ELSEVIER Geomorphology 20 (1997) l-':io

Characteristics of log and clast bed-steps in step-pool streams ofnorthwestern Montana, USA

Ellen Wohl *, Susan Madsen 1, Lee MacDonaldDepartment of Earth Resources, Colorado State University, Ft. Collins, CO 80523, USA

Received 26 August 1996; revised 2 January 1997; accepted 22 February 1997

Abstract

The role of log and clast steps was studied in 53 step-pool reaches in 32 streams in northwestern Montana. In each reach,20 consecutive steps were sampled. Drainage areas ranged from 0.5 to 11.5 km2. Mean step spacing was found to beproportional to channel gradient, width of the bankfull channel, and drainage area; mean step height was proportional to thewidth of the channel. No significant difference exists between clast and log steps with regard to step spacing and step height.This suggested either that (1) woody debris is mobile in even the smallest channels in the study area, or (2) channel flowsarrange clast steps around immobile woody debris. Calculations of theoretical entrainment and transport of sediment suggestthat the coarse clasts forming steps are capable of being entrained at bankfull discharge in most channels. The presence ofimmobile log steps (composed of living roots) in the upstream reaches, an increase in the percentage of log steps withgradient, and a decrease in this percentage with the width of the bankfull channel and drainage area, imply that woody debrisis mobile primarily in the downstream reaches. Calculations of flow resistance suggest that most steps serve to maximizeflow resistance. These results support the second hypothesis, that the height and spacing of clast steps are adjusted aroundimmobile log steps to maintain consistent step characteristics.

Keywords: large woody debris; flow resistance; step-pool channels; mountain channels; bedforms

1. Introduction of clasts (Grant et al., 1990), logs (Heede, 1972;Marston, 1982), or bedrock (Wohl and Grodek, 1994;Duckson and Duckson, 1995). Data from humid-temperate and arid mountain regions indicate that thespacing of clast steps decreases with gradient,whereas step height increases with gradient (Whit-taker, 1987; Chin, 1989; Wohl and Grodek, 1994).These relationships have been explained in terms offlow resistance and sediment transport. At high flowsthat submerge the bedforms, velocities of flow maybe greater over the steps than over the pools (Whit-taker and Jaeggi, 1982); flow energy is dissipated by

High-gradient streams with alternating bed-stepsand pools have been described from a wide varietyof environments (Chin, 1989). Bed-steps, which gen-erally form where channel gradient exceeds 5%(Montgomery and Buffington, 1993), may be formed

. Corresponding author. E-mail: ellenw@cnr.colostate.edu

1Present address: Beak Consultants Inc., 12931 N.E. 126th

Place, Kirkland, WA 98034-7715, USA.

0169-555Xj97j$17.00 @ 1997 Elsevier Science B.Y. All rights reserved.PII SOI69-555X(97)OOO21-4

U~IA\ . A.IO:jSIQ ~sn-pUt!I pUt! 'S~ss~~old ~IQdJOUlO~~

pUt! A~OIOq:lII 'SI°I}UO~ ~n~UlII~ Aq P~YI:j~I}S ~l~M

suI~H .suon~~oI ~uIIdurns ~SI~~ld ~q:I pUt! p~!pmS~q O:j suIs~q ~q:I ~UIAJI:jU~PI 10J Ul~:jSAS uoI:j~~YIss~I~~~~~m1~IQ U p~sn ~M 1~~r01d :jUq:l10d lS~lOd ~Uon

-uN IUU~:jOO)l ~q:I uo sPIoQs~Jq:I PI~IA-1~:jUM ss~ssu

O:j P~u~IS~P :j~~r01d l~~mI ~ JO :jlP.d SUM :>110M S~J,

'~~RWn:J ~Wn!lRW JR~U~UnUO:J R SRq R~m ~ql °UO!~R~-~~~A ~URU!W°P ~q:I S! ~S~lOJ SnOl~J!UO:J pUR 'w OOt?ZO~ O~~ wall ~~URl UO!~~l ~q:I U! SUOnRA~13 0(8t?61

'oJR ~~ uosq!O) SA~IJRA JRU!I:JUAS MOllRU Aq p~~Rl-Rd~s S~~URl u!R~unow JRU!I:JnUR ~U!PU~II ~S~Mq1JOU

-q1J°u p~:Jnpold :J!ozou~J Alm~ o~ :J!OZOS~W ~~RI ~q:I

wall UO!SS~ldwo:J JRUO!~~"M o~~~qS ~:JI URl~Il!PloJ

~u~:Jo~S!~ld ~q:I wall s~!sod~p JR!:JRI~ Aq U!RJl~AO 's~p

-~S ~I~H ~q:I Jo S){:JOl AlmU~W!p~SR~~W URpqWt!:J~ld

Aq p~~RU!W°P S! UO!~~l ~q:I Jo A~OIO~~ ~qJ. 0(1 O~!d)

RUR~UOW W~~S~Mq1JOU U! ~s~lod JRuO!~RN !Ru~~oo)I~q:I U! P~~R:J°I ~m q:J~M Jo IJR Alm~u 'SI~UURq:J Z£

UO S~q:JR~l lood-d~~s £~ Jo S~S!suo:J ~~s R~Rp s!qj.

spoq1aw "£

'(~661 'UUUIJJOH pu~ Pl~uOa:)~W) ~~m ApmS ~q~

JO 1JUd W~~S~M ~tp u! SMOY ~~d ~s~gml ~tp ~~~l~U~g

S~U~A~ MOUS-UO-U!t!ll~~U!MP!UI ~nq '~I~UIMOUS gu!ldsJo ~lns~l ~ S~ ~unf puu A~W U! In:):)O Al~nsn SMOY

~~d 'p~g~gun Al~sOUI ~m Sp~qSl~~~M ~~ 'Sl~A!l){lOd S){ml;) 10 !t!U~~OO)l ~tp ~!A l~Arn ~!qUInl°;)

l~ddn ~tp O~ Am~nq~ ~m pUU 'zUDI ~'11 O~ ~'O

JO s~~m ~g~U!t!lp ~A~q ~~m Apn~S ~tp U! Sp~qSl~~~M

~~ 'SU°!1~A~I~ l~ddn ~tp ~~ UI:) O~Z O~ SA~I~A~tp U! UI:) O~ UIOIJ gU!S~~l:)U! UO!1~!d!:)~ld tp!M

83.18 PI3!.!I "'

':JJodSUUJ1 puu ~u~m

-U~J1u~ ~u~m!p~s IT!~!~~10~~ 10 Suo!~uln~IT!~ q~noJq:l

At!I!qom ~sul~ puu ~01 ~U!SS~SSU puu 's~lqu!lUAl~uuuq~ puu S~!~s!l~~~uJUq~ d~~s U~~M~~q suo~ul~llo~

~~ ~ununIT!A~ puu ~U!~S~~ 'Pl~Y ~~. U! s~~s!l~~~u-JUq~ d~~s puu q~U~l ~u!lnsu~m Aq s~S~~OdAq ~s~~p~~s~~ ~ A\ 's~~s!l~~~uJUq~ d~~s ~U~~S!SUO~ u~~u~m o~

sd~s ~01 ~1!qounu! punoJU sd~~s ~sul~ ~~uulJU SMOgl~uuuq~ (Z) 10 'U~JU ApmS ~~ U! sl~uuuq~ ~s~lIT!ms

~~ U~A~ U! ~1!qom S! S!lq~p ApOOM (I) 1~~!~ U~~

'~lquqS!n~U~SWU! AIIT!~~S~U~S ~q O~ p~A01d sd~~s ~01

pUU ~sul~ 10 ~q~!~q puu ~u!~uds ~~ 1! '~u~ p~UOSU~l

~A\ 'UUU:luoW W~~S~Mq:JJOU U! sl~uuuq~ Z£ ~UOIT!S~OI puu s~sul~ 10 p~sodmo~ sd~~s-p~q p~u!Un!x~ ~ A\

'(17661 'puomq~rn ~q'u0661 'U1q~s~H puu UOS!q°"M~L861 "IT! ~~ UOSS!H ~1861 '~P~~H) l~uuuq~ ~~ ~UOIT!suo~u~ol mopuu1 ~u S~~p~M ~u~mw~s 10 sd~s SUll01

~101~1~~ q~!I1M 'S!lq~p ApOOM ~~JUI :JJodSUUJ1 O~ ~U~-~dmo~ ~OU ~JU sl~uuuq~ 1~p10-MOl U! SMOg ~~ ~U~

p~pnl~uo~ ~AUq SlO~nU IT!l~A~S '~SUJ1UO~ UI "(0661"IT! ~~ ~UU1D) SMOg q~!q ~U!lnP :JJodSUUJ1 ~u~mw~sO~ p~~nq!J1~u AIIT!l~U~~ ~JU sd~~s-p~q ~suD "(1861

'~P~~H) 1~~~~0~ Jn~~O u~Uo sd~~s 10 s~dAt OM~

~s~~ s~u~mq~~u~ u~~unom p~~s~101 U! ~u~ P~uo!~u~m~Auq sl~dud M~1 'sd~~s-p~q ~01 10 sd~~s-p~q ~SU1~

1~~!~ p~U!Un!X~ ~AUq S~wmS IT!l~A~S q~no~IV

"(6861 'U!I1;) sum~J1S ~U~WU1~

-Mol 'IT!!AnnU U! S~U~UJ1SnrpU IT!l~~Ul ~!M P~~U!~OSSUUO~Ud!SSW A~l~U~ ~~ O~ sno~OIT!UU ~q AUm sum~~s

u~unom P~UYUO~ U! SUll01P~q 100d-d~~s 10 ~101

~tU "(0861 'PJUMAUH) Slood ~~ U! ~U!x]1U ~u~lnq1m

~)

~S'JOiI ~UO!~~N !~u,~oo)l

IS'101l \1'UO!~"~ P"'q~"11l

.

I

~

~~qq!l

.~ ""'!if~\'. ( ::. .\

(.

: !. . . . . ...i

...VNV:> iMo911

N06t yNOl-l (L66l) O'l laorOlfdtOIUO;1D /1TJ J;1 IlfoM .:!J

E. Wohl etal./Geomorphology 20 (1997J 1-10

(a) Spacing

Distance Downstream

(b) Spacing

collected systematically across the bankfull channel(Wolman, 1954) at (1) multiple steps within thereach, and (2) multiple pools within the reach. If thestep was formed by large woody debris, the diame-ters of the woody debris were measured, and theclasts forming the step substrate around the woodwere measured. The steps formed by woody debrisincluded: single logs perpendicular to the channeland partly buried by cobble-to-gravel-sized clasts(Fig. 2b); logs attached to one bank that formed aramp down to the channel bed perpendicular to flow;and roots crossing the channel perpendicular to flow.Clasts forming the surface of the channel bed at logsteps were generally smaller in diameter than thoseforming clast steps.

Data from the surveyed channels were used withthe software program XSPRO (Grant et al., 1992) toestimate hydraulic variables (velocity, shear stress,stream power per unit area) for bankfull flow. Di-mensions of bankfull channels were designated inthe field on the basis of changes in bank slope, grainsize, and vegetation. The calculated hydraulic vari-ables were in turn used to estimate proximity toentrainment thresholds for the D84 clast size at eachcross-section using two sets of equations:

Tc = O.17d (1)

Distance downstream

Fig. 2. Schematic diagram of step-pool measurements (not toscale).

Wc = O.O79d1.3 (2)Tc = O.O56d1.213 (3)

each basin, study reaches extending from the head-waters to the mouth of the basin were selected torepresent a range of drainage areas and channelgradients. Three to six reaches 30-60 m long wereidentified within each basin depending on basin size,homogeneity of the valley bottom, and gradientchanges. In each reach, we surveyed channel gradi-ent and two channel cross-sections, and measuredchannel bedforms, percent of exposed bank, woodydebris, and the grain-size distributions of the bedmaterial in the step and pool bedforms.

For each of the 53 study reaches, downstreamspacing and height of 20 consecutive bed-steps weremeasured. Channel gradient was surveyed along thereach using a stadia rod and transit level, and dis-tance from the base of each step-tread to the edge ofthe next step-tread was measured with a metric tape(Fig. 2). Height was measured from the edge to thebase of the tread. Intermediate (b-axis) diameterswere measured for clasts identified as integral com-ponents of the step, and an average size of step-for-ming clast was calculated for each reach. In addition,we measured the intermediate axis of 100 particles

Wc = O.OO9d1.686 (4)

where Tc is critical shear stress (N/m2), and Wc iscritical stream power per unit area (W 1m2), and d isclast size (mm). Eqs. (1) and (2) represent the lowerlimiting line on plots of entrained particle size versushydraulic variables, using a large dataset culled fromthe literature by Williams (1983). Eqs. (3) and (4)describe the best-fit least-squares curve for a datasetfrom high-gradient channels in the Colorado FrontRange (Costa, 1983).

The hydraulic variables calculated with XSPROwere also used to estimate the transport of sedimentat bankfull discharge with the Meyer-Peter andMuller equation for bedload, as modified for high-gradient streams (Smart, 1984).

Ol-l

(L

66l)

Ol.{

Eo/

OIfd

tOU

lOilD

/"lv

Iii

/if o

M .

a

.(U~up P~gUJ~AU-q:>U~J)sd~~s gufUDOJ s!Jq~p f.pOOM JO J~~~W1!!P JO ;IZ!S ~Sl1I:> SnSJ~A

gU!:>uds d~~s-P~1I (q) .(U~up PagUJ~AU-q:>U~J) sd~s gU!UJJoJ s!Jq~p

f.pOOM JO J~~W1!!p JO ~Z!S ~suI:> SnSJ~A ~qg!~q d~~s-P~1I (U) .v .g!d

-~s uuoJ!Unuou 1! JO ' AI;}An:)~S;}l '~q'2!;}M %0£ pUt!

~q'2!;}M %06 q:)~M lOJ 1;}~;}~!P U!t!l'2 ;}m OEp pUt!

06p ':jlodsUt!~ ~U;}W!p;}S SS;}JUO!SU;}w!p S! <1> ;}l;}qM

I J8j8W8IP 501 +ezlS ISep .

(w) BU!"Bde dele pee

g:v O:V g:s OS g'~ O~ g'~ 0'1S'O 0'0-,-_.:_,-' ,-~ ~~. -_:_,.._,_~jO,

0'9

-} :'1

-f"

+ ..:+ .+

-.1;+ + +

.+ + + + ..+ .+ .

..+

.(~I1P p:)gI1J:)AI1-q:m:)J) ~qg!:)qd:)1s-~ SnSJ:)A IpP!M I:)UU\1q;) llnJ:J[U1Jg (;) "(~l1p p:)g\1J:)AI1-q;)I1:)J)

gu!:mds d:)~s-~ snsJ:)A IpP!M I:)UU\1q;) llnJ:J[U1Jg (q) "(~l1p ~gl1

-J:)AI1-q:m:)J) gU!;)l1ds d:)1s-~ SnSJ:)A ~U:)!p1!~ I:)UU\1q;) (11) .£ .g!d

I de18 Boi + d818lseF'. I

'.8-'"

~o

"'~

vO ~...""m.

"'9.0 3:

:[

8.0

, O~

re

09"--+

.+ - -++

--+ -+

.-+++ -1- +

.~ ~(q)

LQ9

(:»

~I

r

q:I!M P;}:IB!:)OSSB SJOJJ;} ;}Z!W!U!W 0:1 J;}pJO U! S{;}UUBq:)

:IS;}:Iq~!BJ:lS ':IS;}~m{ ;}q:I WOlJ SUon:);}S-SSOJ:) ;}Soq:);} A\ .S;}q:)B;}J {;}UUBq:) 9 { WOlJ SUon:);}S-SSOJ:) d;}:js 9 {

pUB SUon:);}S-SSOJ:) {ood 17 { 10 ~U!:IS!SUO:) 'B:lup ;}q:l1°:I;}sqns ;}AnB:lu;}S;}Jd;}J B 0:1 SUO!:IBnOO ;}S;}q:I p;}!{ddB ;}A\

.S{;}UUBq:) {ood-d;}:IS 10 s:)nnBlpAq X;}{dWO:) ;}q:I J01P;}U~!S;}P :I0U ;}J;}M A;}q:I ;}SnB:);}q .:jJOdsUBJ:I pUB W;}W

-U!BJ:lU;} W;}W!P;}S J01 ;}:IBWnS;} ;}pm!U~BW-10-J;}pJOUB ':IS;}q :IB .;}P!AOJd SUonBnb;} ;}S;}q:I .{UJ;}U;}~ UJ

.(17861 .:jJBWS) t;.8 UBq:I SS;}{ O£p /06p °nBJ UO!:1-nq!J:Is!p ;}Z!S-U!BJ~ B pBq SW;}W!P;}S pUB .%OZ UBq:I

SS;}{ ;}J;}M S;}do{S ;}SnB:);}q (t;) .h3 J01 sw;}w;}1!nb;}J ;}q:I

P;}1JS!:IBS B:lBP BUB:IUOW ;}qL "{t;o.o 0:1 £0.0) J;}:1;}UJBmdS,P{;}~S {U:)n!.I:) S! JJ(} pUB .(P{I -S)/ SH = ) SS;}J:lS

m;}qs SS;}{U°!SU;}W!p S! (} .(~.o(SH8 )/11 = ) JO:1:)B1

;}:)UB:lS!S;}J MO{J S! .J 'OOo{S {;}UUBq:) S! S .J;}U1J S! ;}{d

(%) juelpeJB ~uue4:)

~,~ ~~ ~ (2 .~

r

+ ..

+ + s.~ ~

+ .~+ ...~ ~ "',jl .~ ..~ .~ w ,60' -' & ~

i (9) .~ ..~jOO.L ..3 ~~ + .-

.c-(

Ol-l (L66l)O~.{JI°I°l{dtoUloilD/'IVli11l{°M '3

Q

~..,,"..-03

~0.

~"~~-0a.

P~Z!S-T~At!J~ Jo p~sodwo:J Sd~1S J~AO S~!1!:J°T~A MOTJ

J~1t!~J~ p~JnSlJ~W OqM '(Z861) !~~~t!f pUt! J~){t!11~

Jo )fJOM 1t11u~w!l~dx~ ~q:I 1:J!pt!IIuO:J s1Tns~J ~s~~"STood J~AO S~9!:J°T~A MOTJ J~1t!~J& ~q:I wol] P~1:J~dx~

~q PTnoM st! '(8 "~!d) STood ~q1 J~AO 1t!q:I Jo %~~ s~~t!

-J~At! Sd~1S ~q:I J~AO :JJodsUt!II1u~wW~s p~1t!Tn:J1t1;)

"(L O~!d) sT~UUt!q:J1S0W U! ~~Jt!q:JS!P IInJ){Ut!q 1t! p~U!1!IIU~ ~q Ut!:J Sd~1S

pUt! STood JoJ PSa pUt! S1St!T:J ~u!WJ°J-d~1S ~q:I 1t!q:I

~1t!:JWU! su09t!Tn:J1t1:J 1u~UJU!1!IIu~ ~~ "(9 O~!d) S1unO:J

1st!T:J-00I ~q:I JoJ s~n1t1A t8a ~q:I Ut!q:I J~~Jt!T AT1t1J~U~~~J~M s1st!T:J ~u!WJoJ-d~1S Jo SJ~~UJt!W Ut!~w ~~

"(:J~ "~!d) ~st!~J:J~p (6£"0- =.l)t!~Jt! ~~t!U!1!lp pUt! (q~ "~!d) (L£OO - =.l) T~uUt!q:J

IInJ){Ut!q Jo q1P!M St! pUt! '(t!~ O~!d) (0£00 =.l) s~st!~J:J

-U! 1u~Wt!J~ st! s~st!~J:JU! S~oT Aq p~UJJoJ sd~1s-p~q

JO ~~t!1u~:JJ~d ~~ "(qp O~!d) (OZ'O =.l) J~~UJt!!P

~OT q:I!M 10U 1nq '(9Z'O =.l) J~1~UJt!W 1St!T:J q:I!M P~1t!T-~JJO:J AT~A!1!Sod S! ~u!:Jt!ds d~1S St!~J~qM '(t!p O~!d)

Sd~1S ~q:I ~u!WJoJ (LloO=.l) s~OT pUt! (LL'O=.l)

s1st!T:J q:loq Jo SJ~1~UJt!W ~q:I q:I!M P~1t!T~JJo:J AT~A9-!sod S! 1q~!~q d~1S lU~J~JJW ApUt!:JY!U~!S 10U ~Jt!

sd~1s-p~q Jo S~dA1 OM1 ~s~q:I Jo 1q~!~q pUt! ~u!:Jt!dsUt!~w ~q:I 1t!q:I P~1t!:JWU! S1S~-t pUt! 's~q:Jt!~J T~UUt!q:J

~s~q:I 1noq~noJq:l p~x!W ~Jt! sd~1s 1st!T:J pUt! ~O'l

°t!~Jt! ~~t!u!1!lp JO 1U~Wt!J~ T~uUt!q:J

q:I!M 10U 1nq '(:J£ O~!d) q:lP!M IInP1Ul!q q:I!M P~1t!T-~JJO:J AT~A!1!Sod S! 1q~!~q d~1S Ut!~w 't!~Jt! ~~t!u!1!lp

pUt! (q£ O~!d) q:lP!M T~uUt!q:J IInJ){Ut!q q:I!M P~1t!T-~JJO:J AT~A9!Sod pUt! '(t!£ O~!d) 1U~Wt!J& T~UUt!q:J q:I!M

P~1t!T~JJo:J AT~SJ~AU! S! ~u!:Jt!ds d~1S Ut!~w 'SUJt!~IIS

Tood-d~1S 1t1Jn1t!U Jo A1!T!qt!!JUA 1U~J~qu! q~]:-q ~q:I

JO U09!U~0:J~J U! su09t!T~JJo:J ~s~q:I JoJ p~sn S! 01'0

Jo T~A~T ~:Jut!:JY!U~!S V °S~Tqt!!JUA :J!Tnt!lpAq ~q:I pUt!d~s ~q:I u~~M1~q SUO!1t!T~JJo:J ~q:I SMOqS 1 ~lqt!l

~,~ of ~

S:Jlns'a}J '17

'~dOlS l~uURq:> S! S pUR'q:I~u~1 d~~s UR~UI S! 7 '~q~!~q d~~s UR~UI S! H ~J~qM

'(~66I) 'YU ~~ S~~JqV Jo S/7/H O!~t!J ~q:I (Z) pUR

~q:I~U~1 q:>t!~J ~q:I Aq P~P!A!P ~q~!~q d~~s ~Ant!lnum:>~q:I S! a ~J~qM 'a '(~96I) ~sn°'M Jo JO~:>t!J uon

-t!J:jU~:>UO:> ss~uq~noJ ~q:I (I) :~u!sn ~:>UR~S!S~J Molluo sd~~s-p~q JO s~:>~JJ~ ~q:I p~~t!nYUA~ ~M 'AlYUU!d

'SUI~~SAS 100d-d~~s O~ sl~uURq:>

~U~!pt!~ J~MOI JOJ p~dOl~A~P suo!~t!nb~ ~U~Alddt!

0[-[ (L66T) nl ,{JioIOlidtOUiOiJ!> / °IV 1iJ lUOM '"

"U~

~.8"

E. Wohl et al.j Geomorphology 20 (1997) 1-10 7

clasts. The much larger size of the step-formingclasts relative to either step or pool DS4 values alongthe study reaches suggests that the steps are rela-

tively stable features. This is supported by magnetictracer experiments along other step-pool channels ofcomparable drainage area, channel gradient, and clast

>-()cQ):J0-Q)

It

25 75, , , , , , , , , ,

125 175 225 275 325Mean step forming clast size (mm)

375 425

>-ucQ):J0-Q)

It

- , 25 75 125 '175 225 275 325 375 425

Mean 084 (mm)

Fig. 6. (a) Grain-size frequency distribution for the mean size of clasts that are integral components of the steps in a study reach, in mm

(reach-averaged data). Mean for entire population is 239.1 mm, standard deviation is 70.7; sample size is 53. (b) Grain-size frequencydistribution for D84 determined from 100 clasts sampled at multiple steps in a study reach, in mm (reach-averaged data). Mean for entirepopulation is 157.3 mm, standard deviation is 72.7; sample size is 53.

-mus m.J°J!Unuou n JO 'AI~A!1:J~ds~1 '1q~!~M %0£ pun

1q~!~M %06 q:J~M IOJ 1~1~mu!P U~l~ ~m OEp pun

06p ':jlodsunJJ 1u~mw~s SS~IUO!su~Ul!P S! <1> ~1~qM

"(1!}11P ~2111;jA11-q;)11;jl) 1q2!;jqd~s-p~ SnSl;jA qtP!M j;jUU11q;) IInPJU11H (;) "(1!}11P P;j2111;jA11-q;)11;jl)

2U!;)11ds d~s-p;jq SnSl;jA qtP!M j;jUU11q;) IInPJU11H (q) °(11111p P;j211

-l;jA11-q;)11;jl) 2U!;)11ds d;j1S-P;jq SnSl;jA 1U;j!P1lJ2 j;jUU11qJ (11) 0£ "2!d

,~1 (w) 4IP~ teUUB4" IInj'tUB8~~ O~ 8 9 L 9 g v E ~ ::,1 ., .' ~ ' , .,

Ig.o~o.~

,. ~ rn

f:G i g.~

OS

g.s !

., [o.v

l.".. (q) r:::

..++

..+

...'\ ._...) ,,-- " .

.w .+

...+ ~!M p~:Ie!:)oSSe SlOJ.I~ ~Z!ll!U!UI 0:1 l~plO U! sI~uueq:)

:IS~:Iq~!1!IIS ':IS~~mI ~~ WOlJ SUO!:1:)~S-SSOl:) ~Soq:)~ A\ .S~q:)e~l I~uueq:) 9 I WOlJ SUO!:1~S-SSOl:) d~:Is 9 I

pue SU09:)~S-SSOl:) 100d 17 I JO ~U!:IS!SUO:) 'e:lep ~~ JO:I~sqns ~Age:1U~S~ld~l e 0:1 su0genb~ ~S~~ p~ndde ~A\

.sI~uueq:) 100d-d~:Is JO s:)nnelpAq x~Idwo:) ~~ lOJP~U~!S~P :I0U ~l~M A~~ ~sne:)~q ':jlOdsueIl pue :IU~W

-U!1!IIU~ :Iu~W!P~S lOJ ~:Iew9s~ ~pm!U~eW-JO-l~PlOue ':Is~q :le '~P!AOld Su0genb~ ~S~~ '{Ul~U~~ UI

.(17861 ':jJUwS) ~.8 ue~ ss~I o£p /06p °!:Iel UO!:1-nq!.J:IS!p ~Z!S-U!1!ff, e peq S:lU~W!P~S pue '%OZ ue~

ss~I ~l~M S~doIS ~sne:)~q (~) .bH lOJ S:lu~w~l!nb~l ~~

paysges e:lep eUe:lUOW ~~ .(~o.o 0:1 £0.0) l~:1~ummds,PI~~S {U:)9!l:) S! J"(J pue '(P(I -S)/SH = ) SS~IIS

m~qs SS~{U°!SU~WW S! (J '(~.o(SH8 )/ n = ) lO:1:)eJ

~:)ue:lS!S~l MOIJ S! :J '~OIS I~uueq:) S! S 'l~Uy S! ~Id

~0£-£ (L66£)OZ,(E°l°lfdtoUlO;JD/"lVJ;JlIfoM .:!1

E. Wohl et al./ Geomorphology 20 (1997) 1-10

Q'5~

/\/\

to be a primary cause of the observed changes in thenumber of log steps relative to clast steps.

The decrease in the number of log steps withincreasing bankfull width and drainage area suggeststhat large woody debris is more mobile in the down-stream reaches. In the smaller channels we observedthat some of the log steps were formed by live roots,which are clearly immobile. From these results andstudies on the stability of large woody debris instream channels, we reject the hypothesis that woodydebris is mobile throughout the study area, althoughit is probably mobile in the downstream studyreaches.

The alternative hypothesis is that clast steps arearranged in response to the placement of woodydebris, particularly in the upstream channel seg-ments. Although we cannot directly test this hypoth-esis, our calculations of theoretical entrainment andtransport of bedload indicate that the majority of theclasts forming steps in the study area can be mobi-lized during bankfull discharge. We conclude thatclast-steps in these channels are spaced to comple-ment relatively immobile woody debris, particularlyin the high-gradient reaches. This conclusion rein-forces the study by Heede (1981) of mountain chan-nels in Arizona and Colorado, in which he concludedthat the spacing of gravel bars was adjusted to bothchannel slope and to the spacing of log steps.

-.- 6 2 I, 6 6 10 12 14 16

GradiMt (%)

Fig. 10. The ratio of mean step height (m), mean step spacing (m),and reach gradient (m/m), expressed as HILlS, versus reach

gradient.

Acknowledgements

This research was funded by the Montana Cumu-lative Watershed Effects Cooperative and the Koote-nai National Forest. We thank Steve Johnson forfield and technical assistance, and Jim Hoffman andJim Dietzman for field assistance. Doug Thompsonand two anonymous reviewers provided helpful com-ments on the manuscript.

do not fully characterize these channels. We used a90% confidence interval in recognition of this uncer-tainty. Within these constraints, our results demon-strate that, as indicated by previous studies (Whit-taker, 1987; Grant et al., 1990; Wohl and Grodek,1994), mean step spacing is proportional to channelgradient, bankfull channel width, and drainage area,whereas step height is proportional to channel width.We also found that the percentage of log stepsincreased with increasing gradient and decreased withincreasing bankfull width and drainage area. Finally,we found no significant difference in the height orspacing of log versus clast steps. The question iswhether the consistent height and spacing of stepsresults from the spacing of clast steps around immo-bile woody debris, or whether the woody debrismoves through the channel in response to the dis-charge, gradient, and sediment load.

One possibility for the changing percentage of logsteps is the relative abundance of large woody debrisas a function of channel size and gradient. The inputof woody debris is believed to be partly a function ofsideslope gradient (Bisson et al., 1987), and wemight expect higher sideslope gradients along streamswith higher channel slopes. Nearly all of the streams,however, were small relative to tree height, andsample reaches were either in uncut areas or in areaswith a riparian buffer strip. Thus, we would notexpect the abundance or input of lar~e woody debris

References

Abrahams, A.D., Li, G., Atkinson, J.F., 1995. Step-pool streams:Adjustment to maximum flow resistance. Water Resour. Res.

31,2593-2602.Bisson, P.A., Bilby, R.E., Bryant, M.D., Dolloff, C.A., Grette,

G.B., House, R.A., Murphy, M.L., Koski. K.V.. Sedell. J.R..

10 E. Wohl et al./ Geomorphology 20 (1997) 1-10

1987. Large woody debris in forested streams in the PacificNorthwest: past, present, and future. In: E.O. Salo and T.W.Cundy, (Editors), Froc. Symp. Streamside Management:Forestry and Fisheries Interactions. Univ. Wash., Inst. For.Resour., Contrib. 57, 143-190.

Chin, A., 1989. Step pools in stream channels. Frog. Phys. Geogr.

13,391-407.Costa, I.E., 1983. Paleohydraulic reconstruction of flash-flood

peaks from boulder deposits in the Colorado Front Range.Geol. Soc. Am. Bull. 94,986-1004.

De Iong, C., Ergenzinger, P., 1995. The interrelations betweenmountain valley form and river-bed arrangement. In: Hickin,E.I. (Ed.), River Geomorphology. John Wiley, Chichester, pp.55-91.

Duckson, D.W. Ir., Duckson, L.I., 1995. Morphology of bedrockstep pool systems. Water Res. Bull. 31, 43-51.

Gibson, R., Alden, W.C., Pardee, I.T., 1948. Geology and oredeposits of the Libby Quadrangle, Montana. U.S. Geol. Surv.Bull.956, 131 pp.

Grant, G.E.., Swanson, F.I., Wolman, M.G., 1990. Pattern andorigin of stepped-bed morphology in high-gradient streams,Western Cascades, Oregon. Geol. Soc. Am. Bull. 102, 340-352.

Grant, G.E., Duval, I., Koerper, G., Fogg, I., 1992. XSPRO crosssection professional analyzer. Version 1.1, USFS PacificNorthwest Research Station, Corvallis, Ore.

Hayward, I.A., 1980. Hydrology and stream sediments fromTorlesse stream catchment. Tussock Grasslands and MountainLands Inst. Spec. Publ. 17, Lincoln College, New Zealand,236 pp.

Heede, B.H., 1972. Flow and channel characteristics in two highmountain streams. U.S. Department of Agriculture Forest Ser-vice, Res. Pap., RM-96, 12 pp.

Heede, B.H., 1981. Dynamics of selected mountain streams in thewestern United States of America. z. Geomorphol. 25, 17-32.

MacDonald, L.H., Hoffman, I.A., 1995. Causes of peak flows innorthwestern Montana and northeastern Idaho. Water Resour.Bull. 31,79-86.

Marston, R.A., 1982. The geomorphic significance of log steps inforest streams. Ann. Assoc. Am. Geogr. 72, 99-108.

Montgomery, D.R., Buffington, I.M., 1993. Channel classifica-tion, prediction of channel response, and assessment of chan-nel condition. Report TFW-SHI0-93-002, Washington StateTimber/Fish/Wildlife Agreement, 84 pp.

Richmond, A.D., 1994. Characteristics and Function of LargeWoody Debris in Mountain Streams of Northern Colorado.Unpublished MS thesis, Colorado State University, Ft. Collins,Colo., 94 pp.

Robison, E.G., Beschta, R.L., 1990a. Characteristics of coarsewoody debris for several coastal streams of southeast Alaska,USA. Can. I. Fish. Aq. Sci. 47, 1684-1693.

Robison, E.G., Beschta, R.L., 1990b. Coarse woody debris andchannel morphology interactions for undisturbed streams insoutheast Alaska, USA. Earth Surf. Process. Landf. 15, 149-156.

Rouse, H., 1965. Critical analysis of open-channel resistance. I.ASCE Hydraul. Div. 91, 1-25.

Schmidt, K.-H., Gintz, D., 1995. Results of bedload tracer experi-ments in a mountllin river. In: Hickin, E.I. (Ed.), RiverGeomorphology. John Wiley, Chichester, pp. 37-54.

Smart, G.M., 1984. Sediment transport fonnula for steep channels.I. Hydraul. Eng. 110, 267-276.

Whittaker, I.G., 1987. Sediment transport in step-pool streams. In:Thorne, C.R., Bathurst, I.C., Hey, R.D. (Eds.), SedimentTransport in Gravel-Bed Rivers. Wiley, Chichester, pp. 545-579.

Whittaker, I.G., Iaeggi, M.N.R., 1982. Origin of step-pool sys-tems in mountain streams. I. Am. Soc. Civ. Eng., Hydraul.Div. 108, 758-773.

Williams, G.P., 1983. Paleohydrological methods and some exam-ples from Swedish fluvial environments, I. Cobble and boulderdeposits. Geogr. Ann. 65A, 227-243.

Wohl, E.E., Grodek, T., 1994. Channel bed-steps along NahalYael, Negev desert, Israel. Geomorphology 9, 117-126.

Wolman, M.G., 1954. A method of sampling coarse river-bedmaterial. Trans., Am. Geophys. Union 35, 951-956.