1.A Overview of Structure and Scale 1 · Stream channel Floodplain Transitional upland fringe Together they function as dynamic and valued crossroads in the landscape. (Figure 1.1)

11.A Overview of Structure and Scale

• What are the structural components of a stream corridor?• Why are stream corridors of special significance, and why should they be the focus of

restoration efforts?• What is the relationship between stream corridors and other landscape units at broader and

more local scales?• What scales should be considered for a stream corridor restoration?

1.B Stream Corridor Functions and Dynamic Equilibrium• How is a stream corridor structured from side to side?• How do these elements contribute to stream corridor functions?• What role do these elements play in the life of the stream?• What do we need to know about the lateral elements of a stream corridor to adequately

characterize a stream corridor for restoration?• How are the lateral elements of a stream corridor used to define flow patterns of a stream?

1.C A Longitudinal View Along the Stream Corridor• What are the longitudinal structural elements of a stream corridor?• How are these elements used to characterize a stream corridor?• What are some of the basic ecological concepts that can be applied to streams to understand

their function and characteristics on a longitudinal scale?• What do we need to know about the longitudinal elements that are important to stream

corridor restoration?

1

Figure 1.1: Stream corridors func-tion as dynamic crossroads in thelandscape. Water and other materi-als, energy, and organisms meet andinteract within the corridor.

1.A Physical Structure and Time atMultiple Scales

1.B A Lateral View Across the StreamCorridor

1.C A Longitudinal View Along theStream Corridor

stream corridor is an ecosystem thatusually consists of three major ele-

ments:

Stream channel

Floodplain

Transitional upland fringe

Together they function as dynamic andvalued crossroads in the landscape.

(Figure 1.1). Water and other materials,energy, and organisms meet and interactwithin the stream corridor over space andtime. This movement provides critical func-tions essential for maintaining life such ascycling nutrients, filtering contaminantsfrom runoff, absorbing and gradually re-leasing floodwaters, maintaining fish andwildlife habitats, recharging ground water,and maintaining stream flows.

The purpose of this chapter is to definethe components of thestream corridor and intro-duce the concepts of scaleand structure. The chapter isdivided into three subsections.

1–2 Chapter 1: Overview of Stream Corridors

Section 1.A: Physical Structure andTime at Multiple Scales

An important initial task is to iden-tify the spatial and time scales mostappropriate for planning and de-signing restoration. This subsectionintroduces elements of structureused in landscape ecology and re-lates them to a hierarchy of spacialscales ranging from broad to local.The importance of integrating timescales into the restoration process isalso discussed.

Section 1.B: A Lateral View Acrossthe Stream Corridor

The purpose of this and the follow-ing subsection is to introduce thetypes of structure found within

stream corridors. The focus here ison the lateral dimension of struc-ture, which affects the movementof water, materials, energy, and or-ganisms from upland areas into thestream channel.

Section 1.C: A Longitudinal ViewAlong the Stream Corridor

This section takes a longitudinalview of structure, specifically as astream travels down the valley fromheadwaters to mouth. It includesdiscussions of channel form, sedi-ment transport and deposition, andhow biological communities haveadapted to different stages of theriver continuum.

Physical Structure and Time at Multiple Scales 1–3

1.A Physical Structure and Time at MultipleScales

Landscapes,watersheds,stream corri-dors, andstreams areecosystemsthat occur atdifferent spa-tial scales.

A hierarchy of five spatial scales, whichrange from broad to local, is displayedin Figure 1.2. Each element within thescales can be viewed as an ecosystemwith links to other ecosystems. Theselinkages are what make an ecosystem’sexternal environment as important toproper functioning as its internal envi-ronment (Odum 1989).

Landscapes and stream corridors areecosystems that occur at different spa-tial scales. Examining them as ecosys-tems is useful in explaining the basicsof how landscapes, watersheds, streamcorridors, and streams function. Manycommon ecosystem functions involvemovement of materials (e.g., sedimentand storm water runoff), energy (e.g.,heating and cooling of stream waters),and organisms (e.g., movement ofmammals, fish schooling, and insectswarming) between the internal and ex-ternal environments (Figure 1.3).

The internal/external movement modelbecomes more complex when one con-siders that the external environment ofa given ecosystem is a larger ecosystem.A stream ecosystem, for example, has aninput/output relationship with the nexthigher scale, the stream corridor. Thisscale, in turn, interacts with the land-scape scale, and so on up the hierarchy.

Similarly, because each larger-scaleecosystem contains the one beneath it,the structure and functions of thesmaller ecosystem are at least part of thestructure and functions of the larger.Furthermore, what is not part of thesmaller ecosystem might be related to it through input or output relationshipswith neighboring ecosystems. Investigat-ing relationships between structure andscale is a key first step for planning anddesigning stream corridor restoration.

Physical Structure

Landscape ecologists use four basicterms to define spatial structure at aparticular scale (Figure 1.4):

■ Matrix, the land cover that is domi-nant and interconnected over themajority of the land surface. Oftenthe matrix is forest or agriculture,but theoretically it can be any landcover type.

■ Patch, a nonlinear area (polygon)that is less abundant than, and differ-ent from, the matrix.

■ Corridor, a special type of patch thatlinks other patches in the matrix.Typically, a corridor is linear or elon-gated in shape, such as a streamcorridor.

■ Mosaic, a collection of patches, noneof which are dominant enough to beinterconnected throughout the land-scape.

These simple structural element con-cepts are repeated at different spatialscales. The size of the area and the spa-tial resolution of one’s observations de-termine what structural elements one isobserving. For example, at the landscapescale one might see a matrix of matureforest with patches of cropland, pasture,clear-cuts, lakes, and wetlands. Lookingmore closely at a smaller area, onemight consider an open woodland to bea series of tree crowns (the patches)against a matrix of grassy ground cover.

On a reach scale, a trout might perceivepools and well-sheltered, cool, pocketsof water as preferred patches in a matrixof less desirable shallows and riffles, andthe corridor along an undercut stream-bank might be its only way to travelsafely among these habitat patches.

FASTFORWARD

Preview Chap-ter 2, Section Efor a discussionof the six criti-cal functionsperformed bystream corridorecosystems.


Chesapeake Bay Watershed

Region Scale

Valley and Ridge Region

mixed landscape • suburban • agricultural • forest cover

Patuxent River Watershed

Washington, DC

Landscape Scale

Patuxent Stream Corridor

Patuxent Reservoir Watershed

DC

Montgomery Co.

Damascus

Brighton

Stream Scale

Stream Corridor Scale

reach

Patux

entRiver

S tate ParkReach Scale

Figure 1.2: Ecosystems at multiple scales.Stream corridor restoration can occur atany scale, from regional to reach.

At the other extreme, the coarsest of theimaging satellites that monitor the earth’ssurface might detect only patches or cor-ridors of tens of square miles in area,and matrices that seem to dominate awhole region. At all levels, the matrix-patch-corridor-mosaic model provides auseful common denominator for de-scribing structure in the environment.

Figure 1.5 displays examples of the ma-trices, patches, and corridors at broadand local scales. Practitioners shouldalways consider multiple scales whenplanning and designing restoration.

Structure at Scales Broader Thanthe Stream Corridor Scale

The landscape scale encompasses thestream corridor scale. In turn, the land-scape scale is encompassed by the largerregional scale. Each scale within the hier-archy has its own characteristic structure.

The “watershed scale” is another form ofspatial scale that can also encompass thestream corridor. Although watershedsoccur at all scales, the term “watershedscale” is commonly used by many practi-tioners because many functions of thestream corridor are closely tied to drain-age patterns. For this reason, the “water-shed scale” is included in this discussion.

N

ecosystem

input environment output environment

Figure 1.3: A simpleecosystem model.Materials, energy, andorganisms move froman external inputenvironment, throughthe ecosystem, andinto an external out-put environment.

Landscapeecologists usefour basicterms to definespatial struc-ture at a par-ticular scale—matrix, patch,corridor, andmosaic.

matrix

matrix

patch

corridor

patch

matrix

patchmosaic

Figure 1.4: Spatialstructure. Landscapescan be described interms of matrix,patch, corridor, andmosaic at variousscales.



Regional Scale

A region is a broad geographical areawith a common macroclimate andsphere of human activities and interests(Forman 1995). The spatial elementsfound at the regional scale are calledlandscapes. Figure 1.6 includes an ex-ample of the New England region withlandscapes defined both by naturalcover and by land use.

Matrices in the United States include:

■ Deserts and arid grasslands of thearid Southwest.

■ Forests of the AppalachianMountains.

■ Agricultural zones of the Midwest.

At the regional scale, patches generallyinclude:

■ Major lakes (e.g., the Great Lakes).

■ Major wetlands (e.g., the Everglades).

■ Major forested areas (e.g., redwoodforests in the Pacific Northwest).

■ Major metropolitan zones (e.g., theBaltimore-Washington, DC, metro-politan area).

■ Major land use areas such as agricul-ture (e.g., the Corn Belt).

Corridors might include:

■ Mountain ranges.

■ Major river valleys.

■ Interregional development along amajor transportation corridor.

Most practitioners of stream corridorrestoration do not usually plan and de-sign restoration at the regional scale.The perspective is simply too broad formost projects. Regional scale is intro-duced here because it encompasses thescale very pertinent to stream corridorrestoration—the landscape scale.

Figure 1.5: Spatial structure at (a) broad and (b) local scales. Patches, corridors, and matrices arevisible at the broad regional scale and the local reach scale.

(a) (b)

CHESAPEAKEBAY

URB

AN

MAT

RIX

BALTIMORE

WASHINGTON, DC

POOL

POOL

RIFFLE

Practitionersshould alwaysconsider multi-ple scaleswhen planningand designingrestoration.


Landscape Scale

A landscape is a geographic area distin-guished by a repeated pattern of com-ponents, which include both naturalcommunities like forest patches andwetlands and human-altered areas likecroplands and villages. Landscapes canvary in size from a few to several thou-sand square miles.

At the landscape scale, patches (e.g.,wetlands and lakes) and corridors(e.g., stream corridors) are usuallydescribed as ecosystems. The matrix isusually identified in terms of the pre-dominant natural vegetation commu-nity (e.g., prairie-type, forest-type, andwetland-type) or land-use-dominated

ecosystem (e.g., agriculture and urban)(Figure 1.7).

Landscapes differ from one anotherbased on the consistent pattern formedby their structural elements, and thepredominant land cover that comprisestheir patches, corridors, and matrices.

Examples of landscapes in the UnitedStates include:

■ A highly fragmented east coast mosa-ic of suburban, forest, and agricultur-al patches.

■ A north-central agricultural matrixwith pothole wetlands and forestpatches.

■ A Sonoran desert matrix with willow-cottonwood corridors.

■ A densely forested Pacific Northwestmatrix with a pattern of clear-cutpatches.

agricultural

urban

spruce–fir

suburban

northern hardwood

pitch pine–oakoak forest

salt marsh

barrensrivers and lakes

industrial

AdirondackRegion

New YorkRegion

NewEnglandRegion

The MaritimesRegion

Southern Quebec Region

Atlantic

Oce

an

Figure 1.6: The New England region. Structurein a region is typically a function of naturalcover and land use. Source: Forman (1995). Reprinted with the permis-sion of Cambridge University Press.

A landscape isa geographicarea distin-guished by arepeated pat-tern of compo-nents, whichinclude bothnatural com-munities likeforest patchesand wetlandsand human-altered areaslike croplandsand villages.


A woodlot within an agricultural ma-trix and a wetland in an urban matrixare examples of patches at the land-scape scale. Corridors at this scaleinclude ridgelines, highways, andthe topic of this document—streamcorridors.

At the landscape scale it is easy to per-ceive the stream corridor as an ecosys-tem with an internal environment andexternal environment (its surroundinglandscape). Corridors play an impor-tant role at the landscape scale and atother scales. Recall that a key attributeof ecosystems is the movement of en-ergy, materials, and organisms in,through, and out of the system. Corri-dors typically serve as a primary path-way for this movement. They connectpatches and function as conduits be-tween ecosystems and their externalenvironment. Stream corridors in par-ticular provide a heightened level offunctions because of the materials andorganisms found in this type of land-scape linkage.

Spatial structure, especially in corridors,helps dictate movement in, through,and out of the ecosystem; conversely,this movement also serves to changethe structure over time. Spatial struc-ture, as it appears at any one point intime, is therefore the end result ofmovement that has occurred in thepast. Understanding this feedback loopbetween movement and structure is akey to working with ecosystems in anyscale.

“Watershed Scale”

Much of the movement of material, en-ergy, and organisms between the streamcorridor and its external environmentsis dependent on the movement ofwater. Consequently, the watershedconcept is a key factor for planning anddesigning stream corridor restoration.The term “scale,” however, is incorrectlyapplied to watersheds.

A watershed is defined as an area of landthat drains water, sediment, and dis-solved materials to a common outlet atsome point along a stream channel(Dunne and Leopold 1978). Water-sheds, therefore, occur at multiplescales. They range from the largest riverbasins, such as the watersheds of theMississippi, Missouri, and Columbia, to the watersheds of very small streamsthat measure only a few acres in size.

The term “watershed scale” (singular) isa misnomer because watersheds occurat a very wide range of scales. This doc-ument focuses primarily on the water-sheds of small to medium-scale streamsand rivers. Watersheds in this size rangecan contain all or part of a few differentlandscapes or can be entirely encom-passed by a larger landscape.

Ecological structure within watershedscan still be described in matrix, patch,corridor, and mosaic terms, but a dis-cussion of watershed structure is moremeaningful if it also focuses on ele-

Figure 1.7: Structure at the landscape scale.Patches and corridors are visible within an agri-cultural matrix.

WOODLANDPATCH

WOODLANDPATCH

STREAMCORRIDOR

FIELDPATCH

AGRICULTURALMATRIX

A more com-plete broadscale perspec-tive of thestream corridors achieved

when water-shed science iscombined withandscape

ecology.


ments such as upper, middle, and lowerwatershed zones; drainage divides;upper and lower hillslopes; terraces,floodplains, and deltas; and featureswithin the channel. These elements andtheir related functions are discussed insections B and C of this chapter.

In short, watersheds and landscapesoverlap in size range and are defined bydifferent environmental processes.Whereas the landscape is defined pri-marily by terrestrial patterns of landcover that may continue across drainagedivides to where the consistent patternends, the watershed’s boundaries arebased on the drainage divides them-selves. Moreover, the ecologicalprocesses occurring in watersheds aremore closely linked to the presence andmovement of water; therefore as func-tioning ecosystems, watersheds also dif-fer from landscapes.

The difference between landscape scaleand “watershed scale” is precisely whypractitioners should consider bothwhen planning and designing streamcorridor restoration. For decades thewatershed has served as the geographicunit of choice because it requires con-sideration of hydrologic and geomor-phic processes associated with themovement of materials, energy, and or-ganisms into, out of, and through thestream corridor.

The exclusive use of watersheds for thebroad-scale perspective of stream corri-dors, however, ignores the materials, en-ergy, and organisms that move acrossand through landscapes independent ofwater drainage. Therefore, a more com-plete broad-scale perspective of thestream corridor is achieved when water-shed science is combined with land-scape ecology.

The USGS developed a national framework for cata-loging watersheds of different geographical scales. Eachlevel, or scale, in the hierarchy is designated using thehydrologic unit cataloging (HUC) system. At the nationallevel this system involves an eight-digit code thatuniquely identifies four levels of classification.

The largest unit in the USGS HUC system is the waterresource region. Regions are designated by the first twodigits of the code. The remaining numbers are used tofurther define subwatersheds within the region down tothe smallest scale called the cataloging unit. For exam-ple, 10240006 is the hydrologic unit code for the LittleNemaha River in Nebraska. The code is broken down asfollows:

10 Region

1024 Subregion

102400 Accounting code

10240006 Cataloging unit

There are 21 regions, 222 subregions, 352 accountingunits, and 2,150 cataloging units in the United States.The USGS’s Hydrologic Unit Map Series documents thesehierarchical watershed boundaries for each state. Somestate and federal agencies have taken the restoration ini-tiative to subdivide the cataloging unit into even smallerwatersheds, extending the HUC code to 11 or 14 digits.

The Reach File/National Hydrography Dataset (RF/NHD) isa computerized database of streams, rivers, and otherwater bodies in the United States. It is cross-referencedwith the HUC system in a geographic information system(GIS) format so users can easily identify both watershedsand the streams contained within their boundaries.


Structure at the Stream CorridorScale

The stream corridor is a spatial element(a corridor) at the watershed and land-scape scales. But as a part of the hierar-chy, it has its own set of structuralelements (Figure 1.8). Riparian(streamside) forest or shrub cover is acommon matrix in stream corridors. Inother areas, herbaceous vegetationmight dominate a stream corridor.

Examples of patches at the stream corri-dor scale include both natural andhuman features such as:

■ Wetlands.

■ Forest, shrubland, or grassland patches.

■ Oxbow lakes.

■ Residential or commercial develop-ment.

■ Islands in the channel.

■ Passive recreation areas such as pic-nic grounds.

Corridors at the stream corridor scaleinclude two important elements—thestream channel and the plant commu-nity on either side of the stream. Otherexamples of corridors at this scalemight include:

■ Streambanks

■ Floodplains

■ Feeder (tributary) streams

■ Trails and roads

Structure Within the StreamCorridor Scale

At the stream scale, patches, corridors,and the background matrix are definedwithin and near the channel and in-clude elements of the stream itself andits low floodplains (Figure 1.9). At thenext lower scale, the stream itself is seg-mented into reaches.

Reaches can be distinguished in a num-ber of ways. Sometimes they are definedby characteristics associated with flow.High-velocity flow with rapids is obvi-ously separable from areas with slowerflow and deep, quiet pools. In other in-stances practitioners find it useful to de-fine reaches based on chemical orbiological factors, tributary confluences,or by some human influence thatmakes one part of a stream differentfrom the next.

Examples of patches at the stream andreach scales might include:

■ Riffles and pools

■ Woody debris

■ Aquatic plant beds

■ Islands and point bars

Examples of corridors might include:

■ Protected areas beneath overhangingbanks.

Figure 1.8: Structural elements at a stream corridor scale. Patches, corridors, and matrixare visible within the stream corridor.

ROAD CORRIDOR

CHANNEL CORRIDOR

ISLAND PATCH

URBAN MATRIX

STREAM CORRIDOR


■ The thalweg, the “channel within thechannel” that carries water duringlow-flow conditions.

■ Lengths of stream defined by physi-cal, chemical, and biological similari-ties or differences.

■ Lengths of stream defined by human-imposed boundaries such as politicalborders or breaks in land use orownership.

Temporal Scale

The final scale concept critical for theplanning and design of stream corridorrestoration is time.

In a sense, temporal hierarchy parallelsspatial hierarchy. Just as global or re-gional spatial scales are usually toolarge to be relevant for most restorationinitiatives, planning and designingrestoration for broad scales of time isnot usually practical. Geomorphic orclimatic changes, for example, usuallyoccur over centuries to millions ofyears. The goals of restoration efforts,by comparison, are usually described intime frames of years to decades.

Land use change in the watershed, forexample, is one of many factors thatcan cause disturbances in the streamcorridor. It occurs on many time scales,however, from a single year (e.g., croprotation), to decades (e.g., urbaniza-tion), to centuries (e.g., long-term forestmanagement). Thus, it is critical for thepractitioner to consider a relevant rangeof time scales when involving land useissues in restoration planning and de-sign.

Flooding is another natural process thatvaries both in space and through time.Spring runoff is cyclical and thereforefairly predictable. Large, hurricane-in-duced floods that inundate lands far be-yond the channel are neither cyclicalnor predictable, but still should be

planned for in restoration designs.Flood specialists rank the extent offloods in temporal terms such as 10-year, 100-year, and 500-year events(10%, 1%, 0.2% chance of recurrence.See Chapter 7 Flow Frequency Analysisfor more details.). These can serve asguidance for planning and designingrestoration when flooding is an issue.

Practitioners of stream corridor restora-tion may need to simultaneously planin multiple time scales. If an instreamstructure is planned, for example, caremight be taken that (1) installationdoes not occur during a critical spawn-ing period (a short-term consideration)and (2) the structure can withstand a100-year flood (a long-term considera-tion). The practitioner should never tryto freeze conditions as they are, at thecompletion of the restoration. Streamcorridor restoration that works with thedynamic behavior of the stream ecosys-tem will more likely survive the test oftime.

Figure 1.9: Structural elements at a streamscale. Patches, corridors, and matrix are visiblewithin the stream.

Stream corri-dor restorationthat workswith the dy-namic behaviorof the streamecosystem willmore likelysurvive thetest of time.

RIFFLE PATCH

THALWEGCORRIDOR

ALLUVIALDEPOSITPATCH

WATER MATRIX


The previous section described how thematrix-patch-corridor-mosaic modelcan be applied at multiple scales to ex-amine the relationships between thestream corridor and its external envi-ronments. This section takes a closerlook at physical structure in the streamcorridor itself. In particular, this sectionfocuses on the lateral dimension. Incross section, most stream corridorshave three major components (Figure 1.10):

■ Stream channel, a channel with flow-ing water at least part of the year.

■ Floodplain, a highly variable area onone or both sides of the stream chan-nel that is inundated by floodwatersat some interval, from frequent torare.

■ Transitional upland fringe, a portion ofthe upland on one or both sides ofthe floodplain that serves as a transi-tional zone or edge between thefloodplain and the surrounding land-scape.

Some common features found in theriver corridor are displayed in Figure1.11. In this example the floodplain isseasonally inundated and includes fea-tures such as floodplain forest, emer-gent marshes and wet meadows. Thetransitional upland fringe includes anupland forest and a hill prairie. Land-forms such as natural levees, are createdby processes of erosion and sedimenta-tion, primarily during floods. The vari-ous plant communities possess uniquemoisture tolerances and requirementsand consequently occupy distinct land-forms.

Each of the three main lateral compo-nents is described in the following subsections.

Stream Channel

Nearly all channels are formed, maintained, andaltered by the water and sediment theycarry. Usually they are gently roundedin shape and roughly parabolic, butform can vary greatly.

Figure 1.12 presents a cross section of atypical stream channel. The slopedbank is called a scarp. The deepest partof the channel is called the thalweg. Thedimensions of a channel cross sectiondefine the amount of water that can

1.B A Lateral View Across the Stream Corridor

Figure 1.10: The three major components of astream corridor in different settings (a) and(b). Even though specific features might differby region, most stream corridors have a chan-nel, floodplain, and transitional upland fringe.

(a)

(b)

STREAMCHANNEL

STREAMCHANNEL

FLOODPLAIN

FLOODPLAIN

TRANSITIONALUPLAND FRINGE

TRANSITIONALUPLAND FRINGE

A Lateral View Across the Stream Corridor 1–13

pass through without spilling over thebanks. Two attributes of the channel areof particular interest to practitioners,channel equilibrium and streamflow.

Lane's Alluvial Channel Equilibrium

Channel equilibirum involves the interplay of four basic factors:

■ Sediment discharge (Qs)

■ Sediment particle size (D50

)

■ Streamflow (Qw)

■ Stream slope (S)

Lane (1955) showed this relationshipqualitatively as:

Qs• D

50∝ Q

w• S

This equation is shown here as abalance with sediment load on oneweighing pan and streamflow on theother (Figure 1.13). The hook holdingthe sediment pan can slide along thehorizontal arm according to sedimentsize. The hook holding the streamflowside slides according to stream slope.

Channel equilibrium occurs when allfour variables are in balance. If a changeoccurs, the balance will temporarily be

tipped and equilibrium lost. If one variable changes, one ormore of the other variables must increase or decreaseproportionally if equilibrium is to be maintained. For example, if slope is increased and streamflow remainsthe same, either the sediment load or the size of the particlesmust also increase. Likewise, if flow is increased (e.g., byan interbasin transfer) and the slope stays the same, sedimentload or sediment particle size has to increase to maintainchannel equilibrium. A stream seeking a new equilibriumtends to erode more sediment and of larger particle size.

Alluvial streams that are free to adjust to changes in these four variables generally do so and reestablish newequilibrium conditions. Non-alluvial streams such as bedrock or artificial, concrete channels are unable tofollow Lane's relationship because of their inability to

corridor

slough

floodplain

naturallevee

floodplain

island backwater lake

shallo

w, o

pen

water

floo

dp

lain fo

rest

shru

b carr

shallo

w m

arsh

wet m

eado

w

mesic p

rairie

up

land

forest

deep

marsh

wet m

eado

w

up

land

forest

hill

prairie

high river

stage

floodplainlakebluff

low riverstage

mainchannel bluff

Figure 1.11: A cross section of a river corridor. The three main components of the river corridorcan be subdivided by structural features and plant communities. (Vertical scale and channel widthare greatly exaggerated.)Source: Sparks, Bioscience, vol. 45, p. 170, March 1995. ©1995 American Institute of Biological Science.

scarp

stream channel

thalweg

Figure 1.12: Cross section of a stream channel.The scarp is the sloped bank and the thalweg isthe lowest part of the channel.


adjust the sediment size and quantity variables.

The stream balance equation is usefulfor making qualitative predictions con-cerning channel impacts due to changesin runoff or sediment loads from thewatershed. Quantitative predictions,however, require the use of more com-plex equations.

Sediment transport equations, for ex-ample, are used to compare sedimentload and energy in the stream. If excessenergy is left over after the load ismoved, channel adjustment occurs asthe stream picks up more load by erod-ing its banks or scouring its bed. Nomatter how much complexity is builtinto these and other equations of thistype, however, they all relate back to thebasic balance relationships described byLane.

Streamflow

A distinguishing feature of the channelis streamflow. As part of the water cycle,the ultimate source of all flow is precip-itation. The pathways precipitationtakes after it falls to earth, however, af-fect many aspects of streamflow includ-ing its quantity, quality, and timing.Practitioners usually find it useful to di-vide flow into components based onthese pathways.

The two basic components are:

■ Stormflow, precipitation that reachesthe channel over a short time framethrough overland or undergroundroutes.

■ Baseflow, precipitation that percolatesto the ground water and moves slow-ly through substrate before reachingthe channel. It sustains streamflowduring periods of little or no precipi-tation.

FASTFORWARD

sediment size

coarse fine flat steep

stream slope

degradation

Qs • D50 Qw • S

aggradation

Figure 1.13: Factors affecting channel equilibrium. At equilibrium, slope and flow balance the size and quantity of sediment particles the stream moves.

Source: Rosgen (1996), from Lane, Proceedings, 1955. Published with the permission of American Society ofCivil Engineers.

Preview Chap-ter 2, Section Bfor more dis-cussion on thestream balanceequation. Pre-view Chapter 7,Section B fornformation onmeasuring andanalyzing thesevariables andthe use of sedi-ment transportequations.


Streamflow at any one time might con-sist of water from one or both sources.If neither source provides water to thechannel, the stream goes dry.

A storm hydrograph is a tool used toshow how the discharge changes withtime (Figure 1.14). The portion of thehydrograph that lies to the left of thepeak is called the rising limb, whichshows how long it takes the stream topeak following a precipitation event.The portion of the curve to the right ofthe peak is called the recession limb.

Channel and Ground WaterRelationships

Interactions between ground water andthe channel vary throughout the water-shed. In general, the connection isstrongest in streams with gravelriverbeds in well-developed alluvialfloodplains.

Rai

nfa

ll In

ten

sity

(in

ches

/hr)

Stre

am D

isch

arg

e (c

fs)

1

Time (days)

recessionlimb

time of rise

lag time

risinglimb

0 2 3 4

baseflow

stormflow

Figure 1.14: A storm hydrograph. A hydro-graph shows how long a stream takes to risefrom baseflow to maximum discharge and thenreturn to baseflow conditions.

Rai

nfa

ll In

ten

sity

(in

ches

/hr)

Stre

am D

isch

arg

e (c

fs)

Time (hours)

lag time after urbanization

lag time before urbanization

Q after

Q before

runoff before

runoff

after

Figure 1.15: A comparison of hydrographsbefore and after urbanization. The dischargecurve is higher and steeper for urban streamsthan for natural streams.

Change in Hydrology After UrbanizationThe hydrology of urban streams changes as sites are clearedand natural vegetation is replaced by impervious cover suchas rooftops, roadways, parking lots, sidewalks, and driveways.One of the consequences is that more of a stream’s annualflow is delivered as storm water runoff rather than baseflow.Depending on the degree of watershed impervious cover, theannual volume of storm water runoff can increase by up to16 times that for natural areas (Schueler 1995). In addition,since impervious cover prevents rainfall from infiltrating intothe soil, less flow is available to recharge ground water.Therefore, during extended periods without rainfall, baseflowlevels are often reduced in urban streams (Simmons andReynolds 1982).

Storm runoff moves more rapidly over smooth, hard pave-ment than over natural vegetation. As a result, the risinglimbs of storm hydrographs become steeper and higher inurbanizing areas (Figure 1.15). Recession limbs also declinemore steeply in urban streams.


Figure 1.16 presents two types of watermovement:

■ Influent or “losing” reaches lose streamwater to the aquifer.

■ Effluent or “gaining” reaches receivedischarges from the aquifer.

Practitioners categorize streams basedon the balance and timing of the storm-flow and baseflow components. Thereare three main categories:

■ Ephemeral streams flow only during orimmediately after periods of precipi-tation. They generally flow less than30 days per year (Figure 1.17).

■ Intermittent streams flow only duringcertain times of the year. Seasonalflow in an intermittent stream usual-ly lasts longer than 30 days per year.

■ Perennial streams flow continuouslyduring both wet and dry times.Baseflow is dependably generatedfrom the movement of ground waterinto the channel.

Discharge Regime

Discharge is the term used to describethe volume of water moving down thechannel per unit time (Figure 1.18).The basic unit of measurement used inthe United States to describe dischargeis cubic foot per second (cfs).

Discharge is calculated as:

Q = A V

where:

Q = Discharge (cfs)

A = Area through which the water isflowing in square feet

V = Average velocity in the downstreamdirection in feet per second

As discussed earlier in this section,streamflow is one of the variables thatdetermine the size and shape of thechannel. There are three types of char-acteristic discharges:

■ Channel-forming (or dominant) dis-charge. If the streamflow were heldconstant at the channel-forming

water table

water table

(a) Influent Stream Reach (b) Effluent Stream Reach

Figure 1.16: Cross sections of (a) influent and (b) effluent stream reaches. Influent or “losing”reaches lose stream water to the aquifer. Effluent or “gaining” reaches receive discharges fromthe aquifer.

Figure 1.17: An ephemeral stream. Ephemeralstreams flow only during or immediately afterperiods of precipitation.


depth

width

area

velocity

discharge, it would result in channelmorphology close to the existingchannel. However, there is nomethod for directly calculatingchannel-forming discharge.

An estimate of channel-forming dis-charge for a particular stream reachcan, with some qualifications, berelated to depth, width, and shape ofchannel. Although channel-formingdischarges are strictly applicable onlyto channels in equilibrium, the con-cept can be used to select appropriatechannel geometry for restoring a dis-turbed reach.

■ Effective discharge. The effective discharge is the calculated measure of channel-forming discharge. Computation of effective discharge requires long-term water and sediment measurements, either forthe stream in question or for one very similar.

Since this type of data is not often available for stream restoration sites, modeled or computeddata are sometimes substituted. Effective discharge can be computed for either stable or evolvingchannels.

� Bankfull discharge. This discharge occurs when water just begins to leave the channel and spread ontothe floodplain (Figure 1.19). Bankfull discharge is equivalent to channel-forming (conceptual) andeffective (calculated) discharge.

Figure 1.19: Bankfull discharge. This is the flow at which water begins to leave the channel and move onto the floodplain.

To envision the concept of channel-forming discharge, imagine placing awater hose discharging at constant ratein a freshly tilled garden. Eventually, asmall channel will form and reach anequilibrium geometry.

At a larger scale, consider a newlyconstructed floodwater- retardingreservoir that slowly releases storedfloodwater at a constant flow rate.This flow becomes the new channel-forming discharge and will alter chan-nel morphology until the channelreaches equilibrium.

FASTFORWARD


Floodplain

The floor of most stream valleys is rela-tively flat. This is because over time thestream moves back and forth across thevalley floor in a process called lateralmigration. In addition, periodic flood-ing causes sediments to move longitudi-nally and to be deposited on the valleyfloor near the channel. These twoprocesses continually modify the flood-plain.

Through time the channel reworks theentire valley floor. As the channel mi-grates, it maintains the same averagesize and shape if conditions upstreamremain constant and the channel staysin equilibrium.

Two types of floodplains may be de-fined (Figure 1.20):

■ Hydrologic floodplain, the land adja-cent to the baseflow channel residingbelow bankfull elevation. It is inun-dated about two years out of three.Not every stream corridor has ahydrologic floodplain.

■ Topographic floodplain, the land adja-cent to the channel including thehydrologic floodplain and otherlands up to an elevation based on

the elevation reached by a flood peakof a given frequency (for example,the 100-year floodplain).

Professionals involved with floodingissues define the boundaries of afloodplain in terms of flood frequen-cies. Thus, 100-year and 500-yearfloodplains are commonly used inthe development of planning andregulation standards.

Flood Storage

The floodplain provides temporary stor-age space for floodwaters and sedimentproduced by the watershed. This at-tribute serves to add to the lag time of aflood—the time between the middle ofthe rainfall event and the runoff peak.

If a stream’s capacity for moving waterand sediment is diminished, or if thesediment loads produced from the wa-tershed become too great for the streamto transport, flooding will occur morefrequently and the valley floor willbegin to fill. Valley filling results in thetemporary storage of sediment pro-duced by the watershed.

hydrologic floodplain(bankfull width)

topographic floodplain

bankfullelevation

Figure 1.20: Hydrologic and topographic floodplains. The hydrologic floodplain is defined bybankfull elevation. The topographic floodplain includes the hydrologic floodplain and other landsup to a defined elevation.

Preview Chap-ter 7, Section Bfor a discussionof calculating effective dis-charge. Thiscomputationshould be per-formed by aprofessionalwith a goodbackground inhydrology, hy-draulics, andsediment transport.


Landforms and Deposits

Topographic features are formed on thefloodplain by the lateral migration ofthe channel (Figure 1.21). These fea-tures result in varying soil and moistureconditions and provide a variety ofhabitat niches that support plant andanimal diversity.

Floodplain landforms and deposits in-clude:

■ Meander scroll, a sediment formationmarking former channel locations.

■ Chute, a new channel formed acrossthe base of a meander. As it grows insize, it carries more of the flow.

■ Oxbow, a term used to describe thesevered meander after a chute isformed.

■ Clay plug, a soil deposit developed atthe intersection of the oxbow and thenew main channel.

■ Oxbow lake, a body of water createdafter clay plugs the oxbow from themain channel.

■ Natural levees, formations built upalong the bank of some streams thatflood. As sediment-laden water spillsover the bank, the sudden loss ofdepth and velocity causes coarser-sized sediment to drop out of sus-pension and collect along the edge ofthe stream.

■ Splays, delta-shaped deposits ofcoarser sediments that occur when anatural levee is breached. Naturallevees and splays can prevent flood-waters from returning to the channelwhen floodwaters recede.

■ Backswamps, a term used to describefloodplain wetlands formed by nat-ural levees.

splay

oxbowlake

oxbow

clay plugchute

meanderscrolls

backswamp

naturallevee

Figure 1.21: Landforms and deposits of a floodplain. Topographic features on the floodplaincaused by meandering streams.


Transitional Upland Fringe

The transitional upland fringe serves asa transitional zone between the flood-plain and surrounding landscape. Thus,its outside boundary is also the outsideboundary of the stream corridor itself.

While stream-related hydrologic and ge-omorphic processes might have formeda portion of the transitional uplandfringe in geologic times, they are not re-sponsible for maintaining or altering itspresent form. Consequently, land useactivities have the greatest potential toimpact this component of the streamcorridor.

There is no typical cross section for thiscomponent. Transitional upland fringescan be flat, sloping, or in some cases,nearly vertical (Figure 1.22). They canincorporate features such as hillslopes,bluffs, forests, and prairies, often modi-fied by land use. All transitional upland

fringes have one common attribute,however: they are distinguishable fromthe surrounding landscape by theirgreater connection to the floodplainand stream.

An examination of the floodplain sideof the transitional upland fringe oftenreveals one or more benches. Theselandforms are called terraces (Figure1.23). They are formed in response tonew patterns of streamflow, changes insediment size or load, or changes in wa-tershed base level—the elevation at thewatershed outlet.

Terrace formation can be explainedusing the aforementioned stream bal-ance equation (Figure 1.13). When oneor more variables change, equilibriumis lost, and either degradation or aggra-dation occurs.

Figure 1.24 presents an example of ter-race formation by channel incision.Cross section A represents a nonincisedchannel. Due to changes in streamflowor sediment delivery, equilibrium is lost

Figure 1.22: Transitional upland fringe. Thiscomponent of the stream corridor is a transi-tion zone between the floodplain and thesurrounding landscape.

Figure 1.23: Terraces formed by an incisingstream. Terraces are formed in response tonew patterns of streamflow or sediment loadin the watershed.


and the channel degrades and widens.The original floodplain is abandonedand becomes a terrace (cross section B).The widening phase is completed whena floodplain evolves within thewidened channel (cross section C).

Geomorphologists often classify land-scapes by numbering surfaces from thelowest surface up to the highest surface.Surface 1 in most landscapes is the bot-tom of the main channel. The nexthighest surface, Surface 2, is the flood-plain. In the case of an incising stream,Surface 3 usually is the most recentlyformed terrace, Surface 4 the next olderterrace, and so on. The numbering sys-tem thus reflects the ages of the sur-faces. The higher the number, the olderthe surface.

Boundaries between the numbered sur-faces are usually marked by a scarp, orrelatively steep surface. The scarp be-tween a terrace and a floodplain is espe-cially important because it helpsconfine floods to the valley floor.Flooding occurs much less frequently, ifat all, on terraces.

Vegetation Across the Stream Corridor

Vegetation is an important and highlyvariable element in the stream corridor.In some minimally disturbed streamcorridors, a series of plant communitiesmight extend uninterrupted across theentire corridor. The distribution of thesecommunities would be based on differ-ent hydrologic and soil conditions. Insmaller streams the riparian vegetationmight even form a canopy and enclosethe channel. This and other configura-tion possibilities are displayed in Figure1.25.

Plant communities play a significantrole in determining stream corridorcondition, vulnerability, and potentialfor (or lack of) restoration. Thus, the

type, extent and distribution, soil mois-ture preferences, elevation, species com-position, age, vigor, and rooting depthare all important characteristics that apractitioner must consider when plan-ning and designing stream corridorrestoration.

Flood-Pulse Concept

Floodplains serve as essential focalpoints for the growth of many riparian

bankfull channel

A. Nonincised Stream

incised, widening channel

B. Incised Stream (early widening phase)

channel

C. Incised Stream (widening phase complete)

floodplainterrace

terrace

terrace

terrace

terrace

terrace

terrace

terrace

scarp

scar

p

scarp

scarp

scar

p

scar

p

scarp

terrace

scarp s car

p

terrace

scar

pfloodplain

floodplain

Figure 1.24: Terraces in (A) nonincised and (Band C) incised streams. Terraces are abandonedfloodplains, formed through the interplay ofincising and floodplain widening.

FASTFORWARD

Preview Chapter 2, Section D formore informa-tion on plantcommunitycharacteristics.


plant communities and the wildlifethey support. Some riparian plantspecies such as willows and cotton-woods depend on flooding for regener-ation. Flooding also nourishesfloodplains with sediments and nutri-ents and provides habitat for inverte-brate communities, amphibians,reptiles, and fish spawning.

The flood-pulse concept was developedto summarize how the dynamic interac-tion between water and land is ex-ploited by the riverine and floodplainbiota (Figure 1.26). Applicable primar-ily on larger rivers, the concept demon-strates that the predictable advance andretraction of water on the floodplain ina natural setting enhances biologicalproductivity and maintains diversity(Bayley 1995).

Closed Canopy Over Channel, Floodplain,and Transitional Upland Fringe

Open Canopy Over Channel

Figure 1.25: Examples of vegetation structurein the stream corridor. Plant communities playa significant role in determining the conditionand vulnerability of the stream corridor.


Lake and river spawning; young-of-the-year and predators follow moving littoral; fish and invertebrate production high.

Most river-spawning fish start to breed.

Young and adult fish disperse and feed, dissolved oxygen (DO) permitting.

Many fish respond to drawdown by finding deeper water.

Fish migrate to main channel, permanent lakes or tributaries.

input of nutrients, suspended solids; nutrients from newly flooded soil

runoff of nutrients resulting from decomposition

runoff and concentration of nutrients resulting from decomposition

terrestrialshrubs

maximum production of aquatic vegetation

decomposition ofterrestrial and olderaquatic vegetation

aquatic/terrestrial transition zone(floodplain)

maximum biomassof aquatic vegetation

consolidation of sediments

consolidation of sediments; moist soil plantgermination

low dissolved oxygen

flood-tolerant trees

annualterrestrialgrasses

decomposition of stranded aquatic vegetation,mineralization of nutrients

regrowth of terrestrial grasses and shrubs

decomposition of most remaining vegetation

Figure 1.26: Schematic of the flood-pulse concept. A vertically exaggerated section of afloodplain in five snapshots of an annual hydrological cycle. The left column describes themovement of nutrients. The right column describes typical life history traits of fish.Source: Bayley, Bioscience, vol. 45, p.154, March 1995. ©1995 American Institute of Biological Science.


The processes that develop the charac-teristic structure seen in the lateral viewof a stream corridor also influencestructure in the longitudinal view.Channel width and depth increasedownstream due to increasing drainagearea and discharge. Related structuralchanges also occur in the channel,floodplain, and transitional uplandfringe, and in processes such as erosionand deposition. Even among differenttypes of streams, a common sequenceof structural changes is observable fromheadwaters to mouth.

Longitudinal Zones

The overall longitudinal profile of moststreams can be roughly divided intothree zones (Schumm 1977). Some ofthe changes in the zones are character-ized in Figures 1.27 and 1.28.

Zone 1, or headwaters, often has thesteepest gradient. Sediment erodes fromslopes of the watershed and movesdownstream. Zone 2, the transfer zone,receives some of the eroded material. Itis usually characterized by wide flood-plains and meandering channel pat-terns. The gradient flattens in Zone 3,the primary depositional zone. Thoughthe figure displays headwaters as moun-tain streams, these general patterns andchanges are also often applicable to wa-tersheds with relatively small topo-graphic relief from the headwaters tomouth. It is important to note that ero-sion, transfer, and deposition occur inall zones, but the zone concept focuseson the most dominant process.

Watershed Forms

All watersheds share a common defini-tion: a watershed is an “area of land that

1.C A Longitudinal View Along the StreamCorridor

Low-elevation streams merge and flow down gentler slopes. The valley broadens and the river begins to meander.

At an even lower elevation a river wanders and meanders slowly across a broad, nearly flat valley. At its mouth it may divide into many separate channels as it flows across a delta built up of river-borne sediments and into the sea.

Mountain headwater streams flow swiftly down steep slopes and cut a deep V-shaped valley. Rapids and waterfalls are common.

Zone 1HeadwatersZone 2Transfer Zone

Zone 3Depositional Zone

Figure 1.27: Three longitudinal profile zones. Channel and floodplain characteristics change asrivers travel from headwaters to mouth.Source: Miller (1990). ©1990 Wadsworth Publishing Co.

A Longitudinal View Along the Stream Corridor 1–25

drains water, sediment, and dissolvedmaterials to a common outlet at somepoint along a stream channel” (Dunneand Leopold 1978). Form varies greatly,however, and is tied to many factorsincluding climatic regime, underlyinggeology, morphology, soils, and vegeta-tion.

Drainage Patterns

One distinctive aspect of a watershedwhen observed in planform (map view)

is its drainage pattern (Figure 1.29).Drainage patterns are primarily con-trolled by the overall topography andunderlying geologic structure of thewatershed.

Stream Ordering

A method of classifying, or ordering,the hierarchy of natural channels withina watershed was developed by Horton(1945). Several modifications of theoriginal stream ordering scheme have

rela

tive

volu

me

ofst

ored

alluvium

channel width

slopebed

material grain

size

mean flow velocity

characte

ristic

stream

discharge

channel depth

Drainage Area ( downstream distance2)

Incr

ease

Headwaters Transfer Deposition

Figure 1.28: Changes in the channel in the three zones. Flow, channel size, and sedimentcharacteristics change throughout the longitudinal profile.


been proposed, but the modified sys-tem of Strahler (1957) is probably themost popular today.

Strahler’s stream ordering system is por-trayed in Figure 1.30. The uppermostchannels in a drainage network (i.e.,headwater channels with no upstreamtributaries) are designated as first-orderstreams down to their first confluence.A second-order stream is formed belowthe confluence of two first-order chan-nels. Third-order streams are createdwhen two second-order channels join,and so on. Note in the figure that theintersection of a channel with another

channel of lower order does not raisethe order of the stream below the inter-section (e.g., a fourth-order stream in-tersecting with a second-order stream isstill a fourth-order stream below the in-tersection).

Within a given drainage basin, streamorder correlates well with other basinparameters, such as drainage area orchannel length. Consequently, knowingwhat order a stream is can provide cluesconcerning other characteristics such aswhich longitudinal zone it resides inand relative channel size and depth.

Channel Form

The form of the channel can change asit moves through the three longitudinalzones. Channel form is typically de-scribed by two characteristics—thread(single or multiple) and sinuosity.

Single- and Multiple-ThreadStreams

Single-thread (one-channel) streams aremost common, but multiple-threadstreams occur in some landscapes (Fig-ure 1.31). Multiple-thread streams arefurther categorized as either braided oranastomosed streams.

Dendritic Parallel

Trellis Rectangular

Radial Annular

Multi-Basinal Contorted

dry

Figure 1.29: Watershed drainage patterns.Patterns are determined by topography andgeologic structure.Source: A.D. Howard, AAPG © 1967, reprinted bypermission of the American Association of PetroleumGeologists.

4

43

32

2

21

1

1

1

11

1

13

2

2

2

1

1

1

1

1

1

1

11

Figure 1.30: Stream ordering in a drainage net-work. Stream ordering is a method of classify-ing the hierarchy of natural channels in awatershed.


Three conditions tend to promote theformation of braided streams:

■ Erodible banks.

■ An abundance of coarse sediment.

■ Rapid and frequent variations in dis-charge.

Braided streams typically get their startwhen a central sediment bar begins toform in a channel due to reducedstreamflow or an increase in sedimentload. The central bar causes water toflow into the two smaller cross sectionson either side. The smaller cross sectionresults in a higher velocity flow. Givenerodible banks, this causes the channelsto widen. As they do this, flow velocitydecreases, which allows another centralbar to form. The process is then re-peated and more channels are created.

In landscapes where braided streamsoccur naturally, the plant and animalcommunities have adapted to frequentand rapid changes in the channel andriparian area. In cases where distur-bances trigger the braiding process,however, physical conditions might betoo dynamic for many species.

The second, less common category ofmultiple-thread channels is called anas-tomosed streams. They occur on muchflatter gradients than braided streamsand have channels that are narrow anddeep (as opposed to the wide, shallowchannels found in braided streams).Their banks are typically made up offine, cohesive sediments, making themrelatively erosion-resistant.

Anastomosed streams form when thedownstream base level rises, causing arapid buildup of sediment. Since bankmaterials are not easily erodible, theoriginal single-thread stream breaks upinto multiple channels. Streams enteringdeltas in a lake or bay are often anasto-mosed. Streams on alluvial fans, in con-trast, can be braided or anastomosed.

Sinuosity

Natural channels are rarely straight.Sinuosity is a term indicating theamount of curvature in the channel(Figure 1.32). The sinuosity of a reachis computed by dividing the channel

Figure 1.31: (a) Single-thread and (b) braidedstreams. Single-thread streams are most common. Braided streams are uncommon andusually formed in response to erodible banks,an abundance of coarse sediment, and rapidand frequent variations in discharge.

(a)

(b)


centerline length by the length ofthe valley centerline. If the channellength/valley length ratio is morethan about 1.3, the stream can beconsidered meandering in form.

Sinuosity is generally related to theproduct of discharge and gradient.

Low to moderate levels of sinuosity aretypically found in Zones 1 and 2 of thelongitudinal profile. Extremely sinuousstreams often occur in the broad, flatvalleys of Zone 3.

Pools and Riffles

No matter the channel form, moststreams share a similar attribute of al-ternating, regularly spaced, deep andshallow areas called pools and riffles(Figure 1.33). The pools and riffles areassociated with the thalweg, which me-anders within the channel. Pools typi-cally form in the thalweg near theoutside bank of bends. Riffle areas usu-ally form between two bends at thepoint where the thalweg crosses overfrom one side of the channel to theother.

The makeup of the streambed playsa role in determining pool and rifflecharacteristics. Gravel and cobble-bedstreams typically have regularly spacedpools and riffles that help maintainchannel stability in a high-energy envi-ronment. Coarser sediment particlesare found in riffle areas while smallerparticles occur in pools. The pool-to-pool or riffle-to-riffle spacing is nor-mally about 5 to 7 times the channelwidth at bankfull discharge (Leopoldet al. 1964).

Sand-bed streams, on the other hand,do not form true riffles since the grainsize distribution in the riffle area is sim-ilar to that in the pools. However, sand-bed streams do have evenly spacedpools. High-gradient streams also usu-ally have pools but not riffles, but for adifferent reason. In this case, watermoves from pool to pool in a stairstepfashion.

Figure 1.32: Sinuosity: (a) low and (b) extreme.Low to moderately sinuous streams are usuallyfound in Zones 1 and 2 of the longitudinal pro-file. Extremely sinuous streams are more typicalof Zone 3.

(b)

(a)


Vegetation Along the StreamCorridor

Vegetation is an important and highlyvariable element in the longitudinal aswell as the lateral view. Floodplains arenarrow or nonexistent in Zone 1 of thelongitudinal profile; thus flood-depen-dent or tolerant plant communitiestend to be limited in distribution. Up-land plant communities, such as forestson moderate to steep slopes in the east-ern or northwestern United States,might come close to bordering thestream and create a canopy that leaveslittle open sky visible from the channel.In other parts of the country, headwa-ters in flatter terrain may support plantcommunities dominated by grasses andbroad-leaved herbs, shrubs, or plantedvegetation.

Despite the variation in plant commu-nity type, many headwaters areas pro-vide organic matter from vegetationalong with the sediment they export toZones 2 and 3 downstream. For exam-ple, logs and woody debris from head-waters forests are among the mostecologically important features support-ing food chains and instream habitatstructure in Pacific Northwest riversfrom the mountains to the sea (Maserand Sedell 1994).

Zone 2 has a wider and more complexfloodplain and larger channel thanZone 1. Plant communities associatedwith floodplains at different elevationsmight vary due to differences in soiltype, flooding frequency, and soil mois-ture. Localized differences in erosionand deposition of sediment add com-plexity and diversity to the types ofplant communities that become established.

The lower gradient, larger stream size,and less steep terrain in Zone 2 oftenattract more agricultural or residentialdevelopment than in the headwaters

zone. This phenomenon frequentlycounteracts the natural tendency to de-velop broad and diverse stream corridorplant communities in the middle andlower reaches. This is especially truewhen land uses involve clearing the native vegetation and narrowing thecorridor.

Often, a native plant community is re-placed by a planted vegetation commu-nity such as agricultural crops orresidential lawns. In such cases, streamprocesses involving flooding,erosion/deposition, import or export oforganic matter and sediment, streamcorridor habitat diversity, and waterquality characteristics are usually signif-icantly altered.

The lower gradient, increased sedimentdeposition, broader floodplains, andgreater water volume in Zone 3 all setthe stage for plant communities differ-ent from those found in either up-stream zone. Large floodplain wetlandsbecome prevalent because of the gener-ally flatter terrain. Highly productiveand diverse biological communities,

(a)

(b)

rifflepool

thalweg line

pool

riffle or cross over

Figure 1.33: Sequence of pools and riffles in(a) straight and (b) sinuous streams. Poolstypically form on the outside bank of bendsand riffles in the straight portion of the chan-nel where the thalweg crosses over from oneside to the other.


such as bottomland hardwoods, estab-lish themselves in the deep, rich alluvialsoils of the floodplain. The slower flowin the channel also allows emergentmarsh vegetation, rooted floating orfree-floating plants, and submergedaquatic beds to thrive.

The changing sequence of plant com-munities along streams from source tomouth is an important source of biodi-versity and resiliency to change. Al-though many, or perhaps most, of astream corridor’s plant communitiesmight be fragmented, a continuous cor-ridor of native plant communities is de-sirable. Restoring vegetative connectivityin even a portion of a stream will usu-ally improve conditions and increase itsbeneficial functions.

The River Continuum Concept

The River Continuum Concept is an at-tempt to generalize and explain longitu-dinal changes in stream ecosystems(Figure 1.34) (Vannote et al. 1980).This conceptual model not only helpsto identify connections between the wa-tershed, floodplain, and stream systems,but it also describes how biologicalcommunities develop and change fromthe headwaters to the mouth. The RiverContinuum Concept can place a site orreach in context within a larger water-shed or landscape and thus help practi-tioners define and focus restorationgoals.

The River Continuum Concept hypoth-esizes that many first- to third-orderheadwater streams are shaded by the ri-parian forest canopy. This shading, inturn, limits the growth of algae, peri-phyton, and other aquatic plants. Sinceenergy cannot be created through pho-tosynthesis (autotrophic production),the aquatic biota in these small streamsis dependent on allochthonous materials(i.e., materials coming from outside thechannel such as leaves and twigs).

Biological communities are uniquelyadapted to use externally derived or-ganic inputs. Consequently, theseheadwater streams are consideredheterotrophic (i.e., dependent on theenergy produced in the surroundingwatershed). Temperature regimes arealso relatively stable due to the influ-ence of ground water recharge, whichtends to reduce biological diversity tothose species with relatively narrowthermal niches.

Predictable changes occur as one pro-ceeds downstream to fourth-, fifth-,and sixth-order streams. The channelwidens, which increases the amountof incident sunlight and average tem-peratures. Levels of primary productionincrease in response to increases inlight, which shifts many streams to adependence on autochthonous materials(i.e., materials coming from insidethe channel), or internal autotrophicproduction (Minshall 1978).

In addition, smaller, preprocessed or-ganic particles are received from up-stream sections, which serves to balanceautotrophy and heterotrophy within thestream. Species richness of the inverte-brate community increases as a varietyof new habitat and food resources ap-pear. Invertebrate functional groups,such as the grazers and collectors, in-crease in abundance as they adapt tousing both autochthonous and al-lochthonous food resources. Midsizedstreams also decrease in thermal stabil-ity as temperature fluctuations increase,which further tends to increase bioticdiversity by increasing the number ofthermal niches.

Larger streams and rivers of seventh totwelfth order tend to increase in physi-cal stability, but undergo significantchanges in structure and biological func-tion. Larger streams develop increasedreliance on primary productivity by


Stre

am S

ize

(ord

er)

Relative Channel Width

12

11

10

9

8

7

6

5

4

3

2

1

collectors

collectors

collectors

predators

predators

trout

periphyton

vascularhydrophytes

smallmouthbass

perch periphyton

phytoplankton

zooplankton

catfish

shredders

grazers

grazers

coarseparticulatematterfine

particulatematter

fineparticulatematter

coarseparticulatematter

fineparticulatematter

shredders

predators

microbes

microbes

microbes

Figure 1.34: The River Continuum Concept. The concept proposes a relationship betweenstream size and the progressive shift in structural and functional attributes.Source: Vannote et al. (1980). Published with the permission of NRC Research Press.


phytoplankton, but continue to receiveheavy inputs of dissolved and ultra-fineorganic particles from upstream. Inver-tebrate populations are dominated byfine-particle collectors, including zoo-plankton. Large streams frequently carryincreased loads of clays and fine silts,which increase turbidity, decrease lightpenetration, and thus increase the sig-nificance of heterotrophic processes.

The influence of storm events and ther-mal fluctuations decrease in frequencyand magnitude, which increases theoverall physical stability of the stream.This stability increases the strength ofbiological interactions, such as competi-tion and predation, which tends toeliminate less competitive taxa andthereby reduce species richness.

The fact that the River Continuum Con-cept applies only to perennial streams isa limitation. Another limitation is thatdisturbances and their impacts on theriver continuum are not addressed bythe model. Disturbances can disrupt theconnections between the watershed andits streams and the river continuum aswell.

The River Continuum Concept has notreceived universal acceptance due tothese and other reasons (Statzner andHigler 1985, Junk et al. 1989). Never-theless, it has served as a useful concep-tual model and stimulated muchresearch since it was first introducedin 1980.

Documents

1.A Overview of Structure and Scale 1 · Stream channel Floodplain Transitional upland fringe Together they function as dynamic and valued crossroads in the landscape. (Figure 1.1)