1

1.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 function as dynamic crossroads in the

landscape. Water and other materials, energy, and organisms meet and

interact within the corridor.

1.A Physical Structure and Time at

Multiple Scales

1.B A Lateral View Across the Stream

Corridor

1.C A Longitudinal View Along the

Stream Corridor

stream corridor is an ecosystem that

usually consists of three major elements:

Stream channel

Floodplain

Transitional upland fringe

Together they function as dynamic and

valued crossroads in the landscape. 

(Figure 1.1). Water and other materials,

energy, and organisms meet and interact

within the stream corridor over space and

time. This movement provides critical functions essential for maintaining life such as

cycling nutrients, filtering contaminants

from runoff, absorbing and gradually releasing floodwaters, maintaining fish and

wildlife habitats, recharging ground water,

and maintaining stream flows.

The purpose of this chapter is to define

the components of the

stream corridor and introduce the concepts of scale

and structure. The chapter is

divided into three subsections.
1–2 Chapter 1: Overview of Stream Corridors

Section 1.A: Physical Structure and

Time at Multiple Scales

An important initial task is to identify the spatial and time scales most

appropriate for planning and designing restoration. This subsection

introduces elements of structure

used in landscape ecology and relates them to a hierarchy of spacial

scales ranging from broad to local.

The importance of integrating time

scales into the restoration process is

also discussed.

Section 1.B: A Lateral View Across

the Stream Corridor

The purpose of this and the following subsection is to introduce the

types of structure found within

stream corridors. The focus here is

on the lateral dimension of structure, which affects the movement

of water, materials, energy, and organisms from upland areas into the

stream channel.

Section 1.C: A Longitudinal View

Along the Stream Corridor

This section takes a longitudinal

view of structure, specifically as a

stream travels down the valley from

headwaters to mouth. It includes

discussions of channel form, sediment transport and deposition, and

how biological communities have

adapted to different stages of the

river continuum.
Physical Structure and Time at Multiple Scales 1–3

1.A Physical Structure and Time at Multiple Scales

Landscapes,

watersheds,

stream corridors, and

streams are

ecosystems

that occur at

different spatial scales. 

A hierarchy of five spatial scales, which

range from broad to local, is displayed

in Figure 1.2. Each element within the

scales can be viewed as an ecosystem

with links to other ecosystems. These

linkages are what make an ecosystem’s

external environment as important to

proper functioning as its internal environment (Odum 1989).

Landscapes and stream corridors are

ecosystems that occur at different spatial scales. Examining them as ecosystems is useful in explaining the basics

of how landscapes, watersheds, stream

corridors, and streams function. Many

common ecosystem functions involve

movement of materials (e.g., sediment

and storm water runoff), energy (e.g.,

heating and cooling of stream waters),

and organisms (e.g., movement of

mammals, fish schooling, and insect

swarming) between the internal and external environments (Figure 1.3).

The internal/external movement model

becomes more complex when one considers that the external environment of

a given ecosystem is a larger ecosystem.

A stream ecosystem, for example, has an

input/output relationship with the next

higher scale, the stream corridor. This

scale, in turn, interacts with the landscape scale, and so on up the hierarchy.

Similarly, because each larger-scale

ecosystem contains the one beneath it,

the structure and functions of the

smaller ecosystem are at least part of the

structure and functions of the larger.

Furthermore, what is not part of the

smaller ecosystem might be related to 

it through input or output relationships

with neighboring ecosystems. Investigating relationships between structure and

scale is a key first step for planning and

designing stream corridor restoration.

Physical Structure

Landscape ecologists use four basic

terms to define spatial structure at a

particular scale (Figure 1.4): 

■ Matrix, the land cover that is dominant and interconnected over the

majority of the land surface. Often

the matrix is forest or agriculture,

but theoretically it can be any land

cover type. 

■ Patch, a nonlinear area (polygon)

that is less abundant than, and different from, the matrix.

■ Corridor, a special type of patch that

links other patches in the matrix.

Typically, a corridor is linear or elongated in shape, such as a stream

corridor.

■ Mosaic, a collection of patches, none

of which are dominant enough to be

interconnected throughout the landscape.

These simple structural element concepts are repeated at different spatial

scales. The size of the area and the spatial resolution of one’s observations determine what structural elements one is

observing. For example, at the landscape

scale one might see a matrix of mature

forest with patches of cropland, pasture,

clear-cuts, lakes, and wetlands. Looking

more closely at a smaller area, one

might consider an open woodland to be

a series of tree crowns (the patches)

against a matrix of grassy ground cover.

On a reach scale, a trout might perceive

pools and well-sheltered, cool, pockets

of water as preferred patches in a matrix

of less desirable shallows and riffles, and

the corridor along an undercut streambank might be its only way to travel

safely among these habitat patches.

FAST

FORWARD

Preview Chapter 2, Section E

for a discussion

of the six critical functions

performed by

stream corridor

ecosystems.
1–4 Chapter 1: Overview of Stream Corridors

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

Patu

x

ent

River S tate Park

Reach Scale

Figure 1.2: Ecosystems at multiple scales.

Stream corridor restoration can occur at

any scale, from regional to reach.
At the other extreme, the coarsest of the

imaging satellites that monitor the earth’s

surface might detect only patches or corridors of tens of square miles in area,

and matrices that seem to dominate a

whole region. At all levels, the matrixpatch-corridor-mosaic model provides a

useful common denominator for describing structure in the environment.

Figure 1.5 displays examples of the matrices, patches, and corridors at broad

and local scales. Practitioners should

always consider multiple scales when

planning and designing restoration.

Structure at Scales Broader Than

the Stream Corridor Scale

The landscape scale encompasses the

stream corridor scale. In turn, the landscape scale is encompassed by the larger

regional scale. Each scale within the hierarchy has its own characteristic structure.

The “watershed scale” is another form of

spatial scale that can also encompass the

stream corridor. Although watersheds

occur at all scales, the term “watershed

scale” is commonly used by many practitioners because many functions of the

stream corridor are closely tied to drainage patterns. For this reason, the “watershed scale” is included in this discussion.

N

ecosystem

input environment output environment

Figure 1.3: A simple

ecosystem model.

Materials, energy, and

organisms move from

an external input

environment, through

the ecosystem, and

into an external output environment.

Landscape

ecologists use

four basic

terms to define

spatial structure at a particular scale—

matrix, patch,

corridor, and

mosaic. 

matrix

matrix

patch

corridor

patch

matrix

patch

mosaic

Figure 1.4: Spatial

structure. Landscapes

can be described in

terms of matrix,

patch, corridor, and

mosaic at various

scales.

Physical Structure and Time at Multiple Scales 1–5
1–6 Chapter 1: Overview of Stream Corridors

Regional Scale

A region is a broad geographical area

with a common macroclimate and

sphere of human activities and interests

(Forman 1995). The spatial elements

found at the regional scale are called

landscapes. Figure 1.6 includes an example of the New England region with

landscapes defined both by natural

cover and by land use. 

Matrices in the United States include:

■ Deserts and arid grasslands of the

arid Southwest.

■ Forests of the Appalachian

Mountains.

■ Agricultural zones of the Midwest. 

At the regional scale, patches generally

include:

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

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

■ Major forested areas (e.g., redwood

forests in the Pacific Northwest).

■ Major metropolitan zones (e.g., the

Baltimore-Washington, DC, metropolitan area).

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

Corridors might include:

■ Mountain ranges.

■ Major river valleys.

■ Interregional development along a

major transportation corridor.

Most practitioners of stream corridor

restoration do not usually plan and design restoration at the regional scale.

The perspective is simply too broad for

most projects. Regional scale is introduced here because it encompasses the

scale very pertinent to stream corridor

restoration—the landscape scale.

Figure 1.5: Spatial structure at (a) broad and (b) local scales. Patches, corridors, and matrices are

visible at the broad regional scale and the local reach scale.

(a) (b)

CHESAPEAKE

BAY

URBAN MATRIX

BALTIMORE

WASHINGTON, DC

POOL

POOL

RIFFLE

Practitioners

should always

consider multiple scales

when planning

and designing

restoration.
Physical Structure and Time at Multiple Scales 1–7

Landscape Scale

A landscape is a geographic area distinguished by a repeated pattern of components, which include both natural

communities like forest patches and

wetlands and human-altered areas like

croplands and villages. Landscapes can

vary in size from a few to several thousand square miles.

At the landscape scale, patches (e.g.,

wetlands and lakes) and corridors

(e.g., stream corridors) are usually

described as ecosystems. The matrix is

usually identified in terms of the predominant natural vegetation community (e.g., prairie-type, forest-type, and

wetland-type) or land-use-dominated

ecosystem (e.g., agriculture and urban)

(Figure 1.7). 

Landscapes differ from one another

based on the consistent pattern formed

by their structural elements, and the

predominant land cover that comprises

their patches, corridors, and matrices.

Examples of landscapes in the United

States include:

■ A highly fragmented east coast mosaic of suburban, forest, and agricultural patches.

■ A north-central agricultural matrix

with pothole wetlands and forest

patches.

■ A Sonoran desert matrix with willowcottonwood corridors.

■ A densely forested Pacific Northwest

matrix with a pattern of clear-cut

patches.

agricultural

urban

spruce–fir

suburban

northern hardwood

pitch pine–oak

oak forest

salt marsh

barrens

rivers and lakes

industrial

Adirondack

Region

New 

York

Region

New

England

Region

The Maritimes

Region

Southern Quebec 

 Region

Atlantic Ocean

Figure 1.6: The New England region. Structure

in a region is typically a function of natural

cover and land use. 

Source: Forman (1995). Reprinted with the permission of Cambridge University Press.

A landscape is

a geographic

area distinguished by a

repeated pattern of components, which

include both

natural communities like

forest patches

and wetlands

and humanaltered areas

like croplands

and villages.
1–8 Chapter 1: Overview of Stream Corridors

A woodlot within an agricultural matrix and a wetland in an urban matrix

are examples of patches at the landscape scale. Corridors at this scale

include ridgelines, highways, and

the topic of this document—stream

corridors. 

At the landscape scale it is easy to perceive the stream corridor as an ecosystem with an internal environment and

external environment (its surrounding

landscape). Corridors play an important role at the landscape scale and at

other scales. Recall that a key attribute

of ecosystems is the movement of energy, materials, and organisms in,

through, and out of the system. Corridors typically serve as a primary pathway for this movement. They connect

patches and function as conduits between ecosystems and their external

environment. Stream corridors in particular provide a heightened level of

functions because of the materials and

organisms found in this type of landscape linkage.

Spatial structure, especially in corridors,

helps dictate movement in, through,

and out of the ecosystem; conversely,

this movement also serves to change

the structure over time. Spatial structure, as it appears at any one point in

time, is therefore the end result of

movement that has occurred in the

past. Understanding this feedback loop

between movement and structure is a

key to working with ecosystems in any

scale.

“Watershed Scale”

Much of the movement of material, energy, and organisms between the stream

corridor and its external environments

is dependent on the movement of

water. Consequently, the watershed

concept is a key factor for planning and

designing stream corridor restoration.

The term “scale,” however, is incorrectly

applied to watersheds.

A watershed is defined as an area of land

that drains water, sediment, and dissolved materials to a common outlet at

some point along a stream channel

(Dunne and Leopold 1978). Watersheds, therefore, occur at multiple

scales. They range from the largest river

basins, such as the watersheds of the

Mississippi, Missouri, and Columbia, 

to the watersheds of very small streams

that measure only a few acres in size.

The term “watershed scale” (singular) is

a misnomer because watersheds occur

at a very wide range of scales. This document focuses primarily on the watersheds of small to medium-scale streams

and rivers. Watersheds in this size range

can contain all or part of a few different

landscapes or can be entirely encompassed by a larger landscape. 

Ecological structure within watersheds

can still be described in matrix, patch,

corridor, and mosaic terms, but a discussion of watershed structure is more

meaningful if it also focuses on eleFigure 1.7: Structure at the landscape scale.

Patches and corridors are visible within an agricultural matrix.

WOODLAND

PATCH

WOODLAND

PATCH

STREAM

CORRIDOR

FIELD

PATCH

AGRICULTURAL

MATRIX

A more complete broad

scale perspective of the

stream corridor

s achieved

when watershed science is

combined with

andscape 

ecology.
Physical Structure and Time at Multiple Scales 1–9

ments such as upper, middle, and lower

watershed zones; drainage divides;

upper and lower hillslopes; terraces,

floodplains, and deltas; and features

within the channel. These elements and

their related functions are discussed in

sections B and C of this chapter.

In short, watersheds and landscapes

overlap in size range and are defined by

different environmental processes.

Whereas the landscape is defined primarily by terrestrial patterns of land

cover that may continue across drainage

divides to where the consistent pattern

ends, the watershed’s boundaries are

based on the drainage divides themselves. Moreover, the ecological

processes occurring in watersheds are

more closely linked to the presence and

movement of water; therefore as functioning ecosystems, watersheds also differ from landscapes.

The difference between landscape scale

and “watershed scale” is precisely why

practitioners should consider both

when planning and designing stream

corridor restoration. For decades the

watershed has served as the geographic

unit of choice because it requires consideration of hydrologic and geomorphic processes associated with the

movement of materials, energy, and organisms into, out of, and through the

stream corridor.

The exclusive use of watersheds for the

broad-scale perspective of stream corridors, however, ignores the materials, energy, and organisms that move across

and through landscapes independent of

water drainage. Therefore, a more complete broad-scale perspective of the

stream corridor is achieved when watershed science is combined with landscape ecology.

The USGS developed a national framework for cataloging watersheds of different geographical scales. Each

level, or scale, in the hierarchy is designated using the

hydrologic unit cataloging (HUC) system. At the national

level this system involves an eight-digit code that

uniquely identifies four levels of classification.

The largest unit in the USGS HUC system is the water

resource region. Regions are designated by the first two

digits of the code. The remaining numbers are used to

further define subwatersheds within the region down to

the smallest scale called the cataloging unit. For example, 10240006 is the hydrologic unit code for the Little

Nemaha River in Nebraska. The code is broken down as

follows:

10 Region 

1024 Subregion

102400 Accounting code

10240006 Cataloging unit

There are 21 regions, 222 subregions, 352 accounting

units, and 2,150 cataloging units in the United States.

The USGS’s Hydrologic Unit Map Series documents these

hierarchical watershed boundaries for each state. Some

state and federal agencies have taken the restoration initiative to subdivide the cataloging unit into even smaller

watersheds, extending the HUC code to 11 or 14 digits.

The Reach File/National Hydrography Dataset (RF/NHD) is

a computerized database of streams, rivers, and other

water bodies in the United States. It is cross-referenced

with the HUC system in a geographic information system

(GIS) format so users can easily identify both watersheds

and the streams contained within their boundaries.
1–10 Chapter 1: Overview of Stream Corridors

Structure at the Stream Corridor

Scale

The stream corridor is a spatial element

(a corridor) at the watershed and landscape scales. But as a part of the hierarchy, it has its own set of structural

elements (Figure 1.8). Riparian

(streamside) forest or shrub cover is a

common matrix in stream corridors. In

other areas, herbaceous vegetation

might dominate a stream corridor. 

Examples of patches at the stream corridor scale include both natural and

human features such as:

■ Wetlands.

■ Forest, shrubland, or grassland 

patches.

■ Oxbow lakes.

■ Residential or commercial development.

■ Islands in the channel.

■ Passive recreation areas such as picnic grounds.

Corridors at the stream corridor scale

include two important elements—the

stream channel and the plant community on either side of the stream. Other

examples of corridors at this scale

might include:

■ Streambanks

■ Floodplains

■ Feeder (tributary) streams

■ Trails and roads

Structure Within the Stream

Corridor Scale

At the stream scale, patches, corridors,

and the background matrix are defined

within and near the channel and include elements of the stream itself and

its low floodplains (Figure 1.9). At the

next lower scale, the stream itself is segmented into reaches.

Reaches can be distinguished in a number of ways. Sometimes they are defined

by characteristics associated with flow.

High-velocity flow with rapids is obviously separable from areas with slower

flow and deep, quiet pools. In other instances practitioners find it useful to define reaches based on chemical or

biological factors, tributary confluences,

or by some human influence that

makes one part of a stream different

from the next.

Examples of patches at the stream and

reach scales might include:

■ Riffles and pools

■ Woody debris

■ Aquatic plant beds

■ Islands and point bars

Examples of corridors might include:

■ Protected areas beneath overhanging

banks.

Figure 1.8: Structural elements at a stream 

corridor scale. Patches, corridors, and matrix

are visible within the stream corridor.

ROAD CORRIDOR

CHANNEL CORRIDOR

ISLAND PATCH

URBAN MATRIX

STREAM CORRIDOR
Physical Structure and Time at Multiple Scales 1–11

■ The thalweg, the “channel within the

channel” that carries water during

low-flow conditions.

■ Lengths of stream defined by physical, chemical, and biological similarities or differences.

■ Lengths of stream defined by humanimposed boundaries such as political

borders or breaks in land use or

ownership.

Temporal Scale

The final scale concept critical for the

planning and design of stream corridor

restoration is time. 

In a sense, temporal hierarchy parallels

spatial hierarchy. Just as global or regional spatial scales are usually too

large to be relevant for most restoration

initiatives, planning and designing

restoration for broad scales of time is

not usually practical. Geomorphic or

climatic changes, for example, usually

occur over centuries to millions of

years. The goals of restoration efforts,

by comparison, are usually described in

time frames of years to decades.

Land use change in the watershed, for

example, is one of many factors that

can cause disturbances in the stream

corridor. It occurs on many time scales,

however, from a single year (e.g., crop

rotation), to decades (e.g., urbanization), to centuries (e.g., long-term forest

management). Thus, it is critical for the

practitioner to consider a relevant range

of time scales when involving land use

issues in restoration planning and design.

Flooding is another natural process that

varies both in space and through time.

Spring runoff is cyclical and therefore

fairly predictable. Large, hurricane-induced floods that inundate lands far beyond the channel are neither cyclical

nor predictable, but still should be

planned for in restoration designs.

Flood specialists rank the extent of

floods 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 Analysis

for more details.). These can serve as

guidance for planning and designing

restoration when flooding is an issue.

Practitioners of stream corridor restoration may need to simultaneously plan

in multiple time scales. If an instream

structure is planned, for example, care

might be taken that (1) installation

does not occur during a critical spawning period (a short-term consideration)

and (2) the structure can withstand a

100-year flood (a long-term consideration). The practitioner should never try

to freeze conditions as they are, at the

completion of the restoration. Stream

corridor restoration that works with the

dynamic behavior of the stream ecosystem will more likely survive the test of

time.

Figure 1.9: Structural elements at a stream

scale. Patches, corridors, and matrix are visible

within the stream.

Stream corridor restoration

that works

with the dynamic behavior

of the stream

ecosystem will

more likely

survive the

test of time.

RIFFLE PATCH

THALWEG

CORRIDOR

ALLUVIAL

DEPOSIT

PATCH

WATER MATRIX
1–12 Chapter 1: Overview of Stream Corridors

The previous section described how the

matrix-patch-corridor-mosaic model

can be applied at multiple scales to examine the relationships between the

stream corridor and its external environments. This section takes a closer

look at physical structure in the stream

corridor itself. In particular, this section

focuses on the lateral dimension. In

cross section, most stream corridors

have three major components 

(Figure 1.10):

■ Stream channel, a channel with flowing water at least part of the year.

■ Floodplain, a highly variable area on

one or both sides of the stream channel that is inundated by floodwaters

at some interval, from frequent to

rare. 

■ Transitional upland fringe, a portion of

the upland on one or both sides of

the floodplain that serves as a transitional zone or edge between the

floodplain and the surrounding landscape.

Some common features found in the

river corridor are displayed in Figure

1.11. In this example the floodplain is

seasonally inundated and includes features such as floodplain forest, emergent marshes and wet meadows. The

transitional upland fringe includes an

upland forest and a hill prairie. Landforms such as natural levees, are created

by processes of erosion and sedimentation, primarily during floods. The various plant communities possess unique

moisture tolerances and requirements

and consequently occupy distinct landforms.

Each of the three main lateral components is described in the following 

subsections. 

Stream Channel

Nearly all channels are formed, maintained, and

altered by the water and sediment they

carry. Usually they are gently rounded

in shape and roughly parabolic, but

form can vary greatly.

Figure 1.12 presents a cross section of a

typical stream channel. The sloped

bank is called a scarp. The deepest part

of the channel is called the thalweg. The

dimensions of a channel cross section

define the amount of water that can

1.B A Lateral View Across the Stream Corridor

Figure 1.10: The three major components of a

stream corridor in different settings (a) and

(b). Even though specific features might differ

by region, most stream corridors have a channel, floodplain, and transitional upland fringe.

(a)

(b)

STREAM

CHANNEL

STREAM

CHANNEL

FLOODPLAIN

FLOODPLAIN

TRANSITIONAL

UPLAND FRINGE

TRANSITIONAL

UPLAND FRINGE
A Lateral View Across the Stream Corridor 1–13

pass through without spilling over the

banks. Two attributes of the channel are

of 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 relationship

qualitatively as: 

Qs • D50 ∝ Qw • S

This equation is shown here as a

balance with sediment load on one

weighing pan and streamflow on the

other (Figure 1.13). The hook holding

the sediment pan can slide along the

horizontal arm according to sediment

size. The hook holding the streamflow

side slides according to stream slope.

Channel equilibrium occurs when all

four variables are in balance. If a change

occurs, the balance will temporarily be 

tipped and equilibrium lost. If one variable changes, one or

more of the other variables must increase or decrease

proportionally if equilibrium is to be maintained. 

For example, if slope is increased and streamflow remains

the same, either the sediment load or the size of the particles

must also increase. Likewise, if flow is increased (e.g., by

an interbasin transfer) and the slope stays the same, sediment

load or sediment particle size has to increase to maintain

channel equilibrium. A stream seeking a new equilibrium

tends 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 new

equilibrium conditions. Non-alluvial streams such as 

bedrock or artificial, concrete channels are unable to

follow Lane's relationship because of their inability to 

corridor

slough

floodplain

natural

levee

floodplain

island backwater lake shallow, open water

forestuplandmesic prairiewet meadowshallow marshshrub carrfloodplain forestdeep marshwet meadowupland forestprairiehill

high 

river 

stage

floodplain bluff lake

low river

stage

main

channel bluff

Figure 1.11: A cross section of a river corridor. The three main components of the river corridor

can be subdivided by structural features and plant communities. (Vertical scale and channel width

are 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 is

the lowest part of the channel.
1–14 Chapter 1: Overview of Stream Corridors

adjust the sediment size and quantity 

variables.

The stream balance equation is useful

for making qualitative predictions concerning channel impacts due to changes

in runoff or sediment loads from the

watershed. Quantitative predictions,

however, require the use of more complex equations.

Sediment transport equations, for example, are used to compare sediment

load and energy in the stream. If excess

energy is left over after the load is

moved, channel adjustment occurs as

the stream picks up more load by eroding its banks or scouring its bed. No

matter how much complexity is built

into these and other equations of this

type, however, they all relate back to the

basic balance relationships described by

Lane.

Streamflow

A distinguishing feature of the channel

is streamflow. As part of the water cycle,

the ultimate source of all flow is precipitation. The pathways precipitation

takes after it falls to earth, however, affect many aspects of streamflow including its quantity, quality, and timing.

Practitioners usually find it useful to divide flow into components based on

these pathways.

The two basic components are:

■ Stormflow, precipitation that reaches

the channel over a short time frame

through overland or underground

routes.

■ Baseflow, precipitation that percolates

to the ground water and moves slowly through substrate before reaching

the channel. It sustains streamflow

during periods of little or no precipitation.

FAST

FORWARD

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 of

Civil Engineers. 

Preview Chapter 2, Section B

for more discussion on the

stream balance

equation. Preview Chapter 7,

Section B for

nformation on

measuring and

analyzing these

variables and

the use of sediment transport

equations.
A Lateral View Across the Stream Corridor 1–15

Streamflow at any one time might consist of water from one or both sources.

If neither source provides water to the

channel, the stream goes dry.

A storm hydrograph is a tool used to

show how the discharge changes with

time (Figure 1.14). The portion of the

hydrograph that lies to the left of the

peak is called the rising limb, which

shows how long it takes the stream to

peak following a precipitation event.

The portion of the curve to the right of

the peak is called the recession limb.

Channel and Ground Water

Relationships

Interactions between ground water and

the channel vary throughout the watershed. In general, the connection is

strongest in streams with gravel

riverbeds in well-developed alluvial

floodplains.

Rainfall Intensity (inches/hr)

Stream Discharge (cfs)

1

Time (days)

recession

limb

time 

of rise

lag time

rising

limb

0 2 3 4

baseflow

stormflow

Figure 1.14: A storm hydrograph. A hydrograph shows how long a stream takes to rise

from baseflow to maximum discharge and then

return to baseflow conditions.

Rainfall Intensity (inches/hr)

Stream Discharge (cfs)

Time (hours)

lag time after 

urbanization

lag time before 

urbanization

Q after

Q before

runoff before

runoff after

Figure 1.15: A comparison of hydrographs

before and after urbanization. The discharge

curve is higher and steeper for urban streams

than for natural streams.

Change in Hydrology After Urbanization

The hydrology of urban streams changes as sites are cleared

and natural vegetation is replaced by impervious cover such

as rooftops, roadways, parking lots, sidewalks, and driveways.

One of the consequences is that more of a stream’s annual

flow is delivered as storm water runoff rather than baseflow.

Depending on the degree of watershed impervious cover, the

annual volume of storm water runoff can increase by up to

16 times that for natural areas (Schueler 1995). In addition,

since impervious cover prevents rainfall from infiltrating into

the soil, less flow is available to recharge ground water.

Therefore, during extended periods without rainfall, baseflow

levels are often reduced in urban streams (Simmons and

Reynolds 1982).

Storm runoff moves more rapidly over smooth, hard pavement than over natural vegetation. As a result, the rising

limbs of storm hydrographs become steeper and higher in

urbanizing areas (Figure 1.15). Recession limbs also decline

more steeply in urban streams. 
1–16 Chapter 1: Overview of Stream Corridors

Figure 1.16 presents two types of water

movement:

■ Influent or “losing” reaches lose stream

water to the aquifer.

■ Effluent or “gaining” reaches receive

discharges from the aquifer.

Practitioners categorize streams based

on the balance and timing of the stormflow and baseflow components. There

are three main categories:

■ Ephemeral streams flow only during or

immediately after periods of precipitation. They generally flow less than

30 days per year (Figure 1.17).

■ Intermittent streams flow only during

certain times of the year. Seasonal

flow in an intermittent stream usually lasts longer than 30 days per year. 

■ Perennial streams flow continuously

during both wet and dry times.

Baseflow is dependably generated

from the movement of ground water

into the channel.

Discharge Regime

Discharge is the term used to describe

the volume of water moving down the

channel per unit time (Figure 1.18).

The basic unit of measurement used in

the United States to describe discharge

is cubic foot per second (cfs).

Discharge is calculated as:

Q = A V

where:

Q = Discharge (cfs)

A = Area through which the water is

flowing in square feet 

V = Average velocity in the downstream

direction in feet per second

As discussed earlier in this section,

streamflow is one of the variables that

determine the size and shape of the

channel. There are three types of characteristic discharges:

■ Channel-forming (or dominant) discharge. If the streamflow were held

constant 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 from

the aquifer.

Figure 1.17: An ephemeral stream. Ephemeral

streams flow only during or immediately after

periods of precipitation.
A Lateral View Across the Stream Corridor 1–17

depth

width

area

velocity

discharge, it would result in channel

morphology close to the existing

channel. However, there is no

method for directly calculating

channel-forming discharge.

An estimate of channel-forming discharge for a particular stream reach

can, with some qualifications, be

related to depth, width, and shape of

channel. Although channel-forming

discharges are strictly applicable only

to channels in equilibrium, the concept can be used to select appropriate

channel geometry for restoring a disturbed 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 for

the stream in question or for one very similar. 

Since this type of data is not often available for stream restoration sites, modeled or computed

data are sometimes substituted. Effective discharge can be computed for either stable or evolving

channels. 

Bankfull discharge. This discharge occurs when water just begins to leave the channel and spread onto

the floodplain (Figure 1.19). Bankfull discharge is equivalent to channel-forming (conceptual) and

effective (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 channelforming discharge, imagine placing a

water hose discharging at constant rate

in a freshly tilled garden. Eventually, a

small channel will form and reach an

equilibrium geometry. 

At a larger scale, consider a newly

constructed floodwater- retarding

reservoir that slowly releases stored

floodwater at a constant flow rate.

This flow becomes the new channelforming discharge and will alter channel morphology until the channel

reaches equilibrium.
FAST

FORWARD

1–18 Chapter 1: Overview of Stream Corridors

Floodplain

The floor of most stream valleys is relatively flat. This is because over time the

stream moves back and forth across the

valley floor in a process called lateral

migration. In addition, periodic flooding causes sediments to move longitudinally and to be deposited on the valley

floor near the channel. These two

processes continually modify the floodplain.

Through time the channel reworks the

entire valley floor. As the channel migrates, it maintains the same average

size and shape if conditions upstream

remain constant and the channel stays

in equilibrium.

Two types of floodplains may be defined (Figure 1.20):

■ Hydrologic floodplain, the land adjacent to the baseflow channel residing

below bankfull elevation. It is inundated about two years out of three.

Not every stream corridor has a

hydrologic floodplain.

■ Topographic floodplain, the land adjacent to the channel including the

hydrologic floodplain and other

lands up to an elevation based on

the elevation reached by a flood peak

of a given frequency (for example,

the 100-year floodplain). 

Professionals involved with flooding

issues define the boundaries of a

floodplain in terms of flood frequencies. Thus, 100-year and 500-year

floodplains are commonly used in

the development of planning and

regulation standards.

Flood Storage

The floodplain provides temporary storage space for floodwaters and sediment

produced by the watershed. This attribute serves to add to the lag time of a

flood—the time between the middle of

the rainfall event and the runoff peak. 

If a stream’s capacity for moving water

and sediment is diminished, or if the

sediment loads produced from the watershed become too great for the stream

to transport, flooding will occur more

frequently and the valley floor will

begin to fill. Valley filling results in the

temporary storage of sediment produced by the watershed.

hydrologic floodplain

(bankfull width)

topographic floodplain

bankfull

elevation

Figure 1.20: Hydrologic and topographic floodplains. The hydrologic floodplain is defined by

bankfull elevation. The topographic floodplain includes the hydrologic floodplain and other lands

up to a defined elevation.

Preview Chapter 7, Section B

for a discussion

of calculating 

effective discharge. This

computation

should be performed by a

professional

with a good

background in

hydrology, hydraulics, and

sediment 

transport.
A Lateral View Across the Stream Corridor 1–19

Landforms and Deposits

Topographic features are formed on the

floodplain by the lateral migration of

the channel (Figure 1.21). These features result in varying soil and moisture

conditions and provide a variety of

habitat niches that support plant and

animal diversity.

Floodplain landforms and deposits include:

■ Meander scroll, a sediment formation

marking former channel 

locations.

■ Chute, a new channel formed across

the base of a meander. As it grows in

size, it carries more of the flow.

■ Oxbow, a term used to describe the

severed meander after a chute is

formed.

■ Clay plug, a soil deposit developed at

the intersection of the oxbow and the

new main channel. 

■ Oxbow lake, a body of water created

after clay plugs the oxbow from the

main channel.

■ Natural levees, formations built up

along the bank of some streams that

flood. As sediment-laden water spills

over the bank, the sudden loss of

depth and velocity causes coarsersized sediment to drop out of suspension and collect along the edge of

the stream.

■ Splays, delta-shaped deposits of

coarser sediments that occur when a

natural levee is breached. Natural

levees and splays can prevent floodwaters from returning to the channel

when floodwaters recede.

■ Backswamps, a term used to describe

floodplain wetlands formed by natural levees.

splay

oxbow

lake

oxbow

clay plug

chute

meander

scrolls

backswamp

natural

levee

Figure 1.21: Landforms and deposits of a floodplain. Topographic features on the floodplain

caused by meandering streams.
1–20 Chapter 1: Overview of Stream Corridors

Transitional Upland Fringe

The transitional upland fringe serves as

a transitional zone between the floodplain and surrounding landscape. Thus,

its outside boundary is also the outside

boundary of the stream corridor itself.

While stream-related hydrologic and geomorphic processes might have formed

a portion of the transitional upland

fringe in geologic times, they are not responsible for maintaining or altering its

present form. Consequently, land use

activities have the greatest potential to

impact this component of the stream

corridor.

There is no typical cross section for this

component. Transitional upland fringes

can be flat, sloping, or in some cases,

nearly vertical (Figure 1.22). They can

incorporate features such as hillslopes,

bluffs, forests, and prairies, often modified by land use. All transitional upland

fringes have one common attribute,

however: they are distinguishable from

the surrounding landscape by their

greater connection to the floodplain

and stream.

An examination of the floodplain side

of the transitional upland fringe often

reveals one or more benches. These

landforms are called terraces (Figure

1.23). They are formed in response to

new patterns of streamflow, changes in

sediment size or load, or changes in watershed base level—the elevation at the

watershed outlet.

Terrace formation can be explained

using the aforementioned stream balance equation (Figure 1.13). When one

or more variables change, equilibrium

is lost, and either degradation or aggradation occurs. 

Figure 1.24 presents an example of terrace formation by channel incision.

Cross section A represents a nonincised

channel. Due to changes in streamflow

or sediment delivery, equilibrium is lost

Figure 1.22: Transitional upland fringe. This

component of the stream corridor is a transition zone between the floodplain and the

surrounding landscape.

Figure 1.23: Terraces formed by an incising

stream. Terraces are formed in response to

new patterns of streamflow or sediment load

in the watershed.
A Lateral View Across the Stream Corridor 1–21

and the channel degrades and widens.

The original floodplain is abandoned

and becomes a terrace (cross section B).

The widening phase is completed when

a floodplain evolves within the

widened channel (cross section C). 

Geomorphologists often classify landscapes by numbering surfaces from the

lowest surface up to the highest surface.

Surface 1 in most landscapes is the bottom of the main channel. The next

highest surface, Surface 2, is the floodplain. In the case of an incising stream,

Surface 3 usually is the most recently

formed terrace, Surface 4 the next older

terrace, and so on. The numbering system thus reflects the ages of the surfaces. The higher the number, the older

the surface.

Boundaries between the numbered surfaces are usually marked by a scarp, or

relatively steep surface. The scarp between a terrace and a floodplain is especially important because it helps

confine floods to the valley floor.

Flooding occurs much less frequently, if

at all, on terraces.

Vegetation Across the 

Stream Corridor

Vegetation is an important and highly

variable element in the stream corridor.

In some minimally disturbed stream

corridors, a series of plant communities

might extend uninterrupted across the

entire corridor. The distribution of these

communities would be based on different hydrologic and soil conditions. In

smaller streams the riparian vegetation

might even form a canopy and enclose

the channel. This and other configuration possibilities are displayed in Figure

1.25.

Plant communities play a significant

role in determining stream corridor

condition, vulnerability, and potential

for (or lack of) restoration. Thus, the

type, extent and distribution, soil moisture preferences, elevation, species composition, age, vigor, and rooting depth

are all important characteristics that a

practitioner must consider when planning and designing stream corridor

restoration.

Flood-Pulse Concept

Floodplains serve as essential focal

points 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)

floodplain terrace

terrace

terrace

terrace

terrace

terrace

terrace

terrace

scarp

scarp

scarp

scarp

scarp

scarp

scarp

terrace

scarp

s carp

terrace

scarp

floodplain

floodplain

Figure 1.24: Terraces in (A) nonincised and (B

and C) incised streams. Terraces are abandoned

floodplains, formed through the interplay of

incising and floodplain widening.

FAST

FORWARD

Preview 

Chapter 2, 

Section D for

more information on plant

community

characteristics.
1–22 Chapter 1: Overview of Stream Corridors

plant communities and the wildlife

they support. Some riparian plant

species such as willows and cottonwoods depend on flooding for regeneration. Flooding also nourishes

floodplains with sediments and nutrients and provides habitat for invertebrate communities, amphibians,

reptiles, and fish spawning. 

The flood-pulse concept was developed

to summarize how the dynamic interaction between water and land is exploited by the riverine and floodplain

biota (Figure 1.26). Applicable primarily on larger rivers, the concept demonstrates that the predictable advance and

retraction of water on the floodplain in

a natural setting enhances biological

productivity and maintains diversity

(Bayley 1995).

Closed Canopy Over Channel, Floodplain,

and Transitional Upland Fringe

Open Canopy Over Channel

Figure 1.25: Examples of vegetation structure

in the stream corridor. Plant communities play

a significant role in determining the condition

and vulnerability of the stream corridor.
A Lateral View Across the Stream Corridor 1–23

Lake and river 

spawning; 

young-of-theyear and 

predators follow 

moving littoral; 

fish and 

invertebrate 

production high.

Most riverspawning 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

terrestrial

shrubs

maximum production 

of aquatic 

vegetation

decomposition of

terrestrial and older

aquatic vegetation

aquatic/terrestrial transition zone

(floodplain)

maximum biomass

of aquatic vegetation

consolidation 

of sediments

consolidation of 

sediments; 

moist soil plant

germination

low 

dissolved oxygen

flood-tolerant trees

annual

terrestrial

grasses

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 a

floodplain in five snapshots of an annual hydrological cycle. The left column describes the

movement 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.
1–24 Chapter 1: Overview of Stream Corridors

The processes that develop the characteristic structure seen in the lateral view

of a stream corridor also influence

structure in the longitudinal view.

Channel width and depth increase

downstream due to increasing drainage

area and discharge. Related structural

changes also occur in the channel,

floodplain, and transitional upland

fringe, and in processes such as erosion

and deposition. Even among different

types of streams, a common sequence

of structural changes is observable from

headwaters to mouth.

Longitudinal Zones

The overall longitudinal profile of most

streams can be roughly divided into

three zones (Schumm 1977). Some of

the changes in the zones are characterized in Figures 1.27 and 1.28.

Zone 1, or headwaters, often has the

steepest gradient. Sediment erodes from

slopes of the watershed and moves

downstream. Zone 2, the transfer zone,

receives some of the eroded material. It

is usually characterized by wide floodplains and meandering channel patterns. The gradient flattens in Zone 3,

the primary depositional zone. Though

the figure displays headwaters as mountain streams, these general patterns and

changes are also often applicable to watersheds with relatively small topographic relief from the headwaters to

mouth. It is important to note that erosion, transfer, and deposition occur in

all zones, but the zone concept focuses

on the most dominant process.

Watershed Forms

All watersheds share a common definition: a watershed is an “area of land that

1.C A Longitudinal View Along the Stream

Corridor

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 riverborne 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 1

Headwaters

Zone 2

Transfer Zone

Zone 3

Depositional Zone

Figure 1.27: Three longitudinal profile zones. Channel and floodplain characteristics change as

rivers 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 dissolved

materials to a common outlet at some

point along a stream channel” (Dunne

and Leopold 1978). Form varies greatly,

however, and is tied to many factors

including climatic regime, underlying

geology, morphology, soils, and vegetation.

Drainage Patterns

One distinctive aspect of a watershed

when observed in planform (map view)

is its drainage pattern (Figure 1.29).

Drainage patterns are primarily controlled by the overall topography and

underlying geologic structure of the

watershed.

Stream Ordering

A method of classifying, or ordering,

the hierarchy of natural channels within

a watershed was developed by Horton

(1945). Several modifications of the

original stream ordering scheme have

relative

volume

of stored alluvium

channel width

slope

bed

material grain

size

mean flow velocity

characteristic stream discharge

channel depth

Drainage Area ( downstream distance2)

Increase

Headwaters Transfer Deposition

Figure 1.28: Changes in the channel in the three zones. Flow, channel size, and sediment

characteristics change throughout the longitudinal profile.
1–26 Chapter 1: Overview of Stream Corridors

been proposed, but the modified system of Strahler (1957) is probably the

most popular today.

Strahler’s stream ordering system is portrayed in Figure 1.30. The uppermost

channels in a drainage network (i.e.,

headwater channels with no upstream

tributaries) are designated as first-order

streams down to their first confluence.

A second-order stream is formed below

the confluence of two first-order channels. Third-order streams are created

when two second-order channels join,

and so on. Note in the figure that the

intersection of a channel with another

channel of lower order does not raise

the order of the stream below the intersection (e.g., a fourth-order stream intersecting with a second-order stream is

still a fourth-order stream below the intersection). 

Within a given drainage basin, stream

order correlates well with other basin

parameters, such as drainage area or

channel length. Consequently, knowing

what order a stream is can provide clues

concerning other characteristics such as

which longitudinal zone it resides in

and relative channel size and depth.

Channel Form

The form of the channel can change as

it moves through the three longitudinal

zones. Channel form is typically described by two characteristics—thread

(single or multiple) and sinuosity.

Single- and Multiple-Thread

Streams

Single-thread (one-channel) streams are

most common, but multiple-thread

streams occur in some landscapes (Figure 1.31). Multiple-thread streams are

further categorized as either braided or

anastomosed streams.

Dendritic Parallel

Trellis Rectangular

Radial Annular 

Multi-Basinal Contorted

dry

Figure 1.29: Watershed drainage patterns.

Patterns are determined by topography and

geologic structure.

Source: A.D. Howard, AAPG © 1967, reprinted by

permission of the American Association of Petroleum

Geologists.

4

4 3

3 2

2

2

1

1

1

1

1

1

1

1

3

2

2

2

1

1

1

1

1

1

1

1 1

Figure 1.30: Stream ordering in a drainage network. Stream ordering is a method of classifying the hierarchy of natural channels in a

watershed.
A Longitudinal View Along the Stream Corridor 1–27

Three conditions tend to promote the

formation of braided streams:

■ Erodible banks.

■ An abundance of coarse sediment.

■ Rapid and frequent variations in discharge.

Braided streams typically get their start

when a central sediment bar begins to

form in a channel due to reduced

streamflow or an increase in sediment

load. The central bar causes water to

flow into the two smaller cross sections

on either side. The smaller cross section

results in a higher velocity flow. Given

erodible banks, this causes the channels

to widen. As they do this, flow velocity

decreases, which allows another central

bar to form. The process is then repeated and more channels are created.

In landscapes where braided streams

occur naturally, the plant and animal

communities have adapted to frequent

and rapid changes in the channel and

riparian area. In cases where disturbances trigger the braiding process,

however, physical conditions might be

too dynamic for many species.

The second, less common category of

multiple-thread channels is called anastomosed streams. They occur on much

flatter gradients than braided streams

and have channels that are narrow and

deep (as opposed to the wide, shallow

channels found in braided streams).

Their banks are typically made up of

fine, cohesive sediments, making them

relatively erosion-resistant.

Anastomosed streams form when the

downstream base level rises, causing a

rapid buildup of sediment. Since bank

materials are not easily erodible, the

original single-thread stream breaks up

into multiple channels. Streams entering

deltas in a lake or bay are often anastomosed. Streams on alluvial fans, in contrast, can be braided or anastomosed. 

Sinuosity

Natural channels are rarely straight.

Sinuosity is a term indicating the

amount of curvature in the channel

(Figure 1.32). The sinuosity of a reach

is computed by dividing the channel

Figure 1.31: (a) Single-thread and (b) braided

streams. Single-thread streams are most 

common. Braided streams are uncommon and

usually formed in response to erodible banks,

an abundance of coarse sediment, and rapid

and frequent variations in discharge.

(a)

(b)
1–28 Chapter 1: Overview of Stream Corridors

centerline length by the length of

the valley centerline. If the channel

length/valley length ratio is more

than about 1.3, the stream can be

considered meandering in form.

Sinuosity is generally related to the

product of discharge and gradient.

Low to moderate levels of sinuosity are

typically found in Zones 1 and 2 of the

longitudinal profile. Extremely sinuous

streams often occur in the broad, flat

valleys of Zone 3.

Pools and Riffles

No matter the channel form, most

streams share a similar attribute of alternating, regularly spaced, deep and

shallow areas called pools and riffles

(Figure 1.33). The pools and riffles are

associated with the thalweg, which meanders within the channel. Pools typically form in the thalweg near the

outside bank of bends. Riffle areas usually form between two bends at the

point where the thalweg crosses over

from one side of the channel to the

other.

The makeup of the streambed plays

a role in determining pool and riffle

characteristics. Gravel and cobble-bed

streams typically have regularly spaced

pools and riffles that help maintain

channel stability in a high-energy environment. Coarser sediment particles

are found in riffle areas while smaller

particles occur in pools. The pool-topool or riffle-to-riffle spacing is normally about 5 to 7 times the channel

width at bankfull discharge (Leopold

et al. 1964). 

Sand-bed streams, on the other hand,

do not form true riffles since the grain

size distribution in the riffle area is similar to that in the pools. However, sandbed streams do have evenly spaced

pools. High-gradient streams also usually have pools but not riffles, but for a

different reason. In this case, water

moves from pool to pool in a stairstep

fashion.

Figure 1.32: Sinuosity: (a) low and (b) extreme.

Low to moderately sinuous streams are usually

found in Zones 1 and 2 of the longitudinal profile. Extremely sinuous streams are more typical

of Zone 3.

(b)

(a)
A Longitudinal View Along the Stream Corridor 1–29

Vegetation Along the Stream

Corridor

Vegetation is an important and highly

variable element in the longitudinal as

well as the lateral view. Floodplains are

narrow or nonexistent in Zone 1 of the

longitudinal profile; thus flood-dependent or tolerant plant communities

tend to be limited in distribution. Upland plant communities, such as forests

on moderate to steep slopes in the eastern or northwestern United States,

might come close to bordering the

stream and create a canopy that leaves

little open sky visible from the channel.

In other parts of the country, headwaters in flatter terrain may support plant

communities dominated by grasses and

broad-leaved herbs, shrubs, or planted

vegetation.

Despite the variation in plant community type, many headwaters areas provide organic matter from vegetation

along with the sediment they export to

Zones 2 and 3 downstream. For example, logs and woody debris from headwaters forests are among the most

ecologically important features supporting food chains and instream habitat

structure in Pacific Northwest rivers

from the mountains to the sea (Maser

and Sedell 1994).

Zone 2 has a wider and more complex

floodplain and larger channel than

Zone 1. Plant communities associated

with floodplains at different elevations

might vary due to differences in soil

type, flooding frequency, and soil moisture. Localized differences in erosion

and deposition of sediment add complexity and diversity to the types of

plant communities that become 

established. 

The lower gradient, larger stream size,

and less steep terrain in Zone 2 often

attract more agricultural or residential

development than in the headwaters

zone. This phenomenon frequently

counteracts the natural tendency to develop broad and diverse stream corridor

plant communities in the middle and

lower reaches. This is especially true

when land uses involve clearing the 

native vegetation and narrowing the

corridor. 

Often, a native plant community is replaced by a planted vegetation community such as agricultural crops or

residential lawns. In such cases, stream

processes involving flooding,

erosion/deposition, import or export of

organic matter and sediment, stream

corridor habitat diversity, and water

quality characteristics are usually significantly altered.

The lower gradient, increased sediment

deposition, broader floodplains, and

greater water volume in Zone 3 all set

the stage for plant communities different from those found in either upstream zone. Large floodplain wetlands

become prevalent because of the generally flatter terrain. Highly productive

and diverse biological communities,

(a)

(b)

riffle

pool

thalweg 

line

pool

riffle 

or cross over

Figure 1.33: Sequence of pools and riffles in

(a) straight and (b) sinuous streams. Pools

typically form on the outside bank of bends

and riffles in the straight portion of the channel where the thalweg crosses over from one

side to the other.
1–30 Chapter 1: Overview of Stream Corridors

such as bottomland hardwoods, establish themselves in the deep, rich alluvial

soils of the floodplain. The slower flow

in the channel also allows emergent

marsh vegetation, rooted floating or

free-floating plants, and submerged

aquatic beds to thrive. 

The changing sequence of plant communities along streams from source to

mouth is an important source of biodiversity and resiliency to change. Although many, or perhaps most, of a

stream corridor’s plant communities

might be fragmented, a continuous corridor of native plant communities is desirable. Restoring vegetative connectivity

in even a portion of a stream will usually improve conditions and increase its

beneficial functions.

The River Continuum Concept 

The River Continuum Concept is an attempt to generalize and explain longitudinal changes in stream ecosystems

(Figure 1.34) (Vannote et al. 1980).

This conceptual model not only helps

to identify connections between the watershed, floodplain, and stream systems,

but it also describes how biological

communities develop and change from

the headwaters to the mouth. The River

Continuum Concept can place a site or

reach in context within a larger watershed or landscape and thus help practitioners define and focus restoration

goals.

The River Continuum Concept hypothesizes that many first- to third-order

headwater streams are shaded by the riparian forest canopy. This shading, in

turn, limits the growth of algae, periphyton, and other aquatic plants. Since

energy cannot be created through photosynthesis (autotrophic production),

the aquatic biota in these small streams

is dependent on allochthonous materials

(i.e., materials coming from outside the

channel such as leaves and twigs).

Biological communities are uniquely

adapted to use externally derived organic inputs. Consequently, these

headwater streams are considered

heterotrophic (i.e., dependent on the

energy produced in the surrounding

watershed). Temperature regimes are

also relatively stable due to the influence of ground water recharge, which

tends to reduce biological diversity to

those species with relatively narrow

thermal niches.

Predictable changes occur as one proceeds downstream to fourth-, fifth-,

and sixth-order streams. The channel

widens, which increases the amount

of incident sunlight and average temperatures. Levels of primary production

increase in response to increases in

light, which shifts many streams to a

dependence on autochthonous materials

(i.e., materials coming from inside

the channel), or internal autotrophic

production (Minshall 1978). 

In addition, smaller, preprocessed organic particles are received from upstream sections, which serves to balance

autotrophy and heterotrophy within the

stream. Species richness of the invertebrate community increases as a variety

of new habitat and food resources appear. Invertebrate functional groups,

such as the grazers and collectors, increase in abundance as they adapt to

using both autochthonous and allochthonous food resources. Midsized

streams also decrease in thermal stability as temperature fluctuations increase,

which further tends to increase biotic

diversity by increasing the number of

thermal niches.

Larger streams and rivers of seventh to

twelfth order tend to increase in physical stability, but undergo significant

changes in structure and biological function. Larger streams develop increased

reliance on primary productivity by
A Longitudinal View Along the Stream Corridor 1–31 Stream Size (order)

Relative Channel Width

12

11

10

9

8

7

6

5

4

3

2

1

collectors

collectors

collectors

predators

predators

trout

periphyton

vascular

hydrophytes

smallmouth

bass

perch periphyton

phytoplankton

zooplankton

catfish

shredders

grazers

grazers

coarse

particulate fine matter

particulate

matter

fine

particulate

matter

coarse

particulate

matter

fine

particulate

matter

shredders

predators

microbes

microbes

microbes

Figure 1.34: The River Continuum Concept. The concept proposes a relationship between

stream size and the progressive shift in structural and functional attributes.

Source: Vannote et al. (1980). Published with the permission of NRC Research Press.
1–32 Chapter 1: Overview of Stream Corridors

phytoplankton, but continue to receive

heavy inputs of dissolved and ultra-fine

organic particles from upstream. Invertebrate populations are dominated by

fine-particle collectors, including zooplankton. Large streams frequently carry

increased loads of clays and fine silts,

which increase turbidity, decrease light

penetration, and thus increase the significance of heterotrophic processes.

The influence of storm events and thermal fluctuations decrease in frequency

and magnitude, which increases the

overall physical stability of the stream.

This stability increases the strength of

biological interactions, such as competition and predation, which tends to

eliminate less competitive taxa and

thereby reduce species richness. 

The fact that the River Continuum Concept applies only to perennial streams is

a limitation. Another limitation is that

disturbances and their impacts on the

river continuum are not addressed by

the model. Disturbances can disrupt the

connections between the watershed and

its streams and the river continuum as

well.

The River Continuum Concept has not

received universal acceptance due to

these and other reasons (Statzner and

Higler 1985, Junk et al. 1989). Nevertheless, it has served as a useful conceptual model and stimulated much

research since it was first introduced

in 1980.