L ,4N.A-;TtR

. Robert J. Naiman Robert E. Bilby Editors

River Ecology and Management Lessons from the Pacific Coastal Ecoregion

Sylvia Kantor Associate and Managing Editor

With 202 Ilustrations

Springer Robert J. Naiman Robert E. Bilby School of Fisheries Weyerhaeuser Company University of Washington Tacoma. W A 98477 Seattle, W A 98195 USA USA

Cover: Queets River, Olympic National Park. Washington (Photo by Tim Hyatt)

Library of Congress Cataloging-in-Publication Data River ecology and management: lessons from the Pacific coastal ecoregion (edited-by) Robert J. Naiman, Robert E. Bilby. p. cm. Includes index. ISBN 0-387-98323-6 (hc: alk. paper) 1. Stream ecology-Pacic Coast Region (North America) 2. Stream conservation-Pacific Coast Region (North America) I. Naiman. Robert J. II. Bilby, Robert E. QHI04.5.P32R57 1998 577.6' 4' 0979-dc21 97-44766

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1998 Springer-Verlag New York. Inc. All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer-Verlag New York, Inc.. 175 Fifth A venue, New York. NY 10010, USA). except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software. or by similar or dissimilar methodology now known or hereafter developed is. forbidden. The use of general descriptive names, trade names. trademarks, etc.. in this publication, even if the former are not especially identified, is not to be taken as a sign that such names, as understood by the Trade Marks and Merchandise Marks Act. may accrdingly be used freely by anyone.

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ISBN 0-387-98323-6 Spriger-Verlag New York Berlin Heidelberg SPIN 10523806 :';.' , , .

Dynamic Landscape Systems

Lee E. Benda, Daniel J. Miler, Thomas Dunne, Gordon H. Reeves and James K. Agee

Overview prises a population of diverse hilopes that creates spatial varabilty in the sediment and Dynamc landscape processes inuence wood supplied to chanels; (3) chanel net- the supply, storage and transport of water, works, which govern how sediment and wood sediment, and wood, thereby shaping many are routed through a population of lied aspects of riparan and aquatic habitats. These stream reaches and unevenly reditrbuted in processes comprise the disturbance regime of a time and space; and (4) basin history, which watershed. effects the volume of sediment and wood stored The study of natural disturbance (and cu- on hiUslopes and in stream chanels, and which mulative effects) in riverie and riparian areas inuences how sediment and woody debri are . "requires a fundamental shif in focus from redistributed durng storm, fies, wid, and individual landscape elements (such as a forest floods, )3. hislope, and a stream reach) over short . The study of landscapes as systems, focus- ing on the collective behavior of populations of tiescales (year) to populations of landscape e1ements over long time scales (decades to cen- landscape elements over tie, provides the Wres). The study of landscapes as a system necessar framework for investigating natural ands the focus from predictions about exact disturbance and cumulative effects. The field futue states to predictions about the relation- application of thi framework provides inights lbps between large-scale properties of land- into how chanel and riparan morphologies l1pes (i. , cliate, topography, and channel are related to the recent environmental history Detworks) and the long-term behavior of of a watershed. i'quatic systems. Temporal patterns of landscape behavior best described by frequency distributions Introduction :ch estimate the probability of a specific nt occurng. Likewise, describing spatial Powerfl climatic and geomorphic processes ems amongst a population of landscape el- shape the landscape of the Pacifc coastal in any year requires proportionig their ecoregion. Cliatic conditions produce wild- cteristics amongst the range of aUpossible fires and windstorm that modif large tracts 9nmental conditions, and this also is best of forests, enabling new species to contribute to bed by frequency distributions. a diversity of forest ages and structures. Fires Characteristics of landscapes that var in paricular, controlled the age distribution ,.y over time can be described by four of natural forests prior to fie suppression 9nents: (1) climate, which drives environ- throughout most of the mid- and southern pars r., varabilty; (2) topography, which com of the Pacifc coastal ecoregion (Teensma 1987

261 ', ;;

L.E. Benda et al. 262 1990), wind (Borman Morrison and Swanson 1990. Agee 1993). Fires Morrison and Swanson and storms trigger geomorphic processes such et al. 1995). snow avalanches (Hemstrom and rockfalls (Oliver 1981). as bank erosion, surface erosion and gullying, Franklin 1982), and and shallow and deep landslides; processes Although some of the disturbances influencing aquatic systems have been recognized, they which control the supply of sediment and wood quantified (Everest and to streams (Dietrich and Dunne 1978, Swanson have not been well 1981. Sedell and Swanson 1984 1981. Swanston 1991). Once in stream channels, Meehan , Frissell et al. 1986, Resh et sediment and wood are transported episodi- Minshall et al. 1985 cally and redistributed unevenly through the al. 1988. Naiman et al. 1992). As a consequence channel network by floods. descriptions of streams in the context of their spatial deter- Collectively, these climatic and geomorphic watersheds have emphasized minism (which include classification systems processes comprise the disturbance regime of stream biota) a watershed. The term disturbance refers to a of channel morphology and environment that leads to a (Vannote et al. 1980, F rissell et al. 1986. Rosgen disruption in an 1997. Chap- biological response (Pickett and White 1985). 1995, Montgomery and Buffington Fully understanding the role of disturbance ters 2 and 5). Given the importance of distur- in shaping aquatic ecosystems requires estimat- bance in aquatic systems. why has disturbance ing its regime frequencies. magnitudes. and been difficult to define? spatial distributions of landscape processes. Most quantitative theories of landscape pro- Likewise. cumulative effects (because they in- cesses. and their derivative predictive models volve a history of human activities dispersed address the behavior of a single landscape pro- in time and space) can beviewed as a modi- cess (such as fire. flooding, sediment transport, fication of a regime-a shift in frequency. and slope stabilty). or a single landscape magnitude and spa tical distribution of element (such as an individual forest stand hilslope, or stream reach). over short time processes. fire, Disturbance is embodied in the temporal be- scales; for example, responses to a single havior of a single (or set of interacting) land- storm, or flood. Disturbance regimes (and cu- scape element(s). such as forests, hilsides. and mulative effects which can be viewed as an al- streams over decades to centuries. However teration of a regime) in aquatic systems remain in any year. the history of a dynamic climate unquantified largely because theories and mod- els designed to predict numerical solutions or the history of disturbance is represented by processes at the environmental condition of a population about exact future states of single , a few years) are inappropriate of landscape elements (e.g.. hundreds to thou- small scales (e. stream for understanding behavior of populations of sands of forest stands. hilsides. or g.. de- reaches). Therefore, the study of disturbance processes occurring at larger scales (e. I:: cades to centuries). Limitations in data and II' fundamentally involves changes over time and populations of landscape elements. computing power. and the unpredictabilty of the weather. further confound applications I . 11 Natural disturbance is of great interest to resource managers of data-intensive, small-scale theories and . t researchers and natural term because dynamic (temporal) aspects of land- models to the problem of predicting long- ecosystem behavior. scapes are an inherent characteristic of ecosys- distur- t'5 ; :i: . tems in the region (Swanson et al. 1988), and Understanding the consequences of because natural disturbance can be contrasted bance (or cumulative effects) in aquatic sys- reveal tems requires a fundamental shift in focus from with human impacts or disturbances to . ele- the long-term consequences of resource man- individuals to populations (of landscape time scales. For agement. The study of disturbance in land- ments) and from short- to long- scapes of the Pacific coastal ecoregion has example, the behavior of a population of land- sources of focused primarily on processes in terrestrial en- scape elements (such as all point watershed), and the vironments such as revegetation following vol- sediment and wood in a canism (Franklin 1990), fires (Teensma 1987. interaction of that population with other popu- , ;

L.E. Bendel et al. 11. Dynamic Landscape Systems 263

I, wine: (Borman lations (such as a set of linked stream reaches A new system-scale framework is described us- (Hemstrom and comprising a whole network) involves routing ing field data and simulation models to describe s (Oliver 1981). materials over time between highly variable aspects of long-term behavior or natural distur- ances influencing hillslope sources and stream reaches. The bance in the Oregon Coast Range and in south- recognized , they outcome of the collective behavior of such western Washington, ed (Everest and populations, represented, for example, in the Swanson 1984 characteristic frequencies and magnitudes of tal. 1986, Resh et sediment transport and storage in a channel Components of Dynamic network, reflect a system property of a land- \s a consequence Landscape Systems e context of their scape. The term system in this context, refers to :d spatial deter- the interacting group of landscape elements The study of landscapes as systems (in sification systems (i.e., topography, vegetation, soils, fies, rain- term of ld stream biota) storms, channel geometry, and so on) that give populations of upland or riparian forests, hil- sides. and stream channels that vary naturally t al. 1986, Rosgen rise to such long-term patterns of behavior. over time) focuses on four basic n.gton 1997, Chap- Hence, the study of aquatic disturbance (or components: )ortance of distur- cumulative effects) lies within the domain of (1) climate, whieh drives environmental var- abilty and emphasizes the importance of time lY has disturbance the study of landscapes as systems The study of landscapes as systems is differ- in a landscape systems perspective; (2) topogra- of landscape pro- ent than the study of single landscape elements phy, which represents a population of diverse predictive models over short time scales. Viewing the behavior of hilslopes responsible for the spatial variabilty gle landscape pro- a forest, a hilslope, or a stream channel over of sediment and wood sources; (3) channel net- ediment transport decades to centuries requires replacing predic- works, which govern how sediment and wood delivered from interactions between single landscape tions about exact future states with more gen- cliate and topography are transported downstream dual forest stand, eral predictions of long-term patterns of through a population of linked stream reaches; , over short time behavior. This demands a degree of simplifca- ;es to a single fire tion in scientifc analysis and descrptio , re- and (4) basin history, which governs the volume of sediment and wood stored on hilslopes and e regimes (and cu- ' ferred to as coarse graining (Gell-Man 1996), in stream chanels in any year Je viewed as an al- -and the use of estimated probabilty distrbu- , and thereby infuences how sediment and woody debris are ltic systems remain tions to overcome lack of scientifc understand- redistributed durng future storm, fires theori s and mod- idg and data, computing limitations, and , and floods. umerica' solutions ' certinty about future cliate. Such an ap- single processes at oach can be used to parlay empircal knowl- s) are inappropriate dge and theory available at smaler spatial and Cliate r of populations of . poral scaes to produce new uiderstanding er scales (e. , de- Jandscape behavior (theories, models, and In Washington, Oregon, and northern Cal- ations in data and yPotheses) at larger scales. fornia the contemporary climate has been in unpredictability of tLong-term patterns of landscape-behavior place for the past several thousand years result- Jound applicatio cluding disturbance regimes, are best de- ing in relatively stable vegetative communities ;cale theories and bed in terms of frequency distributions from (Heusser 1977, Leopold et al. 1982, Brubaker Jredicting long-term )h-estimates of probabilty of occurrence 1991). Nevertheless, smaller-scale varations :be made. Likewise, descrbing the envion- in climate (decades to centuries), such as the ;equences of distur- tal condition of a population of similar neoglaciation of the seventeenth and eigh- :cts) in aquatic sys- scape elements in any year (govented by teenth centuries, have influenced erosion and al shift in focus from , te hitory and land use) requires propor- channel morphology in mountai areas ; (of landscape ele... g characteristics among the range of all (Church 1983). long-time scales. F ,Ie environmental conditions. Ths also is The following discusion of climate concen- 1 population of land escrbed by frequency distributions, and trates on the effects of precipitation, flooding, all point sources 0 . chapter aspects of long-term patterns of and fire. Although not discussed here, wid- watershed), and th . pe behavior, or natural disturbance are storms are also important controls on the age ion with other popu bed in terms of those statistical measures. distribution of forests along coastal areas and 264 L.E. Benda et al. in northern parts of the region (southeast forest and stream environments. Long-iasting. Alaska). and windthrow may be an important intense rainstorms saturate soils and trigger modifier of soil chemistry and v getation pro- shallow landslides and debris flows, even under ductivity (Borman et al. 1995). forest canopies (Figure 11.) (Pierson 1977 Dietrich and Dunne 1978, Hogan et al. 1995). High seasonal rainfall accelerates movement of Precipitation deep-seated landslides and earth flows. When it Precipitation (rain and snow). as the source of is not raining, forests become dry and the po- groundwater and stream flow, is the primary tential for wildfire increases (Agee 1993). Tem- natural driver of change in Pacific Northwest poral variation in precipitation creates seasonal

FIGURE 11.1. Aerial photograph taken in 1939 of a Hogan et al. (1995) estimated the frequency of such 4th-order basin in the western Olympic Mountains, concentrated landsliding based on precipitati Washington. The storm of record (1934) triggered alone to be between 40 and 70 year in the nort- numerous debris flows (indicated by arrows) that western part of the region. deposited into the main channel. Orme (1990) and . . .' , .. - - , .

11. Dynamic Landscape Systems 265 L.E. Benda et al. ents. Long-lasting. : soils and trigger 32 25 s flows, eveR under !20 .1) (Pierson 1977 5 15 Approximate 1995). threshold Hogan et al. jg 10 rates movement of for landsliding ;arth flows. When it c: 0 1950 :Te dry and the po- 1960 1970 1980 1990 . (Agee 1993). Tem- ion creates seasonal

m 30% 200A Fire potential-J

150A 20% '0 8, 100A 'E 10% 'E 50A

OOA 10 15 20 25 0 1.5 2. Maximum Precipitation Intensity Annual Precipitation within a Given Year (ernday) (m/year)

FIGURE 11.2. (a) A time series records the sequence landslide occurence (Caine 1980), (b) A frequency of rain (and snow) storm over 47 year at the distrbution of daiy maxum precipitation indi- Randle Ranger Station in southwestern Washington. cates the proporton of time that landslides can be The sequence of rainstorms intiates temporal vari- generated (approximately 33% of the time or once abity in the landscape by trggering landslides, every three years). (c) The frequency distribution of generatig floods, and creating drought conditions annual total raiall indicates the amount of tie conducive to widfie. The raial threshold for trg- that wildfies may be triggered due to moisture defi- gerig landslides is estimated from an empirical cits (,::.1.0mlyr occurs less than 3% of the time). relationship among raifal intensity, duration, and

changes in stream flows and anua fluctuations years; and sometimes nearly a decade passes in flood peaks, in sediment delivery to stream with no large storms at all. chanels, and in fire potential. The role of precipitation in landslide or fie A sequence of daily precipitation measured generation is best viewed through a frequency over 47 years for a site in southwestern Wash- distribution. By condensing data in the time ington(Figure 1l.2a) indicates that rainall series of precipitation (Figure 11.2a) to a ditr- vaes widely (0 to 25cm in one day). Despite bution of values, inormation on the temporal th varabilty, these data are useful for evalu- sequence of events is lost but insight as to how !tUng the influence of loca climate on channel often an event of a given magntude occurs is c1 floodplai morphologies. The frequency of gained (Figues 1l.2b and 1l.2c). For example . that trigger landslides, for example , is the frequency of storm that trgger landslides ted by a theshold estiated from an is about once every three year (e. , approxi- lted the frequency of such ca relationship among rainfal intensity mately 33% probabilty in any year, Figue based on precipitati urtion, and landslide occurrence (Caine provides information on nort 1l.2b) and how md 70 year in the Someties several landslide-triggerig often landslide debris might be expected to . occ in one year or in consecutive enter channels in that landscape. Likewise, the ------: ..~~~ ------....

266 L.E. Benda et "I. potential for large . intense fires increases dra- the relationship between floods (potential matically when the annual precipitation total is sediment transport capacity) and sediment less than 1.0m. but such low values of annual supply (the history of erosion) ultimately con- precipitation occur only about 3 % of the time trols the abilty of floods to modify channels (Figure 11.2c). Hence. fire-producing droughts and floodplains, and to create channel refuge are much rarer than landslide-triggering habitats. such as wood jams and side channels storms. (Chapter 2). The temporal variation of flows typical of Pacific coastal streams is illustrated by the time Floods series in Figure 11.3a and the associated fre- Ultimately streambeds, banks, and riparian quency distribution of discharge measured at floodplains are shaped by the temporal se- a gaging station in southwestern Washington quence of flows and sediment loads carried (discharge varies from 2 to 200m /s) (Figure through the channel. Although sediment may 11.3b). Approximate thresholds for bed mobil- arrive directly from adjacent slopes, most mate- zation (i.e.. bedload sediment transport) and rial found in the streambed and floodplain has overbank flows are determined from substrate been carried some distance from upstream. The size. channel width, and bank height for the size and frequency of floods determine the ca- gauged channel with a drainage area of 200 km pacity of a stream to move sediment into or out (Figure 1l.3a). Frequency of channel-forming of a channel and to overflow its banks. Thus events can be defined by representing the time

Approximate threshold ! 180 - for extensive over-bank flow is 120 . Approximate threshold for bed mobilization 60,

1950 1990

I! 20'* : Incrasing potentil for nel modification 15'*

Incrasing potential for 8, 1 0'* flplain moiftion

Q) 0'* a. 30 60 90. 120 150 180 210 240 Peak Annual Discharge (m3/s)

FIGURE 11.3. (a) A time series of channel discharge the channel bed, eroding banks. and inundating dictted by the sequence of rainfall (and snowfall) floodplains. (b) Frequency distribution of floods over previous days or weeks. In this case. the extent derived from time series of stream discharge in of channel and floodplain modification varies greatly Figure 11.3a. Frequency of bedload traport from year to year because only large flows (::I00m about 30% while overbank flows occur 10-15%. of sec) are capable of moving sediment from or onto the time. : j , ':j'j

L.E. Benda et al. 11. Dynamic Landscape Systems 267

floods (potential series of flows as a frequency distribution, For Varation in the frequency of fires across the city) and sediment example, floods transport bedload approxi- Pacific coastal ecoregion is controlled , by the sion) ultimately con- mately once every three years (e. , 33% of frequency of ignitions (i. , by lightning), to modify channels winter floods) (Figure 11.3b). The opportunity the frequency and severity of droughts, the eate channel refuge for floodplai modification is much less, only availabilty of combustible organic material ns and side channels about 7% (once about every 14yrs). This indi- and topography. In addition, ignitions by cates that the frequency of floods is sufcient to Native Americans may have been an important I of flows typical of redistribute the sediment that originates from factor in certain parts of the region lustrated by the time frequent landslides. (Schoonmaker et al. 1995). Low-frequency, j the associated fre- Floods are also important for transporting high-intensity fires associated with high tree ;charge measured at and redistributing wood as well as creating log mortalty are referred to as stand-replacig western Washington jam. Floods large enough to transport wood fires (Agee 1993). The average time interval to 200m /s) (Figure may occur frequently, but transport of wood between stand-replacing fires (the mean fire holds for bed mobil- depends on the wood supply as well as piece recurence interval or fie cycle) at a specifc ment transport) and length and slope (Nakamura and Swanson location in a landscape has been estimated in oined from substrate 1993). Therefore, the transport of woody debris varous par of the Pacific coastal ecoregion bank height for the by stream flow (and the development of debris anQ is about 400 years for cedar (Thuja plicata)/ lnage area of 200 km jam) strongly depends on the temporal se- spruce (Picea sitchensis)/hemlock (Tsuga , of channel-forming quence of flooding magntude and wood sup- heterophylla) forests of the Olympic Peninsula epresenting the time ply, the same way sediment transport depends Washington (Agee 1993); 200 to 300 years for on the temporal sequence of flood magnitude the Douglas-fir (Psuedotsuga mensiezii)/west- and erosion. ern hemlock (Tsuga heterophylla) forests of the central Oregon Coast Range (Teensma et al. Fire 1991, Long 1995); 150 to 200 years for Douglas- fi/western hemlock forests in the western Cas- ate threhold :reriodic droughts , strong east winds, d the cade Mountains of Washigton and Oregon tensive ,nk imasive buidup of woody fuels create condi- (Teensma 1987, Morron and Swanson 1990), ,ximate threhold tions favorable for fire, paricularly in the and 80 to tOO year for lodgepole pine (Pinus )00 mobilizatin southern d middle Pacific coastal ecoregion contorta) forests in southwestern Oregon (Gara (Agee 1993). The frequency and magnitude et al. 1985). (ijtensity and size) of fires strongly influence Varation in fire frequency affects the fre- tIe ditrbution of forest ages, the recritment quency and location of erosion and wood input r",oo to streams, and the frequency and to streams (i.e., by determg rates of mortl- gmtude of erosion. ity and stand age) and therefore the distu- Fife inuences erosion by destroying vegeta- bance regime of the channel. Diferent pars .lion and by sharply decreasing the rate at of the landscape bum at diferent frequencies , '':which water infltrates into soil. Roots of trees depending on the topography. North-facing " shrubs reinforce soils laterally and bind slopes are typicay moister and cooler than soil to parially fractured bedrock. As a south-facig slopes and are therefore less sus- equence landslides and debris flows ceptible to stand-replacing fies, paricularly in more likely to occur afer fies destroy the wetter areas of the ecoregion. Ridges and , etation and when root strength is lowest low-order valleys (e. , Ist- and 2nd-order) are J.ugh 1972). Thus, forest mot13lity by more susceptible to fire than nearby larger val- increases the potential for landslides and ley floors because fies tend to bum upslope. A- flows (Figure 11.4) (Swanson banks, and inundating 1981, fire simulation model, using fie probabilties distribution ' of 800 , and Dunne 1997). Fires in drier areas obtained from field data in a 500 area in )f stream discharge hydrophobic soils can lead to widespread southwest Washigton, ilustrates how difer- f bedload transport " erosion, gulyig, and the release of ent topographic positions result in different fie quatities of sediment to the flows occur 10-15% valley floor regimes (Figue 11.5). Although fie recurrence and Helvey 1976). intervals in the landscape of southwestern L.E. Bendaetal. 268

5km

fires between tion of shallow landslides (small arrows) in the FtGURE 11.4. Several large and intense debris 1933 and 1939 in the north-central Oregon Coast burned area. Some of the failures triggered caused widespread for- flo s in 1st- and 2nd-order channels (large arrow) Range (the Tilamook fires) 4th-order est death and created a forest of standing. dead trees and deposited sediment directly into a 1997a). ina portion of the Kilchus River basin. Aerial photo- channel (Benda and Dunne graphs (1954, 1: 12.00-scale) reveal a concentra-

Washington averages 300 years. fire intervals and wood to streams that varies temporally focuses of 150 years occur on ridges and in low-order well as spatially. Although this chapter stream valleys (e.g.. Ist- and 2nd-order). and on contemporary climate. it also recognizes fire intervals can exceed 500 years for wide and that present erosion rates are influenced by past low-gradient valley floors. climate processes. Sediment Supply

Topography In terms of a landscape system. topography is represented as a population of diverse The second component in the study of land- diverse scapes as dynamic systems is the diverse popu- hilslopes and hence a population of lation of topographic patterns in a watershed erosion sources. Two areas (the Oregon Coast which creates a discontinuous and spatially Range and southern Cascade Mountains of Washington) ilustrate how diverse topography variable supply of sediment and wood to chan- supply nel networks. Interactions between climate and leads to spatially variable sediment topography are fundamental components shap- (Figure 11.6a and 11.6b). Two types of mas movement occur in' the highly dissected, ing landscapes. In the Pacific coastal ecoregion Coast intense rain and wind result in erosion and low-relief topography of the Oregon flooding that form stream and valley floor Range: one is shallow landslides from steep, unchanneled swales (referred to as bedrock morphologies, especially after widespread fire. 1st- Heterogeneous topography coupled wirh a dy- hollows) and the other is debris flows from namic climate results in a supply of sediment and 2nd-order channels spaced along both sides ;, ,

L.E. Benda'et al. 11. Dynamic Landscape Systems 269

of the main river at irregular intervals (Figure to transport material downstream (deposited 11.6a); Landslides or debris flows emanating sediment buries the channel), the average size from anyone site are rare, but mass wasting at of chanel substrate decreases, and riparian the watershed scale (containing thousands vegetation is destroyed. Over time the stream of landslide sites) is a common occurrence gradually removes much of the deposited mate- (ilustrated later in this chapter). rial, larger paricles (boulders) are exposed A population of landslides and debris flows and new plant communities are established over the entire chanel system has a large and (Chapter 12). Persistent, long-term morpho- persistent inuence on the valley floor environ- logic effects include creation of log jams, devel- ment. The imediate short-term effects of opment of terraces, accumulation of boulders mass wasting are readily apparent as the large and construction of debris fan that force infux of sediment and wood overwhelm the the chanel agaist the opposite valley wal capacity of the Oregon Coast Range stream (Benda 1990). The density and location of these

5km mall arrows) in the ures triggered debri annels (large arow) :tly into ' a 4th-order 97a). aries temporally as :his chapter focuses it also recognizes e influenced by past

Relative Probabilty of Burning ,ystem, topography - .:0. ulation of diverse - 0.5-0. pulation of diverse _0.75-1. (the Oregon Coast o-1. .1.25-1. ;ade Mountains of' GJ 1. diverse topography; CJ;:1, e sediment supply Two types of m3ssi 11.5. Topographic controls on fire. Stand- likeliood of fie over tie as a fuction of topogra- , maps obtained from normaled so that highly dissected; 1939 aerial photography phy. Values are the average over . 3 lOOkm area in southwest Washington indi- the entire area equals unity. In area, fire cycles the Oregon Coas th orrelations between forest age and slope aspect are predicted to be less than 200 year on ridge tops dslides from stee west facing slopes bum most frequently) and and greater than 500 year on valey floors, with a :red to as bedr relief (ridge tops bum more frequently than mean of 280 year. Channel network is indicated by bris flows from I' . . floors). These empircal relationships were black li es and ridges and south facing slopes are ced along both sid to construct this map which shows the relative indicated by lighter shading. . . -:.

270 L.E. Benda et at.

Types of, Potential Mass ,Wasting Shallow landslide Debris flow : Deep-seated landslide Inner gorge

FIGURE 11.6. (a) High density of shalow landslides flows and a higher density of small shallow landslides and debris flows characteries the highly dissected along inner gorges. Regional varation in the pro- terrain of the Oregon Coast Rage: (b) In contrast, ceses and spatial patterns of erosion leads to re, pervasive deep-seated landslides have modified the gional differences in the frequency and magnitude of : topography in the southern Caae Mountains ' sediment and wood supply to streams. Washigton, resultig in a lower density of debris

deposits throughout the channel system (Figure Deep-seated landslides likewise have altered 1l.6a) dictates the type, diversity, and distrbu- channel and valley floor morphology. creatig tion of many of the chanel and riparan envi- local low-gradient areas upstream of slides ronments within the watersheds. and local steepening at the downstream slide The southern Cascade Mountains ilustrate a edges. Channels are confined within inner different and more complex topography, one gorges where deep-seated landslides have smoothed by extensive deep-seated landslides created narrow valley floors. (Figure 11.6b). Compared to the Oregon Coast Together, topography and climate create Range, a greater diversity of mass wasting pro- pattern of sediment supply to stream cesses is active, including deep-seated bedrock channels that is variable in space and time. The landslides at a variety of spatial scales and per- status of sediment storage in any particular vasive shallow landslides in inner gorges. The- channel depends on the population of all up- spatial density of shallow landslides adjacent stream sediment sources from the numeroUS to streams is greater than in the Oregon Coast hilslopes. Range parly because steep hiner gorges have formed at the toes of large deep-seated land- Wood Supply slides. In contrast, the spatial density of debris flows is lower, in part, because deep-seated The supply of woody debris to streams is landslides have lowered midslope gradi nts. influenced by climate and topography. The siz ;-- ---

L.E. Benda et al. 11. Dynamic Landscape Systems 271 of trees that fall into streams depends on tree to large fluctuations in the size and number of age which is controlled , in part, by the fre- trees recruited to small channels. quency of fies, windstorm, or erosion. For Concentrating on fire in this example , the example, fires generally do not consume entire change in in-stream wood volume over time trees and the majority of dead trees fall within difers for two different fire regimes (Figure several decades after a fire (Agee 1993) leading 11.7a). Stand-replacing fires every 500 years

High

..-c I \

Low all shallow landslides laration in the pro- 1000 2000 erosion leads to re- 1).:f:g. Time (years) 1cy and magnitude of treams.

High wise have altered )rphology, creatig' lpstream of slides ; downstream slide 'ined within inner landslides have nd climate create supply to stream ;pace and time. The in any particular Low )pulation of all up- Low High rom the numeroUS , Wood Volume 11.7. (a) The changig in-stream volume of wood supply after a fie, and almost al of the time for two diferent fie regies. and (b) their large volumes of wood are present in the channel. In ying frequency distrbutions predicted by a contrast, the 100year fire cycle (-----; no shading) lion model. The 500year fie cycle (- results in lower volumes of wood, and a lower range dig) yields abrupt and large increases in of varabilty in woo loading. ::'

L.E. Benda et al. 272

(similar to the fire frequency in the coastal rain minor volumes of woody debris because of forest of Washington) leads to abrupt and large their low frequencies compared to decay rates increases in wood supply as trees kiled by fire during periods of disturbance wood loading by fall to the ground less than 50 years later. After mass wasting may overwhelm all other sources this initial large input of wood, wood supply of wood. slowly increases from stand mortality as stand- The abilty of woody debris to affect channel ing biomass increases. In contrast, more fre- morphology depends on stream size (Bilby and quent fires every 100 years (similar to drier Ward 1989). Even large rivers can be greatly forests of eastern Washington and Oregon) re- influenced by wood (Sedell and Frogatt 1984 sult in a smaller punctuated supply of wood chapters 12 and 13. this volume). Trees fallng because large amounts of biomass do not accu- into a stream reach significantly modify stream mulate. The difference between the two simu- morphology in several ways (Chapters 2 and lated fire regimes is apparent in the frequency 13). For instance, scouring of the channel bed distributions (Figure 11.7b). The distribution around the downed trees modifies aquatic habi- stl of wood volume for the 1oo-year fire cycle tats as, pools, side channels. and sediment stor- indicates the dominance of low-wood supply age areas are formed (Keller and Swanson conditions. But perhaps more importantly, the 1979, Sullvan et al. 1987). In addition, log jams ti' figure shows that lower volumes of wood are create small terrace-like deposits along valley common. In contrast, the frequency distribu- floors (Kochel et al. 1987) and stabilze portions development of tion for the 5OD-year fire cycle shows signifi- of floodplains allowing cantly higher wood volumes for large portions mature conifers (Abbe and Montgomery 1996). signifi- of the time, and very low volumes of wood are Woody debris jams temporarily store rare. The predicted frequency distributions cant amounts of sediment (Megahan 1982), release from these two fire regimes allow consideration although eventually, decaying jams of how in-stream volumes of wood change as stored sediment. High sediment supply in con- one moves from coastal rain forests to drier cert with debris jams can lead to channel awl- active interior forests. - sions and the isolation of jams from The variabilty in fire frequency for different channels. pars of the Pacific coastal ecoregion also con- cl: tributes to a spatially variable wood supply Hierarchical Patterns of at the watershed scale. Different parts of Channel Networks watersheds burn at diferent frequencies (Figure 11.5), with small, steep valleys having a The third component of landscapes viewed higher fie frequency compared to wider valley systems is the population of linked stream floors. reaches arrayed in a hierarchical chanel Other processes influencing input of woody network. Two aspects of hierarchical chanel debris to streams include bank erosion , land networks are considered: how fluctuations in slides and debris flows. In large streams woody sediment supply (and transport) are mixed and debris is commonly recruited ,by bank erosion modulated downstream through the network which can occur every several years (Keller and and how different sediment transport regies Swanson 1979). Landslides and debris flows are abruptly joined at tributar confluences. (and snow avalanches), with or without fires, sil sit are also important sources of woody debris- Fluctuations in Sediment Supply and (Swanson and Lienkaemper 1978, Chapter 13 Channel Morphology this volume). In the Queen Charlotte Islands. Recall that the first two components of a dy- , British Columbia, landslides under forest cano- topogr- ' pies, with a recurrence interval of approxi- rtamic landscape, climate and diverse mately 40 years, result in large numbers of phy (viewed as a population of erosion and , debris jams (Hogan et al. 1995). Although land- wood sources), result in a punctuated supply of , suddeD slides and debris flows may only contribute sediment and wood to channels. The .' ' '

L.E, Benda et al. 11. Dynamic Landscape Systems 273 debris because of deposition of sediment into a channel previ- and Collins 1984, Roberts and Church 1986), lred to decay rates, ously carrng little sediment may create tem- braided channels (Roberts and Church 1986), :e wood loading by porary zones with greater sediment volume and decreasing pools in conjunction with increasing m all other sources higher sediment transport rates that can either riffes (Madej and Ozaki 1996), and death of remai stationary (such as point bars) or mi- riparan forests (Janda et al. 1975). Passage of is to affect channel grate downstream in the form of a sediment sediment waves on a variety of scales leads to :am size (Bilby and wave (Beschta 1978, Church 1983, Benda and frequent bed fillng and scourg which creates rers can be greatly Dunne 1997). Dimensions of recognizable sedi- a channel environment unfavorable for fish and Frogatt 1984 ment waves range between several hundred to spawning (Nawa et al. 1989). However, sedi- ume). Trees fallng several thousand meters in length and from one ment waves also cause bank erosion and tree ntly modify stream hal to four meters in height (Church 1983 fall which form pools and creates sediment s (Chapters 2 and Pickup et aI. 1983, M:idej and Ozaki 1996). storage areas, including spawnable rifles. of the 'channel bed Sediment waves also may disperse rapidly in )difies aquatic habi- steeper, confned channels, such as in canyons. Tributary Confluences and and sediment stor- Chanel beds that do not undergo major fluc- Discontinuities in Transport Regimes ller and Swanson tuations in sediment supply may exhbit rela- (l addition, log jams , tive stabilty (i.e" no sediment waves and little Diferent sediment transport regies (Le., pro- :posits along valley scur and fi). cess, frequency, magnitude, and parcle sizes) Id stabilize portions Within the channel networks sediment waves are abruptly joined at trbutar confuences. development of (or an accumulation of sediment) may merge For example, a discontinuity in transport pro- Montgomery 1996). downstream because the velocity of bedload cess often occurs where Ist- and 2nd-order ,rarily store signifi- paricles (anual transport distance) generally channels, that are prone to debri flows, join t (Megahan 1982), 'decreases downstream. Accumulations of sedi- higher-order channels. Debri flows transport large volumes and sizs of sediment and wood ying jams release 1nent or large sediment waves have formed in nent supply in con- mastem chanels from erosion in numerous into lower-gradient chanels. As a conse- ad to chanel avul- trbutar subbasins (Jacobson 1995, Ma ej and quence, such confuences may be characteried by boulder deposits, fans, terraces and debris : jams from active Pz 1996). In addition, sediment waves may erge and travel serially downstream when a jam (Benda 1990, Grant and Swanson 1995 lUatic disturbance ( e. , storm or fire) occrs Hogan et al. 1995). Discontinuities in transport at confuences of " r a large area of a watershed and releases regimes also occr large :. ent in numerous adjacent tributares that streams. An increase in the probabilty of sedi- join to become a ' single larger chanel mentation at, and imediately downstream of ndscapes viewed as nda 1994). confluences results from a higher proportion of river (Benda l of linked stream fluctuations in sediment supply are of suff- bedload compared to the mai ierarchical channel' t magnitude, a cycle of scour and fill creates 1994). lierarchical channel .. lais and terraces (Gilbert 1917, Hack how fluctuations in Goodlett 1960, Griffths 1979, Pickup et al. Basin History port) are mixed and f 83). Th cycle, thus, can lead to formation lrough the network :new riparan areas and may reset the age of Basin history is the fourt component influenc- It transport regimes an vegetation. Fluctuations in sediment ing landscape characteristics. The temporal se- Itary confluences. Iy, in conjunction with floods, also creates quence of past climatic and erosional events e. chanels on newly formed terraces and (that has either supplied or removed sediment , Supply and , gh chanels in low-gradient floodplais, and wood from hilsides or channels) influences 'Povide crtica refuge habitats. how sediment and wood are redistributed dur- ttations in sediment supply also lead ing future storms, fies, and floods. omponents of ad es in chanel morphology, Transient The history of soil moisture over a winter md diverse topogr es in sediment supply in the form of period (i. , numerous storm) afects the rate tion of erosion lead to temporar increases in the widths of stream flow and soil stabilty (Pierson 1977) )Unctuated supply els and floodplai (Beschta 1984, which afect stream power and the rate of sedi- annels. The sud 1991), fiing of the substrate' (Coates ment tranport and the volume of sediment . '

274 L.E. Benda et al. 11. supply (through bank erosion and landslides) ture of material fluxes and aquatic morpholo- during any individual storm. Each storm alters gies. The remaining portion of this chapter de- the distribution of sediment, so that the volume scribes some of these relationships, including and location of sediment stored in a channel how effects of scale and channel networks reflects the sequence of storms over several cause shapes of frequency distributions to decades to a century. Hence, the temporal evolve downstream. Biological aspects of land- ordering of storms and associated erosion in a scape systems and general applications to land basin greatly influences channel response and use are also presented. controls cycles of sedimentation and erosion of Observed records (e. , rainfall in Figure valley floors and channels (Beven 1981). 11.2a) are typically too short to fully character- The amount of sediment stored on hilslopes ize relatively rare events such as fires. storms that may ultimately become incorporated into and landslides. Empirical distributions can landslides and debris flows also is affected by be combined with numerical models designed basin history. The timing of a landslide or de- to simplify certain small-scale hilslope and bris flow is influenced by the rate of sediment channel processes to characterize behavior accumulation since the time the site last failed over longer time periods. Use of such coarse- (Sidle 1987, Dunne 1991). Hence, longer peri- grained models allows problems of increasing ods between landslide-triggering events at the complexity to be addressed, leading to watershed scale (e.g., involving a population of testable predictions (hypotheses) on landscape- landslide sites) wil likely result in a greater scale patterns of behavior (Benda and Dunne volume of sediment and wood released to a 1997). channel network. Olaciationstrongly influences both local and Temporal Sequencing of Storms landscape-scale geomorphic processes. Late Fires, and Floods, and Dynamic Pleistocene glaciation (e.g., 12 0015,OOyrs Channel Behavior ago) created large valley floor reservoirs of outwash sand and gravel, and lacustrine de- A large flood may not cause major channel posits of silt and clay in some of the northern changes in the absence of sediment to trans- areas of the Pacific coastal ecoregion. These port. However, in the presence of large quanti- glacial deposits initiated new mechanisms of ties of sediment originating from recent erosion (i.e., deep-seated landslides in glacial erosion, the same flood may cause large modif- sediments) and increased erosion rates for cations to channels and floodplains. The key thousands of years after the glacier retreated to understanding the dynamic behavior of FIGV (Church and Ryder 1972, Benda et al. 1992). channels is the short-term (years to decades) (bh In the 17th and 18th centuries smaller alpine synchronicity of climatic events that produce (c) . glacial events in some areas, such as British erosion and flooding. For example, a sequence Columbia. caused shorter-term changes of stand-replacing fires followed within a de- seu in sediment supply and sediment transport cade or two by large storms may lead to con- (Church 1983). centrated landslides because this is the period of low root strength prior to revegetation set: (Figure 11.4). In drier areas, where fires create ,. Th Dynamic Landscape Systems: soils prone to surface erosion, a fire-storm see set quence may result in massive influx of sediment ' U.s, Populations of Elements to channels and valley floors. and 'Time , Using an example from a computer simul- , ;0: tion of the steep, highly dissected centr) , ane Studying the system properties of landscapes Oregon Coast Range where shallow landslides' JuDb focuses , on relationships among the major at- and debris flows are dominant erosio 'itb tributes of climate, topography, networks, and processes, the sequence of rainstorms (Figu'f l1ek basin history and, hence. the space-time struc- 11.8a) and fires (Figure 11.8b) governs thfr , ,. . '

275 L.E. Benda et al. 11. Dynamic Landscape Systems

,quatic morpholo- )f this chapter de- Cf .- Rainstorms "C Fires Jnships, including 5 E .- 50 :hannel networks Ui m 20 IDN , distributions to II E 30 al aspects of land- ;ij 10 c: .. Q. Jplications to land 400 500 100 200 300 400 500 Year rainfall in Figure to fully character- , storms :h as fires landslides distributions can -; 400 In II I models designed 300 :ale hils lope and acterize behavior :g B 200 Ise of such coarse- j E 100 lems of increasing .. 0 ssed leading to ses) on landscape- Benda and Dunne Sediment Supply

)- 0. Localized )f Storms, sedimentaon Dynamic Sedimentaton of trbutry subasins Basin 1 &. 0. (Weak rock) use major chanel Exsive sedimentaon Basin 2 sediment to tran- ' of mainstem,/ (Stng ro) nce of large quanti- lting f om recent , cause large modif- 100 200 300 Landslides (number/year) Jodplains. The key lamC behavior of UR 11.8. A sequence of raitorm (a) and fies distrbution (d) indicatig how likely varous magn- (year to decades) , generates a sequence of landslides with a basin tudes of sedientation occ. Chanel response de- :vents that produce ; which results in an intermttent sequence of pends on the siz distrbution and durbility of the xample, a sequence diment delivery to the channel system. The time sediment delivered. .lowed withi a de- , uence of sediment sUPI'ly is represented as a lS may lead to con se this is the period or to revegetati uence of erosional events (Figure 11.8c). habitats. By considerig the frequency distribu- , where fies create , series of sediment supply is repre- tion of sediment supply (Figure 11.9a) in con- ion, a fie-storm se- etie ted as a frequency distribution (Figure cert with the frequency distrbution of floods ve influx of sedient ) to predict the long-term pattern of (Figure 11.9b), the frequency ditrbution of Irs. , ent input to a channel network. channel and floodplain changes can be concep- a computer simul iderig the of sedient supply to tually portayed (Figure 11.9c). This distrbu- dissected cent! tig ork (Figue 11.8e) in the context of the tion ilustrates how often certin chanel and re shalow landslid of large floods is necessar to anticipate floodplai changes are and erosi likely to oc domiant modif chan- therefore provides inight on how the chanel f raistorm (Fi ty of erosion or floods to d floodplains, and ultiately aquatic and floodplai wi evolve over tie and space. 11.8b) governs ' . , " : ' ;; : . .

L.E. Benda l' al. 276 11. I

flooc on tJ ff 15% (1 20% been Q) F1 " Floods Sediment Supply :: 15% 1983 '010% ; 1996 & 10% l -- t 5%, "E 5% Q) EffE Q) 0% Q) 0% . 120 180 240 Spa a. 50 100 150 200 250 300 a. 60 Peak Annual Discharge Landslides (m3/s) Viev (numberiyear/200km hilsl nom' of Ii Timing of Sediment Supply and Transport Events spati l2+F1 - :L1+F1:_ L 1 + F2 :-l2+F2- dow! pert also cons land: Proportion distr Time evol' thO!

Top HilL None Channel/Floodplain Modification Large .e The :Litte : Meander migration : Chann 1 avulsion/braiding : .eExens!"e channel bed sco !/depos1t1on chang Floodplain extension na: : Recitment of woody: Exnsl oodplaln deposition debris into channel' Reset npanan forest Creation of new terrcefloodplain surfaces

FtGURE 11.9. The extent to which floods affect chan- the channel system (a), the frequency distribution of nel and floodplain characteristics depends on the flood flows (b), the persistence of sediment in the volume of sediment and the size of the flood (i.e., channel. and (c) the relative timing between thes temporal sequencing). Hence. the frequency with events. Relative timing between flooding (F) and which impacts of a given magnitude occur depends sediment supply from landslides (L) is indicated by on the frequency distribution of sediment input to L1 + Fl and so on.

An example from Idaho ilustrates this con- and the creation of new terrace and floodplai cept. In the summer of 1995, a stand-replacing surfaces (Figure 11.10). In the frequency dj, , 9c)., fire with an estimated recurrence interval of tribution of channel changes (Figure 1l. approximately 150 years was followed very this type of event would be located in tbc; right tail indicating how often new terrce::. shortly thereafter by an extreme rainstorm trig- ri,/ gering widespread surface erosion and gullying, floodplains, and channels (e.g., aquatic and accompanied by floods and fluvial mobilzation parian habitats) are formed along the valer of sediment in channels. The result was wide- floor. In the absence of the fire, a flood of . spread sedimentation throughout an entire lar magnitude likely would be located in the Ie network of chanels, which caused channel hand tail of the distribution in Figure 11 avusions and braiding, floodplain deposition resulting in only minor changes to chanels ' " . . ;:$; . .

L.E. Ikl1d;1 l' al. 11. Dynamic Landscape Systems 277

floodplains. The influence of sediment supply with a simulation model. The model is applied on the geomorphic impacts of floods also has to a 6th-order 200 basin in the Oregon been documented in British Columbia (Church Coast Range where erosion is dominated by 1983), northern California (Madej and Ozaki debris flows (e. , Figure 11.6a) following fires 1996), and New Zealand (Beschta 1984). and storms (Benda and Dunne 1997). Epi- sodes of concentrated landslides occur within burned areas during rainstorms for a decade or Effect of Hierarchical Networks and so after a stand-replacing fire, with fewer land- 240 Spatial Scale on System Properties slides at other times and places. The frequency uge Viewing topography as a population of of concentrated landslides predicted within a hilslopes and sediment sources means that the 3 km 3rd-order basin is about once every 200 number of sediment sources and the number years with 2 to 13 landslides per fie episode of linked stream reaches increases with the (approximately the 20yr period following a fire spatial scale of the basin and with distance with reduced rooting strength) (Figure 11.lla). downstream. In addition, frequency of cliatic As the basin area increases to 2Skm , the fre- perturbations, such as fies and storms quency of landslides increases to once every also increases as basin area increases. As a to 100 years with S to 16Slandslides per fire or consequence the system properties of storm episode (Figure 11.11b). Finally, at a ba- landscapes, as represented in frequency sin area of 200km , landslides are relatively distributions of environmental conditions common with about 10 landslides occurring ev- evolves with scale and distance downstream ery few years (Figue 11.11c). Figure 11.lla-c though a network. show that the likelihood of landslides increases with basin size because a larger' drainage Topography as a Population of area encompasses a greater number of poten- tial landslide sites and the potential for a 'ilillslope Sediment Sources greater number of fies and storms. Thus, the The effects of increasing basin area on the dy- frequency and magntude of sediment supply :eposition namic behavior of channels can be ilustrated to channels increases downstream in a channel lain surfces luencydistribution of e of sediment in the iming between these en flooding (F) and s (L) is indicated by

Tributary influx race and floodplain Post fire the frequency dis- valley floor :Yes (Figure 11.9c), Sedimentation be located In th )ften new terraces, , e g aquatic and ri;;' long the valey, fire, a flood of sim.. Je located in the Ie ()f in Figure 11. " 11.10. Guly erosion afer a fie caused chanel and valey floor sedientation in a basin in Idaho. 1ges to channels an ts correspond to the right hand tail in the distrbution shown in Fig. 1l.9c. := . -

278 L.E. Benda et al. 11.

Point in Landslides Upstream act Channel Network Time Series Frequency Distribution wit 2% Nolandslides bur 98% of all years tior

(Fi nur 200 400 600 800 1000 0% 1 2 3 4 5 6 7 8 91j eas occ

No landslides 100 90% of all years / 5075 :: 1% ..6 250 200 400 600 800 1000

No landslides 68% of all years 200 ' 1% 100 200 400 600 800 1000 0% 50 100 150 200 Years landslides (Number/yr)

FIGURE 11.11. A 1000-year simulation of landslides frequently and in greater numbers within this larger in a 200- basin in the Oregon Coast Range indi- area. (c) Numerous subbasins and a vary large num- cates that the likeliood of landslides and associated ber of potential landslide sites are contained within sediment and wood delivery to the channel system the entire 200- basin. Landslides occur some- increases with increasing basin area. (a) A 3- where within the basin a third of all years. On rare headwater basin experiences infrequent landslides. occasions, when large fires are followed by intense (b) A 25- tributa subbasin contains a greater storms, well over 100 landslides can occur within the total number of potential landslide sites and is more basin in a single year. likely to include a fire: hence, landslides occur more

network. and has important consequences for applied to a 200 basin in the Oregon Coast the dynamic behavior of sediment transport, Range. FiGl channel sediment storage, and channel The model predicts an inherent imbalance and morphology. between sediment supply and transport in 3rd- to t order channels resulting in centuries of sedi- lan ment poor conditions interrupted by decades of se: Channel Networks as a Population of -'oj! num 'Sediment rich conditions after fires and major Linked Stream Reaches men storms (Figure 11.12a). In the Oregon Coast The simulation model also can be used to ilus- Range, sediment poor conditions are character- frOl trate how varations in the supply of sediment ized by bedrock channels with accumulations of gene from landslides and debris flows over long boulders next to debris flow fans and bedrock bur time periods (Figue 11.11a ) interact with outcrops. Woody debris, in single pieces and a converging hierarchical chanel' network jams, creates local areas of sediment storage. (Benda and Dunn 1997). Again, the model is Sediment rich conditions, in contrast, are char- app . .,.,,""" , ~~~

L.E. Benda et al. 11. Dynamic Landscape Systems 279

acterized by gravel and cobble bed channels ever, the magnitude of sediment supply tribution with buried boulders; , some wood may be fluctuations (i. , represented as sediment buried under gravel. The frequency in fluctua- depth in Figure 11.12a-c) decreases down- tions in channel-stored sediment increases with stream in the Oregon Coast Range because of basin area and corresponding stream size mechanical breakdown of weak sandstone (Figure 11.2a-c) because of an increasing bedload and widening channels. number of landslide and debris flow source ar- The many small channels in the branching, eas and increasing probabilty of fires (i. e., fire hierarchical network (represented as 3 occurrence increases with basin area). How- basins in Figure 11.12a) deliver small, disparate

Point in Sediment Depth Channel Network Time Series Frequency Distribution 90'* Sediment poor: 0 " berock clannels Sediment rich: 3km gravel-bed o 0 :ll200 400 600 800 1000

i 50'*

-J. :5 2 umber/yr) c: 10% Seiment rich: ers within this larger 5 5 gravel-bed rId a var large num- I \ ue contained with "C0 0 200 400 600 800 1000 0. 0 0 dslides occur some- of all years. On rare followell by intense can occur within the Berock channels /thin grael shee

o 0 200 400 600 800 1000 Years Sediment Depth (m) the Oregon Coast FIGURE 11.12. Sediment depth as a function of time the time. Since the channel is larger and sedient 1herent imbalance and position in the channel network corresponding must, on average, be cared a longer distace d transport in 3rd- , to the time series and frequency distrbutions of through the channel to thi point, sedient depths centuries of sedi- landsdes shown in Figure 11.11. The volume of rarely exceed 1m. (c) A great number of upstream lpted by decades of sediment found within the channel depends on the sediment sources are integrated at the mouth er fires and major n.umber, frequency, and distance to upstream sedi- the 200km , basin, resultig in a relatively constant ment sources. (a) For a smal channel volume of sediment at ths point. In all caes, the the Oregon Coast driig a headwater basin, upstream sedient inputs time series of sediment depths show fluctuations at tions are character- tto landslides are infrequent and the channel is several time scales: long-term fluctuations are dic- :h accumulations of aeneraly sediment poor, although periods of deep tated by the frequency of large fies. shorter term fans and bedrock \lal ca occur. (b) A larger chanel draing 25- fluctuations are a consequence of anual varabilty I single pieces and has a greater frequency of upstream sediment in flood peaks. The depth of sediment in large par , sediment storage. ts so that the channel here contai sediment determines chanel-bed morphology. contrast, are char- eciable depths (::O.2m) a larger proporton of 280 L.E. Benda et al. sediment pulses to larger channels while de- hierarchical network results in an evolution 'Of creasing stream power in the main channel pro- sediment distributions downstream. motes accumulation of the pulses (Figure 1l.12b). The simulation model predicts that Classification of Landscape 10 to 30% of bed material is transported in System Behavior sediment waves (0.5 m thick and sufficient to change channel morphology) depending on The system behavior of a landscape, embodied stream size, with the greatest proportion car- in the form of frequency distributions, can be ried as waves in the center of the network. In a classified to describe how frequencies and mag- drainage area of 200km , the sediment supply nitudes in the supply and storage of sediment is relatively continuous and the magnitude of and woody debris (or the range and magnitude sediment transport variations is diminished, al- of variabilty in these materials) vary through a though small, biologically relevant fluctuations network. Such a dynamic or disturbance-based in channel-stored sediment continue (Figure classification system explicitly links channel be- 1l.12c). havior to basin climate and to the behavior (in The right-skewed distribution of sediment time and space) of the upstream population of storage at 3 km indicates that the range of vari- sources of sediment and wood. abilty is high but that the most probable condi- The model predictions of frequencies and tion is one of low sediment supply (Figure magnitudes of sediment supply for the 200 km 11.2a). The shif of the distribution towards basin in the Oregon ' Coast Range (Figure the right at 25 km2 indicates a reduction in the ll. lla-c) and sediment routing (Figure 11.12a- range of variabilty but an increase in likeli- c) are used to classify the entire 3rd- through hood of sediment in storage as many more 6th-order network (Table 11.). A similar hilslope sediment sources are integrated (Fig- classification system based on wood loading ure 1l.12b). The more symmetrical distribution and routing also could be developed but is not at 200km (Figure 11. 12c) indicates that varia- included in this example. tion in sediment depth is more evenly distrib- The morphologiCal consequences of the fluc- uted about the mean, although low sediment tuations in sediment supply (Table 11.1) are storage is the most probable condition because controlled partly by certain valley and channel of rapid breakdown of weak sandstone bedload characteristics that vary spatially through the tl: and wide channels. Hence, the branching, channel network, but that are constant in time.

TABLE 11.1. Clasifcation of a 200km channel network in the Oregon Coast Range based on the frequency and depth of sediment deposition causing channel aggradation. The effects of tributary confluences are not included. Low-frequency (fewer than once every two centuries), high-magnitude (thickness greater than Lam) events occur in low-order channels; high-frequency (every five to ten years), low-magnitude (O.3m) tiI events occur in high-order channels. The central part of the network (drainage areas of 20-50km ) has the sit highest probability of encountering significantly aggraded channels. Channel aggradation Magnitude Duration Channel length Draiage area AvglMax AvglMax Avg/Max lal Stream order (km Frequency (m) (yr) (km) Substrte Low 5-1.0 ':0. 612. 210 Bedrock/boulders 512. 5/80 5/10 Gravel/bedrock' 10- 025 512. 5/90 1/12 Gravel/bedrock Ie; 35- 411. 10/120 1/8 Bedrock/boulders High 70-200 0.310. 1/80 214 Bedrock/boulders Adapted from Benda and Dunne (1997). vie ,; ., :; ,,., :' ~~~ ~~~~~~ ~~~

L.E. Benda et al. 11. Dynamic Landscape Systems 281 in an evolution of For example, the response of a channel seg- as a mechanism creating habitat, including lstrear. ment to variations in sediment supply depends refugia. In addition, aquatic organisms living in greatly on channel gradient and confinement. a dynamic environment have evolved a suite Spatial patterns of channel gradient and con- of adaptations for survival (Chapters ' , 10, 12 finement have been used to stratify channel and 17). networks into broad zones of general channel ndscape, embodied types (e. , step-pool channels with steep, bed- Environmental Risk rock floors covered with boulders or flatter istributions, can be Many environmental risks arise naturally from meanderig pool-riffe channels) (Rosgen 1995 quencies and mag- interactions between climate and geomorphol- torage of sediment Montgomery and Buffngton 1997, Chapters ogy. For example, in the absence of 2 and 5, this volume). Spatially determinstic side nge and magnitude channels and woody debris jams (which act as classification schemes predict a general mor- ials) vary through a habitat refugia), annual winter r disturbance-based phological state and aid in estimating the sensi- floods with tivity of channel reaches to change, such as high velocity flows flush juvenile and adult fish cly links channel be- downstream fluctuations in sediment supply. A landscape , resulting in high mortality rates. to the behavior (in Bedload transport systems-level classifcation system estimates durg floods causes bed ream population of scour and fill (often linked with Jod. the frequency, duration, and magnitude of fluctuating sediment volume) that excavates or buries of frequencies and likely changes. In addition, because the sys- tems-level classification scheme is based on the redds (areas within stream gravel beds where pply for the 200 km fish have deposited eggs) (Nawa et al. 1989). In ast ' Range (Figure entire frequency distribution (Figures 11.8 and contrast, low flows prevent access to spawning 11.9), insights into how the formation of ting (Figure 11.12a- areas decreasing available spawnng area entire 3rd- through floodplais and terraces var in the network for adult anadromous fish as well as increasing similar ' alo are gained, 11.1). A temperatures which also may lead to reduced j on wood loading surval (Reiser and Bjorn 1979). Erosion jeveloped but is not Aquatic Biology at the delivers fine sediment that 1f'7L fi inter-gravel , Landscape Systems Level spaces and thus reduces oxygen flux ,,it, , which quences of the fluc- causes fish eggs to suffocate (Everest et al. ,ly (Table 11.1) are uati habitat i n viewed at he haQitat 1t 1987). In addition, mass wasting buries , umt scale (e. , mdlVdual pools, rifes). AI- in- (1 valley and channel stream aquatic habitat (Everest and Meehan patially Jhrough the ::Yf : though this is an important scale of obser- are conStant in time. ;',' vation , understanding landscapes as systems 1981). :cl requires that the focus on individual units shift Habitat Development Including Refugia

e a:Sd Processes creating environmental riks also based on the frequency tated by the dynamc interactions of climate create reach-scale babitats, including refugia. ary confluences are not 0- topography, and chanel networks. Hence, the Wind, ban erosion, landslides, and fire all sup- thickness greater than :oontextual focus is on process linkages and ply woody debris to the chanel, where it accu- low-magnitude (O.3m) ;I ;::'tie , rather than the state of habitat at anyone mulates in jams formng areas with low water IS of 20-50km ) has the :site in a year. velocities (including deep pools). These low . A landscape systems perspective requires velocity areas around debri jam provide )hat the focus of climatic and geomorphic refuge for adult and juvenile fish durg winter gth prcesses (such as storms, fies, floods, and floods (Dolloff 1983, Sullvan et al. 1987). In ;Iandslides) be expanded beyond terms of addition, boulders deposited in channels by Substrate envionmental damage. Poor envionmental mass wasting are used by fish as low-velocity ()ditions associated with these events can be Bedrock/boulders refugia (Everest and Meehan 1981). Gravel/bedock Oft lived. Dynamic landscape processes also Lage flood, often in conjunction with high Gravel/bedrock long-term legacies in the form of habitat sediment suppli s and woody debris jams, Bedock/boulders , elopment in channels, floodplais, and val- cause temporar Bedrock/boulders chanel braiding which cre- floors. Hence, dynamic processes must be ates side chanels (Keller and Swanson 1979), ewed as both an environmental risk but also both in mountain valley floors and along lower- .,,-:

L.E. Benda et al. 11. Dy 282 tains I 1983). Debris western Washington (Figure 11.3b) which sug- gradient floodplains (Church by a g gest that flow velocities capable of scouring jams formed during floods also contribute to dry ur redds and flushing fish downstream (e. , an the formation of side channels and mid-channel an int 1996). Side environmental risk) occur an average of once islands (Abbe and Montgomery the g: every year. However, the same frequency dis- channels, which may persist for decades or depth tribution also reveals that major floods capable longer, are utilzed by fish escaping the high Ea( of bank erosion and tree recruitment (i. , the flows and velocity of the active channel during phol() formation of habitat refugia) occur about 10 to individual storms or during entire winters (Ree' 15% of the time (once every 5 to 7yrs). Hence (Scarlett and Cederholm 1984, Sullvan et al. a sin consideration of cumulative effects at the 1987). system level should seek to understand how Fra At the reach scale, habitat refugia act in the relative proportions of environmental risk strte the context of the environmental risks just versus habitat development have changed due of sp described. However, refugia also occur at a trout variety of spatial scales including subbasins or to land uses. cla watersheds (Sedell et al. 1990). Relatively in- coho frequent but large magnitude events (e. , fires, Biological Adaptations each large storms) trigger widespread erosion and an thou: decreases The abilty of fish to move from risk prone increase in sediment supply which durig floods, and then vertical and lateral channel stabilty areas to low velocity refugia both to utilize changing habitat conditions durig (Figure 11.10). Thus, refugia at the reach scale Li: storms, is based on a suite of behavioral adapta- may not be available or may require time to tions. These adaptations include juvenile and develop, and the environmental risk to fish may adult movements to avoid or utilize changing be high enough to result in local extirpations habitat conditions over days (i.e., a single (Reeves et al. 1995). Nearby basins untouched storm) to years at the reach to watershed scales by fires or storms may have more stable chan- (Quinn 1984, Reeves et al. 1995). In addition nels and pose less environmental risk and, thus, fuction as watershed-scale refugia. Fish may fish encountering potentially hostile conditions durg spawning tend to lay a large numbers of escape to less hostile subbasins of a watershed eggs (Heard 1991), an adaptation to potentialy and, eventually, some may recolonize newly previously impacted high rates of egg loss caused by scour (which forming habitats in the vares from year to year). subbasins. Recall, however, that large quanti- ties of sediment and wood moving into chan- nels during low frequency, high magnitude Dynamic Fish Habitat and storms or fires, and which initially created an Community Structure environmental risk, may evolve over decades to centuries into channel habitats. The variations in channel sediment storage pre- Because processes creating environmental dicted by the simulation model in the Oregon Coast Range (Figure 11.12a-c) cause variations risks also create habitats, the regime of in fish habitat. Fires and large storms occur- ' landscape processes (frequency, magnitude ring asynchronously across the Oregon Coast ' composition, and spatial distribution) is central Range create a spatial diversity of chanel mor- to understanding this duality. Frequency distri- butions of floods (Figure 11.3), erosion (Figure phologies (in several adjacent basins with chao- - nel segments of similar gradient and draiage 11.11), and sediment transport (Figure 11.12) , bas ' help to quantify changes in channel environ- , area). For example, in Knowles Creek , 30km ), an absence of recent wi ments (e.g" Figure 11.9c) and therefore provide (5th-order a measure to understand how environmental fires has contributed to a low sediment supply. : the channel is dominated by bedrock, and poo)s- risk and refugia are related. For example, flows basin, bured by that transport bedload occur approximately are shalow. Harey Creek forest-replacing wildfire in the late 1800, conk 70% of the time in 5th-order channels in south- ";;;:" - - , ,

L.E. Benda et al. 11. Dynamic Landscape Systems 283

. 11.3b) which sug- tains large volumes of sediment, is dominated provides insights into natural disturbance or pable of scouring by a gravel substrate contains deep pools that habitat diversity. This comparison suggests that vnstream (e. , an dry up in the summer. Franklin Creek contains under natural conditions in the central Oregon n average of once an intermediate volume of sediment and has Coast Range, with it's low fire frequency (about lme frequency dis- the greatest diversity of substrates and pool 300yrs), habitat diversity is relatively low. 1jor floods capable depths. Model predictions coupled with field observa- :ruitment (i. , the Each of the three different channel mor- tions also suggest that if the frequency of j occur about 10 to phologies contain diferent fish communities disturbance is slightly higher (a fie cycle 5 to 7yrs). Hence (Reeves et al. 1995). Knowles Creek has only of 150 to 200yrs, for example), the higher sedi- Ie effects at the a single species, coho salmon (0. kisutch). ment (and wood) supply could potentially ) understand how Frank Creek, with a greater diversity of sub- increase habitat diversity. nvironmental risk strate sizes, accommodates a greater diversity have changed due of species: 85% coho salmon, 13% steelhead trout (0. mykiss), and 2% cutthrout trout (0. Applications to Watershed clarki). Harey Creek is also dominated by coho salon, with only incidental portions (1 Science and Management each) of steelhead and cutthrout trout. Al- from risk prone though the relative differences are not great On close examination stream chanels are ex- tremely complex. Valley-floor conditions vary during floods, and there is a significant correlation between reach by reach and are governed by the vaga- conditions during species diversity and sediment supply. behavioral adapta- Ling the field data on channel morphol- ries of local topography and the history of mass :lude juvenile and ogy, fih community strcture, and estimates of wasting. Bed texture and slope var over the )r utilize changing sediment depth to the probabilty distribution scae of meters and are altered by every log that lYS (i. , a single of predicted sediment depth previously dis- fal into the chanel. Sediment tranport, even under steady flow, vares :0 watershed scales cued (Figure 11.12b) indicates how often each across channels and from moment to moment. The sediment under 1995). In addition Of the fish habitat conditions are liely to occur one s feet may have origiated from a fie , hostile conditions " either in a particular stream reach over tUne or a large numbers of across many stream reaches of simar chanel several hundred year ago 20 kilometers up- ltion to potentially slope and draiage area at a single time. The stream or it may have originated from nearby bank erosion the last flood. Hence d by scpur (which -predicted frequency distrbution of sediment durg stream chanels contain inormation over volume (Figure 1l.12b) suggests that the low $tdient supply of Knowles Creek (domi- all length and time scales, providing a classic ,nated by coho salmon) is the most-commonly definition of a complex system (Bak 1994). OCCg environmental state in the central, :Oregon Coast Range (about 70% of the time). A Field Perspective iiment storage pre- e effects of large organic debris were not jdel in the Oregon ronsidered and may include create. Locally A basic rule of streams, paricularly at smal -c) cause variations inerased habitat and fish diversity. The fre- scales, is spatial varabilty and, if one returns to irge storms occur- ncy distrbution in Figure 11.12b also a certai stream reach year after year, temporal the Oregon Coast co' ests that the sediment rich state of Harvey variabilty. Anticipating spatial and temporal ity of channel mor- k would be faily inequent, occurrng varabilty in stream morphology is exceedingly it basins with chan- , aps less than 10% of the time. In addition difcult because of the complex envionmental jient and draiage intermediate sediment storage state of interactions. Compounding this problem Jwles Creek basin Creek with a higher fish diVersity, the uncertaity about recent basin history nce of recent wild'z , ' dicted to occur about 15 to 20% of the that may be responsible for present channel sediment suppl " morphology. bedrock, and pool e- comparson of field studies of habitats Although interpreting the origin and signif- basin, burned by eommunities with model predictions of cance of smal-scale channel and habitat fea- he late 1800, co channel behavior (Figure 11.12b) tures is importnt, the study of landscapes as ' . :,.

L.E. Benda et al. 11. 284 systems provides other insights into channel populations in the Pacific coastal ecoregion are for morphology and the watersheds in which they often condensed into three broad concepts: brc , and are embedded. A systems perspective helps cumulative effects, natural disturbance of- define the range and magnitude of variabilty habitat diversity. Cumulative effects in streams na\ that may be encountered in the field (i. is the accumulation and manifestation of hu- adl through the use of frequency distributions) and, man activities dispersed in space and time at tiv' 19). furthermore, some of the morphological con- any point in a channel network (Chapter sequences of that variabilty (e. , long-term Information on natural disturbance provides an de: legacies in the form of terrace formation). important baseline from which to interpret the Moreover, frequency and probability distribu- ecological significance of cumulative effects tions contain information on how the major or a long history of human impacts. Habitat de. natural disturbance landscape properties of climate, topography, diversity is related to channel networks, and basin history are linked through fires, storms, and floods occurrng tio to channel changes. Interpreting channel con- asynchronously that create a mosaic of habitats ditions using frequency distributions means in watersheds or across landscapes in any year. interpreting the environmental status of the Cumulative effects, natural disturbance. and watershed and its potential for providing habitat diversity have two important things in At: system sediment and wood. common. First, they all pertain to the Applying the landscape systems perspective behavior of a landscape. That is, they pertain to to watersheds requires measurements and the behavior (or the condition in any year) of monitoring that seek to quantify the range of population of landscape elements, such as up- natural characteristics in channels, floodplains, land and riparian for st stands, erosion sources terraces, fans, and forests, In dealing with a channel and valley floor environments, and fish population of channel reaches the complexities stocks, over periods relevant to the governing of an individual reach, or the variation that physical processes (commonly decades to cen- occurs from reach to reach, becomes less im- , turies). Second, they all remain poorly defined portant. The systems approach focuses instead despite significant scientific efforts to address on the range and relative abundance of certain them in the past. For example, a credible scien- channel attrbutes found within the entire tifc method for measuring or predicting cumu- population of reaches. Furthermore, for a lative effects in streams is lacking because of population of channel features, the systems the difficulty in applying existing quantitative perspective endeavors to lin how the shapes theories and models (such as for sediment the frequency distributions are controlled by transport and slope stability) that focus on interactions among climate, topography, and small-scale landscape behavior (e. , a hilslope channel network structure, focusing mostly on or stream reach over a few years) to popula- the recent basin history. A landscape perspec- tions of hilslopes and stream reaches which tive represents a more coarse-grained approach have evolved over long time periods. Further, to stream or watershed interpretation, similar the aquatic or riparian natural disturbance to the coarse grained approach of making sim- regime has never been fully defined for any plifications when applying quantitative theories landscape for the same reason. and models of small-scale processes to larger- The inabilty to make significant progress in ", B scale behavior through the use of simulation defining natural disturbance regimes in terres- trial or aquatic systems, or in measurig or models. predicting cumulative effects, largely is due to an absence of scientific theory explaining land- The Problem of Cumulative Effects, scape behavior in terms of a population ofland- Natural Disturbance, and scape elements over appropriate temporal and : Habitat Diversity spatial scales. Moreover, the avaiabilty of " Concerns over how human activities. including theories addressing landscape processes that: fishing, forestry, and agriculture impact fish act at small spatial and temporal scales often', , , ;,j, , :"'..,'" , ; ', " '

L.E. Benda et al. 11. Dynamic Landscape Systems 285 oastal €:coregionare form the scientific basis for evaluating many Benda, L., and T. Dunne. 1997b. Stochastic forcing e broad concepts: broader environmental issues precisely because of sediment routing and storage in channel net- II disturbance, and of the absence of system-scale theory. Unfortu- , works, Water Resources Research, 33:2865-2880. Beschta, RL. 1978. Long term patterns of sediment ve nately, the specific models cannot successfully effects in streams production following road construction and log- 1anifestation of hu- address system-scale questions of cumula- , and habitat ging in the Oregon Coast Range. Water Resources t space and time at tive effects, natural disturbance Research 14:1011-1016. work (Chapter 19). diversity. The landscape systems framework . 1984. River chanel response to accelerated urbance provides an described in this chapter could potentially be mass soil erosion. 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