WSP05376

• File Number: 8220.101.17
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 2/1/2001
• Title: Monitoring of Coarse Sediment Inputs to the Colorado River in Grand Canyon
• Water Supply Pro - Doc Type: Report/Study

t:xceeding the m;Jximum Aood that h;JS
passed that fan {O date would be caust: LO
again monitor the dt:bris fan for change.
Conversely. changt: in any of tht:st:
parameters. panicularly after large ftoods.
is suggeslive of debris-fan reworking and
additional field-work is necessary. New
measures of fan volume and panicle-size
distribution are necessary to detennine
the fraction of deposit and classes of par-
ticles reworked by a given ~ood discharge
at the site. Effecls on the rapid are again
measured by water-surface fall through
the rapid. surface velocity. and calcula-
tions of slream power. New measures also
reser the baselint: for future measures of
rt:working. Changt:s in fan volume cou-
pled with particle-size dala can be used to
es.imale amount and sizes of coarse parti-
cles redistributed downstream to the pool
or cobble bars.

Fine Sediment

Changes in debris fans can be used to
assess the overall contributions of sand
from small tributaries to the upper reacht:s
of Grand Canyon. where sand bars are
diminished. Sand bars may be covered by
debris nows. eliminating their usefulness
as camping beaches. Monitoring of sand-
bar size downstream from aggraded
debris fans should be a consideration of
any long.lenn monitoring plan for coarse
sediment inpuls. The volume of fint: sedi-
ment in the upper pool could be moni-
tored in concert with debris fans by multi-
beam bathymetric measurt:mt:nts. As
reworking ~oods change the debris fan.
fine sediment is liberated from the upper
pool and is made available for storage
downstream in sand bars.

Benefits of Debris-Fan
Monitoring

Dala collected from the monitoring of
debris Rows will havt: a number of impor-
tant uses in lhe research and management
of the Colorado River ecosystem in Grand
Canyon. Annual monitoring provides
dt:tailed. up.to-dale information on
changes in Grand Canyon rapids and
eddies. MonilOring also provides an
empirical measure of the quantity and
panicle-size distribution of sediment
input to the Colorado River annually hy
debris flow as well as changes ill substral~

that is importanl to the aqualic food hase.
Aqualic planls use coarse paniclt:s as
anchor poinls. and aquatic invenebralt:s
ust: the space~ oetwt:en coarst: sedimt:nt
as habiLat. Identification of stahle vt:rsus
unstable deposil.~ of coarse panicles will
aide in long-term assessmt:nls nf aquaLic
productivity.
Debris-now monitoring should be pan
of an adaptive managemt:nl program. par-
licularly with respt:ct to changes in borh
aquatic and terrt:sLrial habitat. Addressing
the processes of dt:bris-fan reworking and
annoring makes possihle the dett:rmina-
tion of how much and which size classes
of coarse panicles actually enter the river.
This empirical informalion should be
combined with sedimt:nl-transport models
to estimate the zone of in~uence debris
flows and debris-fan rework.ing havt: on
downstream aqualic habilal.
The long-term effects of debris ~ows
on lhe river corridor are also an impol1ant
consideration. particularly in a river sys-
tem with decreasing amounts of fine-sedi-
ment mantling the channel margins.
Debris-flow inpuLS may fonn a significant
pan of fine-grained sediment inputs. as
well as modifying channel geometry to
enhanct: storage of fine-grained stXfi-
menl. To date. these quantities have been
estimated using a probabilily model of
debris-flow occurrence coupled with gen-
eralized ranges of debris-flow volume and
panicle-size distribution (Webb and oth-
ers. 2(00). Building a dalabase of debris
flow input and reworking will permit the
testing of Ihese estimates as well a~ the
development of a more realistic modd of
debris-Row inputs.

-Roberl H. Webh and Peler G. Griffiths

Selected References

Mdis. TS.. ]997. Geomorphology ofdebri<;
flows and allUVial fans Tn Grand Canyon
National Park. and their influence~ on Ihe
Colorado River below Glen Canyon Dam.
Anzona [Ph.D. Dissenation]: Tucson. Um-
versi(~ of AnlOna. 495 p.
Mel". T.S.. Webb. R.H.. GriffiLhs. PG. and
Wise. TJ.. 1994. Magnitude and frequency
dal~ for histOriC debris flows in Grand
Canyon National Park. and vIClnit\'.
Arizona: U.S. Geological Survey Wa'~r
Resources Inwstigatlons Report 94-42]4.
280 p.
Melis. T.S.. Webb. R,H. and Griffiths. P,G..
IQ97. Debris t10w in Grand Canyon

Pro.peel C.nyon
160 8 M.le
1!iHiMllel
8oK1rockCllnyon
1,}76 Mil. .
Specl... Chaam
127.3 Mile
1:l8.!iIMUe
Cry.I.ICrMk
7'1.1"11.
71.2 Mil.
C.tIS.n.. Cr..k
T.nn.r c.nyon
P.li..d..Cr....
63.3 Mil.
e2.5MiI.
leMn.
JaCk... C.nyon

.

_____ 1995
.1993
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1-e191151
I-e 1989
. '5189

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.1995

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.198.4

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t-e 1G87

. '994

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c
~ 50
.! 40
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<! 30

'0 20
.! 10
E
,
z

20 40 50 80
~nl Constriction. C.,

Figure S. Percent constrictions of debris
fans. A. Error bar indicates change in
constriction as a result of the 1996 flood with
date of debris flow (Webb and others. 1999).
B. All debris fans in Grand Canyon.

100

National Park: peak discharges. flow transfor-
mations. and hydrographs. in Chen. e. (ed.).
Debris-flow Hazards and Mitigation: Mechan-. }
ics. prediclion. and assessment. Proceedings o{' '.~
firstlntemallonal Conference: New York.
American Society of Civil Engineers. p. 727-
736.
Webb. R,H.. Griffiths. P.G.. Mel;s. T.S.. ElliOl!.
J.G.. and Pizzuto. J.E.. 1 999a. Reworking of
debris fans by the 1996 controllt:d flood in
Grand Canyon. In Wt:bb. R.H.. Schmidt. J.e..
Valdez. R.A.. and Marzolf. G.R.. The 1996
flood in Grand Canyon: Scientific experiment
and management demonstration: Washington.
D.C. American Geophysical Union. Geophysi-
cal Monograph. p. 37-51.
Webb. R.H.. Melis. T.S.. Griffiths. P.G.. EllioH.
lG.. Cerling. T.E.. Poreda. RJ. Wise. T.W. and
Pizzuto. J.E.. I 999b. Lava Falls Rapid in
Grand Canyon. EffeClS of lale Holoct:ne debris
flows on the Colorado River: U.S. Geological
Survey ProfeSSional Paper 1591. 90 p.
Wehb. R.H.. Griffiths. P.G.. Melis. T.S.. and Han.
ley_ D.R.. 2000. Sediment delivery by ungaged
IribU!aries of Ihe Colorado River in Grand Can-
yon. U.S. Geological Survey Waler-Resources
Invesllgations ReJXlrt 00-4055. 67 p.

For further information, conlact:
Robert H, Wehh
U.S. Geological Survey1675 W. Anklam Road
Tucson. AZ 85745
E-mail: [e-mail]

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