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

530

TOPPING ET AL: COLORADO RIVER SEDIMENT TRANSPORT. I

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SILT & CLAY CONTENT ISILT & CLAY CONTENT AT BASE

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---.- RM -15 (GLEN CANYON)
_____ RM .10 (GlEN CANYON)
___ RM. 4 (GLEN CANYON)
-0- RM 1 (HEAD OF MARIlL.E CYN.)
. X n_ >RM 2 (MARBlE & GRAND CYNS.)

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Figure 8. Vertical trends in the silt and clay content and the median si2e of sand in predam flood deposits
in Glen Canyon, at the head of Marble Canyon, and in Marble (downstream from river mile 2) and Grand
Canyons. Both the silt and clay content and the median size of sand are normaHzed by the values measured
near the base of each deposit. In Glen Canyon and at the head of Marble Canynn the beds did not consistently
coarsen upward with respect to sand, as they did in Marble and Grand Canyons. Thus the upstream supply of
sand probably did not get as depleted during floods in Glen Canyon as it did downstream in Marble and Grand
Canyons.

5.7, Dilrerences in Vertical Grain-Si2e Trends in Predam
Flood Deposits in Marble and Grand Canyons
and Glen Canyons

As suggested by the above series of analyses, predam Glen,
Marble, and Grand Canyons were all annually supply-limited
with respect to silt and clay, whereas predam Marble and
Grand Canyons were annually supply-limited with respect to
sand to a greater degree than was predam Glen Canyon. If
these interpretations are correct, then some difference should
exist between the vertical trends in grain size preserved in
predam flood deposits in Marble and Grand Canyons and
those preserved in predam flood deposits in Glen Canyon.
Though most of Glen Canyon is today inaccessible beneath the
waters of Lake Powell, some predam flood deposits are pre-
served in the 25-km-long reach between Glen Canyon Dam
and Lees Ferry. To compare the vertical trends in grain size
preserved in predam flood deposits in Glen Canyon with those
preserved in predam deposits in Marble and Grand Canyons,
we sampled a total of 10 predam flood deposits vertically for
grain size at three sites in Glen Canyon in 1998 and 1999
(Figures I and 8). .

5.8. Discussion of the Differences Between the Vertical
Grain.Size Trends in Predam Flood Deposits in Marble
and Grand Canyons and Glen Canyon

As in Marble and Grand Canyons. the content of silt and
Clil}' generally decreases upward in the predam flood deposits

in Glen Canyon, with the silt and clay content at the top being
less than that at the base in about 80% of the sampled deposits.
However, in contrast to Marble and Grand Canyons, less gen-
erality exists in the vertical trends in sand grain size in the
predam flood deposits in Glen Canyon. In Glen Canyon the
sand was actually finer at the top than at the base in about 30%
of the deposits (as was the case in the predam deposit at the
head of Marble Canyon). Therefore the sedimentologic record
preserved in the predam flood deposits lends further support
to the interpretation that in the predam era, the majority of
Marble Canyon and all of Grand Canyon were annually sup-
ply-limited with respect to fine sediment to a greater degree
than was Glen Canyon. Thus a predam transition to increased
annual fine-sediment supply limitation may have occurred ne-ar
the head of Marble Canyon.

6. Support for Annual Sediment Supply
Limitation From Late Predam and Early
Postdam Sediment Budgets

In addition (0 displaying the four above-described and doc-
umented effects associated with sediment supply limitation,
the integral constraint on whether a reach is annually supply-
limited with respect (0 fine sediment (i.e., sand and finer ma-
terial) is that the reach must export all of the fine sediment
supplied to it e;ll'!l year. The above analyses provide evidence
Ihat is necessary. but not sufficient, to conclude th,l! the pre-