The Geomorphic Significance of Log Stepsin Forest Streams'

Richard A. Marston

O/C/G/4)44

Oo Nor—

PROTECTION of fisheries and water resources of

forest streams depends in great measure onour understanding of the various functions per-formed by insrream large woody debris. Largewoody debris includes logs, branches, andstumps larger than 10 centimeters in diameter.A log step develops when large woody debrisextends across the entire active channel, creat-ing o change in the water surface elevation asthe water spills over the large woody debris. Byforming a series of vertical falls, log steps (andother foil obstructions) reduce the amount ofpotential energy available for conversion to thekinetic energy of moving water used for sedi-ment routing through the stream network (Fig-ures 1 and 2). By trapping sediment suppliedfrom upstream and adjacent hillslopes, log stepsintroduce an additional storage component intothe sediment budget of forest streams with sub-sequent effects on sediment yield.

It is argued that the distribution of potentialenergy in a stream network is diagnostic of equi-

librium conditions and the general sediment te-gime (aggrading. degrading. or at grade). Byquantifying the dissipation of potential energyand sediment storage by log steps in variousportions of the stream network, the geomorphicsignificance of log steps can be demonstrated. Ifpotential energy dissipation and sediment stor-age by log steps are indeed significant geo-morphic functions, then forest management ac-tivities must address the consequences to streamequilibrium conditions and associated trends insediment production.

Log Step Development

Collection and analysis of data are under-taken for thirteen forest watersheds locatedwithin o 32-by-55-kilometer area of the centralOregon Coast Range (Figure 3), Seven of thethirteen study basins are underlain primarily bysedimentary lithologic units subjected to a history

Department of Geological Sciences. University of Texas at El Paso. El Paso, TX 79968 k REMa V E

P: _Abstract. A functional account of log steps in forest streams is provided by field surveys of 163 kilomet-ers of streams in the central Oregon Coast Range Natural rreefoll, rather than silvicultural activities,accounts for the majority of log steps During low-flow conditions, dissipation of potential streamenergy by log steps ()mounts to 6 percent, approximately equal to that by falls. There are no statisti-cally significant differences regarding spatial distribution of log steps berween study basins with con-trasting silo cultural and natural stream inputs of large woody debris. However. significant spatialdifferences are revealed between streams of various orders, a finding that points to channel flushingcopocity and stream-adjacent topography as dominant controls on log step development Applicationof thermodynamic principles to stream systems demonstrates that neither falls nor log steps cause astatistically significant difference in equilibrium conditions of stream networks. The volume of sedimentstored behind log steps in third-, fourth-, and fifth-order streams is 123 percent of the mean annualsediment discharge (suspended load and bed-load). Depriving some streams of log steps by streamclean-our or repeated harvest of stream-adjacent trees may initiate on episode of progressive erosionby nor dissipating stream energy in excess of that needed to transport imposed sediment suppliesAddition of log steps to streams with energy already insufficient to balance sediment inputs andoutputs may only serve to accentuate progressive deposition. Functions of insrreom large woodydebris not incorporated os log steps must also be addressed in forest management decisions.

Key Words: log steps, forest streams, Oregon Coast Range, instreom woody debris. sediment trans-port, stream energy, entropy, silvicultural practices.

ges o grant fromis research

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American ag-Jnion analysis.2.of commercial

iry a Von Thu-3 Geography

E. 1966. Soilbo Soil Survey.ire Winnipeg:

Soils of theoil Survey, Soils-)ent of Agricul-

99

1111.1111...11.11..mmi p,/4,t5 Ar,s 0 c AI^ . 6-eocre-. '7 7- - / 0E-

100 Marston

tilPE= mgh

04% PE

.0 — oc h

0

., Water Surfoc• Elevation at High flowE PE Dissipation by Log Steps :a

sis

Wafer Surface Elevation at Low Flow.

Cumulative Change of Water Surface °Elevation in Vertical Falls Percent C..,as oof Total Stream Relief

Distance Downstream -.

Figure 1. The potential stream energy per unit moss of water (PEIm) is directly proportional to h, or the relief in aspecific stream segment (g = constant of gravitational acceleration). Potential energy dissipation by log stepscon increose or decrease with changes in river stage.

h

PE DISSIPATION BTFALL OBSTRUCTIONS

PE AVAILABLE c OP CONVERSION TO

KE NEEDED FOR TRANSPORT Of

^ IAPC5E0 SEDIMENT SUPPLIES

1 ,;

KE DISSIPATION BTPARTICLE ROUGHNESS

KE DISSIPATION

15FORM ROUGHNESS

1

ANNEL BCD TOPOGRA B HT1 I ,k

C ANNE. PATTERN

ANNEL -SECTION SHAHE1

I

1

Figure 2. Energy transformations in streams. Log stepscause o reduction in potential energy that would other-wise be convened to a longitudinal component ofkinetic energy actually used for sediment transport(PEim = potential stream energy per unit mass ofwater; KEIm = kinetic energy per unit moss of water;h = relief in o specific stream segment; v = streamvelocity).

of regional tilting. A cuestoform topography re-sults, with gentle slopes in the direction of re-gional dip bounded by steep escarpments.Deep-seated slumps, earrhflows and creep aredominant moss-wasting processes on the dipslopes, while debris slides contribute smalleramounts from the escarpments. Low bedrockfalls are frequent where streams flow overwell-indurated sandstone beds. The six remain-ing study basins are underlain primarily by igne-ous lithologic units with mainstream channelsoriented along fault lineaments. Thin residualsoils are situated on steep convex-up slopes ofthe basaltic terrain or or near the maximumangle of repose Debris torrents are more com-mon in steep headwater channels than in thesandstone terrain.

Streamside vegetation is dominated by Sitkospruce (Picea sitchensis) and western redcedar (Thuja plicate) in moist or swampy ri-parian sires. Red older (Alnus rubra) and big-leof maple (Acer macrophyllum) occupy mesicriporion sites and areas disturbed by fires or log-ging. Individual red older and bigleof mapletrees rarely achieve sizes common in old-growthconiferous stands. Timber harvest in the study re-gion began on a large scale in the 1940's. In thesedimentary study basins. 32.5 percent of thearea has been clearcut; in the igneous study ba-sins. 13.7 percent of the area has been cleorcur.As deciduous species and plantation trees re-place the larger trees lost by harvest of old-

growth stonemay be rerc

Mops of theach basinbend of con'form scale cpersists omorstream netvemethod proobtained wit!using blueration coverchannel deter1957). Guidetogrophic bia.tion method1978). Streonmethods outliare occumulcwith emphos.fifth-order seobstructionsstream orderstored behindrecorded. Thenoted whereHuman-causepresence ofscars related r,

In the 163413 log step!height of 392principally tree

Fet_

Number Su(Total Km St.Total RelictN-Log StepsH-log StepsAll Log SteiPE Dissipori(

N-Boulder FN-Bedrock FH-StructuresAll Falls (htPE Dissipotic

Total PE D,t,

' All heights° Data refer rc i

Flow

or the relief in aon by log steps

oography re-. ection of re-?scorpments.nd creep are, on the dipbure smallerLow bedrock-)s flow overe six remain-only by igne-om channelsThin residual-up slopes ofe maximume more corn-s than in the

aced by Sitkawestern red

'r swampy r1-

)ra ) and big-occupy mesic)y fires or log-gleof maplein old-growththe study re-

1940 . s. In theercent of the, ous study ba->een cleorcut.tion trees re-rvest of old-

Table 1. Summary of Field Dam for All Study Basins'

Feorure

Srreom Order

1 2 3 4 5 Overoll

Number Srreoms SurveyedTotal Km SurveyedTotal Relief Surveyed

12.17158

76.05469

3361.0

3,620

2258.7

1,860

735 4424

70163

6.530N-Log Steps (hr./no.)° 0/0 11.9115 170/202 103/109 2 74/5 288/331H-log Steps (ht./no.)° 0/0 6.67 58.8/44 35 4/27 3.62/5 104/82All Log Steps (ht./no.)° 0/0 18.6/21 229/242 138/136 6.36 1 10 392/413PE Dissipation (%)° 0 3.97 6.33 7 42 1.50 6.00

N-Boulder Falls (ht./no.)° 0/0 5.08/5 123/74 93.2/72 21.4/2 223/153N-Bedrock Falls (ht./no.) 5 3.05/1 10.1/5 64 9/45 65.8/62 13.4/17 157/130H-Structures (ht./no.)° 0/0 0/0 4.57/4 2 13/2 0'0 6 70/6All Falls (ht./no.)° 3.05/1 15.3/10 192/123 161/136 15 8'19 387;289PE Dissipation (%)° 1 93 3.26 5.30 8.66 3.73 5.93

Torol PE Dissipation (%)° 1.93 7.23 11.6 16.1 5.23 11.9

' All heights given in meters to rhree significant digits.° Dora refer to low-flow conditions.

Log Steps in Streams 101

growth stands, full development of log stepsmay be retarded (Likens and Bilby 1979).

Mops of the stream network ore drafted foreach basin delineating streams through thebend of contours on topographic maps of uni-form scale and contour interval. A consensuspersists among geomorphologists that mappingstream networks by the contour crenulotionmethod produces more reliable results thanobtained with aerial photo interpretation or byusing blue lines, especially where mixed vege-tation cover introduces cartographic bias inchannel delineation (Gregory 1966; Morisawa1957). Guidelines ore ovoiloble to reduce car-tographic bias when using the contour crenula-tion method (Marsron 1978; Smith and Stopp1978). Srreom order is designated according tomethods outlined by Strohler (1957). Field dataore accumulated for 163 kilometers of streams,with emphasis on second-, third-, fourth-, andfifth-order segments. The height of all fallobstructions, their location by river mile andstream order, and the volume of sedimentstored behind each log step are measured andrecorded. The probable cause of log steps isnoted where field evidence is available.Human-caused log steps are identified by thepresence of cut ends on logs or by daring logscars related ro mass wasting.

In the 163 kilometers of streams surveyed,413 log steps are identified, with o cumulativeheight of 392 meters (Table 1). Natural causes,principally treefall, account for 80 percent of the

total number and 74 percent of the total heightof log steps Debris torrents (from undisturbedand disturbed sites) are responsible for only 5.3percent of the total number of log steps. How-ever, the mean height of 2.0 meters exceedsvalues for log steps of all other origins. The meanheight of all log steps is 0.98 meters. The meanheight of H-log steps (those entering streams byhuman activities) at 1.3 meters is one and one-half times as great as the mean height of N-logsteps (those entering streams by natural processes).

Prior literature claims that the distribution ofinstreom large woody debris varies inverselywith stream size (Bilby 1979: Keller and Tally1979; Keller and Swanson 1979). When fielddata are stratified by stream order, differences inthe frequency of log steps and relative impor-tance of contributing factors ro log step devel-opment are apparent (Figure 4). The highestconcentration of log steps occurs in third-orderstreams at 3.97 per kilometer. Log steps creoredby natural treefoll are most frequent in third-order streams. Though actual rreefall may occurmost often in first- and second-order streams, logsteps do not fully develop because the typical"V-notch" topography of steep stream-adjacentslopes prevents positioning of logs across thenarrow channels. In fourth- and fifth-orderstreams, competent storm flows tend to removeinstream large woody debris left from rreefollbefore log steps can fully develop. Log stepscreated by blowdown of poorly constructedbuffer strips ore also most frequent in third-order

2FLYNN CREEK

•NORTH FORK BEAVER CREEK •_C_,J,s

.4.00

0,lk

aWELKHORN CREEK

I • 1

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AIM

....)c, r.1-' \

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LOCATION rCAPE CREEK 4•ftrolo c.AND GEOLOGIC

MAP

1 3 ati

,2 • ♦ 11.•

SCOTT CREEKSA

•STUMP CREEK

9CUMMINS CREEK

10SOS CREEK

Sedimentary Litholegi t Units=; I an•ous Lithelagic Units

12RIO CREEK

I.

f 1IL

ROCKCRIIK

13CAPE CREEK

Ilan. C.

10

I

Figure 4. Thf.attributed toorder. The ckrelative frequ.and buffer bit,steps createdH = affected i

steps and 1 'energy dins 1•not approo,for orher for(Colorado Rrof Arizona, H50 to 100by log steps(1979) cite cpotion by lc.nia. Bilby (1cnon in the WiEnergy dissip52 percent a(1979) andtively, for thesingle previoiRange fount:tential enercAside from tf.tween figure.,those previotbe attributed

102 Marston

Figure 3. Location and geologic mop of the study basins. All third-, fourth-. and fifth-order streams are shown.Basins are numbered from north to south according to latitude of basin outlet. Geology simplified after Schlickerand Deacon (1974); and Schlicker et al. (1973).

streams. The frequency of log steps creored bydebris torrents decreases as stream order in-creases Debris torrents originating in first-orderstreams leave log steps that remain stable insecond- and third-order streams but ore oftenbreached by storm flows in fourth- and fifth-order streams.

The Kruskol-Wallis nonparametric rest forcomparing four unmatched sets of doto demon-strates that the difference in frequency of logsteps between second-, third-, fourth-, and fifth-order streams is statistically significant of the p

0.05 level. The Mann-Whitney nonparomerrictest for comparing two unpaired sets of datademonstrates that the difference in frequency oflog steps between sedimentary and igneousstudy basins is not statistically significant at thep 0.05 level.

Dissipation of potential energy at low flow byall fall obstructions amounts to 12.0 percent oftotal stream relief (Table 1). Boulder and bed-rock falls dissipate 5.93 percent of the potentialenergy. Log steps dissipate 6 percent of the po-tential energy, including 4.41 percent by N-log

Log Steps in Srreoms 103

10

E

0zcr)a.w

0

0

>-Uzu j .1

0CCu_

2 3 4

5STREAM ORDER

Figure 4. The frequency of log steps (including thoseattributed to specific causes) os a function of streamorder. The diagram does not necessarily reflect therelative frequency of treefoll, debris torrents, flotation,and buffer blowdown. but rather the frequency of logsteps creored by those processes (N = natural process:H = affected by human activities).

steps and 1.59 percent by H-log steps. Potentialenergy dissipation by log steps of 6 percent doesnor approach figures reported in the literaturefor other forest regions. In forest streams of theColorado Rocky Mountains and White Mountainsof Arizona. Heede (1972; 1975; 1976) reports50 to 100 percent dissipation of stream energyby log steps and gravel bars. Keller and Tally(1979) cite a range from 18 to 60 percent dissi-pation by log steps in Redwood Creek, Califor-nia. Bilby (1979) finds 10 to 52 percent dissipa-tion in the White Mountains of New Hampshire.Energy dissipation of 30 to 80 percent and 32 to52 percent are reported by Keller and Swanson(1979) and by Swanson et al. (1976), respec-tively, for the western Cascades of Oregon. Asingle previous study in the central Oregon CoosrRange found only 4 percent dissipation of po-tential energy by log steps (Dietrich 1975).Aside from the latter study, the discrepancy be-tween figures reported in the present study andthose previously reported in the literature maybe attributed to the failure of past studies to dis-

tinguish dissipation of potential energy by logsteps from dissipation of kinetic energy by in-stream large woody debris os it deflectsstreamflow and increases channel roughness(Figure 2). Insrream large woody debris thatdoes not span the entire active channel andcreate o vertical drop of water surface elevationdoes not dissipate potential energy. It should benoted that log steps were identified and mea-sured in the present study at low flow. However,the effective height of all obstructions, includinglog steps, may change as river stage changes(Figure 1). For example, on impervious log jamwill impound water during peak flows, increasingthe head and dissipation of potential streamenergy. On the other hand, log steps or otherobstructions with low heads become submergedduring peak flows, especially in channels withlow width-to-depth ratios. The variation in ef-fectiveness of foil obstructions remains undocu-mented in the literature. although Heede (1976)reports submergence of log steps to be o rareoccurrence in his study streams.

Potential energy dissipation by fall obstructionsexhibits a spatial pattern contrasting with that oflog step frequency (Figure 5). With respect to log

100

2 3

4

5

STREAM ORDER

Figure 5. The dissipation of potential srreom energy(relief) created by full obstructions as a function ofstream order.

.01

- ,eoms are shownfled after Schlicker

nonparometriced sets of data

in frequency ofiry and igneousignificant at the

y or low flow by12.0 percent of

Dulder and bed-of the potential

?rcent of the po-)ercent by N-log

104 Marston

steps, boulder and bedrock falls, and all fullobstructions, the dissipation of potential energyreaches a maximum in fourth-order streams.Scream downcutting to bedrock in fourth-orderstreams of the sedimentary basins adds anumber of falls, or nickpoints, over beddingplanes The location of fifth-order segments inbroad alluvial lowlands and or the base of collu-vial foorslnpes results in burial of bedrock be-neath unconsolidated deposits, diminishingpotential energy dissipation by spill over verticalobstructions Moreover, the mean height of logsteps in fourth-order streams exceeds that ofthird-order streams, further accounting for thegreatest dissipation of potential energy infourth-order streams in spire of a lower fre-quency of fall obstructions. Many log steps ob-served in fourth-order streams have expandedby trapping material floated from upstream.The Kruskal-Wallis test demonstrates that thedifference in percent of potential energy dissi-pation by log steps between second-, third-,fourth-, and fifth-order streams is statistically sig-nificant or the p 0.05 level The Mann-Whitneyrest demonsrrotes that the difference in percentof potential energy dissipation by log steps be-tween sedimentary and igneous basins is norstatistically significant of the p 0.05 level.

Stream Network Equilibrium

The application of thermodynamic principlesto stream systems allows derivation of criteria toassess the significance of potential energy dissi-pation to equilibrium conditions. The second lawof thermodynamics asserts that a closed or iso-lored system evolves toward the most probablestore of energy within itself, a condition ofmaximum randomness or maximum entropy.Based on on analogy between absolute tem-perature in a thermal system and stream relief ino fluvial system (both forms of potential energyper unit moss), stream systems should evolvetoward a condition of maximum entropy, withan equal distribution of relief throughout thestream network. Yong's (1971) "low of overagestream for states that the mean relief forstreams of all orders in the some network shouldbe equivalent. Although the deviation of po-tential energy from o uniform distribution in astream network can increase (negentropy)with certain changes in stream relief, theclosed-system concept of thermodynamic en-

tropy provides on explanation for the tendencytoward a stream-relief ratio of unity that hasbeen long noticed in Horton-Strahler relationships.

A principal shortcoming of Yang's equilibriumcomputations is the reliance on topographicmop measurements of stream relief with nocompensation for reduction of effective poten-tial energy by fall obstructions. Ar the sometime, existing techniques for evaluating channelequilibrium rely excessively on subjective in-terpreration - during a limited time span, and donor consider energy dynamics of fluvial systemscontrolling channel morphology. To overcomethese problems, o quantitative measure ofequilibrium conditions is proposed using the chi-square test statistic:

x 2 = (0 - E) 2/E, (1)

where 0 and E are the observed mean reliefand equilibrium relief, respectively, for streamsof each order (in meters). The use of ordinaldata (stream relief) rather than the usual fre-quency dory in chi-squore calculations is accept-able because no statistical inferences will bemode regarding the chi-square values per se.Chi-square calculations are mode using threesets of relief (potential energy) measurements:

mean relief for each stream order mea-sured directly from mops;mean relief for each srreom order afteradjusting values downward by a fractionequal to the percent of potential energydissipated by bedrock and boulder foils;and

(3) mean relief for each srreom order afteradjusting values downward by a fractionequal to the percent of potential energydissipated by falls and log steps together.

An examination of the chi-square values forthe study basin reveals the influence of foils andlog steps on equilibrium conditions (Table 2).Falls improve the equilibrium condition ofstream networks in the following watersheds:Scott Creek, Cape Creek—Lincoln County. BobCreek, and Rock Creek. In these stream net-works, foils are located where needed ro dissi-pate potential energy in excess of that neededto balance sediment inputs and outputs. Chi-square values decrease accordingly. Log srepsimprove the equilibrium condition in the fol-lowing watersheds: North Fork Beaver Creek,Scott Creek, Cope Creek—Lincoln County,Cummins Creek, Bob Creek, Rock Creek, and

rp (El < '-<L,-)DL-) ?", rD i 0 5 --T- '^ 6"-) D ' 1.) '- 2 3 j =-. ■ u -i-, t uo aL, bu 0_1_ ‘e) _■- l( 1 ct):.n r., r:;:r . D 0 ,-(6 D ,c) a D S b , u Q., rti b- '5' w (15 <-

7 ((i:j -, FT) ri cf- 3 0- ii 0- 3 3 mi ma - o< " ° O. 5-. ' ' 'n 1

a m _ m E(T) 0 L0 rD

C ,..<- a D to - (t) cc : 1 05 _ o< D , . ,:7 c0 a ma. , on on , rt) _r) D

D IS. 3 51 E -6 c ° Z' ) (°- 7; , ̂ 7,', D . 5-o D --< 2,0 (pc, C En D o D-;,..' 3

D .--, 0 '< 0 - 0 tn 5 ‹. o ',CI 5.

., (1) ,, c 0 L) rt.(t. 52 5. 16 ° (I'D (D4 c 0 a , -.. <n -oc b o5Fti=--0 6 (5.0 6. 67 00 a D EA.

7 0 n D- to ,-,.. rDo

6 3 - - .7 ,T, = . 0- a Ee t.0 rt) 71 ' I) fD, T rET 0 1)-1.' (ii k o c ,-, ° rD, ,, 0„_, 9_m 2, g. 551m , . 0 D - 0 iT Cr - 5.) M D U 3 lj . rg, 0 --• D0 0 D m

6 _oir,- 9 a 0- D u) ro D o 3-• (1) 0 0 6 r_Ds 0, 0 3 m - m

D- - =_- n - 6_. (1) ;i3 . : 2 89( : Q .23 ( DrR :3 3- 7-81)-. : opau S; : : 27, _ rp rD '-^ (1) 0 0- n N3 D r,--- m ti3 5. g fT> ,:', rD 0 n

, L1 '6. CT" -0TD7 ° rD3 ,3 a ' u2 ( D3 4 ) o? p F,i 7 e3 15" ,93- -2D

-.< -a - 1' -7- cT, T . Q_ . 7 CT" !". - --- 0_ - •-<---.•q:() .9, 9 m m r rf) 8 J ("TT .•

Table 2. Equilibrium Conditions for Study Basins'

sa, 152

i 8 1° 8, 1

l2 ci-),El? C

0 L.,), -5E a s.x E _. _r

O.

a) 0N u0 a,

0 8

t Vi = i.. .... b i E i L., i:50, '' Q. ir

0 C1,' E „a"

,.,; 0 3 6 ', 1 A '' '' 10 c k c

Z c:, LL_ 0 G.71 l..) Y., ‘. u 0 a. 5 ti U. el E u o..J

1 2 3 4 5 6 7 8 9 10 11 12 13

From Map Data OnlyMean Relief 1st-Order Streams 792 579 762 945 762 671 1,280 884 1,160 1,340 1,040 1.190 1.100Meon Relief 2nd-Order Streams 671 396 732 1,160 640 549 732 549 1.280 1.220 1.010 1.130 1,070Mean Relief 3rd-Order Streams 701 305 701 1.310 671 792 1,650 792 1.280 2.190 1,040 914 1, 100Mean Relief 4th-Order Streams 914 274 671 2,070 792 274 - 335 1.580 671 2.500 305 579Mean Relief 5th-Order Streams 427 - - 366 884 91.4 - 305 - - - 1,250 914Equilibrium Relief 701 389 717 1,170 750 475 1,220 573 1.325 1,360 1,400 958 953Chi-Square Value185 145 6.45 1,300 50.9 699 350 478 72.7 870 1,160 623 208

Adjusted for FallsMean Relief 1st-Order Streams 792 579 762 945 762 671 1,280 884 1,160 1,340 1.040 1,190 1.080Mean Relief 2nd-Order Streams 591 375 732 1,160 640 549 732 530 1,280 1.220 1,010 1,130 927Mean Relief 3rd-Order Streams 646 283 683 1,090 625 777 1,620 792 1,230 2,150 716 860 1.020Mean Relief 4th-Order Streams 884 262 616 1,260 771 216 - 335 1,580 640 2,170 287 552Mean Relief 5th-Order Srreoms 381 - - 323 872 91.4 - 296 - - - 1.230 890Equilibrium Relief 659 375 698 956 734 461 1,210 567 1,310 1.340 1.230 939 894Chi-Square Value 228 168 17.5 578 57.1 756 332 493 78.4 866 1,000 655 189

Adjusted for Falls and Log StepsMean Relief 1st-Order Streams 792 579 762 945 762 671 1,280 884 1,160 1,340 1,040 1,190 1,080Mean Relief 2nd-Order Streams 536 363 732 1,160 640 549 732 506 1.280 1,220 1,010 1,130 893Mean Relief 3rd-Order Streams 570 271 631 1.040 613 674 1.570 783 1.190 1.990 664 814 930Mean Relief 4th-Order Streams 783 262 582 933 762 107 - 329 1.410 604 2,020 241 509Mean Relief 5th-Order Streams 375 - - 323 869 88 4 - 296 - - - 1, 190 869Equilibrium Relief 611 369 677 880 729 418 1,190 560 1,260 1,290 1,180 913 856Chi-Square Value 205 177 31 6 479 59.2 842 304 501 30.0 750 865 725 207

'All heights given in decimeters to three significant digits

Feature

106 Marston

Big Creek—Lone County. In the stream networksof these seven watersheds, log steps ore locatedwhere needed to dissipate excess potentialenergy, with chi-square values decreasing fromthose after adjusting for falls. Falls and log stepstogether improve the equilibrium condition ofstream networks in the following watersheds:Scott Creek, Cope Creek—Lincoln County,Cummins Creek, Bob Creek, and Rock Creek. Inthese five watersheds, fall obstructions are lo-cated in the stream network where needed todissipate excess potential energy, with chi-squore values less than values derived frommap darn only The Wilcoxon nonparametrictest for paired measurements demonstrates thatneither falls nor log steps cause a significantoverall difference (or the p 0.05 level) in theequilibrium condition of stream networks in thethirteen study basins. In other words, the dissipa-tion of potential energy by falls and log steps isinsufficient to overcome the influence ofgeologic structure and lithologic controls on thedistribution of relief among different orderstreams.

Sediment Storage

To compare the volume of sediment storedby log steps to the mean annual sediment dis-charge, an estimate of the latter is needed.Maxwell and Morsron (1980) have derived amorphometric index of mean annual sedimentyield (suspended and bedlood) using data fromfive experimental watersheds in mountain re-gions of western Oregon (when using SI units):

MASY= 17.9 + (18.3)(DD x B x SRR x TRS/P),

(2)

where MASY is mean annual sediment yield(kilograms/hectare), DD is drainage density(kilometers/square kilometer), B is bifurcationratio, SRR is stream relief ratio, TRS is total reliefof streams in the basin (kilometers), and P isbasin perimeter (kilometers). The morphometricerosin factor is calculated for each study basin togive on index of mean annual sediment yield inkilograms per hecrore per year, which is thenconvened to cubic meters per year (i.e., sedi-ment discharge) assuming a specific weight of1,600 kilograms per cubic meter and multiply-ing sediment yield by basin area The meanannual sediment discharge may then be ex-

pressed os a percent of the sediment stored bylog steps in third-, fourth-, and fifth-orderstreams. Sediment stored in first- and second-order streams may not be extrapolated fromvolumes measured in larger streams because ofdifferences in channel capacity and the relativelack of field data for the smaller streams.

Log steps store a total of 14,000 cubic metersof sediment in the streams surveyed, includingboth fine and coarse-size particles. The total vol-ume of sediment stored by log steps in all third-,fourth-, and fifth-order streams from all study ba-sins is estimated of 15,600 cubic meters. Theestimated mean annual sediment dischargefrom the thirteen study basins is 12.700 cubicmeters per year derived by the morphometricerosion factor. A comparison of the figures re-veals the volume of sediment stored behind logsteps in third-, fourth-, and fifth-order streams tobe 123 percent of the mean annual sedimentdischarge (including both suspended and bed-load). Note that first- and second-order streamsare nor included in calculations of sediment stor-age. Nevertheless, the relative percent of sedi-ment stored behind log steps in the study areagenerally exceeds values repOrted in the litera-ture for studies that also consider effects of in-srreom debris not incorporated as log steps(Megohan and Nowlin 1976; Swanson andLienkaemper 1978).

Conclusions: ForestManagement Implications

The research is based on the premise thatpotential energy expenditure in the streamnetwork has a determinant effect on channelmorphology and that log steps (with other fallobstructions) reduce the amount of potentialenergy available for conversion to kineticenergy used for sediment routing. A quantitativemeasure of the equilibrium condition is pro-posed to examine potential energy distributionwithin the entire stream network of a basinrather than within isolated reaches or cross-sections. In this context, stream network equilib-rium is considered o time-independent condi-tion. Comparisons among the chi-squore valuesdemonstrate that neither falls nor log steps causea significant difference in the equilibrium condi-tion of stream networks in the study region.Degradation is inferred for streams of an orderwith excess potential energy compared to the

equilibrium reprogressive dcgrade is genet(of bank erosio,des in the chcadjacent to strdowncutting tsource of log st(cess potential estreams of an •energy comporinferred trend vstreams below (field evidencechannel and pcdance of fine-ssnore. Preserv;below grade v.position by incrcenergy by log s'

The volumesteps in all third-is 123 percentdischarge. Adoage is provideOincorporated ascomponent of slsediment rourinLog steps creoiisedimentation iruated by thewoody debrissedi mentor] onwhere log stepsediment are rgrade.

Ar this time,more on contr.lithology thanral processes ordecodes. The d•position in the s'pend on inditeand lithology.majority of logaccounting for ctionol to the pebasins. It mustcreated by sikcumulate becotinstream largesignificant impcditions. In any c,sedimentation

Log Steps in Streams 107

equilibrium relief. The inferred trend towardprogressive downcutring for streams abovegrade is generolly supported by field evidenceof bank erosion and absence of fine-size parti-cles in the channel substrate. Preserving treesadjacent to streams above grade will counterdowncutring by maintaining the dominantsource of log steps needed for dissipation of ex-cess potential energy. Aggradation is inferred forstreams of on order with insufficient potentialenergy compared to the equilibrium relief. Theinferred trend toward progressive deposition forstreams below grade is generolly supported byfield evidence of overbonk flows, extensivechannel and point bar deposits. and an abun-dance of fine-size particles in the channel sub-strate. Preserving trees adjacent to streamsbelow grade will serve only to aggravate de-position by increasing the dissipation of potentialenergy by log steps added from treefall.

The volume of sediment stored behind logsteps in all third-, fourth-, and fifth-order streamsis 123 percent of the mean annual sedimentdischarge. Additional short-term sediment stor-age is provided by insrream woody debris notincorporated as log steps. A sediment storagecomponent of such high magnitude deceleratessediment routing through the stream network.Log steps create a buffer against downstreamsedimentation impacts, on effect that is accen-tuated by the long duration of instreom largewoody debris (Beschto 1979). Downstreamsedimentation impacts will be most severewhere log steps storing abundant volumes ofsediment ore removed from streams abovegrade.

At this time, equilibrium conditions dependmore on controls by geologic structure andlithology than on log steps contributed by natu-ral processes and silviculturol activities in recentdecodes. The distribution of log steps varies withposition in the stream network and does nor de-pend on indirect controls by geologic structureand lithology. Natural treefall accounts for themajority of log steps with silviculturol activitiesaccounting for only 20 percent, a figure propor-tional to the percent area clearcut in the studybasins. It must be noted that future log stepscreated by silvicultural activities will likely ac-cumulate because of the long residence time ofinstreom large woody debris, possibly causingsignificant impacts on channel equilibrium con-ditions. In any case, the equilibrium criteria andsedimentation impacts of log steps provide only

two standards by which to manage instreomlarge woody debris. The biological functions ofinstreom large woody debris and the geomor-phic functions of instream large woody debrisnot incorporated os log steps must also be ad-dressed in forest management decisions. Furtherresearch is encouraged to measure the changein water surface elevation caused by log steps asriver stage fluctuates. The Siuslow Notional Forestand Southwest Regional Office of the U.S. ForestService currently use the stream networkequilibrium-evaluation technique as a planningtool for scheduling timber harvest and wa-tershed rehabilitation projects.

Acknowledgments

The author gratefully acknowledges support for theresearch provided in a grant from the Siuslow NationalForest and administered through the Pacific NorthwestForest and Range Experiment Station. All figures weredrown by Linda M. Marston

Notes

1. Award-winning presentation for the 1981 WarrenNystrom Fund Competition.

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