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

Alternatively, because of the changing flow regime
from Glen Canyon Dam, sediment in the system is
undergoing constant change so that the measurement
also may have included existing sediment within the
pool or upstream of the pool that was reworked.
Sediment also can be supplied by mass failure of
sandbars, as documented by Cluer ( 199]), or from
un gaged tributaries.

CHANGES IN CROSS-SECTIONAL AREA

To detennine whether cross sections were gaining
or losing sediment over time. the net change in area
(running total or accumulation) was calculated by
summing the non-normalized change in area between
measurements for each cross section in the matching-
date data set (table 4). The lirst date, considered the
initial measurement for these analyses, is assigned to
be the baseline with a net area equal to zero. Changes
in area then are calculated between subsequent
measurements and summed over time to form the net
change in area or the nmning total. Using data from a
sequence of dates for which the same cross sections
were measured provides a way to observe how
individual cross sections respond to similar hydrologic
conditions over time. The two study reaches with the
longest and most complete time-series data are the
reaches below the mouths of the Paria and Little
Colorado Rivers.
For the 5 groups of cross sections downstream
from the Paria River, there were 13 matching dates or
12 periods (matching-date data set, Paria group)
between August 24, 1992, and August 19, 1996, for
which the mnning totals were calculated for all cross
sections except p02 and p32 (fig. 17). All the cross
sections in the lirst group (pOI-pOS) had a negative
running total, ranging from -22 to -519 m", for the first
period. This lirst-period value represents the change in
area between the August 24, 1992, and the January 21,
1993, measurement dates. The initial measurement on
August 24, 1992, was shortly after large sediment
inputs from the Paria River, and it is likely that the
cross sections just below the contluence still contained
sediment from this tributary input during this
measurement. Because the sediment deposited before
the August 24, 1992, measurement had scoured from
the cross sections before the January 21, 1993.
measurement. there is a large decrease in the running
total for the first period for these cross sections.

All but two of the cross sections downstream from
the Paria showed net losses between August 24, ] 992,
and August 19, 1996. Cross section p2] had a minimal
gain of70 me, and loss or gain could not be determined
o
at p 19 because the standard error, :!:2.S m- (95-percent
level of confidence. table 5). added to the net change in
area, 2 me, is too close to zero. Cross section p06
o
showed the largest net loss, 562 m-. Overall there was a
wide range of responses among cross sections; cross
section p06 showed a large capacity to store and lose
sediment. whereas cross sections p27-p31 did not
show large net changes.

For the 6 groups of cross sections downstream
from the Little Colorado River. there were
II matching-measurement dates, or 10 periods
(matching-date data set. Little Colorado River group),
for which all cross sections, except la4, laS, Ib I b. IdS,
and If5, were measured between January 29, 1993, and
April 16, 1997 (fig. ] 8). Cross sections Id2 and Id3
were excluded because data sets were incomplete.
The first measurement in January] 993 was made
approximately 2.5 weeks after a large flood (recurrence
interval of 15 years) occurred on the Little Colorado
River. Unlike cross sections just downstream from the
confluence of the Paria River, the cross sections just
downstream from the conlluence of the Little Colorado
River changed little until after the 1996 controlled
flood (fig. 18).

All the cross sections just downstream from the
Little Colorado River showed net losses for the time
between January 31, 1993. and April ]6, 1997. Cross
section 1c2 showed the largest net loss, ] ,400 m2, and
cross section Idl showed the smallest net loss,S m2.
Similar to the cross sections in the Paria reach, the
cross sections in the lower portion of the Little
Colorado River reach consistently showed less net
change than upstream cross sections.

FACTORS AFFECTING SEDIMENT STORAGE
CHANGES

An analysis was done to detennine if there was a
correlation between changes in cross-sectional area and
the corresponding hydrologic conditions (factors) that
occurred between cross-section measurements. The
hydrologic conditions considered included high flow,
low tlow, mean tlow. range ofllow, and sediment input
from tributaries.

~~ lChanges in Cross-Sectional Area 31