debris flows huachucamtns

8/6/2019 Debris Flows Huachucamtns

1/49

CONTROLS ON THE ORIGIN ANDRECURRENCE OF DEBRIS FLOWS IN

THE HUACHUCA MOUNTAINS,

SOUTHEASTERN ARIZONAby

Ellen E. W ohl and Philip A. Pearthree

Arizona Geological SurveyOpen-File Report 90-6

October 1990


2/49


3/49


4/49

Where the t r i b u t a r y channels en t e r l a rg e r, t runk channels ,

th e debr i s flows se rve as the main source o f very coarse

sediment . The l oca l s lope and coarse p a r t i c l e d i s t r ibu t ion of

th e t runk channel depends on the competence of th e channel to

t r a n s p o r t th e mate r i a l in t roduced by debr i s f lows. Where th e

sm a l l e r channels dra in d i rec t ly to th e mountain f ron t , debr i s

flows c rea t e ex tens ive debr i s fans which dominate th e morphology

o f the basin-range boundary.

Recurrence i n t e r v a l s between debr i s f lows in th e dra inage

bas ins of the Huachuca Mountains are probably cont ro l led by

complex i n t e r ac t ions among cl imate ,fo re s t

f i r e s , an d s lope

processes . F i r e s des t roy the pro tec t ive vege ta t ion t h a t

s t a b i l i z e s the upper catchment s lopes an d i n h i b i t s eros ion .

However, not every f i r e t h a t burns a catchment causes debr i s

f lows, because s u f f i c i e n t weathered mate r i a l must accumulate in

th e upper channel reaches to i n i t i a t e a l a rge debr i s f low. I f

such accumulation has n ot occurred, th e mate r i a l in t roduced to a


5/49

Ju ly 1988. I n i t i a l l y, the i n t en t of our s tudy was to

cha rac t e r i ze t h i s s ing l e debr i s f low, but a reconnaissance of

other drainages in the area revealed evidence o f many

contemporary and o lde r deb r i s flows. By examining evidence l e f t

by h i s t o r i c a l deb r i s f lows, which occurred in 1988 and 1977, and

debr i s - f low depos i t s going back to the Ple i s tocene , we were able

to draw in fe rences about recur rence i n t e r v a l s an d the geomorphic

ro l e of debr i s flows in t h i s semiar id mountain range.

Debris flows are s lu r ry flows of poor ly-sor ted rock and s o i l

deb r i s mixed with 10-20% water (Costa, 1984). Debris f lows have

a s t r eng th an d v i s c o s i t y much higher than those of water flows,an d are respons ib le fo r s i g n i f i c a n t eros ion an d depos i t ion in a

wide range o f environments (Pat ton, 1988). They have been

repor ted in humid temperate mountains (P ie r son , 1980; Osterkamp

an d o the r s , 1986; Church an d Miles , 1987; Kochel, 1987; Mil l s ,

1989), a lp ine regions (Eisbacher an d Klague, 1984; Kotarba and

o the r s , 1987; Van S t e i j n an d o the r s , 1988), an d a r id an d semiarid


6/49

during the f i r e s , are swif t ly gul l ied an d des t ab i l i zed (Wells,

1 9 8 7 ) . With the except ion of the work of Beaty (1990) in the

White Mountains, s tud ie s of debr i s flows in southern Cal i forn ia

have concentrated pr imar i ly on th e h i l l s l o p e processes ac t ive in

a bas in immediately following a f i r e , r a t h e r than on the long

term recur rence i n t e r v a l s of debr i s f lows, an d t h e i r ro le in

channel an d s lope evolu t ion . Our work in the Huachuca Mountains

d i f f e r s in t h a t we are concentrat ing on the long-term geomorphic

e f f e c t s of debr i s flows in a t ec ton ica l ly s t ab le , semiar id

mountain reg ion .

PHYSICAL SETTING

The phys ica l c h a r a c t e r i s t i c s of th e drainage basins an d the

c l imate of the area are important var iab les t h a t cont r ibu te to

the occurrence of debr is flows in the Huachuca Mountains o f

southeas te rn Arizona (Figure 1 ). The s tudy area ranges in

e leva t ion from 1300 to 2300 meters, with vegeta t ion grading


7/49

eas t - f ac ing s lopes , reaching th icknesses o f on e meter, while i t

seldom reaches a h a l f meter in th ickness on southward s lopes .

Rela t ive ly in tense , sho r t - l i ved prec ip i t a t ion events are

f a i r l y common in an d around the Huachuca Mountains. Annual

prec ip i t a t ion over the area averages 64 cm, most of which f a l l s

in the l a t e summer ra iny season (Se l le rs an d H i l l , 1974). The

high i n t ens i ty "summer monsoon" r a in s o f ea r ly Ju ly t o ear ly

September t yp ica l ly occur as sca t te red convect ional thunders torms

t r i gge red and enhanced by in tense sur face heat ing an d orographic

e f f e c t s . Summer moisture comes from two sources ; (1)

southeas te r ly flow around the western limb of th e Bermuda high,

which en t r a in s moist a i r from the Gulf of Mexico into Arizona,

and (2) t r o p i c a l storms an d assoc ia ted cu to ff lows or low

pressure t roughs from the eas te rn North Pac i f i c t h a t move in land

o r d i s s i p a t e o f f o f Baja Cal i forn ia (Hirschboeck, 1985; Reyes an d

Cadet, 1988). A secondary prec ip i t a t ion maximum occurs in

November-April, from cyclonic storms and f r o n t a l systems


8/49

t r i gge red a s e r i e s of debr i s flows in smal l t r i b u t a r y canyons

affec ted by th e f i r e (Figure 2) . Two of these debr i s flows, in

Dorothy Ryan an d Manzanita Canyons, are r ep re sen ta t i ve of the

types of canyons in which debr i s flows occurred, and the e f f e c t

of the f lows on the surrounding geomorphic systems.

Veloc i t ies and peak discharges of debr i s flows can be

est imated from the superelevat ion of flow a t channel bends.

Elevat ion d i f f e r ence across the bend (he), e f f e c t i v e channel

width (W), and th e radius of curva ture (Re) , may be used to

ca l cu l a t e mean veloc i ty (v) around a bend:

v = [gRehe/ W)0.5

where g i s grav i t a t iona l acce le ra t ion (Johnson, 1979; Webb and

others , 1987). The r e su l t an t discharge, Q, i s then es t imated

from

Q = A v

where A i s c ros s - sec t iona l area. As noted by Webb and others


9/49

dra in s the nor thern s lope of Bob Thompson Peak. The 1.25 km 2

catchmenti s

formed on s i l i c eous volcan ics , with a r e l i e f r a t i o(H/L) of 0 .33. Channel s lopes range from 1.5 to 40, but tend t o

be in th e range o f 6-15. Hil l s lopes in the upper drainage basin

a re s t eep , t y p i c a l l y 30 to 40. The channel dra ins d i r e c t l y to

th e mountain f ron t , r a t h e r than in to a l a rge r channel , and has

formed a deb r i s fan a t the basin-range boundary. Measurements of

th e debr i s depos i ted on t h i s fan by th e 1988 flow i nd i ca t e t h a t

i t had a volume of approximately 13,000 m Est imated peak

v e l o c i t i e s ranged from about 2 to 4 ml s , with peak discharges of

48-75 ~ / s (Table 1).Manzanita Canyon dra ins the nor th s lope o f Ash Peak, and i s

t r i b u t a r y to th e South Fork of Ash Canyon. The 0.60 km 2

Manzanita catchment i s underla in by s i l i c eous volcan ics an d Naco

Group carbona tes , with a r e l i e f r a t i o of 0.38. Channel s lopes

average 6-10. The 1988 debris flow had a volume o f about 10,000

m3 , with peak v e l o c i t i e s of approximately 2-7 m/s; peak


10/49

systems. Evidence of debr i s flows f a l l s in to th ree genera l

ca t ego r i e s : (1) very young depos i t s represen t ing deb r i s flowst h a t probably occurred subsequent to a major f i r e in 1977; (2)

o lde r deb r i s flow depos i t s in fo res ted areas t h a t have n o t burned

i n s e v e r a l decades o r more; and (3) sequences of deb r i s flow

d e p o s i t s along th e drainages t h a t exper ienced deb r i s f lows in

1988.

A major f i r e burned much of the cen t r a l Huachuca Mountains

in th e summer of 1977 (see f igure 2 ) . A des t ruc t i ve " f lood" was

repor ted on a t r i b u t a r y of Mi l le r Creek in 1977 fol lowing th e

f i r e (Na t iona l Fores t Service , unpubl ished r epo r t , 1978) .We

examined photographs of f lood damage an d depos i t s taken in 1977,

an d v i s i t e d th e s i t e o f the f lood ing . Based on th e very coarse

an d very poor ly so r t ed depos i t s , an d the ex ten t o f channel scour

f a r t h e r upstream, we conclude t h a t t h i s " f lood" was ac tua l ly a

deb r i s f low. Simi la r phys ica l evidence o f r e cen t deb r i s flow

a c t i v i t y was found in the steep upper por t ions of Stump an d


11/49

drainages t o Brown Canyon and in th e upper reaches of Brown Creek

i t s e l f . The ages of these debr i sflows

are no t known, but maturet r ee s growing on the deposi ts ind ica te t h a t they are of some

an t iqu i ty.

Several debr i s - f low depos i t s with s o i l p r o f i l e s developed in

them are exposed in Dorothy Ryan Canyon a t a s i t e about 200 m

upstream from th e head of the a l l u v i a l fan. The t ime t h a t these

depos i t s were a t the surface p r i o r t o bu r i a l , and the approximate

ages of these depos i t s , can be estimated by comparing s o i l

prope r t i e s with s o i l s of other chronosequences developed in

southernNew

Mexico and Arizona ( fo r example, Gile an d others ,1981; Pear three an d Calvo, 1987).

Four d i s t i n c t pre-1988 sedimentary un i t s , probably ranging

in ag e from l a t e Ple i s tocene to l a t e Holocene, a re exposed in th e

channel wal l a t on e loca t ion along Dorothy Ryan Creek ( f igure 4;

t ab l e 2) . The lowermost depos i t (Unit 4) i s c l e a r l y a debr i s

flow depos i t , conta in ing la rge boulders in a f ine matr ix . This


12/49

developed on i t , with a s u r f i c i a l horizon (3A) and a weak

a r g i l l i c horizon (3Bt) , suggest ing t h a t i t was exposed fo r

severa l thousand years or more pr io r to bur i a l ; i t may be of

ea r ly Holocene age. Unit 2 i s predominantly pebbles an d cobbles ,

and c l a s t s exhib i t a weak imbricat ion, implying t h a t i t i s a

wate r- l a id depos i t . There i s no c lea r evidence of a s o i l p r o f i l e

s p e c i f i c a l l y assoc ia ted with t h i s un i t , al though t he re i s a

s l i g h t t e x t u r a l inc rease between the upper (2C1) and lower (2C2)

horizons. The lack of s o i l - p r o f i l e development sugges t s t h a t

t h i s u n i t was buried by un i t 1 f a i r l y soon a f t e r i t was

depos i ted .

Evidence fo r the t ime s ince th e l a s t l a rge debr is flow in

Dorothy Ryan Canyon i s contained in the youngest preh i s to r i c

debr i s flow depos i t in t h i s s t r a t i g r a p h i c exposure (Unit 1).

This depos i t has a weak s o i l p r o f i l e developed in i t , composed of

a s u r f i c i a l A horizon an d cambic (weak s t ruc tu ra l ) B horizon.

So i l p r o f i l e s l i ke t h i s develop over periods of seve ra l hundred


13/49

t h a t i n t e r v a l s between o c c u r r e n c e s o f l a r g e d e b r i s f l ows i n t h i s

a r e a a r e f a i r l y l o n g . There i s p h y s i c a l ev idence o f o n l y t h r e e

Holocene d e b r i s f l ows (two i n t h e exposure and t h e 1988 e v e n t ) ,

s u g g e s t i n g a s e v e r a l t housand y e a r r e c u r r e n c e i n t e r v a l f o r l a r g e

d e b r i s f l o w s . S o i l p r o f i l e s deve loped i n t h e d e b r i s f low

d e p o s i t s s u g g e s t hundreds t o a fe w t housand y e a r s o f

n o n d e p o s i t i o n between d e b r i s f lows . C l e a r l y, howeve r, t h i s one

exposure ma y n o t p r o v i d e a comple t e r e c o r d o f Holocene d e b r i s

f l o w s . A t t h i s t i m e , an e s t i m a t e o f s e v e r a l hundred t o s e v e r a l

t housand y e a r s f o r d e b r i s f low r e c u r r e n c e seems r e a s o n a b l e .

Proposed r a d i o c a r b o n d a t i n g and deve lopmen t o f t r e e - r i n g

c h r o n o l o g i e s ma y p r o v i d e more a c c u r a t e e s t i m a t e s o f r e c u r r e n c e

i n t e r v a l s . The e s t i m a t e d r e c u r r e n c e i n t e r v a l f o r t h e Huachuca

d e b r i s f l ows i s l o n g e r t h a n t h e more t i g h t l y - c o n s t r a i n e d 300-350

y e a r r e c u r r e n c e i n t e r v a l f o r d e b r i s f l ows i n t h e White Moun ta in s

o f C a l i f o r n i a (Bea ty, 1990) .


14/49

Tr ibu ta ry channels

Debris flows occur in r e l a t ive ly smal l , s teep drainage

basins l i ke Dorothy Ryan and Manzanita Canyons. Examination of

th e Dorothy Ryan catchment soon a f t e r the 1988 deb r i s flow

ind ica ted t h a t the flow did not have a d i sc re t e f a i l u r e source ,

such as a slump or a l ands l ide . Rather, th e s teep upper por t ion

of the bas in , which was severely burned, ha d widespread r i l l i n g

an d s o i l creep. Debris flow mobil izat ion from r i l l i n g has a lso

been descr ibed in cen t r a l Cal i forn ia (Johnson and Rodine, 1984).

The rap id in t roduc t ion of mater ia l i n to t he uppermost channels of

Dorothy Ryan canyon as a r e s u l t of r i l l i n g an d slopewash

apparen t ly des tab i l ized the weathered mater ia l prev ious ly

accumulated there , t r i gge r ing the debris f low.

As th e flow moved downward, i t scoured th e channel to

bedrock, c rea t ing a cross-sec t iona l ly t r apezo ida l t r ench (Figure

Sa) . Deposi t ion occurred l oca l ly where slope and/or confinement

decreased. In the lower port ion of the catchment th e flow


15/49

va l l ey s ide walls an d the upstream reaches o f th e catchment. The

percentage of bedrock exposed in th e channel decreases with t ime ,

and th e percentage of mater ia l sand-sized or f i n e r inc reases

(Figure 5d) .

Debris flows appear to be the channel-forming events on

these t r i b u t a r y drainages . Subsequent water f lows gradua l ly

modify th e upper reaches of th e channel, an d i n c i s e th e lower

reaches , where th e debr i s f lows are depos i t i ona l . Channel c r o s s

sec t i ons during an d immediately a f t e r the debr i s flow are l imi ted

on th e bed by the e ros iona l r e s i s t ance of th e bedrock, an d in th e

banks by bedrock, th e binding e f f e c t of t r e e an d shrub roo t s , and

the l a rge p a r t i c l e s i ze an d indura t ion of o lde r deb r i s flow

depos i t s .

Trunk channels

Where th e t r i b u t a r y channels dra in in to t runk channels ,

deb r i s flows se rve as th e main source of coarse (g rea t e r than 1/2

m diameter) sediment. The degree to which t h i s coarse sediment


16/49

weeks of format ion (Figure 6) , bu t i t caused aggrada t ion an d

subsequent i n c i s i o n fo r more than a hundred meters upstream along

Ash Creek. The boulders in t roduced by t h i s and o the r f lows have

n ot been t ranspor ted any s i g n i f i c a n t dis tance downstream, so t h a t

the South Fork has a nonuniform d i s t r i b u t i o n of coarse sediments

(Figure 7 ) . This i s analogous to the c rea t ion and maintenance of

rap ids on th e Colorado River in Grand Canyon Nat iona l Park

through the in t roduc t ion o f l a rge boulders by t r i b u t a r y debr i s

f lows (Webb an d o the r s , 1987).

Stump Creek dra ins 4 km2 an d has seasona l flow. Like Ash

Creek, i t hasa

low r e l i e f r a t i o (0 .19) . TheStump

Creek

drainage was burned in th e 1977 f i r e , an d t r i b u t a r y debr i s f lows

depos i ted coarse sediment which subsequent water f lows have not

been powerful enough to re-work. These e leven-year-o ld boulder

accumulat ions preserve boulder lobe an d levee morphology

c h a r a c t e r i s t i c of debr i s flow depos i t s , al though the f ine r

sediments have been winnowed by water flows (Figure 4 ) .


17/49

f ac to r ; bas in e longa t ion , r e l i e f r a t i o , aspec t , l i t ho logy,

average channel s lope , and drainage dens i t y do not d i f f e r

s i g n i f i c a n t l y among t he t h r ee dra inages .

There i s thus a continuum in terms of th e degree t o which

deb r i s flow depos i t s a f f e c t th e morphology of secondary channels .

Water f lows in the sma l l e s t channels , as exempl i f i ed by stump

Creek, are completely unable to re-work th e debr i s flow depos i t s ,

so t h a t c h a r a c t e r i s t i c debr i s flow depos i t morphologies, such as

boulder lobes and levees , are preserved. The water f lows in

l a rg e r channels , l i k e the South Fork o f Ash Creek, are competent

t o t r anspor t th e coarse sediment in t roduced by deb r i s f lows sho r t

d i s t ances downstream, al though the channel r e t a i n s an uneven

longi tudina l d i s t r i b u t i o n of boulders . Th e l a r g e s t channels ,

such as Mil l e r Creek, completely r e d i s t r i b u t e the boulders

in t roduced by deb r i s flows, so t h a t evidence of debr i s f lows i s

preserved only in t r i b u t a r y channels .

Mountain f ron t a l l u v i a l fans


18/49

1988 debr i s f low depos i ted boulders up to one meter in diameter

on th e d i s t a l po r t i ons o f the fan. Following a deb r i s f low, th e

d e b r i s fans are i n c i s ed by water f lows, so t h a t subsequent deb r i s

f lows tend to be channeled along th e inc i sed pa ths . The l o ca t i on

of debr i s depos i t ion s h i f t s across th e fan with t ime , however,

c rea t i ng a complex mosaic of depos i t s o f va r ious ages . Older

depos i t s a re wel l - indura ted due to secondary carbona te an d c lay

accumulat ion.

Geomorphic work an d e ff e c t i venes s of debr i s f lows

Debris f lows in th e Huachucas can be cons idered in te rms o f

geomorphic work, which Wolman an d Mi l l e r (1960) equated wi th th e

amount o f sediment t r anspor ted during ind iv idua l even ts , and

geomorphic e ff ec t i venes s , def ined as th e pe r s i s t ence o f the

e f f e c t s of an i nd i v i d u a l event (Wolman and Gerson, 1978) . For

th e t r i b u t a r y catchments an d debr i s fans , deb r i s f lows are

c l e a r l y the dominant even ts in terms of both geomorphic work an d

e ff ec t i venes s ; they t r a n s p o r t the major i ty o f coarse sediment


19/49

J a r r e t t , 1981), an d in the Oregon coas t Range, where episodic

debr i s flows are the dominant sed iment - t ranspor t ing agent in

f i r s t - and second-order basins (Benda an d Dunne, 1987).

For the l a rge r channels , the e f f e c t of moderate, more

f requent water flows becomes increasingly important as catchment

s i ze increases . Debris flows perform l i t t l e geomorphic work in

these channels, but r a t h e r in t roduce coarse sediment t h a t may be

d i f f i c u l t fo r water f loods to t r anspo r t .

CONTROLS ON DEBRIS FLOW RECURRENCE

Several fac tors i n t e r a c t to cont ro l the occurrence of debr is

flows in the Huachuca Mountains. These inc lude ; (i) basin

l i tho logy, ( i i ) cl imate, par t i cu l a r ly p r e c i p i t a t i o n frequency,

i n t ens i ty and dura t ion , ( i i i ) basin s lope , ( iv) fo re s t f i r e

frequency, an d exten t an d sever i ty of burn, (v) th ickness and

ex t en t of s lope r ego l i t h , (vi) vegetat ion cover, par t i cu l a r ly

f o r e s t maturat ion t ime an d character of ground cover, (v i i )


20/49

uppe r b a s i n s l o p e s , expos ing t h e r e g o l i t h t o e r o s i o n . Charcoa l

was found i n abou t o n e - h a l f o f t h e o l d e r d e b r i s f low d e p o s i t s

exposed a l o n g Dorothy Ryan wash, s u g g e s t i n g a p e r s i s t e n t l i n k

between d e b r i s f low o c c u r r e n c e and f i r e s . T h e r e f o r e , t h e

dynamics o f f o r e s t f i r e s a r e o f pr ime impor t ance i n u n d e r s t a n d i n g

d e b r i s f low o c c u r r e n c e .

Occur rence o f F o r e s t F i r e s

F o r e s t f i r e s i n t h e Huachuca Mounta ins t y p i c a l l y occur

d u r i n g t h e summer and autumn due t o l i g h t n i n g s t r i k e s . The h o t ,

d r y e a r l y summer i s t h e main f i r e season because l i g h t n i n g

s t r i k e s a s s o c i a t e d wi th t h e beg inn ing o f t h e summer monsoon

season o f t e n p r e c e d e t h e more s U b s t a n t i a l r a i n f a l l s t h a t o c c u r

l a t e r i n t h e monsoon s e a s o n .

The f i r e h i s t o r i e s o f s e v e r a l moun ta in r a n g e s i n

s o u t h e a s t e r n Ar izona have been compi led from F o r e s t S e r v i c e

documents and f i r e - s c a r r e c o r d s on t r e e s (Swetnam and B e t a n c o u r t ,

1 9 9 0 ) . These f i r e h i s t o r i e s i n d i c a t e t h a t r e g i o n a l c l i m a t e


21/49

a s s o c i a t e d w i t h enhanced s p r i n g and f a l l p r e c i p i t a t i o n (Andrade

and S e l l e r s , 1988) due t o a more s i n u o u s , o r m e r i d i o n a l , f low o f

t h e w e s t e r l i e s , which a l lows d i s s i p a t i n g t r o p i c a l c y c l o n e s o f f

t h e c o a s t o f Baja C a l i f o r n i a t o move n o r t h e a s t i n t o t h e s t u d y

r e g i o n (Webb and B e t a n c o u r t , i n p r e s s ) . T h i s i n f l u x o f m o i s t u r e

d u r i n g t h e c r i t i c a l burn ing p e r i o d r e d u c e s f i r e f r equency d u r i n g

p e r i o d s o f ENSO c i r c u l a t i o n (Swetnam, i n p r e s s ) .

F o r e s t f i r e s i n t h e Huachuca Mounta ins a r e a l s o i n f l u e n c e d

by t h e management p r a c t i c e s o f t h e u n i t e d s t a t e s F o r e s t S e r v i c e .

R i g o r o u s f i r e s u p p r e s s i o n began i n t h e e a r l y 1930s , and was

a s s o c i a t e d w i t h l a r g e r and more s e v e r e , b u t l e s s f r e q u e n t , f i r e s .

I n t h e 1970s p o l i c y s h i f t e d toward c o n t r o l l e d b u r n s , a c l o s e r

a p p r o x i m a t i o n o f t h e n a t u r a l l y - o c c u r r i n g , h igh f r equency, lo w

i n t e n s i t y b u r n s (Swetnam, i n p r e s s ) .

The e f f e c t s o f management p r a c t i c e s a r e supe r imposed on t h e

l a r g e r - s c a l e c o n t r o l s o f f o r e s t m a t u r a t i o n and c l i m a t e . Swetnam

( i n p r e s s ) found t h a t l a r g e f i r e y e a r s t e n d t o o c c u r a y e a r o r


22/49

however, because the t r e e s were essen t i a l ly complete ly burned in

much of th e f i r e area . Recurrence i n t e rva l s between f i r e s in th e

oak- jun iper woodlands on the lower slopes are probably c lose r t o

f i f t y to a hundred years , based on analogy with the chapar ra l

a reas o f southern Cal i forn ia . His to r i ca l l y, even l a rge f i r e s

have not extended over the e n t i r e Huachuca range, crea t ing a

mosaic e f f e c t with burned and unburned areas .

Prec ip i t a t ion Charac t e r i s t i c s

The r a i n f a l l t h a t t r iggered the debr i s flow in Dorothy Ryan

Canyon apparent ly was not extreme. Figure 8a shows th e da i ly

r a i n f a l l t o t a l s fo r th e summer of 1988 from Coronado National

Monument headquar te r s , located about 3 km sou thea s t of Dorothy

Ryan Canyon. The debr i s flow occurred on Ju ly 11; al though

recorded r a i n f a l l on t h i s date was modest, i t was the f i f t h

consecut ive da y with measurable prec ip i t a t ion , so s o i l moisture

conten t may have been high. Measurements of r a i n f a l l i n t ens i ty

are not ava i l ab l e , but loca l res idents" repor ted t h a t i t ra ined


23/49

reached fo r southwestern Br i t i sh Columbia (Church and Miles ,

1987), New Zealand (Pierson, 1980), southern Ca l i fo rn i a (Weirich,

1987; Wells and others , 1987), an d Colorado (Costa and J a r r e t t ,

1981). Caine (1980) has expressed the th reshold for debr i s flow

occurrence numerical ly through a l imi t ing curve r e l a t i n g r a i n f a l l

i n t e n s i t y and dura t ion . Unfortunate ly, p r e c i p i t a t i o n i n t e n s i t y

an d dura t ion values are not ava i l a b l e fo r th e Huachuca Mountains.

Channel Sediment Accumulation

Accumulation of sediment in th e channel , p a r t i c u l a r l y in th e

upper reaches , i s probably th e primary l im i t i ng con t ro l on deb r i s

f lows. We have reached t h i s conclus ion because th e f i r e s in an y

one basin t h a t heads in th e upper e leva t ions o f th e range occur

f a i r l y f requent ly, an d r e l a t i v e l y in tense r a i n f a l l i s qu i t e

common dur ing th e summer "monsoon" season in sou theas te rn

Arizona.

Rain fa l l fol lowing a f o r e s t f i r e wi l l n ot necessar i ly

t r i g g e r a deb r i s f low. We observed a t r i b u t a r y dra inage on th e


24/49

d e s t a b i l i z a t i o n and debr i s flow. cos ta an d J a r r e t t (1981) noted

t h a t th e r a t e s o f weathering an d colluvium formation are an

important con t ro l on debr i s flow occurrence in the small mountain

bas ins of Colorado, an d s imi l a r conclusions were reached fo r

southern Cal i forn ia (Florsheim and Kel le r, 1987), southwestern

Norway ( Innes , 1985), China (Zicheng and J ing , 1987), B r i t i s h

Columbia (Bovis an d Dagg, 1987), and t he Sco t t i sh highlands

(Bal lantyne, 1986; Braz ie r and Ballantyne, 1989).

Sediment l i k e l y accumulates in channels in upper watershed

areas through the gradual in t roduc t ion of weathered h i l l s l o p e

mate r i a l s by processes l i ke r i l l i n g and s o i l creep . These s lope

processes opera te cont inuously, but they a re grea t ly enhanced

fol lowing a fo re s t f i r e , when the ground cover of l e a f l i t t e r and

smal l shrubs i s destroyed. We observed the formation of 5 cm

deep r i l l s within minutes during a heavy thunders torm r a i n f a l l on

the burned upper s lopes of the Dorothy Ryan drainage in september

1988. The enhancement o f r i l l i n g i s probably due to a


25/49

t ab l e dur ing heavy thunderstorm r a in s , f a c i l i t a t i n g s o i l movement

(Howard an d others , 1982; Innes , 1983; Van s t e i j n an d o the r s ,

1988). The rapid development of ex tens ive r i l l networks on

burned s lopes in sou thern Cal i forn ia due t o a hydrophobic s o i l

l ayer has been descr ibed by Wells (1987).

When s lope m a te r i a l i s in t roduced in to th e channel ,

p a r t i c u l a r l y th e s teep upper reaches , i t may; (1) remain

e s s e n t i a l l y in place , because the r e s i s t ance t o movement caused

by roughness o r o the r r e t a rd ing fac tors i s g r e a t e r than th e

t r anspo r t i ng a b i l i t y of the water flow, (2) flow a sho r t way down

th e channel as a water flow or a smal l deb r i s f low t h a t s tops a t

th e f i r s t l oca l reduc t ion in s lope , o r (3) become incorpora ted in

a f u l l - s c a l e debr i s flow which sweeps down th e e n t i r e l eng th of

th e channe l , scouring th e channel to bedrock as i t goes. The

d i s t i n c t i o n between these t h r ee a l t e r n a t i v e s w i l l depend on; ( i )

th e amount of water an d sediment en te r ing the channel from th e

s lopes , and thus th e an teceden t s o i l mois tu re cond i t i ons , th e


26/49

the f i r e - r a i n f a l l events (Figure 9) .

This requirement imposes a minimum recurrence i n t e r v a l on

debr i s flows, and i s presumably more of a l im i t i ng f ac to r than

e i t h e r s u f f i c i e ~ tr a i n f a l l , which occurs f requent ly, o r fo re s t

f i r e s , which m a ~ recur every few decades on th e basin h i l l s i d e s .

Based on surveys of channels affec ted by debr i s flows in 1977, a t

l e a s t t en years , and probably many more, are requ i red before

s u f f i c i e n t mater ia l accumulates in a channel fol lowing a debr i s

flow.

The s lope of th e channel, and th e amount of exposed bedrock,

may also be c ruc ia l in t r igger ing l a rge debr i s f lows. The upper

reaches of t r i b u t a r y channels in th e Huachucas have i r r e g u l a r,

s tepped gradien ts , with very s teep bedrock reaches a l t e rna t ing

with reaches of lower slope. These s teep reaches may se rve as

co l l ec t i ng or concent ra t ing areas fo r water an d sed iment , which

acce le ra te down the s teep reaches an d can then r i p out mate r i a l

s t ab i l i zed by vege ta t ion in the reaches of lower s lope . Erosion


27/49

a normal geomorphic process in t h i s region of t ec ton ic s t a b i l i t y

and r e s i s t a n t bedrock. Debris flows probably occur very

in f requent ly in ind iv idua l drainage bas ins . Because of th e

tremendous amount of work they accomplish, however, they are the

major channel-forming events in the t r i b u t a r y catchments,

scouring to bedrock an d crea t ing channels t h a t a re t rapezoida l in

c ros s - sec t ion . The angles between the bed and banks, and th e

percentage of exposed bedrock, gradual ly decrease as weathered

slope mater ia l accumulates, but the channel r e t a i n s i t s ove ra l l

t rapezoida l shape. Where these t r i bu ta ry channels dra in d i r e c t l y

to th e in te r- range basin, debr i s flow depos i t s form fans an d

aprons of varying s teepness an d s i ze . Where th e t r i bu ta ry

channels en te r l a rge r channels , debr i s flow depos i t s a f f e c t

channel morphology through the in t roduc t ion of very coarse

sediment . The degree to which channel s lope and the downstream

d i s t r i b u t i o n of coarse p a r t i c l e s are con t ro l l ed by debr i s flow

depos i t s depends on the s i ze and discharge of the main channel .


28/49

p o s s i b i l i t i e s . I f th e frequency of debr i s f low occurrence has

var ied through t ime, then presumably th e r a t e and na ture of

channel adjus tment to debr i s flows has a l so va r i ed . I t may be

t h a t c l imate an d f i r e cont ro l the tempo o f deb r i s - f l ow- re l a t ed

geomorphic change in the Huachucas. Periods o f enhanced ENSO

c i r c u l a t i o n and reduced f i r e frequency produce fewer, bu t l a rg e r,

f i r e s , which may f a c i l i t a t e th e i n i t i a t i o n of deb r i s f lows i n t he

s t e ep upper reaches of drainage bas ins ; per iods of f a i r l y

f requen t , smal l f i r e s may r e s u l t in more r ap id r a t e s o f channel

an d h i l l s lope eros ion an d more rap id accumulation of weathered

mate r i a l in th e channels , leading to a s h o r t e r recur rence

i n t e r v a l fo r debr i s f lows. Inc reased unders tand ing o f th e

f ac to r s t h a t c o n t r o l o f debr i s flow occurrence in t h i s semiar id

montane environment would provide a base l ine fo r comparison with

deb r i s f lows in other reg ions .


29/49

REFERENCES CITED

Andrade, E.R. and Se l l e r s , W.O., 1988. El Nino and i t s e f f e c t onprec ip i t a t i on in Arizona an d western New Mexico. Journa l ofClimatology, 8: 403-410.

Ballantyne, C.K., 1986. Landsl ides and s lope f a i l u r e s inScot land: A review. s c o t t i s h Geographical Magazine, 102:134-150.

Beaty, C.B. 1990. Anatomy of a White Mountain debr i s - f low - Themaking of an a l l u v i a l fan. In , A.H. Rachocki and M. Church,eds . , Al luv i a l fans: A f i e l d approach. John Wiley and Sons,New York, p . 69-89.

Benda, L. and Dunne, T. 1987. Sediment rou t ing by deb r i s f low.In , R.L Beschta , T. Blinn, G.E. Grant , G.G. Ic e and F. J .Swanson, eds . , Erosion and sed imenta t ion in the Pac i f i c Rim.proceedings of th e Corva l l i s symposium, IAHS Publ no 165,p.213-223.

Blackwelder, E ., 1928. Mudflow as a geologic agen t in semiar idmountains. Geological Society of America B u l l e t i n , 39: 465-484.

Bovis, M.J. and Dagg, B.R. 1987. Mechanisms of debr i s supply tos teep channels along Howe Sound, southwest B r i t i s h Columbia.In , R.L. Beschta , T. Blinn, G.E. Grant , G.G. Ic e and F. J .Swanson, eds . , Erosion an d sed imenta t ion i n t he Pac i f i c Rim.Proceedings o f the Corva l l i s Symposium, IARS Publ no 165,p.191-200.

Braz ie r, V. and Ballantyne, C.K., 1989. Late Holocene debr i s conel t i i Gl F hi t i g M t i


30/49

Church, M. an d Miles, M.J. , 1987. Meteorological antecedents todebr i s flow in southwestern Br i t i sh Columbia; Some cases tud ie s . In , J .E . Costa and G.F. Wieczorek, eds . , Debrisflow/avalanches: Process , recogni t ion , an d mit iga t ion .Geological Society of America, Reviews in EngineeringGeology, VII: 63-79.

Costa , J . E . , 1984. Physical geomorphology o f deb r i s flows. In,J .E . Costa an d P. J . Fle isher, eds . , Developments andappl ica t ions of geomorphology. spr inger-Ver lag , Ber l in , p.268-317.

Costa , J .E . an d J a r r e t t , R.D., 1981. Debris flows in smal lmountain stream channels of Colorado an d t h e i r hydrologicimpl ica t ions . Bul l e t i n of the Associat ion of EngineeringGeologis ts , XVIII: 309-322.

Eaton, E.C., 1935. Flood an d eros ion con t ro l problems and t h e i rso lu t ion . American soc ie ty o f c i v i l Engineers Transact ions,101: 1302-1330.

Eisbacher, G.H. and Clague, J . J . , 1984. Dest ruc t ive massmovements in high mountains: Hazard and management.Geological Survey of Canada, Paper 84-16, 229 p.

Florsheim, J .L . an d Kel le r, E.A. 1987. Rela t ionsh ips betweenchannel morphology, u n i t stream power, and sediment rou t ingan d s torage in a s teep , bedrock cont ro l led channel . In , R.L.Beschta, T. Blinn, G.E. Grant, G.G. Ice an d F. J . Swanson,eds . , Erosion an d sedimentat ion in the Pac i f i c Rim.Proceedings o f th e Corva l l i s symposium, IAHS Publ no 165,p.279-280.

Gil L H H l J W d G R B 1981 S i l d


31/49

Innes , J . L . , 1983. Debris f lows. Progress in Phys ica l Geography,7: 469-501.

Innes , J . L . , 1985. Lichenometric dat ing of debr i s - f low depos i t son a lp ine c o l l u v i a l fans in southwest Norway. Earth SurfaceProcesses an d Landforms, 10: 519-524.

Johnson, A.M., 1979. Fie ld methods for es t imat ing rheologica lprope r t i e s o f debr i s flows. Unpublished manuscr ipt ,Department o f Geology, Univers i ty o f Cinc inna t i , Cinc inna t i ,Ohio, 36 p.

Johnson, A.M. an d Hampton, M.A., 1969. Subaer ia l an d subaqeousflow of s l u r r i e s . Stanford Univers i ty, School o f EarthSc iences , Stanford, c a l i f o r n i a , 137 p.

Johnson, A.M. and Rodine, J .R . , 1984. Debris flows. In , D.Brunsden and D.B. Pr ior, eds . , Slope i n s t a b i l i t y . Wiley, NewYork, p . 257-361.

Kochel, R.C. , 1987. Holocene debr i s flows in c e n t r a l Virg in ia .In , J .E . Costa and G.F. Wieczorek, e d s . , Debrisf lows/avalanches: Process , recogni t ion , an d mi t iga t i on .Geological Society of America, Reviews in Engineer ingGeology, VII .

Kotarba, A., Kaszowski, L. and Krzemien, K., 1987. High-mountaindenudat ional system of the Pol i sh Ta t ra Mountains. Pol i shAcademy of Sc iences , I n s t i t u t e of Geography an d Spa t i a l

Organizat ion, Geographical Studies Spec ia l I ssue N o . 3 , 106p.

Mil l s , H.H., 1989. Hollow form as a func t ion of boulder s i z e inh V ll d d i h Vi i i


32/49

1990, Geomorphic assessment of f l u v i a l behav ior an d f loodprone on the southern piedmont of the To r t o l i t a Mountains,Pima County, Arizona: Arizona Geological Survey Open-FileReport 90- , 50 p.

Pierson , T.C. , 1980. Erosion and depos i t ion by deb r i s f lows a tMt. Thomas, North Canterbury, New Zealand. Ear th SurfaceProcesses and Landforms, 5: 227-247.

Rasmusson, E.M. and Wallace, J .M. , 1983. Meteoro log ica l a spec t sof th e El Nino/southern Osc i l l a t i on . Science, 222: 1195-1202.

Reyes, S. and Cadet, D.L. , 1988. The southwest branch of th eNorth American monsoon during summer 1979. Monthly WeatherReview, 116: 1175-1187.

Rodine, J . D . , 1974. Analysis of th e mobi l iza t ion of deb r i s flows.PhD d i s s e r t a t i o n , Stanford Univers i ty, Stanford , Cal i fo rn i a ,226 p.

Sco t t , K.M., 1971. or ig in an d sedimentology of 1969 deb r i s flowsnear Glendora, Cal i fo rn i a . USGS P ro fe s s iona l Paper 750-C,C242-C247.

S e l l e r s , W.D. an d H i l l , R.H., 1974. Arizona c l ima t e , 1931-1972.Univers i ty o f Arizona Press , Tucson, 616 p .

swetnam, T.W., in p re s s . Fire h i s to ry and c l imate in th esouthwestern uni ted Sta t e s . proceedings of

Symposiumon

e f f e c t s of f i r e in management of southwestern n a t u r a lr e s o u r c e s , Tucson, AZ, November 14-17, 1988: US Departmentof Agr icu l tu re , Fores t Serv ice , General Technica l Report .


33/49

and f lood frequency o f the san ta Cruz River, Pima County,Arizona. Open-Fi le Repor t 89- .

Weirich, F.H. 1987. Sediment t r anspo r t and depos i t i on by f i r e -r e l a t e d debr i s f lows in southern Cal i fo rn i a . In , R.L.Beschta , T. Bl inn , G.E. Grant , G.G. Ic e and F. J . Swanson,e d s . , Erosion and sed imenta t ion in the P a c i f i c Rim.Proceedings of the Corva l l i s symposium, IAHS Publ no 165,p.283-284.

Wells , W.G., 1987. The e f f e c t s of f i r e on th e genera t ion ofdeb r i s f lows in sou thern Cal i fo rn i a . In , J . E . cos ta and G.F.Wieczorek, eds . , Debris f lows /ava lanches : Process ,r ecogn i t i on , and mi t iga t i on . Geological Socie ty of America,Reviews in Engineer ing Geology, VII: 105-114.

Wells , W.G., Wohlgemuth, P.M. and Campbell, A.G. 1987. Pos t f i r esediment movement by d e b r i s f lows in the Santa YnezMountains, Cal i fo rn i a . In , R.L. Beschta , T. Blinn, G.E.Grant , G.G. Ice and F. J . Swanson, e d s . , Erosion andsedimentat ion

inth e

Pac i f i c Rim.Proceedings of

th eCorva l l i s Symposium, IAHS Publ no 165, p.275-276.

Wolman, M.G. and Gerson, R ., 1978. Rela t ive s c a l e s of t ime ande ff ec t i venes s of c l imate in watershed geomorphology. EarthSurface Processes , 3: 189-208.

Wolman, M.G. and Mil l e r, J . C . , 1960. Magnitude and frequency offo rces in geomorphic processes . Journa l o f Geology, 68 : 54-

74.Zicheng, K. and J ing , L. 1987. Erosion processes and e f f e c t s of

deb r i s f low. In , R.L. Beschta , T. Blinn, G.E. Grant, G.G.d h


34/49

Canyon. Four d i s t i n c t pre -1988 d e p o s i t i o n a l u n i t s a r e a p p a r e n t ,t h r e e o f which a r e i n t e r p r e t e d t o be d e b r i s f low d e p o s i t s . Af a i r l y d i s t i n c t s o i l p r o f i l e deve loped i n each o f t h e d e b r i s f lowd e p o s i t s , i n d i c a t i n g t h a t i t s uppe r s u r f a c e formed t h e l ands u r f a c e f o r a s u b s t a n t i a l p e r i o d . S o i l p r o f i l e s ands e d i m e n t o l o g i c c h a r a c t e r i s t i c s o f t h e v a r i o u s u n i t s a r esummar ized i n t a b l e 2 .

5 . The dependence o f channe l g e o m e t r i e s on t h e t ime s i n c e t h el a s t d e b r i s f l ow.

a . Sample ups t r eam (U o r S) and downst ream (A) c h a n n e lc r o s s - s e c t i o n s f o r Dorothy Ryan Canyon.

b . Pho to showing t r e n c h - l i k e , s c o u r e d c h a n n e l , a f t e r t h e1988 d e b r i s f low i n Manzan i t a Canyon.

c . Pho to o f s w a l e channe l i n u p p e r Brown Canyon, which h asn o t been burned f o r d e c a d e s , a t l e a s t .

d . P a r t i c l e s i z e graphs f o r c h a n n e l s a f f e c t e d by t h e 1988and 1977 d e b r i s f l o w s , and a channe l t h a t h a s n o t had ar e c e n t d e b r i s f l ow.

6. Photoo f t h e d e b r i s

f low fana t t h e j u n c t i o n o f

Manzan i t aCanyon and Ash Creek . The t o e o f t h i s f an t e m p o r a r i l y dammed AshCreek , b u t wa s o v e r t o p p e d f a i r l y r a p i d l y and h as s u b s e q u e n t l ybeen removed by Ash Creek .

7 . I l l u s t r a t i o n o f clumped b o u l d e r d i s t r i b u t i o n a l o n g t h e Sou thFork o f As h Creek . Medium and l a r g e b o u l d e r s i n Ash Creek a r ec o n c e n t r a t e d a t o r j u s t downst ream from t r i b u t a r y d e b r i s f low fanl o c a l i t i e s , i n d i c a t e d by a r rows on t h e l o n g i t u d i n a l p r o f i l e

segmen t s o f Ash Creek .8 . a . D a i l y p r e c i p i t a t i o n f o r t h e Coronado N a t i o n a l Monumenth e a d q u a r t e r s , l o c a t e d 2 km s o u t h o f Doro thy Ryan Canyon, f o r


35/49

d i s c h a r g e e s t i m a t e s f o r each r e a c h , b u t t h e a v e r a g e s from eachs e c t i o n a l o n g Dorothy Ryan Canyon a r e r e a s o n a b l y c o n s i s t e n t . Thes c a t t e r i n t h e d a t a from Manzani ta Canyon i s s e v e r e , making t h ed i s c h a r g e e s t i m a t e s u s p e c t .

2 . D e s c r i p t i o n o f s o i l p r o f i l e s and s t r a t i g r a p h i c r e l a t i o n s h i p sfrom t h e e x p o s u r e i n Dorothy Ryan Canyon i l l u s t r a t e d i n f i g u r e 3 .A b b r e v i a t i o n s a r e as f o l l o w s : s o i l s t r u c t u r e s i z e , f i s f i n e , mi s medium; s t r u c t u r e s t r e n g t h o f development , wk i s weak, mod i smodera t e ; s t r u c t u r e t y p e , sbk i s subangu la r b locky ; d ryc o n s i s t e n c e , so i s s o f t , sh i s s l i g h t l y h a r d ; w et c o n s i s t e n c e ,n s , np a r e non- ( s t i c k y, p l a s t i c ) , v s s , vsp a r e v e r y s l i g h t l y( s t i c k y, p l a s t i c ) , s s , sp a r e s l i g h t l y ( s t i c k y, p l a s t i c ) , s i ss t i c k y ; c a r b o n a t e s t a g e s a r e from Mache t t e (1985) .


36/49

PRECIPITATION RECORDS''-

...... , DRAINAGE DIVI DE

_MOUNTAINS

o I miI , , , !

o I km


37/49

(J ) (J )- : : ; :W W::;: '"J)3: z z

0 0 0-.oJ N NlL.

Q(J )W w

0: z za::D a::

w ~ ~Q CD CDen en I"-en en I"-Q) Q) ~

) DO

\ i

\-"'-. " . .-.

10-:::

~


38/49


39/49

terrace surface

1 mstream bed

- 5 m

f lJ b1re 4-

If '


40/49

- -I)

'0:s-;::

debris flows huachucamtns

Documents