WSP00684

• File Number: 8220.101.16
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 1/1/2000
• Author: Webb et al.
• Title: Sediment Delivery by Ungaged Tributaries of the Colorado River in Grand Canyon Arizona
• Water Supply Pro - Doc Type: Report/Study

Grand Canyon respond similarly and developed a
climatic variability relation between summer
rainfall (fig. 4a) and sediment yield.
We evaluated the effects of climatic variability
on streamflow in eight small drainages on Black
Mesa for which annual sediment-yield data are
available for 10-12 years of record (table 4).
Equation (2) was used to develop a standardized
summer (July-September) rainfall index for 1985-
1997, which corresponds to the period of record for
sediment yield for the Black Mesa gaging stations.
Data came from weather stations surrounding Black
Mesa - Betatakin National Monument, Chinle,
Kearns Canyon, and Monument Valley - instead
of the weather stations in the Grand Canyon region
(table 3). We then standardized the sediment-yield
data for Black Mesa with the regression equation:

Q,BM= -0.046 + 0.888 . PBM, R2 = 0.41, (13)

where Q,BM = standardized Black Mesa annual
sediment yields and P 8M is the standardized
precipitation index for the weather stations near
Black Mesa. Similar regression equations
developed for winter precipitation and annual
precipitation were not significant.
We assume that equation (14) represents the
expected sediment-yield response to climatic
variability in Grand Canyon, and that the
interannual variability of sediment yield in Grand
Canyon was of a similar magnitude to the
interannual variability of sediment yield on Black
Mesa. The time series of sediment yield from the
ungaged tributaries in a reach was calculated as:

Qitl = Q,' (Cv' (-0.046 + 0.888' P,(t)) + I), (14)

where Q,(t) = the time series of sediment yield for
each year t, Q, = the average annual sediment yield
for a reach (table 6), Cv = a coefficient of variation
for the sediment data, and P,(t) = the time series of
standardized precipitation presented in figure 4a.
The average Cv for the eight sediment records from
Black Mesa is 1.3; the Cv for the Paria River is 0.9.
Using this method, we calculated the annual
variation in streamflow sediment yield for Reaches
AandB(fig.15). The larger peaks (1940 and 1980)
and troughs (mid 1970s) replicate a pattern seen in
Topping's (1997) calculations for fine sediment
input from the Paria River. Total sediment yield is
increased 5 to 20 percent when accounting for

debris flows, (table 11) which are assumed not to
vary with climate.
This approach clearly has problems that
diminish its usefulness. Among them, we assume
that climate can be adequately measured by rain
gages that are fairly distant from a watershed, and
that the variability in summer rainfall is reasonably
synchronous across the region of interest. Given the
localized nature of the summer thunderstorms, that
may not always be the case. We also assume that the
relation between sediment yield and summer
precipitation for Black Mesa, which is relatively
weak, is directly transferable to the Grand Canyon
region and adequately describes the relation
between sediment yield in ungaged tributaries and
regional climate. In addition, we assume that the
coefficient of variation of sediment yield in Grand
Canyon tributaries is similar to that of Black Mesa.
Finally, we assume that annual variation in
sediment yield is completely controlled by
variability in summer precipitation.

DISCUSSION AND CONCLUSIONS

Sediment input to the Colorado River in Grand
Canyon, Arizona, is a valuable resource required to
sustain terrestrial and aquatic ecosystems. Since the
closure of Glen Canyon Dam in 1963,_sediment
enters Grand Canyon from 4 major tributaries with
gaging stations and 768 small, ungaged tributaries.
Estimation of sediment yield from ungaged
tributaries is a critical element in the development
of a sediment mass balance for Grand Canyon.
However, estimation of sediment yield is
complicated by the fact that sediment is delivered
by debris flows as well as by streamflow floods,
requiring development of new techniques for
assessing sediment yield.
A total of768 tributaries deliver sediment to the
Colorado River between Glen Canyon Dam and the
Grand Wash Cliffs (river miles -15 to 276). The 32
tributaries between the dam and Lee's Ferry
produce only streamflow floods whereas 736
tributaries in Grand Canyon produce streamflow
floods and debris flows. We used three techniques
to estimate annual streamflow sediment yield from
ungaged tributaries to Grand Canyon, all of which
gave very similar results. The flood-frequency
technique depends on numerous untested

DISCUSSION AND CONCLUSIONS 35