Recent Floods in Grand Canyon Tributaries


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 (rhwebb@usgs.gov, 520-670-6671 ext 238) or Tillie Klearman (tklearma@usgs.gov, 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