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

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Figure 3. The sand bar downstream from Tapeats Creek (mile 133.9-R) (continued).

B. March 1, 1995. Large rocks and boulders are now exposed because of severe beach erosion. New sand
was deposited here during the 1996 controlled flood but was quickly removed (Steve Tharnstrom, Stake
2676).

1961 flood may have had a small debris-flow
component to it; the rapid was filled in on the top right
side.

Rockfalls

~

Rockfalls were rarely noted or observed in the
pre-dam era. Reilly recorded any rockfalls he observed;
he particularly mentions a 1,500-lb boulder falling at
Diamond Creek in 1949. On one of his last river trips in
1982, Reilly observed that many rockfalls had occurred
since his previous trip in 1964. Rigg also mentioned the
lack of rockfalls when he ran the river. This was
thought to be significant because we witnessed a
rockfall at Eminence Camp (mile 44-L) on the Old
Timers Trip in 1994, which Shoemaker noted was the
first he had seen in Grand Canyon. Many recent
rockfalls were pointed out by trip participants. Since
the Old Timers' trip, a major rockfall in 1999,
significantly limited navigation through the right side
of President Harding Rapid (mile 43.3).

Rapids

The Old Timers recognized some obvious and
some subtle changes in rapids. The change in Crystal
Rapid (mile 98.3), which many of the Old Timers had
not seen before, is the most dramatic of any noted,
although Rigg was also impressed with the changes in
Lava Falls Rapid. Cross II had a wealth of information
on changes to rapids in the first decade after closure of
Glen Canyon Dam (1963), and shared his notes with us
of what he saw during the first river trip after the
December 1966 storm. Lava Falls Rapid changed on
the left side in 1966 owing to many large boulders
being deposited in a former "sneak" run; we
subsequently identified the cause as a debris flow from
Prospect Canyon (mile 179.4-L). Bright Angel Rapid
(mile 87.9) also changed slightly in 1966.
Although the Old Timers could not agree when
House Rock Rapid (mile 16.9) changed, the body of
observations points to more than one event. Litton
associated the change with the 1966 storm but had no
specific evidence to support the observation. Dierker
claims the largest change occurred before 1969, when

SPECIFIC CHANGES OBSERVED IN GRAND CANYON 13