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

536

TOPPfNG ET AL: COLORADO RIVER SEDIMENT TRANSPORT. I

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JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN

JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN

Figure 10. (continued)

the approximate threshold between sand accumulation and
conveyance (Le., 250 m'js) only 27.0% of the time and ex-
ceeded the approximate threshold between sand conveyance
and erosion (i.e., 450 m'js) only 8.8% of the time. During the
season of sand erosion (April-June), flows exceeded the ap-
proximate threshold between sand accumulation and convey-
ance 90.4% of the time and exceeded the approximate thresh-
old between sand conveyance and erosion 69.1% of the time
(Figure II).
When compared to that in the predam era, the hydrology of
the postdam Colorado River during sediment years 1966-1970
was closer to that during the predam season of sand erosion
(April-June) than it was to the hydrology during the predam
season of sand accumulation and storage (July-March). Fur-
thermore, given that the hydrology of the Colorado River
during sediment years 1966-1970 was similar to that during the
rest of the postdam era (sediment years 1966-1998), interpre-
tations of the magnitude and duration of sand storage based on
this limited period of sediment data should be valid for the
entire postdam era (Figure 11). Indeed, flows were slightly
higher during sediment years 1966-1998 than they were during
sediment years 1966-1970, suggesting that the data from sed-
iment years 1966-1970 provide an upper bound on the mag-
nitude and duration of seasonal sand accumulation and storage.
During sediment years 1966-1970, flows exceeded the approxi-
mate predam threshold between sand accumulation i] nd con.
veyance 68.1% of the time; during sediment years 196(j-1998,

flows exceeded the approximate predam threshold between
sand conveyance and erosion 73.6% of the time (Figure 11).

6.3. Discussion of Ihe Annual and Seasonal Sediment
Budgets

Analysis of the annual sediment budget does not invalidate
the hypothesis that in both the predam and postdam era the
Colorado River in Marble Canyon and upper Grand Canyon
was an annually supply-limited system with respect to fine
sediment (i.e., sand and finer material). During the 19 years of
complete sediment data the magnitudes of the annual fine-
sediment supply and export are equal to within the uncertain-
ties in the sediment budget in aU but four years. The fine-
sediment export can be shown to have exceeded the supply in
only sediment years 1949 and 1965. Sediment year 1949 was a
year in which a large snowmelt flood occurred (Figure 9b), and
sediment year 1965 was a year with abnormally high dam
releases (Figure 9b). The high flows in 1965 scoured the reach
immediately downstream of the dam [Pemberlon, 1976; Wil-
liams and Wolman, 1984; Burkham, 1986] and, as suggested by
this sediment budget, scoured sediment from much of Marble
and Grand Canyons. The fine-sediment supply can be shown to
have exceeded the export in only sediment years 1954 and 1963
(Table 1). Sediment year 1954 was the predam year in the
period of sediment record with the lowest flows (Figure 9b).
and sediment year 1963 was the year in which the dam was
closed and in which the naturally erosive snowmelt flood was