The monsoon season of 2001 brought
a number of changes to the tributaries in Grand Canyon. During our debris-fan
monitoring research trip in October and November, we counted about 18
new debris flows that reached the Colorado as well as a number of significant
streamflow floods. This would make the 2001 season, which will not be
known as a significant monsoon in terms of the total amount of precipitation,
as the producer of the largest number of debris flows in recorded history
of Grand Canyon.
Two very significant changes that you might want to make note of include
debris flows at Comanche Creek (mile 67.2-L) and Monument Creek (mile
93.5-L). Kirk Burnett passed by Comanche Creek just after the flow ceased
on August 7th and observed the recessional flood that followed the debris
flow. This is the same storm that caught and killed George Mancuso and
his hiking companion in the Little Colorado River. The new rapid, already
enlarged after the 1999 debris flow, now has a higher fall and bigger
waves in the center but is not a significant navigational hazard.
The entire debris fan at Monument Creek was covered by a debris flow that
reached the Colorado River over a wide area but did not significantly
affect Granite Rapid. In terms of amount of deposition, this is the largest
debris flow here since 1984, eclipsing the 1996 event. The beach below
the rapid is covered with gravel and cobbles. The rapid is narrower in
the middle, and the tailwaves appeared to extend through the downstream
eddy and into Lower Granite Rapid, a change consistent with the idea that
dam releases move cobbles from the debris fan into the pool just downstream
instead of into the secondary rapid or the island. Rachel Schmidt hiked
Monument Creek on September 19th and did not notice anything significant;
she thought the flow might have occurred during the widespread and severe
storms in the region on October 6th.
Most of the new debris flows are small and did not have significant effects
on the Colorado River. A good example of this is the debris flow at Little
Nankoweap Creek (mile 51.7-R). The debris fan here has a large area but
a low slope, and most of the deposition is hidden upslope of a dense thicket
of tamarisks, willows, and Baccharis.
75-Mile Canyon, which creates
Nevills Rapid, also had its fourth debris flow since 1987. This flow deposited
sediment over perhaps one third of the area of the fan affected directly
by debris flows but only reached the river in the middle of the rapid.
Ted Melis observed that the channel in the Shinumo Quartzite gorge upstream
from the fan was deeply entrenched, creating a significant climb at the
first waterfall. .Finally, Salt Creek (mile 93.8-L) had a debris flow
that covered most of its fan with cobbles and boulders. This flow likely
occurred at the same time as the one in Monument Creek (October 6th).
In addition to the 2001 debris flows, we noted a number of debris flows
from the 2000 monsoon season. The most significant of these is at Granite
Park, where a debris flow deposited considerable sediment into the entrance
of the former left channel. The depositional area of this debris flow
was considerable, and it is possible that in the absence of a significant
flood release from Glen Canyon Dam that this channel will be cut off permanently.
The 33,000 ft3/s release of fall 2000 overtopped the debris flow slightly,
indicating it occurred during the summer of 2000, but otherwise we do
not know the date for this debris flow. If you witnessed it, please contact
us (see below). Some members of our monitoring trip thought the hole in
209-Mile Rapid was briefly filled in after this event, and we’d
like to know if you saw that also.
If you see significant changes to tributaries, particularly at their debris
fans at the Colorado River, or if you are lucky enough to witness a debris
flow in 2002, we are interested in hearing from you. Contact either Bob
Webb (firstname.lastname@example.org, 520-670-6671 ext 238) or Tillie Klearman (email@example.com,
520-670-6671 ext 267). We are also interested in any eyewitness accounts
of debris flows or other large floods in the last 6 years. Dates, times,
and other observations would be greatly appreciated for any significant
floods that you might have seen. We’d love to hear from you.
Bob Webb and the
Debris Flow Monitoring Crew