Bedrock
failures

Firehose
effect

Failures in
colluvium

Combinations

of failures
1 1 1 1 1
0 10 20 30 40 50 60 70
Percent of debris flows

Figure 3. Initiation mechanisms for selected debris flows between 1939 through 2002 in
Grand Canyon (n = 68; modified from Griffiths and others, 1997).

preexisting colluvial wedges at the base
of cliffs, and presumably repeated debris-
flow occurrence is at least partially
dependent on regeneration of these
wedges following their partial or total
removal during an event. Direct failure of
colluvium owing to saturation or under-
cutting adjacent to channels accounts for
16% of debris flows in this period. Direct
failure of weathered Paleozoic shales and
sandstones, most often in either the Her-
mit Formation or Supai Group, is trig-
gered by intense, localized rainfall from
convective summer thunderstorms or pro-
longed winter precipitation (Cooley and
others, 1977); debris flows initiated by
bedrock failures tend to be the least
numerous but among the largest in Grand
Canyon (Webb and others, 1988).

MANAGEMENT
IMPLICATIONS

Debris flows occur, on average, five
times per year in Grand Canyon. Most
debris flows occur during the summer,
when the river is most heavily used.
Therefore, debris-flow activity may be a
significant threat to the safety of recre-
ationists along the river corridor. Educa-
tion about the causes and frequency of
debris flows in Grand Canyon may help
reduce the potential for life-threatening
situations along the river corridor.

Debris flows are a natural process,
unaffected by land-use practices in the

tributary drainages within Grand Can-
yon. No watershed management methods
are known that would alter the probability
of debris flows in Grand Canyon,
although wildland fire might temporarily
increase the probability of an event if a
storm with sufficient intensity followed
closely after a burn. Debris flows, com-
bined with streamflow floods, are the
source for gravel that lines the bed of
much of the river corridor, providing opti-
mal spawning habitat for nonnative
salmonids, which prey on endangered
native fish. Reduction of this gravel can
only be achieved by periodic flushing
flows released from Glen Canyon Dam.

REFERENCES CITED

Cooley, M.E., Aldridge, B.N., and Euler, R.C.,
1977, Effects of the catastrophic flood of
December, 1966, North Rim area, eastern
Grand Canyon, Arizona: U.S. Geological
Survey Professional Paper 980, 43 p.

Fenton, C.R., Webb, R.H., Pearthree, P.A.,
Cerling, T.E., and Poreda, R.J., 2001, Dis-
placement rates on the Toroweap and Hur-

ricane faults: implications for Quaternary
downcutting in Grand Canyon, Arizona:
Geology, v. 29, no. 11, p. 1035-1038.
Griffiths, P.G., Webb, R.H., and Melis, T.S.,
1997, Initiation of debris flows in bedrock
canyons of the Colorado River, USA, in
Chen, Cheng-lung, ed., Debris-flow haz-
ards mitigation: Mechanics, prediction, and

assessment: New York, American Society of
Civil Engineers, p. 12-20.

Hereford, R., Thompson, K.S., and Burke, K.J.,
1998, Numerical ages of Holocene tributary
debris fans inferred from dissolution pitting on
carbonate boulders in the Grand Canyon of
Arizona: Quaternary Research, v, 50, p. 139-
147.

Melis, T.S., Webb, R.H., Griffiths, P.G, and Wise,
TJ., 1994, Magnitude and frequency data for
historic debris flows in Grand Canyon
National Park and vicinity, Arizona: U.S. Geo-
logical Survey Water Resources Investigations
Report 94-4214, 285 p.

Stephens, H.G., and Shoemaker, E.M.
the footsteps of John Wesley Powel
Colorado, Johnson Books, 286 p.

Turner, R.M., and Karpiscak, M.M., 1980, Recent
vegetation changes along the Colorado River
between Glen Canyon Dam and Lake Mead,
Arizona: U.S. Geological Survey Professional
Paper 1132, 125 p.

Webb, R.H., 1996, Grand Canyon, a century of
environmental change: Tucson, University of
Arizona Press, 290 p.

Webb, R.H., Pringle, P.T., Reneau, S.L., and Rink,
G.R., 1988, Monument Creek debris flow,
1984 — Implications for formation of rapids
on the Colorado River in Grand Canyon
National Park: Geology, v. 16, p. 50-54.

Webb, R.H., Pringle, P.T., and Rink, G.R., 1989,
Debris flows in tributaries of the Colorado
River in Grand Canyon National Park, Ari-
zona: U.S. Geological Survey Professional
Paper 1492, 39 p.

Webb, R.H., Melis, T.S., Griffiths, P.G., Elliott,

J.G., Cerling, T.E., Poreda, R.J, Wise, T.W, and
Pizzuto, J.E., 1999a, Lava Falls Rapid in
Grand Canyon, effects of late Holocene debris
flows on the Colorado River: U.S. Geological
Survey Professional Paper 1591, 91 p.

Webb, R.H., Melis, T.S., Griffiths, P.G., Elliott,
J.G., 1999b, Reworking of aggraded debris
fans, in Webb, R.H., Schmidt, J.C., Marzolf,
G.R., and Valdez, R.A., eds., The controlled
flood in Grand Canyon: Scientific experiment
and management demonstration: Washington,
D.C. American Geophysical Union, Geophysi-
cal Monograph 110, p. 37- 511.

Webb, R.H., Griffiths, P.G., Melis, T.S., and Hart-
ley, D.R., 2000, Sediment delivery by ungaged
tributaries of the Colorado River in Grand
Canyon, Arizona: U.S. Geological Survey
Water-Resources Investigations Report 00-
4055, 67 p.

1987, In
: Boulder,

Robert H. Webb
Peter G. Griffiths
Tillie A. Klearman

@® Printed on recycled paper



Prepared in cooperation with the Grand Canyon Monitoring and Research Cente,

Frequency of Debris Flows in Grand Canyon

Debris flows from 740 tributaries
within Grand Canyon, Arizona, contrib-
ute coarse-grained sediment to the Colo-
rado River (Webb and others, 2000). Dis-
tributed along 444 km (kilometers) of
river between the Paria River and the
Grand Wash Cliffs, these tributaries drain
12,000 km? of steep terrain, most of
which lies between the North and South
Rims of the canyon, and are little affected
by land-use practices. Grand Canyon
debris flows are more than 80% sediment
by weight and deposit poorly sorted sedi-
ment in the river that ranges in size from
clay to large boulders (Melis and others,
1994). Debris flows constrict the Colo-
rado River at tributary junctures, raising
the river bed until floods rework debris
deposits and remove or reposition boul-
ders (Webb and others, 1989). The large
boulders deposited in the river by debris
flows form the core of rapids that modify
the longitudinal profile and locally con-
trol the geomorphic framework of the
Colorado River in Grand Canyon (Webb,
1996).

Rapids account for most of the verti-
cal drop of the river through Grand Can-
yon, 66% of which occurs in only 9% of
the river’s length. Debris flows can be
hazardous to the recreational community,
affecting navigation of white water and
potentially trapping hikers in narrow can-
yons. Debris flows have repeatedly dam-
aged the water-supply pipeline along
Bright Angel Creek, destroyed hiking
trails, destroyed vehicles, and threatened
lives. This Fact Sheet presents debris-
flow frequencies for the periods of
observed debris flows (1984-2002) and
the historic photographic record (1871-
1984). A total of 1,365 historical photo-
graphs of the river corridor taken between
1871 and 1984 were matched between
1989 and 1995 and were analyzed for the

occurrence of debris flows that reached
the Colorado River. A frequency distri-
bution also is given for the four debris-
flow initiation mechanisms observed
between 1984 and 2003 in Grand Can-
yon.

DEBRIS-FLOW
FREQUENCY

Direct Observations (1984 —
2002)

We directly observed and compiled
notes from river runners on when debris
flows, rockfalls, or significant stream-
flow floods occurred in Grand Canyon
from 1984 through 2002, updating the
work of Melis and others (1994) and
Webb and others (2000). In several
cases (e.g., the 1993 Tanner Canyon

debris flow), eyewitnesses described the
floods (Melis and others, 1994). From
1984 through 2002, an average of 4.9
debris flows occurred per year for a total
of 93 events in 81 tributaries (fig. 1, table
1). A total of 14 debris flows occurred in
2001 and 13 in 2002, the most prolific 2-
year period in the record. The number of
debris flows in a given year is not directly
related to the total amount of summer pre-
cipitation (fig. 1) but instead is a function
of the number of intense precipitation
events and where they occur in Grand
Canyon (Webb and others, 1999a).
Several tributaries had more than one
debris flow between 1984 and 2002. For
example, 75 Mile and Monument Creeks
each had three debris flows, suggesting
that storms are not randomly distributed
in Grand Canyon, at least during this time
period. Multiple debris flows from a sin-
gle drainage also suggest that slope and

@ Number of Debris Flows
=e Summer Precipitation

15 , : + r 300
g 4250 »
g 5
2 3
@ 10 4200 8
$ 2
@ a
4 4150 =
° z
3 s
2 5F jioo $
ee 3
450 2
0 0
1980 1985 1990 1995 2000 2005
Year

Figure 1. Number of debris flows in Grand Canyon and summer precipitation at Grand
Canyon National Park Airport. Rainfall records are missing for the summer of 1998 and 1999.

U.S. Department of the Interior
U.S. Geological Survey

USGS Fact Sheet 065-03
‘August 2003


i a he

Figure 2. Replicate photographs of the debris fan at Bright Angel Creek (river mile 87.8-R). (A) (February 5, 1890) Bright Angel Creek, a perennial
stream, has its headwaters on the North Rim. In 1890, the channel is distinct, relatively narrow, and hooks right to join the Colorado River
beyond the cliff at lower right (photograph by Robert Brewster Stanton). (B) (February 13, 1991) Beginning in December 1966, several debris
flows have significantly changed the debris fan, moving the edge of the debris fan closer to the Colorado River (photograph byTed Melis).

channel destabilization caused by the ini-
tial event may lead to repeated events
until either the destabilized sediment is
removed or sufficient respite from severe
storms allows stabilization. Several debris
flows can result from a single local thun-
derstorm, which may be of sufficient size
to cover adjacent tributary canyons simul-
taneously. As a result, debris-flow fre-
quency in Grand Canyon does not appear
to be uniformly distributed in space. Of
the 19 debris flows in 2000 and 2001,
many of the deposits extended into the
river and 5 created major changes in the
river.

Repeat Photography (1890 —
1983)

Repeat photography has been used
successfully in Grand Canyon in numer-

ous studies to document long-term
changes in both terrestrial ecology and
geomorphology (Turner and Karpiscak,
1980; Stephens and Shoemaker, 1987;
Webb, 1996; Webb and others, 1989,
1999b). This type of scientific photogra-
phy is particularly useful for evaluating
the types of landscape changes associated
with debris flows (fig. 2). For Grand Can-
yon, the year with the most abundant
widespread coverage is 1890, when the
well-documented Stanton _ expedition
occurred (Webb, 1996). Debris flows that
occurred between 1984 and the year a
photograph was matched were not
included in this analysis, yielding a
debris-flow record from 1890 to 1983.
By developing a time series of histori-
cal photographs for specific debris fans,
we were able to bracket when debris
flows occurred at the mouths of tributar-

ies. For example, at Prospect Canyon, 121
of 232 historical photographs were
matched to provide detailed reconstruc-
tion of debris-flow occurrence (6 events
between 1939 and 1995) and changes to
Lava Falls Rapid (Webb and _ others,
1999a). For some tributaries, the dates of
debris flows could be determined to
within 1 year; for others, we know only
that one or more debris flows occurred
during the period of record (1890-1983).
Detailed descriptions of the repeat pho-
tography collection and the criteria used
to identify debris flows are given in Melis
and others (1994), Webb (1996), and
Webb and others (1999b).

We also analyzed several sets of low-
altitude aerial photographs taken between
1935 and 1984 for evidence of debris
flows. In 1935, aerial photographs of river
miles 0 to 61 and 225 to 280 were taken at



Table 1. Frequency of debris flows in Grand Canyon, Arizona

Period Interval Debris Tributaries in sample Percent of Tributaries per year Debris flows per year
(yrs) flows “Debris flows Nodebrisflows Total POPUIation “So pie Population | Sample Population
1890-1983 94 93 84 63 147 19.9 0.89 45 10 5.0
1984-2002 19 93 81 659 740 100.0 43 43 49 49

a scale of 1:31,800; a second set of unre-
lated photographs, taken in November
1935, recorded river miles 87 to 129; and
a third set was taken in 1938 at a scale of
about 1:20,000 and recorded river miles
211 to 280. Spatially continuous aerial
photography of the river corridor was
taken in 1965, 1973, and annually
between 1980 and 2002.

The combination of repeat and aerial
photography resulted in the detection of
108 debris flows in Grand Canyon tribu-
taries that occurred between 1890 and
1983. Combining these results with
observations between 1984 and 2002, 201
debris flows are known to have occurred
in Grand Canyon tributaries since 1890.
In order to determine the frequency of
debris flows before 1984, we evaluated
the 445 Stanton photographs taken in
1890 and matched between 1990 and
1995 (Webb, 1996). These photographs
provide upstream and downstream views:
of the river corridor at approximately 1
km intervals, including images of many
major rapids. This remarkable set of pho-
tographs is an essentially unbiased sample
of tributary debris fans, 147 of which are
depicted with sufficient coverage for
proper evaluation of significant geomor-
phic change. One important attribute of
this data set is that the photographic
record captures only the mouths of tribu-
taries, therefore the resulting frequency
estimates refer only to debris flows large
enough to reach the Colorado River.

The photographic evidence reveals
that 93 debris flows occurred at 84 of 147
tributaries between 1890 and 1984 (table
1), indicating that 57% of the tributaries
with photographic data had at least one
historical debris flow. In the remaining
43% of tributaries, debris flows are
assumed to have occurred at a frequency
of less than one per century. Only 6% of
tributaries had 2 or more debris flows in
the last century, up to a maximum of 6 at
Lava Falls Rapid (including the 1995
event; Webb and others, 1999a). Taking

this data set as an unbiased sample of the
entire population of 740 tributaries, we
calculate a rate of occurrence of 5.0/yr in
the entire canyon for the period of 1890 to
1983, which is statistically identical to the
4.9/yr frequency observed between 1984
through 2002. The rate of debris flows
before 1984 may actually have been
higher because smaller events may not
have been recorded owing to reworking
during the pre-dam (before 1963) part of
the record, when annual floods could eas-
ily remove the evidence of smaller events.

Radiogenic and Cosmogenic
Dating

Radiogenic and cosmogenic dating
methods have been used to date prehis-
toric Grand Canyon debris flows (Melis
and others, 1984; Hereford and others,
1998). The oldest radiocarbon date on a
Grand Canyon debris flow is 5,410+175
yrs BP (before present) from a small can-
yon at river mile 63.3 (Melis and others,
1994), and dates of 2 ka (kiloannum,
thousands of years ago) or younger are
common (Hereford and others, 1998).
Cosmogenic 5He dating (age-dating using
Helium-3 that has been produced by cos-
mic rays) has been applied to Holocene
debris-flow deposits that contain olivine-
rich basalts (Webb and others, 1999a).

The oldest debris flow known in
Grand Canyon occurred in Whitmore
Canyon (river mile 189) and was dated at
8.50.2 ka using cosmogenic *He (Fenton
and others, 2001); this debris flow did not
reach the Colorado River. The largest
Holocene debris flow to reach the Colo-
rado River, from Prospect Canyon at river
mile 179.4, was dated at 3.00.5 ka using
this method; several smaller flows at this
site were dated at 1-2 ka (Webb and oth-
ers, 1999a). Hereford and others (1998)
used radiogenic and cosmogenic dates to
calibrate a dissolution-pit dating method
for Grand Canyon debris-flow surfaces,
thereby estimating frequency data for 26
tributaries. These studies suggest that his-

toric debris-flow activity in Grand Can-
yon is a continuation of prehistoric
activity that dates back to the early
Holocene. Further research on prehistoric
debris flows may address the question of
whether debris-flow frequency in the his-
toric period represents a change from con-
ditions in the rest of the Holocene.

INITIATION MECHANISMS
FOR DEBRIS FLOWS

Debris flows in Grand Canyon are ini-
tiated by a combination of intense precipi-
tation and subsequent slope failure in
source sediment. Detailed discussions of
the hydroclimatology that contributes to
debris-flow initiation are presented in
Melis and others (1994), Griffiths and
others (1997), and Webb and others
(1999a). Intense summer or prolonged
winter precipitation causes slope failures
in either bedrock or colluvium, and these
failures mobilize into debris flows that
can flow as far as 22 km (Webb and oth-
ers, 1989).

In Grand Canyon, Melis and others
(1994) identified four debris-flow initia-
tion mechanisms: (1) direct failure of
weathered bedrock, (2) streamflow runoff
falling directly onto colluvium at the base
of cliffs (the “firehose effect,” Johnson
and Rodine 1984), (3) direct failure of
colluvium by saturation or undercutting
along channels, and (4) combinations of
these mechanisms.

Observed initiation mechanisms for
selected debris flows in Grand Canyon
from 1939 to 2002 appear in Figure 3.
Owing to the steep, commonly inaccessi-
ble topography, initiation points for many
recent debris flows could not be deter-
mined. These data include 51 debris flows
occurring between 1984 and 2001 and 17
debris flows occurring between 1890 and
1983 with known initiation mechanisms
(Cooley and others, 1977). The firehose
effect is the most common. initiation
mechanism (62%) for debris flows in
Grand Canyon. This mechanism requires