WSP05649

• File Number: 8220.101.10.D
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 2/1/2000
• Author: Topping et al.
• Title: Colorado River Sediment Transport
• Water Supply Pro - Doc Type: Report/Study

WATER RESOURCES RESEARCH, VOL. 36, NO.2, PAGES 54>-570, FEBRUARY 2000

Colorado River sediment transport
2. Systematic bed-elevation and grain-size effects
of sand supply limitation

David J. Topping,' David M. Rubin,> Jonathan M. Nelson,' Paul J. Kinzel Ill,'
and Ingrid C. Corson4

Abstract. The Colorado River in Marble and Grand Canyons displays evidence of annual
supply limitation with respect to sand boIh prior to [Topping el aI., this issue] and after Ihe
closure of Glen Canyon Dam in 1963. Systematic changes in bed elevation and systematic
coupled changes in suspended-sand conrentration and grain size result from this supply
limitation. During floods, sand supply limitation either causes or modifies a lag between
the time of maximum discharge and the time of either maximum or minimum (depending
on reach geometry) bed elevation. If, at a cross section where the bed aggrades with
increasing flow, the maximum bed elevation is observed to lead the peak or the receding
limb of a flood, then this ohserved response of Ihe bed is due 10 sand supply limitation.
Sand supply limitation also leads to the systematic evolution of sand grain size (both on
the bed and in suspension) in the Colorado River. Sand input during a tributary flood
travels down the Colorado River as an elongating sedimenI wave, wiIh the finest sizes
(because of their lower settling velocities) traveling the fastest. As the fine front of a
sediment wave arrives at a given location, the bed fines and suspended-sand
concentrations increase in response to the enhanred upstream supply of finer sand. Then,
as the front of the sediment wave passes that location, the bed is winnowed and
suspended:sand concentrations decrease in response to the depletion of the upstream
supply of finer sand. The grain-size effects of depletion of the upstream sand supply are
most obvious during periods of higher dam releases (e.g., the 1996 flood experiment and
the 1997 test flow). Because of substantial changes in the grain-size disIribution of the
bed, stable relationships between the discharge of water and sand-transport rates (i.e.,
stable sand rating curves) are precluded. Sand budgets in a supply-limited river like the
Colorado River can only be construcIed through inclusion of the physical processes that
couple changes in bed-sediment grain size 10 changes in sand-transport rates.

1. Introduction

In some rivers the upstream supply of sediment is in equi-
librium with the upstream supply of water, whereas in others,
the upstream supply of sediment is decoupled, either com-
pletely or partially, from the upstream supply of water. In the
first type of river, changes in sediment transport are controlled
by changes in the discharge of water, whereas in the second
(and perhaps more common) type of river, changes in sedi-
ment transport are also coupled to changes in sediment grain
size. In this paper we investigate the systematic changes in bed
elevation, sediment transport, and sediment grain size that
occur in response to changes in the upstream supply of sand in
a river with an intermillentlimited supply of sand, specifically
the Colorado River in Marble and Grand Canyons (Figure 1).
To develop an intuitive understanding of the linkage be-
tween sediment grain size and the upstream supply of sediment
in a river, it is informative to first examine sediment-transport

IU.S. Geological Survey, RestoD, Virginia.
2U.S. Geological Survey, Menlo Park., California.
JU.S. Geological Survey, Denver, Colorado.
4Department or Geography, University or Colorado, Boulder.
Copyright 2000 by the American Geophysical Union.
Paper number 1999WR 900286.
0043-13971OO/1999W R 900286S09.00

Dume experiments. N discussed by Parker and Wikock [1993],
these experiments typically fall into two categories: (I) those
using sediment-recirculating flumes (in which the water and
sediment are reintroduced at the upstream end of the flume at
the same rate that they leave the downstream end) and (2)
those using sediment-feed flumes (in which the sediment is
supplied at the upstream end of the flume in a manner de-
coupled from the rate at which the sediment leaves the down-
stream end). In sediment-recirculating flumes the sediment
supply at the upstream end of the flume equals the sediment
export at the downstream end of the flume. Thus these flumes
are like rivers in which the upstream supply of sediment is in
equilibrium with the upstream supply of water. In these exper-
iments, no substantial change occurs in the grain size of the
sediment on the bed of the flume [e.g., Guy el aI., 1966]. In
sediment-feed flumes, however, the upstream supply of sedi-
ment is decoupled from the upstream supply of water. In these
experiments the grain-size distribution of the bed sediment
and the sediment-transport rate are free to change substan-
tially as a function of the interaction between the rates of
sediment feed and downstream transport (as described by Wi/.
cock and McArdeU [1993]).
Prior to closure of Glen Canyon Dam in March 1963, the
Colorado River in Marble Canyon and upper Grand Canyon
(Figure 1) was annually supply-limited with respect to sand but

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