Regulation of sand transport in the Colorado River by changes

in the surface grain size of eddy sandbars over multi-year

timescales

DAVID J. TOPPING*, DAVID M. RUBIN and JOHN C. SCHMIDT

*U.S. Geological Survey, 2255 N. Gemini Dr., Flagstaff, AZ 86001, USA (E-mail: dtopping@usgs.gov)

U.S. Geological Survey, 400 Natural Bridges Dr., Santa Cruz, CA, 95060, USA

Department of Aquatic, Watershed, and Earth Resources, Utah State University, Logan, UT, 84322-5210,

USA

ABSTRACT

In settings where the transport of sand is partially or fully supply limited,

changes in the upstream supply of sand are coupled to changes in the grain

size of sand on the bed. In this manner, the transport of sand under the supplylimited case is ‘grain-size regulated’. Since the closure of Glen Canyon Dam in

1963, the downstream reach of the Colorado River in Marble and Grand

Canyons has exhibited evidence of sand-supply limitation. Sand transport in

the river is now approximately equally regulated by changes in the discharge

of water and changes in the grain sizes of sand on the channel bed and eddy

sandbars. Previous work has shown that changes in the grain size of sand on

the bed of the channel (driven by changes in the upstream supply of sand

owing to both tributary floods and high dam releases) are important in

regulating sand transport over timescales of days to months. In this study,

suspended-sand data are analysed in conjunction with bed grain-size data to

determine whether changes in the grain size of sand on the bed of the channel

or changes in the grain size of sand on the surface of eddy sandbars have been

more important in regulating sand transport in the post-dam Colorado River

over longer, multi-year timescales. The results of this study show that this

combined theory- and field-based approach can be used to deduce which

environments in a complicated setting are the most important environments

for regulating sediment transport. In the case of the regulated Colorado River in

Marble and Upper Grand Canyons, suspended-sand transport has been

regulated mostly by changes in the surface grain size of eddy sandbars.

Keywords Colorado River, grain size, sand bars, sediment transport, suspended sediment.

INTRODUCTION

The transport of a given size of sediment is

controlled by: (i) hydraulics (i.e. the boundaryshear-stress and velocity fields), and (ii) the

upstream supply of that size class. In the case

where the upstream supply of the given size class

is sufficiently large, changes in hydraulics are the

dominant controllers of transport. This case can

be referred to as the ‘equilibrium-upstream-supply case’. In the case where the upstream supply

of the given size class is not sufficiently large to

maintain transport in equilibrium with the

hydraulics, changes in the upstream supply of

that size class result in changes in the transport of

that size class. This latter case is typically

referred to as ‘supply-limited transport’. In most

situations, the transport of a given size of sediment

is probably controlled by a combination of changes

in the hydraulics and changes in the upstream

supply. In the equilibrium-upstream-supply

case, no substantial changes in the grain-size

Sedimentology (2005) 52, 1133–1153 doi: 10.1111/j.1365-3091.2005.00738.x

 2005 International Association of Sedimentologists, No claim to original US government work 1133
distribution of the bed sediment occur, whereas,

in the supply-limited case, substantial changes in

the grain-size distribution of the bed sediment

accompany changes in the upstream supply of

sediment (Topping et al., 2000a,b). Sediment

transport under the equilibrium-upstream-supply

case is ‘flow-regulated’ and sediment transport

under the supply-limited case is ‘grain-size regulated’ (Rubin & Topping, 2001).

Rubin & Topping (2001) developed a generalized analytical technique for determining the

relative degrees to which sediment transport in

a given setting is regulated by changes in bedsediment grain size and changes in hydraulics.

They determined that both processes are equally

important in controlling sand transport in the

Colorado River downstream from Glen Canyon

Dam in Grand Canyon. In addition, they found

that changes in bed-sand grain size can play

important roles in regulating sand transport not

only in bedrock-canyon rivers but also in alluvial

rivers, e.g. the Mississippi River.

In this paper, the dam-regulated Colorado River

is examined to determine the relative roles of

different parts of the bed in regulating sand

transport. Sand occurs in two key environments

on the bed of this river. It occurs in the channel as

patches on gravel, colluvium, and bedrock, and

also in eddies as sandbars that may be more than

several meters thick. Although the sand in eddies

is typically thicker than on the bed of the

channel, it probably covers less area because

eddies comprise only about 20% of the area of the

riverbed (Schmidt et al., 2004; Hazel et al., in

press). As described in this paper, b-analyses of

suspended-sand data suggest that the bed environment that is the dominant regulator of sand

transport in the Colorado River has fined over the

past several decades. To determine which riverbed environment, channel or eddy, is the dominant regulator of sand transport, b is calculated for

discrete river-discharge intervals and all available

surface grain-size data from the channel and eddy

bed environments are analysed. These analyses

show that the smallest part of the bed, i.e. the

eddy sandbars, is the dominant regulator of sand

transport in this river. This approach of using

suspended-sediment data in conjunction with

field observations of surface grain-size change in

different bed environments should be of use in

other settings to determine whether sediment

transport is regulated by the sediment comprising

large portions of the bed or by the sediment found

in discrete environments that may comprise only

a small part of the bed.

Goal of this study

Sand transport in the post-dam Colorado River is

regulated by both changes in bed-sand grain size

and changes in the discharge of water. Previous

studies have concluded that changes in the grain

size of the sand on the bed of the channel are the

dominant regulator of sand transport in the

Colorado River in Marble and Upper Grand

Canyons over timescales of days to months. The

goal of this study is to evaluate the relative roles

of the sand on the bed of the channel and the sand

on the surface of eddy sandbars in regulating sand

transport in the Colorado River in Marble and

Upper Grand Canyons over longer, multi-year

timescales.

Study Area

The study area is the Colorado River in Marble

and Upper Grand Canyons (Fig. 1). Discharge and

sediment data from the following U.S. Geological

Survey (USGS) gaging stations are analysed:

Colorado River at Lees Ferry, Arizona, station

Fig. 1. Map of the study area showing the locations of

selected USGS gaging stations, sampled eddy sandbars,

and trenches excavated through 1983 flood deposits.

1134 D. J. Topping et al.

 2005 International Association of Sedimentologists, No claim to original US government work, Sedimentology, 52, 1133–1153
number 09380000 (herein referred to as ‘the Lees

Ferry gage’), Colorado River above Little Colorado

River near Desert View, Arizona, station number

09383100 (herein referred to as ‘the Lower Marble

Canyon gage’), and Colorado River near Grand

Canyon, Arizona, station number 09402500 (herein referred to as ‘the Grand Canyon gage’). By

convention, locations along the Colorado River

are referred to by river mile, as measured downstream from the Lees Ferry gage (Stevens, 1983).

Marble Canyon extends from the Lees Ferry gage

99 km to the mouth of the Little Colorado River at

river-mile 61Æ5; Grand Canyon extends from the

mouth of the Little Colorado River 346 km to

the Grand Wash Cliffs at river-mile 276 (at

the upstream end of Lake Mead reservoir). The

upstream 42 km of Grand Canyon, from the

mouth of the Little Colorado River to the Grand

Canyon gage at river-mile 87Æ4, is herein referred

to as Upper Grand Canyon. The study area begins

25Æ5 km downstream from Glen Canyon Dam,

which impounds the Colorado River to form Lake

Powell reservoir, and has regulated the discharge

of water in the study area since its closure in

March 1963 (Topping et al., 2003). Closure of the

dam has greatly reduced the supply of sand to the

study area. The only substantial supplier of sand

to the upstream end of the study area is now the

Paria River, which supplies about 6% of the predam sand load at this location (Topping et al.,

2000a).

The longitudinal profile of the Colorado River

in Marble and Grand Canyons is characterized

by long, gently sloping pools separated by short,

steep drops in rapids (Leopold, 1969; Magirl

et al., 2005). The banks of the river are

composed mostly of talus and bedrock, with

lesser amounts of sand and finer material

(Howard & Dolan, 1981). The rapids in which

most of the elevation loss occurs are formed by

debris fans composed of coarser sediment

delivered to the Colorado River by debris flows

and floods from relatively small tributaries

(Cooley et al., 1977; Webb et al., 1989; Melis

et al., 1994). These debris fans give rise to the

dominant geomorphic element in this reach,

the fan-eddy complex (Schmidt & Rubin, 1995).

The typical fan-eddy complex consists of a

backwatered pool in the channel upstream from

the debris fan, constricted flow through a rapid

over the toe of the debris fan, and a large

expansion in flow area downstream from the

debris fan, where the downstream flow separates from the bank to form a lateral recirculation eddy along the bank (Fig. 2). Sandbars form

in the eddy and are largely composed of sand

deposited from suspension (Howard & Dolan,

1981; Rubin et al., 1990, 1994; Schmidt, 1990;

Schmidt & Graf, 1990). If the upstream supply

of suspended sand is sufficient, sandbars may

fill most of the eddy to a relatively high

elevation (Wiele et al., 1996).

Though only a small percentage of the river

area is composed of eddies, the total area of

sand in eddies is only about 33–50% less than

the total area of sand on the bed of the channel.

Analyses of historical aerial photography and

recent surveys indicate that about 20% of the

river in Marble Canyon is composed of lateral

recirculation eddies (Schmidt et al., 2004; Hazel

et al., in press). Side-scan-sonar data collected

in the study area between 1994 and 2000

suggest that, on average, the channel bed is

composed of by area 40% finer gravel (pebbles

and cobbles of probable fluvial origin), 20–30%

sand, 20–30% bedrock, and 10–20% large,

immobile boulders that are either derived from

tributaries or the hillslopes and cliffs adjacent to

the river (Anima et al., 1998; Wong et al., 2003).

These data also suggest that, on average, sandbars occupy at least 60% of the eddy area, with

the remainder of the eddy area composed of

finer gravel or boulders. Thus, the area of sand

in eddies is equivalent to about 12% of the total

river area and the area of sand in the channel

is equivalent to about 16–24% of the total

river area.

Although it begins 20 years after closure of

Glen Canyon Dam, the study period 1983–2000 is

the first multi-year post-dam period with suspended- and bed-sand data adequate for this study.

The analyses presented in this paper require

time-series of suspended-sand concentration

and grain size (preferably from more than one

location), and time-series of bed-sand grain size

from multiple locations and environments. The

study period includes both the largest post-dam

flood (2750 m3 sec)1 peak discharge) in 1983 and

one of the longest post-dam periods of sustained

low discharge in 2000 (Fig. 3). The period from

1983 through 1986 was characterized by higher

than average dam releases because of full reservoir conditions in Lake Powell. The period from

1987 through July 1991 was characterized by

large daily fluctuations in discharge for power

generation. Beginning in August 1991, dam releases were constrained in an attempt to minimize

the downstream effects of dam operations (National Research Council, 1996). In March–April

1996, an experimental 7-day controlled flood

Regulation of sand transport 1135

 2005 International Association of Sedimentologists, No claim to original US government work, Sedimentology, 52, 1133–1153
with a peak discharge of 1270 m3 sec)1 (the 1996

controlled flood) was released from the dam

(Webb et al., 1999). In November 1997, a second

experimental dam release was conducted consisting of a 2-day 890 m3 sec)1 powerplant-capacity

dam release (Topping et al., 2000b). In 2000, a

third experimental dam release was conducted

consisting of 3 months of constant 227 m3 sec)1

discharge between two 4-day powerplant-capacity flows.

Previous Work

Beginning in the 1970s, concerns were raised

about the effect of Glen Canyon Dam on the

downstream ecosystem of the Colorado River in

Marble and Grand Canyons (Dolan et al., 1974).

Sandbar erosion downstream from the dam led

to the early sediment-transport and geomorphologic studies of Dolan et al. (1974); Howard

(1975); Laursen et al. (1976), and Howard &

Dolan (1981). Dolan et al. (1974) and Laursen

et al. (1976) concluded that, under normal

powerplant releases from Glen Canyon Dam,

the transport capacity of the river exceeded the

supply of sand, and that the eddy sandbars

would gradually erode away. In contrast to the

results of these earliest studies, Howard & Dolan

(1981), Randle & Pemberton (1987), Andrews

(1990, 1991), and U.S. Department of the Interior

(1995) concluded that the tributary supply of

sand downstream from the dam exceeded the

transport capacity of the river under normal

releases from the dam. These early sediment

budgets relied on stable relationships between

the discharge of water and sand transport.

Following the experimental 1996 controlled

flood, Topping et al. (2000b) showed that,

because the grain size of the sand on the bed

of the channel changes substantially over time in

response to both tributary activity and dam

releases, stable relationships between the discharge of water and sand transport do not exist.

Rubin & Topping (2001) concluded that, under

normal powerplant releases from the dam, sand

transport in the Colorado River is equally

regulated by changes in bed-sand grain size

and changes in hydraulics. From their analysis

Fig. 2. A typical eddy sandbar in Marble Canyon, the right-bank sandbar in the fan-eddy complex at river-mile 22.

White arrows indicate direction of flow in the channel; the discharge of water is 227 m3 sec)1

. This sandbar is

completely inundated at a discharge of approximately 1,200 m3 sec)1

.

1136 D. J. Topping et al.

 2005 International Association of Sedimentologists, No claim to original US government work, Sedimentology, 52, 1133–1153
of only suspended-sand data, however, it is

impossible to know which riverbed environment, channel or eddy, was most important

in regulating sand transport.

Rubin et al. (2002) analysed recent sand-transport and geomorphic data, and determined that

the sand budget for the study area was negative

over the previous several years of normal powerplant releases from Glen Canyon Dam. Topping

et al. (2000a) showed that, given the uncertainties in the sediment budget constructed using

USGS historical daily sediment-transport data,

one could not conclude that tributary-supplied

sand was stored in the Colorado River in Marble

and Upper Grand Canyons for more than a few

months. Flynn & Hornewer (2003) showed that

the amount of sediment decreased in 55 of 57

cross-sections surveyed repeatedly by the USGS

between 1992 and 1999. Additionally, Schmidt

et al. (2004) showed that, between the mid-1980s

and 2000, the amount of sand in the channel and

eddies in Marble and Upper Grand Canyons

decreased by about 25%. More recently, Hazel

et al. (in press), estimated that, by volume,

between 51 and 94% of the sand in Marble

Canyon is stored in eddies, and that, as suggested

by Schmidt (1999), eddies are the primary source

of sand deposited at higher elevations during

high dam releases. Thus, under normal dam

releases over multi-year timescales, the sand

budget of the Colorado River in Marble and

Upper Grand Canyons is negative and most of

the sand in multi-year storage occurs in eddy

sandbars that are getting smaller over time. This

result suggests that, although the area of sand in

eddies is about 50–67% of the area of sand in the

channel, the sand stored in the eddy sandbars

may play a dominant role in regulating sand

transport in the post-dam Colorado River over

longer, multi-year timescales.

0

1,000

500

1,500

2,000

2,500

3,000

Calendar Year

1983 Flood

1996 Controlled flood

1997 & 2000 

PowerplantCapacity

releases

Sustained low discharge

Capacity of GCD powerplant

Instantaneous discharge (m3 sec–1)

19821983198419851986198719881989199019911992199319941995199619971998199920002001

Fig. 3. Hydrograph of the Colorado River at the Lees Ferry gage between 1982 and 2001. Shown are the capacity of

the Glen Canyon Dam (GCD) powerplant, the1983 flood (i.e. the post-dam flood of record), the 1996 controlled flood,

the three 1997 and 2000 powerplant-capacity flows, and the three months of sustained low discharge during June–

August 2000.

Regulation of sand transport 1137

 2005 International Association of Sedimentologists, No claim to original US government work, Sedimentology, 52, 1133–1153
Shorter-term coupled changes in sandtransport and bed-sand grain size in the

channel

Recent work on the role that bed-sand grain size

plays in regulating sand transport in the Colorado

River has focused on the channel. Over shorter

timescales, the grain size of the sand on the

channel bed evolves quickly in response to large

inputs of sand from the tributaries and in response

to high releases from Glen Canyon Dam (Topping

et al., 2000b). Because the median size of the sand

supplied by tributaries is about 1/4 to 1/2 of the

median size of the sand typically comprising the

channel bed, tributary-supplied sand is more

mobile than the sand on the bed of the channel.

Therefore, newly input sand travels downstream

in the Colorado River as an elongating sediment

wave, with a component in the suspended load,

bedload, and bed sediment. As the front of a

sediment wave propagates downstream, the bed of

the channel fines, and suspended-sand concentrations increase in response to the enriched

upstream supply of finer sand. Under normal

powerplant releases from Glen Canyon Dam, these

sediment waves propagate downstream quickly.

Following the passage of a wave front, the sand on

the bed is winnowed, and suspended-sand

concentrations decrease in response to the depletion of the upstream supply of finer sand. Observations made by Topping et al. (2000b) in Marble

Canyon during 1998 and 1999 suggest that the

sand on the channel bed coarsens back to its pretributary-input antecedent condition within about

6 months during moderate dam releases.

High clear-water releases from Glen Canyon

Dam cause the sand on bed of the channel to

coarsen rapidly. During the 1270 m3 sec)1 1996

controlled flood, the sand on the channel bed and

in suspension coarsened by about a factor of 1Æ5

as the suspended-sand concentrations decreased

(Rubin et al., 1998; Topping et al., 1999, 2000b).

Though the Grand Canyon gage was the only

place where measurements of bed-sand grain size

were made during this flood, the systematic

progressive coarsening and depletion of the sand

in suspension were observed everywhere where

measurements were made (spanning a streamwise distance of over 170 km). During the

2750 m3 sec)1 flood released from Glen Canyon

Dam in 1983, the sand on the bed of the channel

at the Grand Canyon gage coarsened by about a

factor of two (Fig. 4). As during the 1996 controlled flood, the Grand Canyon gage was the only

location where bed-sand grain-size measurements

were made during the peak part of the 1983 flood

1,000

1,500

2,000

2,500

3,000

0·1

0·2

0·3

0·4

0·5

0·6

0·7

0·8

0·9

1·0

1·1

20 21 22 23 24 25 26 27 28 29 30 1 2 3 4 5 6 7 8

Discharge of water

Mean D50 of bed sand

Instantaneous discharge (m3 sec–1)

Cross-sectional mean D50 of 

sand on channel bed (mm) 

June July

1983

Fig. 4. Mean of the median size of the channel-bed sand over the central two-thirds of the cross-section at the Grand

Canyon gage during the post-dam flood of record in June 1983. Gray-shaded region shows the full pre-dam range in

the median size of the bed sand at the Grand Canyon gage. Error bars are one standard error. Data are from Garrett

et al. (1993).

1138 D. J. Topping et al.

 2005 International Association of Sedimentologists, No claim to original US government work, Sedimentology, 52, 1133–1153
(Garrett et al., 1993). Unlike during the 1996

controlled flood, however, measurements of suspended sand concentration and grain size were

not measured at any location until the receding

limb of the 1983 flood.

CHANGES IN BED-SAND GRAIN SIZE

INFERRED FROM b-ANALYSES OF

SUSPENDED-SAND DATA

Between 1983 and 2000, there were many more

measurements of suspended-sand concentration

and grain size made at the Lower Marble Canyon

and Grand Canyon gages than there were direct

measurements of the grain size of the sand on the

bed of the river. Thus, it is advantageous to use

the suspended-sand data to infer temporal changes in bed-sand grain size. Furthermore, because

suspension processes in the river effectively

provide an average ‘sample’ of the sand on the

upstream bed of the channel and the underwater

portions of the eddy sandbars, an appropriate

analysis of the suspended-sand data can yield

information on changes in the grain size of the

sand on the surface of the channel bed and eddy

sandbars upstream from the suspended-sand

measurement location. Rubin & Topping (2001)

developed such a technique to analyse suspended-sediment data based on theory and tested

this technique against data from flumes and

rivers. Their parameter ‘b’ is a nondimensional

measure of the average bed-surface grain-size that

interacts with the suspended sand in the flow. b

uses the concentration and grain size of the sand

in suspension to compute the average upstream

grain size of the sand on the bed of the channel

and the underwater portions of the eddy sandbars. b is defined as

b 1⁄4 Db

Dbm

ð1Þ;

where Db is the median grain diameter of the bed

sand at an instance in time and Dbm is the average

of Db over a specified time interval at the same

location. For broad and narrow bed-sand grainsize distributions and cases with and without

dunes on the bed, Rubin & Topping (2001) found

that

b 1⁄4 C

Cm

 01 Ds

Dsm  02

ð2Þ;

where C is the concentration of sand in suspension at an instance in time, Cm is the average of C

over a specified time interval at the same location,

Ds is the median grain diameter of sand in

suspension at an instance in time, and Dsm is

the average of Ds over a specified time interval at

the same location. This result was computed

using suspended-sediment theory reviewed by

McLean (1992).

In the case where the sand on the channel bed

and eddy sandbars is not armoured by coarser

sediment, changes in b at a given location may

correlate with changes in the volume of sand

stored in the reach upstream. In this case, reductions in b indicate fining of the channel-bed and

eddy-sandbar surfaces interacting with the flow

upstream and suggest an increase in the amount

of sand in storage upstream. Conversely, increases in b indicate winnowing of the inundated

upstream sand deposits and suggest an overall

decrease in the amount of sand in storage

upstream. In the case where the sand on the

channel bed or eddy sandbars may be armoured

or inversely graded, however, changes in b may

be inversely correlated with the overall sand

budget. If the bed is armoured or inversely

graded, erosion will expose finer sand underneath the surface and cause b to decrease even

though the amount of sand in storage has also

decreased.

To determine the relative changes in sand grain

size on the bed of the river in Marble and Upper

Grand Canyons, b was computed using the

method of Rubin & Topping (2001) for all of the

1983–2000 suspended-sand data from the Lower

Marble Canyon and Grand Canyon gages (Fig. 5).

Analysis of variance indicates that the fining

trends in b at the Lower Marble Canyon and

Grand Canyon gages are significant at the

<1Æ0 · 10)16 level and 4Æ9 · 10)10 level, respectively. For this and subsequent statistical analyses,

it is determined that trends or differences in

means are significant when the computed level of

significance is <0Æ05. The fining trends in b at the

two gaging stations are therefore significant.

Segregation of the b time-series at the two gaging

stations into two parts, 1983–1986 (i.e. the period

following the 1983 flood) and 1991–2000, and

analysis of these two parts using a Student’s t-test

comparing the means of two groups with unequal

variance yield similar results. For this and subsequent t-tests, the Student’s t-test comparing the

means of two groups with equal variance is used

when an F-test indicates that the variances of

the two groups cannot be determined to be

different at the 0Æ05 level, and the Student’s t-test

comparing the means of two groups with unequal

Regulation of sand transport 1139

 2005 International Association of Sedimentologists, No claim to original US government work, Sedimentology, 52, 1133–1153
variance is used when an F-test indicates that the

variances of the two groups are different at the 0Æ05

level. The only difference between these two tests

is in the degrees of freedom. The t-test comparing

two groups with unequal variance has many fewer

degrees of freedom than the t-test comparing two

groups of equal variance. At the Lower Marble

Canyon gage, the 1983–1986 b-computed bed sand

was coarser than the 1991–2000 bed sand at the

<1Æ0 · 10)16 level of significance. At the Grand

Canyon gage, the 1983–1986 b-computed bed sand

was coarser than the 1991–2000 bed sand at the

4Æ0 · 10)15 level of significance. The next step in

this study was to therefore determine which bed

environment, the sand on the bed of the channel or

the sand on the bed in the eddies, was most

responsible for this fining trend in b.

METHODS

b-inferred changes in the grain size of bed sand

as a function of elevation

To determine the relative changes in sand grain

size on the bed of the channel and eddies in three

elevation zones in Marble and Upper Grand

Canyons, b was computed using the method of

Rubin & Topping (2001) for all of the 1983–2000

suspended-sand data from the Lower Marble

Canyon and Grand Canyon gages, and then segregated into three discharge intervals (Fig. 6). These

discharge intervals were: (i) flows <250 m3 sec)1

,

(ii) flows from 250 to 700 m3 sec)1

, and (iii) flows

>700 m3 sec)1

. These discharge intervals were

chosen on the basis of river morphology and to

ensure that sufficient suspended-sand data could

be analysed over the entire study period in each

interval. During the study period, flows in the

lowest discharge interval (<250 m3 sec)1

) occurred 6% of the time, flows in the middle

discharge interval (250–700 m3 sec)1

) occurred

65% of the time, and flows in the highest

discharge interval (>700 m3 sec)1

) occurred 29%

of the time (though mostly during the early part of

the study period). By using this approach, changes

in b over time in the lowest discharge interval

could be related to changes in the grain size of the

sand on the bed of the channel and on the lowelevation parts of the eddy-sandbar surfaces.

Changes in b in the middle discharge interval

could be related to a combination of the changes in

0·4

0·5

0·6

0·7

0·8

0·9

1·0

1·1

1·2

1·3

1·4

1·5

1·6

1·7

1·8

1·9

2·0

198319841985198619871988198919901991199219931994199519961997199819992000

Lower Marble Canyon gage

Grand Canyon gage

b

Calendar Year

Fining Coarsening

Fig. 5. b time-series computed from all 1983–2000 suspended-sand data collected at the Lower Marble Canyon and

Grand Canyon gages. The solid line is the linear regression through b at the Lower Marble Canyon gage and the

dashed line is the linear regression through b at the Grand Canyon gage. Both of these fining trends are significant.

1140 D. J. Topping et al.

 2005 International Association of Sedimentologists, No claim to original US government work, Sedimentology, 52, 1133–1153
the grain size of sand on the low-elevation parts of

the bed, plus changes in the grain size of the sand

on the surface of the eddy sandbars within the

range of normal powerplant discharges. Similarly,

changes in b in the highest discharge interval

could be related to a combination of the changes in

the grain size of sand on the low- and midelevation parts of the bed, plus changes in the

grain size of the sand on the surface of the eddy

sandbars that are inundated by only the highest

powerplant releases and in discharges that exceed

powerplant capacity (890 m3 sec)1

) at Glen Canyon Dam.

Changes in the grain size of the sand on the bed

of the channel and on the surface of eddy

sandbars determined from direct observations

Changes in the grain size of the sand on the bed of

the channel were evaluated using two independent datasets: cross-sectionally averaged datasets

collected using USGS BM-54 samplers under the

cableways at the Lower Marble Canyon and

Grand Canyon gages between 1983 and 2000

(Fig. 1), and pipe-dredge datasets collected in

the center of the channel at various locations in

Marble and Upper Grand Canyons on river trips

in September 1984, November 1997, March 1998,

September 1998, May 1999, September 1999, and

May 2000. Each BM-54 measurement consists of a

cross-sectional average of the grain size of the bed

sand sampled at two or more locations across the

channel. These data provide a robust measure of

how the grain size of the sand on the channel bed

evolved over time at two locations, i.e., the

cableways at the Lower Marble Canyon and

Grand Canyon gages. The pipe-dredge data,

though they contain fewer observations at fewer

times than the BM-54 data, have the advantage of

providing a more complete spatial measure of the

sec–1

sec–1

sec–1

sec–1

sec–1

sec–1

Fig. 6. (A) Cartoon showing a typical fan-eddy complex in Marble and Upper Grand Canyons indicating the relative

positions of ‘channel’ and ‘eddy’ environments. Arrows indicate direction of flow. Modified after Schmidt (1990). (B)

Cross-section through A-A¢ in (A) showing the approximate elevations of the three discharge intervals used in the

b-analysis immediately after a high dam release (e.g., the 1983 flood or 1996 controlled flood). (C) Cross-section

through A-A¢ in (A) showing the approximate elevations of the three discharge intervals used in the b-analysis after

subsequent powerplant releases have partially eroded the sandbar.

Regulation of sand transport 1141

 2005 International Association of Sedimentologists, No claim to original US government work, Sedimentology, 52, 1133–1153
changes in the grain size of the sand on the bed of

the channel between the Lees Ferry and Grand

Canyon gages.

Pipe-dredge samples collected during river

trips in September 1984, September 1998, May

1999, September 1999, and May 2000 were used

to evaluate changes in the grain size of the sand

on the lowest-elevation part of the eddy sandbars.

Pipe-dredge data were collected in the centers of

the same 11 eddies in Marble Canyon in September 1998, May 1999, September 1999, and May

2000. During the September 1984 trip, only one

eddy was sampled in the study area. The only

eddy in which data were collected in each of the

five trips between 1984 and 2000 was the eddy on

the left side of the river at river-mile 60Æ5, herein

referred to as the river-mile 60Æ5 eddy.

Changes in the surface grain size of the sand

on the mid- and higher-elevation parts of the

eddy sandbars between 1982 and 2000 were

evaluated using grain-size data that we collected

on river trips in 1996, 1997, and 2000, and using

the eddy-sandbar surface grain-size data of Beus

et al. (1983), Lojko et al. (1984), Lojko (1985,

1987), McKay (1991), and McCutcheon (1992).

The dates, locations, and numbers of samples

used from each dataset are shown in Table 1. To

be sure that samples included in this analysis

were from sandbar surfaces that had recently

interacted with flow in the river, only those

samples from these datasets that were collected

at elevations that had been inundated by dam

operations in the months preceding each sampling trip were used.

Samples collected from four trenches excavated

through 1983 flood deposits were used to help

evaluate the changes in the surface grain size of

eddy sandbars during the course of the 1983

flood. The deposits from this flood comprise a

bench that typically rises 1–2 m above the bench

created by the more recent 1996 controlled flood.

RESULTS AND DISCUSSION

b-inferred changes in the surface grain size of

bed sand as a function of elevation

Analyses of the discharge-binned b time-series

indicate that the greatest universal fining of the

bed sand between 1983 and 2000 occurred at midand high-elevations (Fig. 7). The average significant fining in b among the Lower Marble Canyon

and Grand Canyon gages was 9% in the lowest

discharge interval, 19% in the middle discharge

interval, and 20% in the highest discharge interval. Therefore, the surfaces of the eddy sandbars

probably fined to a greater degree than did the

sand on the bed of the channel.

In the lowest discharge interval, b at the Lower

Marble Canyon gage fined by about 18%, whereas

b at the Grand Canyon gage remained constant between 1985 and 2000 (Fig. 7A). Analysis

of variance indicates that the fining trend in

Table 1. Dates, locations, numbers of samples, and sources of data for sampled eddy-sandbar surfaces in Marble and

Upper Grand Canyons.

Sampling date

River-mile locations of

sampled sandbar surfaces Source of data

Number of

samples

7-24-1982 to 8-11-1982 8, 20, 22, 29Æ3, 43, 53, 68, 76 Beus et al. (1983) 8

7-29-1983 to 8-7-1983 8, 20, 29Æ3, 34Æ8, 43, 53, 59, 61Æ5,

65Æ5, 72Æ2, 75Æ5, 81Æ3

Lojko et al. (1984) 32

8-1-1984 to 8-11-1984 8, 20, 29Æ3, 34Æ7, 43, 53, 53Æ5, 61Æ8,

65Æ5, 72Æ2, 75Æ5, 81Æ3

Lojko (1985) 23

July–August 1985 8, 20, 29Æ3, 34Æ7, 53, 75Æ5 Lojko (1987) 7

July–August 1986 8, 20, 29Æ3, 34Æ7, 53, 75Æ5, 81Æ3 Lojko (1987) 8

July–August 1991 43 McKay (1991) 6

July–August 1992 8, 34Æ7, 53, 61Æ5, 75Æ5, 81Æ3 McCutcheon (1992) 10

April 1996 48, 63, 69Æ5 Rubin et al. (1998) 3

11-7-1997 to 11-14-1997 8, 22, 30Æ5, 43, 47, 55Æ5,

62Æ6, 68, 81Æ3

Topping et al. (2000b) 12

5-12-2000 8Æ8, 21Æ8, 30Æ5 Topping & Rubin at

http://www.gcmrc.gov

3

8-19-2000 to 8-26-2000 1, 29Æ3, 29Æ9, 30Æ1, 43, 43Æ3,

43Æ4, 44Æ4, 59Æ9, 60Æ2, 63Æ3

Topping & Rubin at

http://www.gcmrc.gov

13

9-8-2000 to 9-16-2000 1, 29Æ3, 43, 44, 60, 60Æ3, 62Æ5, 63, 65 Topping & Rubin at

http://www.gcmrc.gov

17

1142 D. J. Topping et al.

 2005 International Association of Sedimentologists, No claim to original US government work, Sedimentology, 52, 1133–1153
low-discharge b at the Lower Marble Canyon gage

is significant at the 7Æ9 · 10)12 level. Segregation of

the b time-series at the Lower Marble Canyon gage

into two parts, 1985–1986 and 1991–2000, and

analysis of these two parts using a Student’s t-test

comparing the means of two groups with equal

variance yields a similar result: low-discharge b

was coarser in 1985–1986 than it was in 1991–2000

b

Lower Marble Canyon gage

Grand Canyon gage

b

Calendar Year

Calendar Year

250-700 m3 sec–1 Discharge 1983 1984 1985 1986 1987 1988 1989

19901991199219931994199519961997199819992000

Lower Marble Canyon gage

Grand Canyon gage

b

Calendar Year

>700 m3 sec–1 Discharge 

(A)

(B)

(C)

0·4

0·5

0·6

0·7

0·8

0·9

1·0

1·1

1·2

1·3

1·4

1·5

1·6

1·7

1·8

1·9

2·0

Lower Marble Canyon gage

Grand Canyon gage

< 250 m3 sec–1 Discharge 1983 1984 1985 1986 1987 1988 1989

19901991199219931994199519961997199819992000

1983

1984

1985

1986

1987

1988

1989

19901991199219931994199519961997199819992000

0·4

0·5

0·6

0·7

0·8

0·9

1·0

1·1

1·2

1·3

1·4

1·5

1·6

1·7

1·8

1·9

2·0

0·4

0·5

0·6

0·7

0·8

0·9

1·0

1·1

1·2

1·3

1·4

1·5

1·6

1·7

1·8

1·9

2·0

Fining Coarsening Fining Coarsening Fining Coarsening

Fig. 7. 1983–2000 b time-series from

Fig. 5 segregated into three discharge

intervals. Solid lines are the linear

regressions through b at the Lower

Marble Canyon gage; dashed lines

are the linear regressions through b at

the Grand Canyon gage. (A) b timeseries for the lowest discharge interval (<250 m3 sec)1

). The fining trend

at the Lower Marble Canyon gage is

significant. At the Grand Canyon

gage, no significant trend exists in

b at discharges less than

250 m3 sec)1

. (B) b time-series for

the middle discharge interval

(250–700 m3 sec)1

). The fining

trends at both gaging stations are

significant. (C) b time-series for the

highest discharge interval

(>700 m3 sec)1

). The fining trends at

both gaging stations are significant.

Regulation of sand transport 1143

 2005 International Association of Sedimentologists, No claim to original US government work, Sedimentology, 52, 1133–1153
at the 1Æ8 · 10)12 level of significance. Analysis of

variance indicates that the fining trend in lowdischarge b at the Grand Canyon gage is significant

at only the 0Æ36 level, and is therefore not significant. Segregation of the b time-series at the Grand

Canyon gage into two parts, 1985–1986 and 1991–

2000, and analysis of these two parts using a

Student’s t-test comparing the means of two groups

with unequal variance yields the identical result:

low-discharge b in 1985–1986 is different than

low-discharge b in 1991–2000 at only the 0Æ36 level

of significance. Because low-discharge b remained

constant at the Grand Canyon gage, the average

significant fining of the low-elevation bed sand

among the two gaging stations was about 9%.

In the middle discharge interval, b at the Lower

Marble Canyon gage fined by about 21%, and b at

the Grand Canyon gage fined by about 17%

between 1983 and 2000 (Fig. 7B). Analysis of

variance indicates that the fining trends in middischarge b at the Lower Marble Canyon and

Grand Canyon gages are both significant at the

<1 · 10)16 level. Segregation of the b time-series

at the two gaging stations into two parts, 1983–

1986 and 1991–2000, and analysis of these two

parts using a Student’s t-test yield identical

results. Based on the results of F-tests, a t-test

comparing the means of two groups with unequal

variance was used on the Lower Marble Canyon

gage data; and a t-test comparing the means of two

groups with equal variance was used on the

Grand Canyon gage data. At both the Lower

Marble Canyon and Grand Canyon gages, middischarge b was coarser in 1983–1986 than it was

in 1991–2000 at the <1 · 10)16 level of significance. The average significant fining of the

combined low- and mid-elevation bed sand was

therefore about 19%, based on the changes in

mid-discharge b at the two gaging stations.

In the highest discharge interval, b at the Lower

Marble Canyon gage fined by about 16%, and b at

the Grand Canyon gage fined by about 23%

between 1983 and 2000 (Fig. 7C). Analysis of

variance indicates that the fining trend in highdischarge b at the Lower Marble Canyon gage is

significant at the 3Æ8 · 10)12 level, and the fining

trend in high-discharge b at the Grand Canyon

gage is significant at the <1 · 10)16 level. Segregation of the b time-series at the two gaging

stations into two parts, 1983 and 1996–2000, and

analysis of these two parts using a Student’s t-test

comparing the means of two groups with unequal

variance yield similar results. At the Lower

Marble Canyon gage, high-discharge b was coarser in 1983 than it was in 1996–2000 at the

6Æ2 · 10)6 level of significance; at the Grand

Canyon gage, high-discharge b was coarser in

1983 than it was in 1996–2000 at the <1 · 10)16

level of significance. The average significant

fining in the combined low-, mid-, and highelevation bed sand was therefore about 20%,

based on the changes in high-discharge b at the

two gaging stations.

In addition to the fining in b during the late

1980s and early 1990s, b fines with increasing

discharge at any given time (Fig. 8). This occurs

because, as the discharge in the river increases,

more finer sand on the bed at higher elevations is

inundated by the flow and is therefore available

to go into suspension. Rubin & Topping (2001)

reported this type of behaviour for the Colorado

River (pre- and post-dam) and two alluvial rivers

(the Paria and Mississippi Rivers). Also evident

in Fig. 8 is the result that, although b fined at low

discharges at the Lower Marble Canyon gage, it

remained constant at low discharges at the Grand

Canyon gage between 1986 and 1991.

Changes in the grain size of the sand on the bed

of the channel

Despite the short-term large changes in the grain

size of sand on the bed of the channel described

in the introduction, the grain size of the sand on

the channel bed has either not changed or has

coarsened by about 10–20% during the 17 years

following the 1983 flood. Although this result is

in agreement with the result from the lowdischarge b-analysis at the Grand Canyon gage

(that showed no significant change in the grain

size of the low-elevation bed sand), it is in

disagreement with the fining observed in the

low-discharge b-analysis at the Lower Marble

Canyon gage. Thus, the average 9% fining

between 1983 and 2000 observed in the lowdischarge b-analyses between the two gaging

stations was probably not due to fining of the

sand on the channel bed.

Analyses of variance of the cross-sectionally

averaged BM-54 datasets from the Lower Marble

Canyon and Grand Canyon gages indicate that the

sand on the channel bed at both of these locations

coarsened slightly (by 10–20%) from 1983

through 2000 (Fig. 9). The large variability in

channel-bed grain size at each gaging station

within each year results from the short-term

changes in channel-bed grain size described in

the Introduction, with fining of the bed during

periods of upstream tributary resupply of finer

sand and winnowing of the bed either after

1144 D. J. Topping et al.

 2005 International Association of Sedimentologists, No claim to original US government work, Sedimentology, 52, 1133–1153
cessation of upstream tributary activity or during

periods of high dam releases. During 1983–2000,

the cross-sectional mean of the median size of the

sand on the channel bed at the Lower Marble

Canyon gage coarsened from about 0Æ36 mm to

0Æ40 mm and the cross-sectional mean of the

median size of the sand on the channel bed at the

Grand Canyon gage coarsened from about

0Æ37 mm to 0Æ43 mm. Owing to the large number

of observations, these slight coarsening trends are

significant at the 0Æ0073 and 2Æ4 · 10)7 levels,

respectively. Segregation of the time-series at the

two gaging stations into two parts, 1983–1986 and

1996–2000, and analysis of these two parts using

a Student’s t-test comparing the means of two

groups with unequal variance yield similar

results. The sand on the channel bed at the

Marble Canyon gage was coarser in 1996–2000

than it was during 1983–1986 at the 0Æ00011 level

of significance, and the sand on the channel

bed at the Grand Canyon gage was coarser in

1996–2000 than it was during 1983–1986 at

the 0Æ00043 level of significance. Furthermore,

although comparable with the coarsest pre-dam

value (0Æ4 mm), the cross-sectional mean of

median size of the channel-bed sand at the Grand

Canyon gage during 1983–2000 was 90% coarser

than the finest pre-dam value (0Æ2 mm) and 27%

coarser than the mean pre-dam value (Fig. 9).

Analyses of the seven pipe-dredge datasets

shows that, despite substantial short-term fining

of the channel-bed sand in uppermost Marble

Canyon following floods on the Paria River, there

is no significant difference between the grain size

of the sand on the channel bed measured on each

of the river trips from September 1984 through

May 2000 (Fig. 10). Analysis of variance of the

data in Fig. 10A indicates that, at the 0Æ11 level of

significance, variance is minimized when the

1984–2000 pipe-dredge data are treated as a

single dataset rather than seven different datasets.

In other words, at the 0Æ11 level of significance,

0·4

0·5

0·6

0·7

0·8

0·9

1·0

1·1

1·2

1·3

1·4

1·5

1·6

1·7

1·8

1·9

2·0

0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 2,200 2,400 2,600

1983–1986 

1991–2000 

b

Instantaneous Discharge (m3sec–1)

P < 1x10–16

0·4

0·5

0·6

0·7

0·8

0·9

1·0

1·1

1·2

1·3

1·4

1·5

1·6

1·7

1·8

1·9

2·0

0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000

1983–1986 

1991–2000 

b

Instantaneous Discharge (m3sec–1)

P < 1x10–16

P = 2·1x10–8

(A)

(B)

P < 1x10–16

Fining Coarsening Fining Coarsening

Fig. 8. b as a function of the

discharge of water at (A) the Lower

Marble Canyon gage, and (B) the

Grand Canyon gage. Solid lines are

the linear regressions through the

1983–1986 data; dashed lines are

the linear regressions through the

1991–2000 data. Levels of significance (p) of the trends in b as a

function of discharge are indicated.

Regulation of sand transport 1145

 2005 International Association of Sedimentologists, No claim to original US government work, Sedimentology, 52, 1133–1153
the variance about a single linear regression fit to

all of the 1984–2000 data is less than the variance

about different linear regressions fit to the data

from each of the seven trips. Therefore, because

this level of significance is >0Æ05, it cannot be

concluded that the 1984–2000 data in Fig. 10A

are seven different datasets. Although the longitudinal mean of the median sizes of the channelbed sand was slightly finer in the late 1990s than

it was in September 1984, analysis of variance

indicates that the slight fining trends in the

longitudinal mean of the median grain sizes of

the channel-bed sand in Marble and Upper Grand

Canyons are not significant (Fig. 10B).

Segregation of the data in Fig. 10B into two parts,

1984 and combined 1997–2000, and analysis of

these two parts from Marble and Upper Grand

Canyons using a Student’s t-test yields a slightly

different result. In Marble Canyon, analysis using a

t-test comparing the means of two groups with

equal variance indicates that the longitudinal

mean of the median grain size of the channel-bed

sand in the combined 1997–2000 data set is slightly

finer than it was during the 1984 river trip, at the

0Æ046 level. Because this level of significance is

essentially identical to the 0Æ05 level set as the

threshold for significance in the analyses, this

difference may or may not be significant. In any

case, the slight difference between the 1984 and

combined 1997–2000 Marble Canyon data is the

result of the short-term fining of the channel bed in

September 1999 following large Paria River floods.

In Upper Grand Canyon, analysis using a t-test

comparing the means of two groups with unequal

variance indicates that the longitudinal mean of

the median grain size of the channel-bed sand in

the combined 1997–2000 data set is slightly

finer than it was during the 1984 river trip, but

at only the 0Æ71 level (a difference that is not

significant).

0·1

0·2

0·3

0·4

0·5

0·6

0·7

0·8

0·9

1983198419851986

1987

198819891990

1991

1992

19931994

19951996

1997

1998

1999

2000

Calendar Year

Cross-sectional mean D50 of

sand on channel bed (mm)

Calendar Year

(A)

(B)

198319841985

1986

1987

1988

19891990

1991

19921993

199419951996

19971998

1999

2000

0·1

0·2

0·3

0·4

0·5

0·6

0·7

0·8

0·9

Cross-sectional mean D50 of

sand on channel bed (mm)

Fig. 9. Temporal variation in sand

grain size on the channel bed at the

Lower Marble Canyon and Grand

Canyon gages from 1983 through

2000. The cross-sectional mean of

the median sizes is computed by

averaging the median grain sizes of

the sand from two or more samples

collected across the central twothirds of the cross-section. Error bars

are one standard error. (A) Crosssectional mean of the median sizes

of the sand on the bed at the Lower

Marble Canyon gage. Solid line is

the linear regression through these

data. (B) Cross-sectional mean of the

median sizes of the sand on the bed

at the Grand Canyon gage. Grayshaded region shows the full predam range in the median size of the

bed sand at this location. Solid line

is the linear regression through

these data. All data in (A) and (B)

were collected using a BM-54

sampler except for the data in September 1998 and May 1999, which

were collected using a pipe dredge.

Data are from Garrett et al. (1993)

and http://www.gcmrc.gov.

1146 D. J. Topping et al.

 2005 International Association of Sedimentologists, No claim to original US government work, Sedimentology, 52, 1133–1153
Changes in the surface grain size of sand on

the lowest-elevation parts of eddy sandbars

Pre-1997 data from the lowest-elevation parts of

eddy sandbars are extremely sparse, and it is

impossible to know whether any major change in

the grain size of sand on the lowest-elevation

parts of eddy sandbars occurred between 1984

and 2000 (Fig. 11). Analysis of the data from the

one eddy sampled throughout the study period

(i.e. the river-mile 60Æ5 eddy) indicates that the

grain size of sand on the lowest-elevation part of

this eddy sandbar was relatively coarse and of

similar size during the September 1984, September 1998, and September 1999 river trips, and was

relatively fine and of similar size during the May

1999 and May 2000 river trips (Fig. 11B). Finally,

despite the observation in Fig. 11A that sand

0·2

0·3

0·4

0·5

0·6

0·7

0·8

198319841985

19861987

19881989

1990

1991199219931994

19951996

1997

1998

19992000

Marble Canyon 

Upper Grand canyon 

Longitudinal mean D50 of

sand on channel bed (mm)

Calendar Year

P = 0·078

P = 0·23

0·1

0·2

0·3

0·4

0·5

0·6

0·7

0·8

0·9

1·0

1·1

1·2

1·3

1·4

0 10 20 30 40 50 60 70 80 90

Sept 1984

Nov 1997

March 1998 

Sept 1998

May 1999

Sept 1999

May 2000

D50 of sand on channel bed (mm)

River Mile

(A)

(B)

Fig. 10. Longitudinal and temporal variation in sand grain size on the channel bed between the Lees Ferry and

Grand Canyon gages from 1984 through 2000 determined from pipe-dredge data. (A) Longitudinal variation in the

median size of the channel-bed sand between the Lees Ferry and Grand Canyon gages during seven river trips

between 1984 and 2000. Solid and dashed lines are the linear regressions through the data from each of the river

trips. Analysis of variance indicates that the data from the seven trips are not significantly different and that variance

is minimized when the 1984–2000 pipe-dredge data are treated as a single data set. Gray-shaded region shows the

pre-dam range in the median size of channel-bed sand interpolated between the measurements at the Lees Ferry and

Grand Canyon gages. (B) Temporal variation in the longitudinal mean of the median grain sizes in (A) for Marble and

Upper Grand Canyons. Error bars are one standard error. Solid line is the linear regression through the longitudinal

mean of the median grain sizes of the channel-bed sand in Marble Canyon and the dashed line is the linear regression

through the longitudinal mean of the median grain sizes of the channel-bed sand in Upper Grand Canyon. Levels of

significance (p) of the slight fining trends are indicated. At the 0Æ05 level, no significant trend exists in the longitudinal mean of the median grain sizes of the channel-bed sand between 1984 and 2000.

Regulation of sand transport 1147

 2005 International Association of Sedimentologists, No claim to original US government work, Sedimentology, 52, 1133–1153
sampled on the lowest-elevation parts of eddy

sandbars in uppermost Marble Canyon tended to

be finer in September after new inputs of sand

from the Paria River (as was the case on the

channel bed in the previous section), there is no

statistically significant difference in the longitudinal mean of the median grain sizes on the lowestelevation parts of the eddy sandbars among the

11 eddies sampled in Marble Canyon between

September 1998 and May 2000 (Fig. 11B).

Changes in the surface grain size of sand on

the mid- and higher-elevation parts of eddy

sandbars

Substantial changes in the mean median grain size

of the eddy-sandbar surfaces at mid- and highelevations occurred in Marble and Upper Grand

Canyons between August 1982 and September

2000 (Fig. 12A). The eddy sandbar surfaces

coarsened during the 1983 flood, remained coarse

through 1986, fined between 1986 and 1991, and

then remained relatively fine through 2000. The

mid- and high-elevation parts of the eddy sandbars are therefore the only riverbed environment

that tracks with the fining observed in b between

1983 and 2000. In addition, the discharge-binned

b-analyses presented above indicate that the

greatest amount of fining over time in the bed

sand ‘sampled’ by the suspended sand occurred at

the highest elevations. The grain size of the eddysandbar surfaces was thus the most important

regulator of sand transport during the study

period.

The 1983 flood deposited coarsening-upward

flood deposits, causing the surface grain size of

the eddy sandbars to coarsen substantially and

significantly. Subsequent surface grain-size data

collected by Lojko (1985, 1987), McKay (1991),

and McCutcheon (1992) indicate that the regularly inundated surfaces on the eddy sandbars

remained coarse through at least August 1986,

0·1

0·2

0·3

0·4

0·5

Longitudinal mean among 11 Marble Canyon eddies 

River-mile 60·5 eddy

Sept. 1998

Sept. 1999

Sept. 1984

May 1999

May 2000

0·1

0·2

0·3

0·4

0·5

0·6

0·7

0 10 20 30 40 50 60 70 80 90

Sept 1984

Sept 1998

May 1999

Sept 1999

May 2000

River Mile

1983

19841985

1986

19871988

19891990

1991

1992

1993

19941995

1996

1997

19981999

2000

Calendar Year

D50 of sand on the lowest-elevation

parts of eddy sandbars (mm)

D50 of sand on the lowest-elevation

parts of eddy sandbars (mm)

(A)

(B) Fig. 11. Longitudinal and temporal

variation in sand grain size on the

lowest-elevation parts of eddy

sandbars in Marble Canyon determined from pipe-dredge data. Error

bars are one standard error.

(A) Longitudinal variation in the

median size of the sand on the

lowest-elevation parts of the 11

eddy sandbars sampled between

1984 and 2000. Two samples were

collected in September 1984 from

the sandbar in the river-mile 60Æ5

eddy. (B) Temporal variation in the

longitudinal mean of the median

grain sizes in (A), and the median

grain size of the sand on the lowestelevation part of the sandbar in the

only eddy sampled in 1984 and in

1998–2000, the river-mile 60Æ5 eddy.

1148 D. J. Topping et al.

 2005 International Association of Sedimentologists, No claim to original US government work, Sedimentology, 52, 1133–1153
and began fining in the late 1980s and early

1990s. In August 1982, the mean median grain

size of the sand on the surface of the eddy

sandbars was 0Æ14 mm (Beus et al., 1983). This

value is within the 0Æ12–0Æ19 mm range of median

grain sizes of the sand on the surfaces of eddy

sandbars in the mid-1970s reported by Howard &

Dolan (1981). Following the 1983 flood, the mean

median grain-size of sand on the eddy sandbars in

Marble and Upper Grand Canyons coarsened by a

factor of 1Æ8 to a value of 0Æ25 mm (Beus et al.,

1983; Lojko et al., 1984). Analysis of the 1982 and

1983 data using a Student’s t-test comparing the

means of two groups with equal variance indicates that the coarsening of the eddy sandbar

surfaces during the 1983 flood was significant at

the 3Æ3 · 10)7 level. An identical degree of average coarsening was observed in four trenches

excavated through 1983 flood deposits in Marble

and Upper Grand Canyons (Fig. 12B).

The similarity of the magnitude and direction

of the changes in the grain size of the eddysandbar surfaces with the observed factor of two

coarsening of the channel-bed sand at the Grand

Canyon gage during the 1983 flood (Fig. 4) suggests that the sand supplied to the eddy sandbars

during the 1983 flood coarsened through time as

the bed of the channel was winnowed. Thus, it is

likely that the coarsening of the eddy sandbar

surfaces during the 1983 flood was due to the

same processes observed by Rubin et al. (1998)

and Topping et al. (1999) during the 1996 controlled flood and Topping et al. (2000b) during

the 2-day 890 m3 sec)1 powerplant-capacity release in November 1997. Coarsening-upward

eddy deposits were also produced during two

4-day powerplant-capacity releases in 2000 (data

available from http://www.gcmrc.gov). During all

of these high-discharge releases from the dam, the

upstream supply of sand decreased, causing the

bed to winnow. This coarsening of the channel

bed, in turn, caused the sand available to be

deposited in eddies to coarsen, resulting in

coarsening-upward flood deposits and a general

coarsening of the eddy-sandbar surfaces.

Grain-size data collected from eddy-sandbar

surfaces during the late 1990s indicate that, after

the eddy-sandbar surfaces fined between 1986

and the early 1990s, these surfaces remained

relatively fine through 2000. Analysis of the

combined 1983–1986 and combined 1991–2000

data using a Student’s t-test comparing the means

of two groups with unequal variance indicates

that the fining of the bar surfaces between 1986

and 1991 was significant at the 2Æ6 · 10)11 level.

The fining of the bar surfaces in the late 1980s

was probably due to three different processes: (i)

lateral erosion through the coarsening upward

1983 flood deposits, (ii) reworking or addition of a

thin layer of finer sand supplied from upstream

eddy sandbars and tributaries, and (iii) less

depletion of the upstream supply of finer sand

during the 1996 controlled flood, and 1997 and

2000 powerplant-capacity releases relative to

during the much larger 1983 flood.

The area of eddy sandbars decreased by gradual

lateral erosion from the mid-1980s to just prior to

the 1996 controlled flood (Schmidt et al., 2004).

Rubin et al. (1994) and Barnhardt et al. (2001)

showed that the basal contact of the 1983 flood

deposit generally slopes offshore at a gentle angle

0·10

0·15

0·20

0·25

0·30

0·35

1982

1983

1984

1985198619871988198919901991

1992

1993

1994

19951996199719981999

2000

 Mean D50 of sand on surface of 

eddy sandbars (mm)

Calendar Year

(A)

(B)

0·5 1·0 1·5 2·0 2·5 3·0

0·0

0·1

0·2

0·3

0·4

0·5

0·6

0·7

0·8

0·9

1·0

River-mile 43·2

River-mile 51·8

River-mile 61·0

River-mile 70·4

Sand D50 normalized by sand D50 at base of deposit Line of zero grain-size change

Normalized height within deposit

(measured from base to top)

Fig. 12. (A) Mean median size of the sand on the surface of eddy sandbars in Marble and Upper Grand

Canyons from 1982 through 2000. Error bars are one

standard error. (B) Normalized median grain size of

sand measured vertically through four 1983 flood

deposits in Marble and Upper Grand Canyons. Median

grain sizes at each elevation were normalized by dividing by the median grain size at the base of the deposit.

These four deposits coarsened upward by a mean factor

of 1Æ8, in agreement with the observed factor of 1Æ8

coarsening of the eddy sandbar surfaces between 1982

and 1983 in (A).

Regulation of sand transport 1149

 2005 International Association of Sedimentologists, No claim to original US government work, Sedimentology, 52, 1133–1153
in the eddy sandbars, and that the 1983–1986

high-flow deposits overlay older pre-1983 deposits. Therefore, as lateral erosion progressed during

the late 1980s and early 1990s, the coarser

surfaces of the 1983–1986 eddy sandbars were

gradually removed exposing the finer sand at

progressively greater depths and lateral distances

within the 1983 flood deposits (Fig. 13). This

newly exposed finer sand was then available for

downstream transport and could be added to

downstream eddy sandbars. Similarly, new finer

sand supplied from upstream tributaries and

deposited on the eroding 1983–1986 eddy-sandbar surfaces caused additional fining of the

sandbar surfaces (e.g. the deposits from the 1993

Little Colorado River floods described by Rubin

et al., 1994).

Available USGS bed grain-size data collected

during the 1996 controlled flood, and during the

1997 and 2000 powerplant-capacity releases indicate that the sand on the bed of the channel never

coarsened during these smaller floods to the

degree that it did during the 1983 flood. Likewise,

available USGS suspended-sand data indicate

that, throughout the 1996 controlled flood, suspended-sand concentrations were higher and

suspended-sand grain sizes were finer than during the larger 1983 flood. Similarly, USGS suspended-sand data indicate that, throughout the

peaks of the much lower 1997 and 2000 powerplant-capacity releases, suspended-sand concentrations were comparable with those measured

during the peak of the 1983 flood, and suspendedsand grain sizes were finer than during the peak

of the 1983 flood. Thus, although the upstream

sand supply decreased during all five high clearwater dam releases, the upstream supply of sand

became more depleted during the 1983 flood than

during the 1996 controlled flood or the 1997 and

2000 powerplant-capacity releases. Therefore, the

sand available to be transported from the channel

into eddies at the peak of the 1983 flood was

much coarser than that available to be transported

into eddies during the 1996, 1997, and 2000 highdischarge releases.

CONCLUSIONS

This study shows that b-analyses of suspendedsediment data can be used in conjunction with

analyses of surface grain-size data to deduce

which environments in a complicated setting

are the most important environments for regulating sediment transport, regardless of whether

these environments comprise a relatively large

or small part of the total environment. In the

case of the sand-supply-limited Colorado River

in Marble and Upper Grand Canyons, the bed

environment that is the dominant regulator of

sand transport in the river over multi-year

timescales, the eddy environment, comprises

only a small percentage of the total area of

the river.

In Marble and Upper Grand Canyons, grain-size

data indicate that the only environment in which

the grain size of the bed sand substantially

changed over multi-year timescales was the surpre-1983 sand

colluvium

Initial sandbar surface after 1983 flood

Sandbar surface (dashed)

after partial erosion of 

coarsening-upward 

1983 flood deposit 

exposes underlying 

finer sand

coarsening-upward 

 1983 flood deposit

Colorado River

Fig. 13. Cartoon cross-section idealized from Rubin et al. (1994) and Barnhardt et al. (2001) showing an eroding

eddy sandbar capped by a coarsening-upward 1983 flood deposit. The initial sandbar surface after the 1983 flood is

relatively coarse. Subsequent lateral erosion of the sandbar exposes the finer sand at progressively greater depths

within the 1983 flood deposit.

1150 D. J. Topping et al.

 2005 International Association of Sedimentologists, No claim to original US government work, Sedimentology, 52, 1133–1153
face of the eddy sandbars. As the upstream

supply of sand became depleted during the 1983

flood, the sand in suspension and on the bed of

the channel coarsened. This led to the production

of coarsening-upward flood deposits in eddies.

As a result of this process, the eddy-sandbar

surfaces coarsened by about a factor of 1Æ8 during

the 1983 flood and remained relatively coarse

through the high flows of 1984, 1985, and 1986.

Then, as the eddy sandbars eroded, exposing

underlying finer sand (and mixing in finer sand

supplied from upstream), the eddy-sandbar surfaces fined such that the sandbar surfaces in

1991–2000 were not substantially coarser than

they were in either 1982 or the mid-1970s.

b-analyses at both the Lower Marble Canyon

and Grand Canyon gages suggest that the dominant signal in the suspended-sand data collected

since 1983 has been the fining of the eddysandbar surfaces as the bars eroded between the

mid-1980s and the early 1990s. This finding has

major implications for sand transport in Marble

and Upper Grand Canyons. Between 1986 and the

early 1990s, the median size of the sand on the

bar surfaces decreased by about 30–40%. Topping

et al. (2000b, fig. 18) showed that, for a narrow

grain-size distribution in deeper water, this magnitude of change in bed grain size corresponds to

about a factor of two increase in the concentration

of sand in suspension over this surface. For the

more general case in shallower water, Rubin &

Topping (2001) showed that this effect would be

only slightly smaller. Thus, as the eddy-sandbar

surfaces became finer, substantially more sand

could be carried in suspension over the eddy

sandbars. This increase in flux over the eddysandbar surfaces as the bar surfaces fined can lead

to greater deposition and erosion rates in the

eddies depending on the details of the flow fields

in the eddies (Schmidt et al., 1993; Nelson et al.,

1994, 2003; Nelson & McDonald, 1995; Rubin

et al., 1998; Wiele et al., 1999). Analysis of aerial

photographs (Schmidt et al., 2004) and side-scansonar data (Anima et al., 1998; Wong et al., 2003)

indicate that eddy sandbars comprise only about

12% of the riverbed area (sandbars comprise

somewhat greater than 60% of the eddies by area,

which comprise about 20% of the river by area).

Despite covering a small percentage of the riverbed area, the eddy sandbars in Marble and Upper

Grand Canyons are the dominant storage environment for finer sand in the regulated post-dam

river (Hazel et al., in press). Furthermore, the

grain size of the surfaces of these eddy sandbars is

the dominant multi-year regulator of suspendedsand transport under normal powerplant releases

from Glen Canyon Dam.

ACKNOWLEDGEMENTS

Funding for this study was provided by the Bureau

of Reclamation through the USGS Grand Canyon

Monitoring and Research Center. This study was

motivated by conversations with Randy Peterson

(Bureau of Reclamation). Richard Wilson and

Julie Graf (USGS) collected the September 1984

pipe-dredge data used in this study. Sara Goeking

(Utah State University) provided her analyses of

side-scan sonar data collected in 2000 by Roberto

Anima (USGS). Ted Melis and Steve Wiele provided valuable input and help during the 1997–2000

fieldwork. Logistical support for fieldwork on the

Colorado River in Grand Canyon was provided by

Humphrey Summit Support. We would like to

thank Amy Draut, Ted Melis, Jon Nelson, and Ned

Andrews for helpful reviews of earlier versions

of this manuscript.

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