fine-sediment accumulation for the average predam year

(13 106 Mg).

[22] In a year when 7 106 Mg of fine sediment

accumulated between July and the next March and where

half of the fine sediment accumulated in eddies and half in

the main channel, the average elevation change in eddies

would have been about 0.6 m and the average change in the

main-channel bed elevation would have been 0.5 m, assuming that all main-channel bed storage occurred in 30% of

the channel area (Table 3). The estimate for average mainchannel bed scour and fill is less than the range of annual

scour and fill measured at the Lees Ferry and Grand Canyon

gauges. The bed scoured and filled between 3 and 6 m and

between 1 and 3 m, respectively, in the pools at the two

gauges in the predam era [Topping et al., 2000a]. We

suspect that the magnitude of the scour and fill measured

in these pools occurred only in the pools immediately above

or below the rapids formed by the debris fans and not on the

majority of the channel bed, because these larger magnitudes of bed change could not have occurred over large

parts of the channel under any scenario of seasonal sediment

accumulation and a reasonable proportion of eddy storage

[Schmidt et al., 2004].

[23] Scenarios where a larger proportion was stored in

either the main-channel bed or in eddies yield unrealistically

small values of scour and fill in the other storage environment. If one assumes that more than 50% of fine sediment

was stored on the main-channel bed and less was stored in

eddies, then the estimated average range of change in active

eddy storage is unrealistically low and less than that

indicated by the repeat photography of Webb [1996]. For

example, if eddies were the repository of only 10% of the

total accumulation, then average bed-elevation changes in

eddies would have been about 0.1 m, an unreasonably small

value. Although there is no way to confirm our estimates,

Table 1. Summary of Sediment Storage Environments in Marble Canyon

Sediment Budget Components

GIS Reaches

Marble Canyon

Average

Lees

Ferry

Roaring

Twenties

Redwall

Gorge

Point

Hansborough

Tapeats

Gorge

Reach length,a km 14.0 7.5 10.0 10.8 2.9 99.0

Number of eddies larger than 1000 m2

/kma 2.2 4.4 3.3 3.8 5.7 3.5

Average eddy area,a m2 9230 4980 4760 10660 7000 7430

Average eddy area inundated by the 1996 controlled flood,a m2 6210 3230 3750 6240 5160 4830

Average new channel-margin deposit area in 1996,a m2

/km 520 780 770 3930 1440 1760

Average new channel-margin deposit area in 2000, a m2

/km NAb NAb NAb 1110 NAb 460

Channel area excluding eddies at 227 m3

/s, a m2/km 132,840 58,460 70,200 86,330 73,740 84,850

Percentage of channel composed of rapids and rifflesc 6 10 16 9 10 10

Percentage of postdam channel composed of gravel, boulders, and bedrockd 93 NAb 88 85 NAb 88

a

From Schmidt et al. [2004]. b

No data available.

c

From Magirl al. [2005] using a water surface slope of 0.002 or greater to delineate rapid/riffle length. d

Estimated by S. Goeking, Utah State University (written communication, 2003) using the data of Wong et al. [2003].

Figure 4. Diagram showing the method of calculation of

eddy deposition zones (EDZs) from aerial photographs. The

EDZ is shown at bottom of figure [from Schmidt et al.,

2004].

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the greatest agreement with the sparse historical data is the

scenario where half of the fine sediment was stored in a

relatively small proportion of the total bed area and half the

fine sediment was stored in eddies (Table 3).

4. Sediment Storage in the Postdam River

4.1. Data Sources and Methods

[24] Our strategy to examine fine-sediment storage during

postdam conditions was to first quantify the total volume of

active storage within the two main storage environments,

eddies and the main channel; and second, develop sediment

budgets for two periods when dam releases were high and

when influx and efflux of sediment were measured. These

two periods were the 1996 Controlled Flood, between

26 March and 2 April 1996, and the September 2000

Powerplant Capacity Flow between 5 and 8 September

2000 (Figure 6). The 1996 Controlled Flood consisted of

a 7-day steady discharge of 1274 m3

/s, and the September

2000 Powerplant Capacity Flow consisted of a 4-day steady

discharge of 878 m3

/s.

[25] We used two different techniques to estimate changes

in fine-sediment storage during the 1996 Controlled Flood

and September 2000 Powerplant Capacity Flow. The first

estimate was derived from direct measurements of

topographic change and the GIS database described in

Table 2. Estimates of the Potential Fine-Sediment Storage Volume in Nine EDZs

EDZ Name

EDZ Area,a

m2

Minimum

Surface Area,b m2

EDZ and Minimum

Surface Overlap, %

Area of Comparison,

m2

Void Volume,c

m3

Void Volume

Thickness, m

Cathedral 11,658 8392 72 7124 25,122 3.53

Fence Fault 11,479 9448 82 4954 8949 1.81

South Canyon 10,837 9536 88 4316 11,877 2.75

Anasazi Bridge 25,348 11,318 45 4545 12,412 2.73

Eminence Break 80,259 30,377 38 12,884 34,776 2.70

Saddle Canyon 44,977 29,935 67 21,831 92,797 4.25

Crash Canyon 20,103 17,816 89 14,878 92,787 6.24

Carbon 20,253 18,123 89 10,971 24,451 2.23

Tanner 11,476 9422 82 4269 11,822 2.77

a

From Schmidt et al. [2004]. b

Computed from all topographic surveys conducted at each site 1990 – 2000.

c

Represents the potential storage volume of fine sediment in the EDZ between the minimum surface and the stage associated with a discharge of 100 m3/s.

Figure 5. Maximum predam potential scour and fill in EDZs determined from the minimum surface at

long-term study site 47 (location shown on Figure 1). (a) Thickness change based on subtracting the

minimum surface from the 100 m3

/s stage elevation within the area of EDZ overlap. Minimum surface

contour interval is 1 m. (b) Cross section A-A0 constructed from the minimum surface. For comparison,

the 24 August 2000 profile is shown. The gray shaded area illustrates the overlap of the EDZ and the

minimum surface up to the 100 m3

/s stage elevation.

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section 3.1. The second estimate of change in storage was

derived by partitioning the sediment export from Marble

Canyon into main channel- and eddy-derived components,

on the basis of the characteristic grain sizes of each storage

environment.

4.1.1. Sediment Transport

[26] Fine-sediment influx and efflux was computed using

streamflow and sediment-transport measurements at the

Lees Ferry gauge and one at the downstream end of Marble

Canyon (Colorado River near Desert View, Arizona, hereafter called the lower Marble Canyon gauge) (Figure 2).

Influx was set equal to zero during both the 1996 Controlled

Flood and the September 2000 powerplant flow, because

both dam releases had zero sediment content, and sediment

supply from the Paria River during both releases was

negligible (Figure 7). Measurements of suspended-sediment

concentration at the Lees Ferry gauge indicate that the reach

between Glen Canyon Dam and the Paria River supplied

little sediment to the study reach. The contributions of

sediment from the smaller, ungauged tributaries in Marble

Canyon were also ignored; the annual magnitude of the

fine-sediment supply from these tributaries is only 5– 20%

of that from the Paria River [Webb et al., 2000].

[27] Sediment transport at the lower Marble Canyon

gauge was calculated using suspended-sediment samples

collected on 27 and 29 March and 2 April 1996 (days 1, 3,

and 7 of the 1996 Controlled Flood), and daily during the

September 2000 Powerplant Capacity Flow (Figure 8).

Uncertainties associated with calculating sediment loads

from measured sediment concentrations for the Colorado

River are at least 5% [Topping et al., 2000b]. In this paper, a

more conservative uncertainty of 10% was assigned to the

measured sediment loads of the Colorado River for both

high flows. Given that discharge remained constant and

there was no tributary flooding during the budget computation periods, the error associated with estimating sediment

load over each flood is primarily that associated with

estimating the suspended sediment concentrations. The

Table 3. Estimates of the Average Predam Change in Fine-Sediment Thickness in Marble and Upper Grand Canyonsa

Seasonal Sediment

Accumulation, Mg

Equivalent

Volume,b m3

Equivalent Thickness,c m

Eddies 0.1

Channel

[0.9] (0.3) 0.9 Eddies 0.5

Channel

[0.9] (0.3) 0.5 Eddies 0.9

Channel

[0.9] (0.3) 0.1

1,000000 640,000 0.02 [0.04] (0.13) 0.08 [0.02] (0.07) 0.15 [0.00] (0.01)

7,000,000 4,460,000 0.11 [0.30] (0.91) 0.57 [0.17] (0.51) 1.03 [0.03] (0.10)

13,000,000 8,280,000 0.21 [0.56] (1.69) 1.06 [0.31] (0.94) 1.91 [0.06] (0.19)

a

Under various scenarios of seasonal sediment accumulation, relative proportion of accumulation stored in eddies and the main channel, and proportion

of the main channel where storage occurs. The two assumptions about the proportion of the channel that can store fine sediment are shown in brackets and

parentheses as 90% and 30%, respectively. Boldface indicates the most likely scenario for predam seasonal storage of fine sediment. b

Assumes bulk specific weight of deposited fine sediment is 1570 kg/m3. c

Assumes area of eddies is 3.9 106 m2, and area of channel is 14.7 106 m2.

Figure 6. Instantaneous discharge of the Colorado River at the Lees Ferry gauge 1995 – 2000. Shown

are the 1996 Controlled Flood and the September 2000 Powerplant Capacity Flow, the times of

the Northern Arizona University (NAU) surveying trips, and the times of the USGS bed-sediment

sampling trips.

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Figure 7. Estimated sediment supplied by the Paria River between October 1994 and September 2000.

The amount and grain sizes of sediment were calculated using the predictive flow and sediment-transport

model of Topping [1997]. The cumulative mass of total sand (0.0625 – 2.0 mm), fine sand (0.0625 –

0.25 mm) and coarse sand (0.25– 2.0 mm) with 20% uncertainties is shown. Also shown are the times of

the 1996 Controlled Flood and the September 2000 Powerplant Capacity Flow. The mean annual sand

supply for this 6-year period, 1.2 ± 0.3 million Mg, was average compared to the entire period of gauge

record estimated by Topping [1997]. Nearly two thirds of this sediment supply occurred during floods in

1997 and 1998. In addition, the median size (D50) of the sand inflow was about the same as the estimated

D50 of 0.13 mm for the period of record.

Figure 8. Measured suspended-sediment concentrations at the Lower Marble Canyon gauge. (a) The

1996 Controlled Flood. (b) The September 2000 Powerplant Capacity Flow. Note that the peak

suspended-sand concentration measured during the September 2000 Powerplant Capacity Flow is greater

than that during the 1996 Controlled Flood. This is probably not a result of a greater sediment supply but

simply a result of the measurements being made more closely in time to the arrival of the peak discharge

in 2000. Travel time of each high dam release has been removed, so that zero corresponds to the arrival

time of the released flood peak.

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1996 samples and techniques for determining suspended

sediment concentration are reported by Konieczki et al.

[1997], grain-size analyses of the 1996 samples are reported

by Topping et al. [1999], and similar analyses of the 2000

samples are available electronically from http://www.

gcmrc.gov/.

4.1.2. Sediment Storage and Characteristic Grain Sizes

[28] Changes in fine-sediment storage were estimated

from field measurements of topographic change, estimates

of sediment storage area from aerial photographs and sidescan sonar surveys, and measurements of sediment grain

size. For the purposes of this study, sediment stored above

the elevation reached by the 1996 Controlled Flood were

not considered active storage and were excluded from the

sediment budget.

4.1.2.1. Topographic Data

[29] Detailed field measurements were made at 11 longterm study sites that had been surveyed annually or more

frequently between 1990 and 2002 [Beus et al., 1992;

Kaplinski et al., 1995; Hazel et al., 1999] (Figure 2). The

sites comprise approximately 4% and 5% of the eddy and

main-channel area, respectively, in the study area. Each site

includes an eddy and adjacent main-channel bed (Figure 9).

Surveys were conducted immediately before and after each

high release to define changes in storage over the same

periods during which sediment transport was measured

(Figure 6). We combined bathymetric surveys of submerged

areas with ground surveys of emergent areas and produced a

triangulated irregular network (TIN) model using Delaunay

triangulation [see McCullagh, 1988, 1998]. Topographic

change caused by erosion or deposition of fine sediment

was determined by direct TIN comparison.

[30] We estimated the volume of sediment stored in each

eddy at specific times as the difference between each TIN

and the same minimum surface used to calculate the predam

active storage volume (Figure 5). We separated the mainchannel bed from each eddy by establishing a fixed boundary at the base of the eddy bars because this topographic

feature remained relatively stable throughout the 1990s

(Figure 9). Although the area of the eddy defined in this

way is somewhat different than the area of the associated

EDZ (Figure 5), it does not affect the interpretation of our

Figure 9. (a) Triangulated irregular network (TIN) model fit to the combined topographic and

hydrographic data collected on 24 August 2000 at long-term study site 47. The thickness of erosion and

deposition (b) following the 1996 Controlled Flood, (c) between the 1996 Controlled Flood and the

September 2000 Powerplant Capacity Flow, and (d) following the September 2000 Powerplant Capacity

Flow. The dashed white line in Figure 9a is the location of the boundary used for volumetric changes in

eddy and main-channel environments. P, main-channel pool; HEB, high-elevation eddy bar; LEB, lowelevation eddy bar. The sequence shows a pattern of deposition at high elevations in the eddy and erosion

at low elevations in the eddy and main channel during high flows, and the opposite pattern of high

elevation erosion and low elevation deposition during the intervening low flows.

F01025 HAZEL ET AL.: DAM-INDUCED STORAGE CHANGE

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F01025
results. We also distinguished between the high-elevation

and low-elevation part of each eddy bar as the area above or

below the stage of 227 m3

/s, respectively.

[31] Channel-margin deposits were not directly surveyed

before or after the 1996 Controlled Flood, and sedimentthickness changes were based on observations of excavated

trenches (D. Rubin, USGS, and J. C. Schmidt, Utah State

University, unpublished data, 1999). During the September

2000 Powerplant Capacity Flow, channel-margin deposits

were surveyed at 6 cross-sections previously established by

Graf et al. [1995] and at 18 cross sections established in

March 2000 by Cain et al. [2001].

4.1.2.2. Area Measurement of Sediment-Storage

Environments

[32] Results from the five GIS reaches were used to

determine the aerial proportion of eddy, channel-margin

deposit, and main-channel storage in Marble Canyon

(Table 1). We multiplied these areas by the average change

in deposit thickness in order to estimate the total volume

change in each storage environment. We assumed that all

bed change in the storage environments involved fine

sediment. The total area of the main channel (8.5 106 m2

)

was scaled by the percentage of bed in the main channel

composed of gravel, boulders, and bedrock (where fine

sediment does not occur) and rapid/riffle area (Table 1),

which leads to an aerial extent of fine sediment on the mainchannel bed as approximately 0.94 106 m2 (or 11% of the

total channel area excluding eddies). The percentage of bed

in the main channel composed of gravel, boulders, and

bedrock was extrapolated from GIS analysis (S. Goeking,

Utah State, unpublished data, 2003) of side-scan sonar

surveys conducted before and after the September 2000

Powerplant Capacity Flow by Wong et al. [2003]. Although

sand in interstices of gravel is a potentially important

component of the sediment budget in some rivers [Lisle,

1995], changes in the storage of this component were

ignored in this analysis, because (1) interstitial sand comprises a much smaller volume than either the volume of

sand in patches on the channel bed or in the eddy bars, and

(2) video images of interstitial sand indicate that this sand is

typically very coarse.

4.1.3. Sediment Grain Size

[33] The uncertainties in extrapolating the detailed measurements to the entire study area led us to develop an

alternative technique with which to partition sediment

storage changes. There is strong evidence that fine-sediment

sizes are segregated among depositional environments.

Generally, fine sediment on the main-channel bed is medium

and coarse sand with little or no silt or clay [Wilson, 1986;

Schmidt and Graf, 1990; Topping et al., 2000b]. In contrast,

the sizes of sediment in eddy bars and channel-margin

deposits are finer [Howard and Dolan, 1981; Schmidt and

Graf, 1990; Budhu and Gobin, 1994; Rubin et al., 1998;

Topping et al., 1999; Topping et al., 2000b].

[34] The fine-sediment grain size in each of the storage

environments was determined from pipe-dredge samples.

Sand patches on the main-channel bed were sampled in the

center of downstream flow, and eddies were sampled in

their center. These samples were collected throughout the

study area in November 1997, March 1998, September

1998, and May 1999 (Figure 6). Field methods and grainsize analyses for the 1997 and 1998 samples were previously reported by Topping et al. [2000b]. Channel-margin

deposits were not systematically sampled in this study

because the average grain-size was found by Schmidt and

Graf [1990] to be similar to that of eddy bars. Because the

average differences between the 1997, 1998, and 1999 data

were relatively small, we assumed that the mean 1997–

1999 grain-size distributions for the main-channel and eddy

storage environments were good approximations for the

grain-size distributions in these storage environments in

other years.

4.2. Results

4.2.1. Analysis of Fine-Sediment Storage in the

Postdam River

[35] The mean thickness of sediment stored in eddies at

the detailed study sites was 2.02 ± 0.10 m in March 1996

(Figure 10). The total eddy area below the stage of the 1996

Controlled Flood is approximately 1.7 106 m2

, which

leads to an estimate of the volume of fine sediment in active

storage in Marble Canyon eddies as about 3.4 106 m3

, or

5.9 million Mg. The average thickness of sediment stored

in the main channel at the detailed study sites was 0.92 ±

0.11 m in March 1996 (Figure 10), which leads to an estimate

of the volume of fine sediment stored on the main channel

bed as about 0.8 106 m3 (or 1.4 million Mg). These results

are probably accurate to within a factor of 2 and, given this

uncertainty, 51–94% of the sediment in Marble Canyon is

stored in eddies; despite the fact that eddies only comprise

17% of the total surface area of the flow field.

4.2.2. Variation in the Size of Fine Sediment in

Storage and in Transport

[36] The mean grain-size of sediment in eddies was much

finer than that on the bed of the main channel during 4 years

of measurements, despite short-term local fining of the

channel bed following Paria River floods (Figure 11).

Between 1997 and 1999, the average D50 of fine sediment

on the main-channel bed was about 0.40 mm, and the

average D50 of eddy sediment was about 0.18 mm. On

average, 70% of the fine sediment in the eddies during the

study period was finer than 0.25 mm, and 2.4% was finer

than sand, whereas only 17% of the fine sediment in the

channel was finer than 0.25 mm, and 0.16% was finer than

Figure 10. Temporal sequence of average eddy and mainchannel bed sediment thickness changes at the 11 long-term

monitoring sites 1996 – 2000. Error bars are standard error.

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F01025
sand. This distinct difference in grain-size distribution

between eddy and main-channel storage environments

proves crucial in computing a fine-sediment budget based

on grain size, because about 92% of the influx of sand from

the Paria River is finer than 0.25 mm (Figure 7).

[37] Depletion of the upstream supply of fine sediment

resulted in rapid decrease in the total sediment concentration

during the first day of both the 1996 Controlled Flood and

the September 2000 Powerplant Capacity Flow (Figure 8).

The sediment exported from the study area during each high

flow was eroded from the sediment stored in Marble

Canyon because there was negligible fine-sediment influx

from either the Paria River or the Colorado River upstream

from the Lees Ferry gauge during these two releases. The

sediment export at the lower Marble Canyon gauge during

the 1996 Controlled Flood was approximately 0.67 million

Mg of sand and 0.12 million Mg of silt and clay. Of the

sand-sized sediment, about 41% was very fine sand

(0.0625 – 0.125 mm), about 38% was fine sand (0.125 –

0.25 mm), about 19% was medium sand (0.25– 0.5 mm),

and only about 2% was coarser than 0.5 mm [Topping et al.,

1999]. During the September 2000 Powerplant Capacity

Flow, approximately 0.22 million Mg of sand and 0.065

million Mg of silt and clay were exported past the lower

Marble Canyon gauge. Of the sand-sized sediment, about

62% was very fine sand (0.0625 – 0.125 mm), about 32%

was fine sand (0.125 – 0.25 mm), about 5% was medium

sand (0.25– 0.5 mm), and only about 1% was coarser than

0.5 mm.

4.2.3. Sediment Budgets for the 1996 Controlled Flood

and the September 2000 Powerplant Capacity Flow

[38] The two different techniques used to estimate

changes in fine-sediment storage during the 1996 Controlled Flood and September 2000 Powerplant Capacity

Flow each have their own respective strengths and provide

independent checks on the other. Measurement of topographic change at selected sites provides a direct accounting

of the depositional and erosional environments, but relies on

substantial extrapolation. The grain-size technique provides

a more global accounting of the source environments of the

sediment eroded from Marble Canyon but involves no actual

measurements of topographic change.

4.2.3.1. Sediment Budgets Developed

From Topographic Measurements

[39] Measurements during both high flows indicate that

high-elevation deposition in eddies and as channel-margin

deposits was accompanied by scour of the low-elevation

parts of eddy bars and those parts of the main-channel bed

where fine sediment was stored (Figure 9) [also see

Andrews et al., 1999; Hazel et al., 1999; Schmidt, 1999].

In both high flow events, the average increase in fine

sediment thickness at high elevation was greatly exceeded

by losses in fine-sediment thickness at low elevation

(Table 4). The topographic-based sediment budget for

Marble Canyon was determined as

DS 1⁄4 AeddiesTeddies þ Acm depositsTcm deposits þ AchannelTchannel; ð1Þ

where DS is the total sediment-storage change in Marble

Canyon, A is the storage area in Marble Canyon for eddies,

channel-margin deposits, and the main-channel bed (Table 1),

and T is the average thickness change of eddy, channel

margin deposits, and the main-channel bed (Table 4). The

accuracy of the sediment budget was evaluated by comparing

DS to the difference in sediment flux at the upstream and

downstream ends of the study area,

DS 1⁄4 Ssupply  Sexport; ð2Þ

Figure 11. Average cumulative grain-size distributions for the sand on the main-channel bed and in

eddies in the Colorado River in Marble Canyon. Error bars are 1 standard deviation. The patches of sand

on the bed of the channel are predominantly composed of sand coarser than 0.25 mm; eddies are

composed of sand predominantly finer than 0.25 mm.

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F01025
where Ssupply is the sediment supply from the Paria River (set

equal to zero) and Sexport is the sediment export past the

Lower Marble Canyon gauge.

[40] The difference between DS and Sexport during the

two high releases represents an imbalance in the budgets

(Table 5). The difference is approximately 0.94 106 Mg

and 0.17 106 Mg for the 1996 Controlled Flood and

September 2000 Powerplant Capacity Flow, respectively.

The budget imbalance reflects errors in our extrapolation

methods and indicates that one or more of the storage

environments was not accurately characterized. Although

the error could be a result of an inaccurate estimate of Sexport,

we attribute the inaccuracy to the fact that the estimates of DS

in the main-channel bed are based on measurements made in

pools immediately upstream or downstream from rapids.

Thus, as in the case of the predam observations of scour

and fill in the pools at the Lees Ferry and Grand Canyon

gauges, the magnitudes of scour and fill measured in pools

probably overestimates the average magnitude of scour and

fill elsewhere in the main channel. Also, as observed at the

Grand Canyon gauge [Topping et al., 2000b] and elsewhere

in selected reaches during the 1996 Controlled Flood [Anima

et al., 1998], scour of sediment from main-channel pools

during high flows is partially balanced by deposition of

sediment outside of the deeper pools. Another and perhaps

more likely possibility is that the overestimation is a result of

coarser material mobilized in areas of high sheer stress

produced by the high flows [Wiele et al., 1999]. Thus bed

thickness changes may not be solely due to changes in fine

sediment.

4.2.3.2. Sediment Budgets Developed From

Measurements of the Grain-Size Distribution

of Exported Sediment

[41] The grain-size budgeting technique was developed to

independently evaluate the errors in the topographic budget

described above. The grain-size budgeting technique involved using the measured size distribution of sand, silt, and

clay exported past the lower Marble Canyon gauge in

conjunction with (1) the 1997 –1999 mean grain-size distributions of the fine sediment stored in the eddy and mainchannel environments (Figure 11) and (2) the computation

that between 51 and 94% of the sand, silt, and clay in

Marble Canyon is stored in eddies. The change in the mass

of fine sediment was approximated by

DS 1⁄4 XN

i1⁄41

aeddiesbeddiesi

aeddiesbeddiesi þ achannelbchanneli

DSi



þ achannelbchanneli

aeddiesbeddiesi þ achannelbchanneli

DSi



; ð3Þ

where N is the total number of size classes of sand, silt, and

clay, aeddies is the proportion of sand, silt, and clay stored in

the eddies in Marble Canyon (given the uncertainty in this

estimate, this value ranges from 0.51 to 0.94), achannel is the

fraction of sand, silt, and clay stored in the main channel

(given the uncertainty in this estimate, this value ranges

from 0.06 to 0.49), beddiesi is the fraction of each sediment

size class i stored in eddies (computed from the data in

Figure 9), achanneli is the fraction of each sediment size class

i stored in the main channel (computed from the data in

Figure 11), and DSi is the change in the mass of each

sediment size class i in Marble Canyon (as measured at the

lower Marble Canyon gauge). Thus the change in the mass

of the sand, silt, and clay in the eddies in Marble Canyon is

DSeddies 1⁄4 XN

i1⁄41

aeddiesbeddiesi

aeddiesbeddiesi þ achannelbchanneli

DSi

; ð4Þ

and the change in the mass of the sand, silt, and clay in the

main channel in Marble Canyon is

DSchannel 1⁄4 XN

i1⁄41

achannelbchanneli

aeddiesbeddiesi þ achannelbchanneli

DSi

: ð5Þ

The mass of eddy- and main-channel-derived sediment

exported from Marble Canyon is shown in Figure 12 for

each grain-size class. Given the uncertainties in the methods

described above, these results indicate that nearly all of the

silt and clay and the majority of the very fine to fine sand

(0.0625 – 0.25 mm) exported during the 1996 Controlled

Flood and the September 2000 Powerplant Capacity Flow

were eroded from eddies, whereas the medium to coarse

Table 4. Summary of Average Sediment-Thickness Changes

Derived from Topographic Data

Topographic Storage

Components

1996 Controlled

Flood,a m

September 2000

Powerplant Capacity

Flow,a m

High-Elevation Eddy 0.18 ± 0.05 0.03 ± 0.02

Low-Elevation Eddy 0.56 ± 0.18 0.15 ± 0.08

Channel Margin 0.30 ± 0.10 0.15 ± 0.07

Main-Channel Bed 0.49 ± 0.13 0.08 ± 0.07

a

Uncertainties are 1 standard error.

Table 5. Topographic-Based Sediment Budget for the Colorado River in Marble Canyona

Budget Components

1996 Controlled

Flood, 106 Mg

September 2000

Powerplant Capacity

Flow, 106 Mg

Supply from the Paria River (Ssupply) Negligible Negligible

High-elevation sediment in eddies, >227 m3

/s 0.54 ± 0.14 0.09 ± 0.06

Low-elevation sediment in eddies, <227 m3

/s 1.63 ± 0.52 0.44 ± 0.23

Channel-margin deposits, >227 m3

/s 0.09 ± 0.02 0.01 ± 0.01

Main-channel bed sediment, <227 m3

/s 0.73 ± 0.20 0.12 ± 0.11

Total sediment storage change, DS 1.73 ± 0.57 0.46 ± 0.28

Export past Lower Marble Canyon gauge, Sexport 0.79 ± 0.08 0.29 ± 0.03

a

Uncertainties in storage components are based on the standard error of the mean sediment-thickness change and do not

include errors associated with the GIS maps and extrapolation of component area to the reach scale. Uncertainty in the export

component is based on the 10% uncertainties associated with the suspended-sediment measurements.

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F01025
sand (0.25 – 2.0 mm) was eroded from both eddies and the

main channel. About 0.68 ± 0.08 million Mg (86%) of

the sediment mobilized and transported downstream, during

the 1996 Controlled Flood, was eroded from eddies,

whereas during the September 2000 Powerplant Capacity

Flow, about 0.26 ± 0.02 million Mg (92%) of the

sediment in transport was eroded from eddies. The

magnitudes of eddy and channel-derived sand in each size

class were relatively similar between the two experimental

releases. Very fine and fine sand was the dominant portion

of the sediment eroded from eddies varying from 71%

during the 1996 Controlled Flood, to 73% during the

September 2000 Powerplant Capacity Flow. Of the sediment eroded from the main channel, fine to medium sand

was the dominant portion, varying from 79% during the

1996 Controlled Flood, to 60% during the September 2000

Powerplant Capacity Flow.

4.2.3.3. Reconciliation Between the Topographic- and

Grain-Size-Based Sediment Budgets

[42] Although there is potential for large error in a

sediment budget constructed from a variety of data sources,

the estimates of DS in eddies by the two independent

methods provides an estimate of the accuracy of the

extrapolation of the topographic data to the reach scale.

Subtracting the sediment transferred to high-elevation eddy

bars and channel-margin deposits from that eroded from

lower elevations in eddies, yields the total sediment efflux

from Marble Canyon supplied by eddies (Table 5). This

estimate of eddy-derived sediment during the 1996 Controlled Flood (1.0 ± 0.36 million Mg) agrees, within

estimated error, with the grain-size based estimate in

Figure 12 (0.68 ± 0.08 million Mg). These findings support

the conclusion of Schmidt [1999] that eddies were the

primary source of most of the sediment in transport during

the 1996 Controlled Flood. Similar to the results for the

1996 Controlled Flood, the topographic-based estimate of

the export of eddy-derived sediment during the September

2000 Powerplant Capacity Flow (0.34 ± 0.17 million Mg)

agrees, within error, with the grain-size based estimate in

Figure 12 (0.26 ± 0.02 million Mg). These results suggest

that the main-channel storage environment in the topographic-based budget was overestimated and accounts for

the budget imbalance in Table 5.

[43] The topographic-based estimate of eddy-derived sediment and the grain-size estimate of channel-derived sediment can be reconciled into a complete sediment budget that

is balanced between DS and Sexport during the two high dam

releases. During the 1996 Controlled Flood, about 1.74 ±

0.6 million Mg of sediment was eroded from eddies and the

main channel bed. About 94% of this amount was eroded

from the lowest parts of eddy bars. Of this eroded mass,

36% was deposited as high-elevation bank deposits above

the stage of 227 m3

/s. Of the sediment deposited at high

elevation, about 86% was deposited in eddies and the

remainder as channel-margin deposits. Using the same

reasoning as for the 1996 Controlled Flood, the sediment

budget constructed for the September 2000 Powerplant

Capacity Flow indicates that about 0.46 ± 0.25 million

Mg of sediment was eroded from eddies and the main

channel. About 95% of this amount was eroded from lowelevation eddy bars, and 21% of this amount was deposited

at high elevation. About 91% of the sediment redistributed

to high elevation was deposited in eddies. Although the

pattern of storage change was similar to the budget for the

1996 Controlled Flood, 6 times less sediment was deposited

as high-elevation eddy bars and channel-margin deposits,

during the lower-discharge September 2000 Powerplant

Capacity Flow, and a greater percentage of sediment was

exported from Marble Canyon.

5. Discussion

[44] Glen Canyon Dam releases clear water into a river

channel formerly adjusted to extremely large suspended

sediment loads, resulting in long-term changes in sediment

storage. Whereas fine sediment was stored in both the main

channel and in eddies in the predam environment, eddies are

now the dominant temporary storage site. Thus eddies are

the primary sink and source term in the postdam sediment

budget; a result suggested by previous studies but never

quantified [Rubin et al., 1994; Schmidt and Rubin, 1995]. A

fine-sediment deficit exists in Marble Canyon and downstream response of the channel includes degradation of eddy

Figure 12. The proportions of eddy- and channel-derived

sediment eroded from Marble Canyon. (a) The 1996

Controlled Flood. (b) The September 2000 Powerplant

Capacity Flow. Error estimates incorporate uncertainties

associated with the suspended-sediment and grain-size

measurements and the estimate that 51 to 94% of the

fine sediment in Marble Canyon is stored in eddies (see

section 4.2.1).

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F01025
storage and textural segregation of fine sediment into

distinct grain-size distributions.

[45] Calculations of average bed-elevation change suggest that about half of the fine sediment stored in Marble

Canyon in the predam era was stored on the main-channel

bed (Table 3). Parts of the main-channel bed that once

scoured during the three months of the annual snowmelt

flood and refilled during the nine months of the finesediment accumulation season are now permanently devoid

of fine sediment [Schmidt et al., 2004]. The magnitude and

proportion of fine sediment evacuated from storage during

the two experimental dam releases in this study indicates

that about 90% of the fine sediment in the riverine ecosystem is now stored in eddies (Table 5).

[46] During the short periods when the Paria River is in

flood, eddies and the main channel are blanketed with fine

sediment and neither environment predominates as a sink

for new inputs. However, when tributary flooding ceases

and suspended-sand concentrations begin to decrease this

material is rapidly eroded from the bed under normal dam

operations. The bed coarsens, especially in the main channel

[Topping et al., 2000b]. Within months, active storage of

fine sediment is almost exclusively in eddies, and only

small amounts of the finer sand sizes (<0.25 mm) are stored

in sandy patches on the main-channel bed, where the

median size is much coarser (Figure 11). This downstream

response to the transport capacity of the river and the

available sediment supply results in a marked similarity of

bed-material grain sizes between the sediment stored in

eddies and that supplied by the Paria River (Figure 7).

[47] The results presented here are of more than scientific

interest. The long-term fate of sandbars depends on the

ability of dam managers to utilize special dam releases for

redistribution of the relatively limited amount of sand

supplied by the Paria River [Rubin et al., 2002]. Dam

managers want to know how much sand is delivered from

the Paria River, how long that sand remains in Marble

Canyon, where that sand resides before it is transported

downstream, and how much will be mobilized during these

special dam releases. Because they are infrequently inundated, higher-elevation flood deposits have a longer response time to normal dam operations than subaqueous

deposits in low-elevation eddies or on the main-channel

bed. These higher-elevation deposits erode or adjust

over a period of years to decades [Schmidt et al., 2004].

Unfortunately, high dam releases that occur when the

channel bed is not temporarily enriched in fine sediment

scour the sediment in active eddy storage. During the 1996

Controlled Flood, the percentage of sand redistributed

to higher elevations was small (36%), relative to that

transferred out of eddies, and this ratio was even smaller

during the September 2000 Powerplant Capacity Flow

(26%).

[48] Rubin et al. [1994] made generalized predictions of

the long-term fate of bars at different distances downstream

from Glen Canyon Dam. The model that presented the

greatest difficulties in managing the scarce sand resources in

GCNP turns out to be the case that actually exists: Most

sand is stored in eddies. Prolonged periods without significant input of sand from the Paria River will result in

continued erosion of sandbars in Marble Canyon. The area

of high-elevation eddy deposits in 2003 was smaller than

the area of these deposits in 1984 or 1990 [Schmidt et al.,

2004]. The dilemma for the future is that in sedimentdeficient reaches near Glen Canyon Dam, and for some

undetermined distance downstream, controlled floods will

export sediment from the same eddies where bar building is

desired.

6. Conclusions

[49] The combination of a greatly reduced sediment

supply and the operations of Glen Canyon Dam have

changed the pattern of sediment transport and storage in

Marble Canyon. The predominant proportion of fine sediment in this canyon river is now stored in eddies, rather than

stored in both eddies and the main-channel bed. Because

eddies comprise only 17% of the total channel area in

Marble Canyon, the supply available for entrainment by

high releases is limited. In addition, the spatial distribution

of sizes of sediment is different between eddy and mainchannel deposits, with the grain-size distribution in eddies

being far more similar to the median size of the long-term

supply of sand from the Paria River. Both the sediment

budgets and field data indicate that floods scheduled in

seasons other than the short period of time when sediment is

shown to accumulate in this system (during periods of

tributary flooding) will erode sediment from long-term eddy

storage, resulting in continued degradation of the finesediment deposits in Marble Canyon.

[50] Acknowledgments. This study was funded by the U.S. Bureau

of Reclamation through the Glen Canyon Environmental Studies and later

through the USGS Grand Canyon Monitoring and Research Center. The

authors thank Mark Gonzales, Keith Kohl, and Mark Manone for assistance

in the field and Sara Goeking, Mike Breedlove, and Jairo Hernandez for

GIS support. Thanks also go to Paul Grams, Ted Melis, Rod Parnell, and

Steve Wiele for helpful and constructive comments on the initial version of

this manuscript. A subsequent version was reviewed by Peter Ashmore,

Yvonne Martin, and Robert Jacobson. We thank Peter Wilcock, John

Pitlick, and an anonymous reviewer for suggestions on improving the final

version of the manuscript.

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J. E. Hazel Jr. and M. Kaplinski, Department of Geology, Northern

Arizona University, P.O. Box 4099, Flagstaff, AZ 86011-4099, USA.

(joe.hazel@nau.edu; matt.kaplinski@nau.edu)

J. C. Schmidt, Department of Aquatic, Watershed, and Earth Resources,

Utah State University, Logan, UT 84322-5210, USA. (jschmidt@cc.usu.edu)

D. J. Topping, U.S. Geological Survey, 2255 North Gemini Drive,

Flagstaff, AZ 86001, USA. (dtopping@usgs.gov)

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