9390

• UCREFRP Catalog Number: 9390
• Author: Webb, R. H., T. S. Melis and R. A. Valdez.
• Title: Observations of Environmental Change in Grand Canyon, Arizona.
• USFW Year: 2002.
• USFW - Doc Type: 02-4080,
• Copyright Material: NO

I

sand bars, while not noticing other aspects, such as
changes in wildlife populations. These observations
must be taken in aggregate to provide an accurate
accounting of environmental changes in Grand
Canyon.
Much has changed along the Colorado River
since most of the Old Timers and other historic river
runners were in Grand Canyon. The following changes,
many of which are supported using photographic or
other evidence, are probably significant:

· The low water temperature, increase in riparian
vegetation, lack of substantial sediment in the
river, noise from aircraft, and deteriorated air
quality are considered the largest changes in
Grand Canyon.

· The largest changes in the river corridor, other
than rapids, were observed just downstream from
Nankoweap Creek (mile 52.2-R), and at the mouth
of the Little Colorado River (mile 61-L), Elves
Chasm (mile 116.5-L), and Tapeats Creek (mile
133.8-R).

· The Old Timers saw little evidence of debris flows
before closure of Glen Canyon Dam. The
frequency of debris flows may have increased
after the early 1960s, as speculated by Melis and
others (1994) and Webb (1996). Certain rapids --
such as Crystal (mile 98.3), Bedrock (mile 130.5),
and House Rock (mile 16.9) -- are now more
difficult than in the pre-dam era. Lava Falls Rapid
(mile 179.5), which was always large, also was
discussed as having changed. Other rapids are
unchanged or are less difficult.

· Erosion of camping beaches used by pre-dam river
runners is severe. Now, some formerly popular
camping sites are not easily used for camping.

· Fires had a significant impact on the amount of
driftwood along the river corridor. Fires were set
randomly and specifically at certain popular
places, particularly the mouth of Nankoweap
Creek (mile 52.2-R) and Tanner Canyon (mile
68.5-L).

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· Riparian vegetation, mostly tamarisk, is much
denser along the river, whereas cottonwoods and
tree-form willows decreased in the post-dam era.
Tamarisk slowly increased, particularly in the
1940s and 1950s, but accelerated after 1963.

· Perennial marsh vegetation was not observed or
photographed along the pre-dam river except near
spnngs.

· Catfish were the most common fish caught in the
pre-dam river, although rainbow trout were also
present. Some of the Old Timers caught native
fish, mostly before about 1950, and the species
they observed could not be accurately determined
unless the catch was photographed.

· Ducks and other water birds were commonly
observed along the unregulated river, but they
were not as abundant as they are now, in accord
with the findings of Stevens and others (1997).

Other changes mayor may not be significant, if
in fact changes even occurred: (1) Bats and some
species of birds may have decreased along the river,
whereas swallows may have increased. (2) Bighorn
sheep possibly are more numerous; they certainly are
observed more often now. No changes can be inferred
in the population of deer or other large mammals along
the river. (3) It is uncertain whether the population of
beavers has changed, and otters remain an uncommon
sighting. (4) Bothersome insects were common before
Glen Canyon Dam, although it is unknown if specie.~ or
abundances may have changed. ' "~
Some of the issues raised in this report have ..~
significant implications for dam management. The .....\
increase in frequency of debris flows will hasten a
long-postulated change in the longitudinal profile of
the river; without dam releases significantly higher
than power plant capacity, the river will eventually
attain an exaggerated pool-drop configuration.
Implications of such a trend might be that navigation of
rapids becomes generally more difficult, and in some
cases, the river may become impassable. Continued
aggradation of the river channel by coarse sediment
delivered during debris flows will likely fill-in deeper
pools above and below rapids, while also enhancing the
size of eddies. On the basis of photographic evidence
and sediment-transport studies, bars are predicted to
continue decreasing in size (Laursen and others, 1976).
Tamarisk was increasing despite pre-dam flooding.
Although dam releases could be used to thin some of
the population, tamarisk would have become a
common, although exotic, resident of Grand Canyon
even if the dam had not been built. Marshes are an
artifact of flow regulation, not a natural feature of the
pre-dam environment (Webb, 1996). Native fish may

CONCLUSIONS 31