7423

• UCREFRP Catalog Number: 7423
• Author: U.S. Bureau of Reclamation.
• Title: Glen Canyon Environmental Studies Draft Report, September 1987.
• USFW Year: 1987.
• USFW - Doc Type: Washington, D.C.
• Copyright Material: NO

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Water temperature no longer changes seasonally but is
relatively. constant year-round. Water which passes
through the powerplant is drawn from a level in Lake
Powell where the temperature varies little. River
temperature at Lees Ferry now ranges from 46 to 54
degrees F.

other Resources Have Changed In Response To Changes
In The River

Beaches. Examination of historical photographs shows
that locations of camping beaches and sand substrate
for vegetation have remained much the same throughout
this century. The amount of sand stored in the main
channel riverbed apparently gradually decreased between
1940 and the start of flow regulation in 1963 in
response to large scale climate changes. Available
evidence is not sufficient to allow us to determine if
camping beaches gradually decreased in size during the
same time period. Studies of post-dam changes have
shown that camping beaches have apparently decreased in
area and volume since flow regUlation.

Veaetation. The riverbanks which were scoured nearly
every year by spring floods are now vegetated (Figure
IV-3 and IV-4). Before flow regulation, the vegetation
community now called the "Old High Water Zone" (OHWZ)
had stablized above the level of peak summer floods.
The area below this zone was scoured by annual floods
and supported only a sparse growth of short-lived
herbaceous and shrubby plants. As a result of the
decrease in peak flow, significant amounts of veg-
etation have become established in the former flood
zone. This new zone of vegetation is called the "New
High Water Zone" (NHWZ) and is composed of
newly-established native species such as willow,
seep-willow, and arrowweed; other native species that
are colonizing from the OHWZ such as mesquite and
acacia; and exotic species such as tamarisk. Tamarisk,
which is a major component of the new zone, was found
along the river before 1963, but has greatly increased
in area since that time. Thi$ increase may not be due
entirely to flow regulation--great increases in
tamarisk occurred in many riparian. zones throughout the
Southwest over the same time periOd. (See Appendix B,
Sections II and. IV.)

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