8128

• UCREFRP Catalog Number: 8128
• Author: U.S. Bureau of Reclamation.
• Title: Operation of Glen Canyon Dam\
• USFW Year: 1995.
• Copyright Material: NO

Affected Environment and Environmental Consequences
of this document. Also, all the linkages among resources of the Grand
Canyon ecosystem are not fully understood at this time. A program of
monitoring and adaptive management is required to expand our
understanding of how changes in processes affect this system.
The Glen Canyon Dam EIS focuses on the following processes, resources,
and their linkages:
• Water release and sediment transport patterns
• Aquatic and terrestrial "indicator resources" within the system
The system of concern in this study is the Colorado River corridor-from
Glen Canyon Dam through Grand Canyon to Lake Mead-and includes
resources located in the river channel and in a narrow band of adjacent
land (figure 4). Resources within this system depend on factors outside
these operationally defined boundaries, including the physical and
biological constraints of Lake Powell and, to a lesser extent, Lake Mead
and tributaries such as the LCR.
The Grand Canyon ecosystem originally developed in a sediment-laden,
seasonally fluctuating environment. The construction of Glen Canyon
Dam altered the natural dynamics of the Colorado River. Today, the
ecological resources of Grand Canyon depend on the water releases from
the dam and variable sediment input from tributaries. The alternatives
evaluated through this EIS must take into account not only the short-term
needs of the environment but also the long-term requirements for
maintaining and supporting the ecological elements of Grand Canyon.
Lake Powell traps water, sediment, and associated nutrients that
previously traveled down the Colorado River. Interruption of riverflow
and regulated release of lake water now support aquatic and terrestrial
systems that did not exist before Glen Canyon Dam. Some changes are
lamented while others are valued. The following discussion addresses the
current systems, their resources, and how dam operations affect them
either directly or through linkages among resources. The present
interactions among water volume and release patterns, sediment transport,
and downstream resources have created and support a complex system
much different from predam conditions.
Water Volume and Pattern
of Release
The major function of Glen Canyon Dam (and Lake Powell) is water
storage. The dam is managed to release at least 8.23 million acre-feet (maf)
of water annually to the Lower Basin. In this EIS, riverflows below the
dam are referred to as releases or discharge. The measure of riverflow is in
cubic feet per second (cfs). Annual and monthly volumes are measured in
acre-feet. To put these relationships in perspective, Glen Canyon Dam
Glen Canyon Dam EIS Summary 37