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

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Particle Size ($)

Figure 14. Particle-size distributions of 41 historical debris flows in Grand Canyon.

range from 1.5 to 2.0 Mg/m3. The larger value of
this estimate most likely results from
underestimation of macro-pore spaces between
larger par1icles. We therefore used the upper limit
of measured densities, 2.0 Mg/m3 (60 percent solids
by volume), to calculate debris-flow sediment
yields.

Reworking of Debris Fans by the
Colorado River

Reworking of debris fans by the Colorado
River is a complicated process that has been rarely
documented in Grand Canyon (Kieffer, 1985;
Webb and others, 1999a, 1999b; Pizzuto and others,
1999). Reworking releases sediment stored in
debris fans - particularly sediment finer than
gravel size - into the river, which rearranges the
sediment into sandbars and debris bars. In order to
understand sediment yield to the Colorado River
from debris flows, some estimate must be made of
the amount of sediment that is stored in debris fans

versus what is expected to be released into the
Colorado River. The technique must be such that all
debris fans in Grand Canyon can be considered, if
only on a reach-average basis. In the pre-dam
Colorado River, the 10-yr flood had a discharge of
3,940 m3 Is, which is more than sufficient to overtop
and rework most historic debris fans in Grand
Canyon (Melis, 1997). Therefore, we assume that in
the pre-dam era all fine sediment delivered by
debris flow entered the river every decade.

After completion of Glen Canyon Dam,
discharges decreased substantially and powerplant
releases caused only minor reworking of debris fans
(Melis and others, 1994). Large dam releases, such
as the 1983 discharge and the 1996 controlled
flood, significantly reworked debris fans (Kieffer,
1985; Webb and others, 1989, 1999a, 1999b). The
10-yr flood on the post-dam river is 1,430 m3/s,
about the size of the 1996 controlled flood. Using
debris fan volume data contained in Kieffer (1985)
and Webb and others (1989, 1999a, and 1999b), we
estimated that 25:1:30 percent of the volumes of 13

32 Sediment Delivery by Ungeged Tributaries of the Colorado River In Grand Canyon