Executive Summary: 2000 - 2001
The Adopt-a-Beach (AAB) program has completed its sixth year as a study
that monitors annual camping beaches in Grand Canyon. This program, sponsored
by Grand Canyon River Guides, Inc., is implemented by a 100% volunteer
force of river guides, scientists, and NPS personnel. Results are submitted
to various agencies such as the Cultural Resources Program of the Grand
Canyon Monitoring and Research Center (GCMRC). Furthermore, results are
presented to the Adaptive Management Program so that private and commercial
recreational interests are represented as stakeholders in Colorado River
management as reported to the Secretary of the Interior.
Adopt-a-Beach is a program of repeat photography that documents the condition
of a selected set of Grand Canyon camping beaches from April through October
of each year. The program assesses visible change to beaches resulting
from changing regulated-flow regimes, rainfall, wind, and human impacts.
Volunteers for this program are unique in that they run the Colorado River
many times in one season, and they are able to provide sets of repeat
photographs for each study beach. To date, guides have produced over 2000
repeat photographs and associated field sheets having recorded the sequential
condition of beaches throughout the commercial boating season, year after
year. Research results include total change to beaches after being impacted
by certain flow regimes, longevity of the 1996 beach/habitat building
flow (BHBF) deposits, change to individual beaches between monitoring
seasons, and primary and secondary processes that cause a decrease in
camping beach area.
Flood experiments were conducted during spring, summer, and fall of 2000.
Habitat maintenance flows (HMF) of 30,000 cfs were released for 4 days
in early May and again in early September. The intervening period was
subjected to moderately high fluctuating flows of 16,000-18,000 cfs throughout
May, followed by low steady summer flows (LSSF) of 8000 cfs until early
September.
The spring HMF showed that 63% of studied beaches (n = 32) gained area
within the 20,000 to 30,000 cfs zone. Most of the changes, determined
from photographs, resulted in extension of beachfront area toward the
river. About half of the beaches showed a small increase in elevation
of about 0.10 meters (as estimated from reference points in photos). The
rest of beaches remained the same size and only one beach, Stone Creek,
decreased in area. The highest percent of increases (87%) appeared in
the Marble Canyon reach (n = 8), although the other reaches showed increases
of 50% and 65%, respectively. Many guides reported that their adopted
beaches gained new sand primarily below the 20,000 cfs line. Morphological
changes resulted in new low-elevation benches and sand bars that covered
pre-existing gravel-and-boulder bars.
The fall HMF did not show as much gain above 20,000 cfs compared to the
spring HMF. Fifty-five percent of beaches (n = 20) gained campsite area.
Mostly, beach areas increased if they had been eroded by the high fluctuating
flows, a 31/2-week duration, immediately following the spring HMF, or
if recent rainfall from the monsoon season caused flash flooding and gully
cutting. Beaches not affected by these factors showed very little change
in beach area after the fall HMF. In assessing cumulative changes from
both HMFs, Muav Gorge showed the highest percentage of increases.
Observations from the 2000 summer season show an overall increase in beach
size from the previous years of 1997-1999 but a substantial decrease from
the BHBF deposit of 1996. This result implies that the spike flow conditions
help to maintain an acceptable beach size for camping when low steady
flows or low fluctuating flows are imposed. However, they are inadequate
for maintaining overall beach elevation above the 30,000 cfs line.
In analyzing longevity of the HMF deposits , over 70% of beaches had lost
most of this deposit by September of 2001. Beach size decreases were reported
to be from primarily fluctuating flows, secondly impacts from rain, and
thirdly impacts from people.
The longevity of the BHBF deposit since 1996 shows varying results. As
of fall 1999, 59% of camps had returned to their pre-BHBF condition (O'Brien
and others 2000). Results of the 2000 HMF flows showed that 78% of beaches
were again larger than their pre-BHBF condition, within the 20,000 to
30,000 cfs zone. Then by September 2001, 45% of the adopted beaches analyzed
in 2001 had returned to conditions similar to that before the BHBF. Campsite
areas within the 30,000 and 45,000 cfs zone have continued to decrease
overall throughout the years. At least 30% of beaches have apparently
developed quasi-stable deposits within this zone, as they show no to very
little change. The status of 20% of beaches could not be determined due
to inconsistent data.
The Low Steady Summer Flows (LSSF) provided more diverse camping, both
upstream and downstream of campsites within the study set, and within
the campsite itself, according to guide response for 31 beaches. The combination
of the HMF followed by the LSSF proved beneficial to 78% of all studied
beaches.
These results contrast with those of the 1999 river season, during which
a high percentage of beaches lost area due to flash floods, and a small
percentage were affected by fluctuating flows. Before 1999, beaches have
been eroding at a decreasing rate, mostly from fluctuating flows, as reported
by guides and supported by visual cutbank retreat in photographs (O'Brien
and others 2000). Typically, rapid adjustment of newly aggraded beaches
to fluctuating flows following a high release leads to initial high rates
of erosion. These rates then fall off over time (Hazel and others 2001).
According to many guide remarks, campsite beaches were "primed and
ready" for the HMF and LSSF regime of 2000. Then by September 2001,
guides reported that camping had become harder on about 50% of the beaches.
This loss of campsite quality directly relates to beach erosion. Other
reported influences include tamarisk encroachment .
These results suggest that any newly deposited sand will be quickly eroded
if subsequent high fluctuating flows are released from Glen Canyon Dam.
This was evidenced by the fall HMF of 1997. Over the winter, high flows
stripped away the new deposit and any benefit to improved camping by spring
1998 went largely unseen by river guides and recreationalists. To date,
at least 30% of beaches show evidence of high-elevation sand (above 30,000
cfs line) deposited by the 1996 BHBF. However, the amount of sand appears
to be diminishing from year to year. Annual implementation of HMFs in
spring and in fall would help preserve this deposit by maintaining the
beachfront. A regimen of Beach Habitat Building Flows is needed periodically
to rebuild campsite areas above the 30,000 cfs line.
For questions or comments please contact Kate Thompson or Lynn Hamilton
Grand Canyon River Guides, Inc., Flagstaff, Arizona (928) 773-1075.
