WSP04447

• File Number: 8220.101.10.D
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 1/1/1999
• Author: Schmidt/John C.
• Title: Summary & Synthesis of Geomorphic Studies Conducted During the 1996 Controlled Flood in Grand Canyon
• Water Supply Pro - Doc Type: Report/Study

332 SUMMARY AND SYNTHESIS OF GEOMORPHIC STUDIES

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Figure 2. Graphs showing area.weighted (A) deposition and (B) erosion in 5 eddies at different limes during the flood. calculated
from the data of Andrews et oJ. [thiS volume]

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because the change in thickness of the emire eddy was
much less than the change in thickness at high eleyation.
Within 2 weeks, Broud" et al. [this volume] observed that
backwaters created by the flood were no longer available as
potential rearing habitat for native fishes.

6. SCIENTIFIC AND MANAGEMENT ISSUES THAT
ARISE BECAUSE OF THE RESEARCH FINDINGS

6./. Mainstem SedimeTlt Trampon. Computation o/Sedi-
ment Budgets, and Ihe TIming of Restoralioll Floods

Analysis of the suspended.sedimenl measurements
demonstrates that some of the fundamental ideas under-
pinning management in Grand Canyon may need to he
revised. One such idea is that the average annual rate of
sediment delivery from the Paria River, Little Colorado
River, and smaller tributaries downstream from Glen
Canyon Dam exceeds the transport capacity or Ihe
Colorado River in those years when releases from the dam

are not unusually large and that accumulating sand can be
stored on the bed for a number of years. To reach this idea,
Howard and Dolan [1981] analyzed sediment-transport
data for the Colorado River and iLs major tributaries for
1947-1970, assuming unifonn sedimem-deIivery rates from
ungaged lributaries and that measurement uncertainty was
not significant. Pemberton [1987J and E. D. Andrews [cited
by Smillie el a/.. 1993] calculated transport relations from
'sand-transport data collected by the Geological Survey in
1983.1985, and 1986 [Garretlelal.. I 993J. These relalions
were used by Ralldle el a/. [1993] to calculate that the bed
and banks of the Colorado River had accumulated sediment
belween 1965 and 1982, because the mass of sediment
entering Grand Canyon had exceeded the mass exported
downstream. Smillie e/ al. 11993] calculated mass balances
for ditTerent operational scenarios and suggested that sand
accumulates in the river when tributary inflows are at least
average and there are not wide daily fluctuations in
discharge. Based on lhese findings, the Glen Canyon Dam
EIS 1 US Departlllen/ of Ihe IIl/erior, 1995, fig. Ill-IS I
propused Ihat Beach/Habitat.Building Flows, such as the