WSP03492

• File Number: 8220.101.10.A
• Description: Colorado River-Water Projects-Glen Canyon Dam/Lake Powell-Glen Canyon Adaptive Management-AMWG
• State: AZ
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 3/3/2004
• Author: USGS
• Title: AMWG Meeting Attachments-March 3-4 2004-Variations in Sand Storage Measured at Monumented Cross Sections in the Colorado River Between Canyon Dam and Lava Falls Rapid-Northern Arizona 1992-99
• Water Supply Pro - Doc Type: Report/Study

CROSS-SECTION VARIABILITY

Because of channel geometry and hydraulics, some
cross sections have a larger capacity to store and
subsequently lose sediment. This capacity is reflected
in the range of change-in-area values.

Spatial Variability in Changes in Cross-Sectional
Area

Nonnalized and non-normalized change in area
calculated between successive measurement dates for
cross sections in the primary data set (table 4) between
the Paria River and Bright Angel Creek (fig. I) are
shown in figures II and 12, respectively. All measure-
ments were used in the change-in-area calculations,
except for those made during the 1996 controlled flood,
and some cross sections were measured more
frequently than others. Measurements made during the
1996 controlled flood were not included because flows
dUling the flood were extreme and not representative of
nOl1nal hydrologic conditions. Some cross sections
were more dynamic than others, showing a wider range
of sediment deposition and scour. Cross sections p05
and p06 are examples of more dynamic cross sections
(fig. 13). Both cross sections are immediately below
the mouth of the Paria River and are able to store
sediment in a large eddy, as well as in the deep main
channel. fora limited period of time (fig. 13).
For example. the largest increase in sediment was
measured for p06 on September 14, 1998 (fig. 13),
after significant sediment input from the Paria River
(fig. 3); however, the next measurement at p06,
April 13, 1999 (fig. 13), indicated the largest decrease
in sediment. These sections, farthest upstream, also are
the first to be exposed to clear water flowing from the
dam and thus are more susceptible to winnowing and
erosion. Cross sections p 12 and Id I are examples of
less dynamic cross sections; unlike sections p05 and
p06, they do not have large eddies and thus have a
limited capacity to store sediment in the main channel
(fig. 14).
Although cross section la I is immediately down-
stream from the confluence of the Little Colorado and
Colorado Rivers and has a channel geometry (tig. 15)
similar to that of cross sections p05 and p06 (fig. 13), it
does not show the same range of deposition and scour
(figs. II and 12). The largest gain for cross section la I
was measured on January 29.1993 (Iig. 15).
immediately following significant inputs of water and

sediment from the Little Colorado River (fig. 3);
however. only a small loss was measured when cross
section lal was measured on April 22. 1993 (Iig. 15).
A negative change of comparable magnitude was not
measured until the April measurement following the
1996 controlled flood. A possible explanation is that
the 1993 flood on the Little Colorado River deposited
large-sized sediment immediately downstream from
the confluence, armoring the channel at cross section
lal (Anima and others, 1998: Wiele and others, 1999).
Releases from Glen Canyon Dam were such that the
large-sized sediment was not moved downstream until
the controlled flood occurred in 1996.

Temporal Variability in Cross-Sectional Area

The effects of large suspended-sediment inputs
from tributaries on changes in the normalized cross
sections are evident in the cross-section measurements
when viewed chronologically (fig. 16). The cross
sections downstream from the confluence of the Paria
and Colorado Rivers measured in September 1998
show a significant increase in area in the component of
the cross section occupied by sediment the day after the
storm that produced an input of 1.7 million metric tons
of sediment from the Paria River (Iig. 3). The subse-
quent measuring trip in April 1999. which occurred
after 7 months of almost no suspended-sediment
inputs. reported decreases in cross-sectional area,
indicating decreases in sediment. from these same
cross sections. Large increases in cross-sectional area
also were measured during September 1999 following
a storm. The results from this sequence of measure-
ments demonstrate the rapid response of these cross
sections to tributary inputs and indicate that because
sediment storage in these cross sections is only tempor-
ary, the timing of the measurements is critical in terms
of capturing the full effect of a tributary input.

Positive changes in cross-sectional area were also
measured in the absence of known tributary inputs
(fig. 16). For example, around September 1996, there
was little sediment input from the Paria River, yet
some cross sections in the h l-h5 and the w l-w5
groups (fig. 6) showed net positive changes in area
(fig. 16D). Cross sections in the la 1-11'5 group (Iig. 7)
also showed net positive changes in area around
March 1999, when there was little sediment input
from either the Paria River or the Little Colorado River
(fig. 16G). The increase in sediment may be the result
of a sediment pulse from a previous tributary input.

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Cross-Section Variability 23