WSP00684

• File Number: 8220.101.16
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 1/1/2000
• Author: Webb et al.
• Title: Sediment Delivery by Ungaged Tributaries of the Colorado River in Grand Canyon Arizona
• Water Supply Pro - Doc Type: Report/Study

Chasm (river miles 87 to 129) and western Grand
Canyon from about river mile 211 to Snap Canyon
(river mile 280) in 1938. The scale of these
photographs is unknown but probably is about
1:20,000. The 1965 aerial photography is available
from the EROS Data Center in Sioux Falls, South
Dakota, and 1973 aerial photography is stored at the
U.S. Geological Survey in Tucson, Arizona. Aerial
photography flown annually or more frequently
between 1980 and 1998 is stored at the Grand
Canyon Monitoring and Research Center in
Flagstaff, Arizona.

Temporal and Spatial Distribution of
Debris Flows

We documented 196 historical debris flows in
Grand Canyon (appendix 2; fig. II). Of these, 51
debris flows significantly affected the Colorado
River during the past one-hundred years by creating
rapids or increasing constrictions (table 14). From
1984 through 1998, four rapids or riffles were
created and t 3 were enlarged by debris flows.
Sixty-two debris flows (32 percent) can only be
dated as "historic" (1890-1990) and cannot be
analyzed further for temporal distribution. With a
few notable exceptions (e.g., the debris flow in
Badger Canyon that occurred between 1897 and
1909), most documented debris flows occurred
after 1960 (table 14, fig. 5). The occurrence of
debris flow is not random in Grand Canyon; debris
flow activity is par1icularly concentrated in Marble
Canyon and other reaches where the river trends
towards the southwest (fig. II). Where the river
trends northwest, few historical debris flows were
documented (Griffiths and others, 1996).
Repeat photography documented the
distribution of debris flows during the last century
in 171 tributaries (Webb, 1996; Griffiths and
others, 1996). We identified 97 debris flows in
historic photographs, indicating that 57 percent of
the tributaries with repeat-photography records had
debris flows sometime during the last 127 years.
Debris flow in the remaining 43 percent of
tributaries in Grand Canyon occur at a frequency of
fewer than one per century. Using time series of
repeat photography and other information, we
found that approximately 10 percent of tributaries
had two or more debris flows in the last one-

hundred years, with a maximum of six debris flows
at Lava Falls Rapid (Melis and others, 1994; Webb
and others, I 999b). Twelve steep-angle chutes in
extra areas outside of tributaries had debris flows
during the last century indicating that these areas
can be occasionally be active producers of small
debris flows.

Climatic Variability and Debris-Flow
Initiation

Historically, most Grand Canyon debris flows
have occurred during localized, convective summer
thunderstorms that affect only one or two drainage
basins at a time. These storms typically occur in
July through October (Melis and others, 1994).
Rainfall from summer thunderstorms typically is
intense, but localized, and has a duration of less
than several hours. Debris flows in summer months
are not related to the level of seasonal precipitation;
instead, debris flows are equally likely to occur in
wet, average, or dry summers (Griffiths and others,
1996; Webb and others, 1999b). In contrast, a few
of the largest debris flows have occurred during
prolonged winter precipitation from unusually
warm frontal systems (Cooley and others, 1977;
Griffiths and others, 1996; Webb and others,
I 999b). Winter storms mostly affect large drainage
basins (Cooley and others, 1977; Webb and others,
1989), and the occurrence of winter debris flows is
unrelated to the amount of seasonal precipitation.
Although monthly precipitation was high when
most historic debris flows occurred (Webb and
others, I 999b), seasonal precipitation was not
consistently high. Grand Canyon debris flows do
not necessarily require season-long buildup of
antecedent soil moisture; however, the importance
of above-average rainfall in the days preceding the
debris flow is reflected in the recurrence intervals
for storm precipitation (Griffiths and others, 1997).
Recurrence intervals of daily precipitation for
summer debris flows are not well known because
summer storms are localized and weather stations
typically are kilometers from affected drainage
basins. Storms that produce debris flows typically
end in a strong microburst of intense rainfall, thus
hourly precipitation data are required to determine
triggering rainfall (Griffiths and others, 1996;
Webb and others, 1999b). Because of these

FREQUENCY OF HISTORICAL DEBRIS FLOWS IN GRAND CANYON 23