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A Stream Evolution Model Integrating Habitat and Ecosystem Benefits

February 2014
River Research and Applications 30(2)

DOI:10.1002/rra.2631

Authors:

Brian Cluer

National Oceanic and Atmospheric Administration

Colin R. Thorne

University of Nottingham

For decades, Channel Evolution Models have provided useful templates for understanding morphological responses to disturbance associated with lowering base level, channelization or alterations to the flow and/or sediment regimes. In this paper, two well‐established Channel Evolution Models are revisited and updated in light of recent research and practical experience. The proposed Stream Evolution Model includes a precursor stage, which recognizes that streams may naturally be multi‐threaded prior to disturbance, and represents stream evolution as a cyclical, rather than linear, phenomenon, recognizing an evolutionary cycle within which streams advance through the common sequence, skip some stages entirely, recover to a previous stage or even repeat parts of the evolutionary cycle.

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A STREAM EVOLUTION MODEL INTEGRATING HABITAT AND

ECOSYSTEM BENEFITS

B. CLUER

*AND C. THORNE

Fluvial Geomorphologist, Southwest Region, NOAA’s National Marine Fisheries Service, Santa Rosa, California, USA

Chair of Physical Geography, University of Nottingham, Nottingham, UK

ABSTRACT

For decades, Channel Evolution Models have provided useful templates for understanding morphological responses to disturbance associated

with lowering base level, channelization or alterations to the ﬂow and/or sediment regimes. In this paper, two well-established Channel

Evolution Models are revisited and updated in light of recent research and practical experience. The proposed Stream Evolution Model

includes a precursor stage, which recognizes that streams may naturally be multi-threaded prior to disturbance, and represents stream

evolution as a cyclical, rather than linear, phenomenon, recognizing an evolutionary cycle within which streams advance through the common

sequence, skip some stages entirely, recover to a previous stage or even repeat parts of the evolutionary cycle.

The hydrologic, hydraulic, morphological and vegetative attributes of the stream during each evolutionary stage provide varying ranges

and qualities of habitat and ecosystem beneﬁts. The authors’personal experience was combined with information gleaned from recent

literature to construct a ﬂuvial habitat scoring scheme that distinguishes the relative, and substantial differences in, ecological values of

different evolutionary stages. Consideration of the links between stream evolution and ecosystem services leads to improved understanding

of the ecological status of contemporary, managed rivers compared with their historical, unmanaged counterparts. The potential utility of the

Stream Evolution Model, with its interpretation of habitat and ecosystem beneﬁts includes improved river management decision making with

respect to future capital investment not only in aquatic, riparian and ﬂoodplain conservation and restoration but also in interventions intended

key words: Stream Evolution Model (SEM); channel evolution; freshwater ecology; habitat; conservation; river management; restoration; climate resilience

Received 1 November 2012; Accepted 13 November 2012

INTRODUCTION

It is now generally accepted that river engineering and

management that works with rather than against natural pro-

cesses is more likely to attain and sustain the multi-functional

goals (e.g. land drainage, ﬂood risk management, ﬁsheries,

conservation, biodiversity, and recreation) demanded by local

stakeholders and society more widely (Wohl et al., 2005;

Thorne et al., 2010). This, coupled with growing recognition

that the range and value of ecosystem services provided by

rivers increase with the degree to which they are allowed to

function naturally, fuels the drive for restoration of ﬂuvial

systems degraded by past management and engineering

actions that have proven, in the long term, to be unsustainable

(Palmer et al., 2005).

However, complete restoration of a river to some former

condition is seldom possible, nor always desirable (Downs

and Gregory, 2004), and deciding whether partial restor-

ation, rehabilitation or environmental enhancement is the

best way to treat a damaged stream raises fundamental ques-

tions for river managers responsible for achieving increased

biodiversity or the protection and recovery of endangered

species. Speciﬁcally, serious questions arise concerning the

nature of the pre-disturbance condition to which a given

river should be restored, the likely sequence (and habitat

impacts) of channel adjustments associated with post-pro-

ject evolution and the merits of restoring the river to some

former condition rather than facilitating, or even enhancing,

its progression to a conﬁguration that is, ﬁrst, better adjusted

to the prevailing hydrological and sediment regimes and,

second, more resilient to the unavoidable impacts of future

climate change and/or land use.

In this paper, these questions are addressed by

1. revisiting well-established Channel Evolution Models

(CEMs) for streams that respond to disturbance through

incision,

2. updating these CEMs in light of recent research, including

that on pre-disturbance channel forms in Europe and North

America, to propose a more broadly based Stream Evolu-

tion Model (SEM),

3. linking the evolutionary stages of stream adjustment to

indicators of habitat and lotic ecosystem beneﬁts and

*Correspondence to: B. Cluer, Fluvial Geomorphologist, Southwest

Region, NOAA’s National Marine Fisheries Service, 777 Sonoma Ave.,

Suite 325, Santa Rosa, California 95404, USA.

E-mail: brian.cluer@noaa.gov

RIVER RESEARCH AND APPLICATIONS

River Res. Applic. 30: 135–154 (2014)

Published online 10 January 2013 in Wiley Online Library

(wileyonlinelibrary.com) DOI: 10.1002/rra.2631

4. considering how the SEM linked to ecosystem beneﬁts

might be used to better understand, strategically manage

and sustainably restore freshwater aquatic systems.

Channel evolution models help us conceptualize how

single-thread alluvial channels may respond to disturbances,

through a series of morphological adjustments, which can

be generalized into an evolutionary sequence common to

streams in different physiographic settings. On this basis,

past evolutionary changes can be explained and future ones

predicted through space-for-time substitution within the

affected ﬂuvial system. The utility of CEMs (originating

from Schumm et al., 1984; Simon and Hupp, 1986) to

inform interventions for managing the impacts of channel

instability endures, and subsequent authors have expanded

the concept (e.g. Doyle and Shields, 2000; Simon and

Darby, 2002; Beechie et al., 2008; Hawley et al., 2011). In

contrast, there has been little application and even less evalu-

ation of CEMs in the contexts of aquatic conservation and

ecologically led river restoration. An unintended conse-

quence of the broad acceptance of CEMs as conceptual mod-

els for alluvial stream behaviour has been to help perpetuate

the assumption that a single-thread, meandering channel

represents the natural conﬁguration of a dynamically stable

alluvial stream and that this, consequently, represents a

universally appropriate target morphology for restoration

(see Kondolf (2009) for extended discussion of this point).

Reﬂecting on the history of human land use suggests

that we should not be surprised that single-thread channels

predominate in more economically developed countries since

the late 19th and early 20th centuries. By then, anthropogenic

disturbance of many multi-channel systems had already

triggered widespread channel metamorphosis into single-

thread conﬁgurations (Marston et al., 1995; Surian and

Rinaldi, 2003). Actually, channel transformations to simpler

conﬁned forms were the speciﬁc intentions of many early

settlement river management measures. Manipulating a

multi-channel reach into a single-threaded channel not only

improved waterway commerce but also enhanced drainage,

opened bottom land for agriculture, facilitated construction

of small dams for water abstraction or hydropower and

allowed building of fewer, shorter bridges.

In the USA, beginning two centuries ago, ﬂoodplain

wetland complexes were systematically drained and devel-

oped, the transformative engineering supported by public

programmes (e.g. the Swamp Land Act of 1850) as a means

of ceding ‘waste’lands to the States. Two hundred thousand

miles of streams were systematically channelized or embanked

into single-thread conﬁgurations that were deeper, simpler and

narrower (Schoof, 1980). These approaches to wetland, ﬂood-

plain and stream management prevailed in the USA until

the late 20th century (Dahl and Allord, 1996), when wetland

restoration began (Lewis, 2001). Centuries earlier, similar

wetland and river management had begun in Europe

(Brookes, 1988). The outcome is that most streams in the

global North currently have channel forms and relations to

their ﬂoodplains that are the legacy of a century or more of

systematic manipulation and inadvertent impacts on channel

processes and fresh water ecology (Brown and Sear, 2008).

Recognizing that the single-thread channel is perhaps not

necessarily the ‘natural channel’that restoration would seek

to emulate, Montgomery (2008, p.292) stated that ‘[T]he

ﬁrst step in a river-restoration program should instead be

to develop a solid understanding of what the targeted rivers

were actually like before the changes that restorationists

seek to undo or mitigate’. Indeed, ﬂuvial geomorphologists

have for some time questioned the notion that stable,

equilibrium channel forms exist at all (Phillips, 1992;

1999; 2009). Similarly, recognition of the ecological bene-

ﬁts of frequent and prolonged ﬂoodplain inundation, driven

by ﬂooding several times a year, has initiated the discour-

aging of 2-year (bankfull) ﬂow event as the prime design

discharge for stream restoration in Europe (Habersack and

Piégay, 2008) and the USA (Doyle et al., 2007).

Accepting that a multi-channel conﬁguration and increased

ﬂoodplain inundation better represent the pre-disturbance

condition of many alluvial streams, it may be argued that the

CEMs could be extended by including a precursor stage.

Recognition that multi-threaded channels and ﬂoodplains

inundated several times per year may provide a great range

of more valuable habitats, and so represent a valid design

template for restoration, suggests that links between evolu-

tionary stage and habitat attributes could be explored. Also,

the ecological values provided by streams during different

evolutionary stages need to be properly evaluated to facilitate

river management and restoration decision making that is led

ecologically, rather than morphologically.

FRAMEWORKS FOR UNDERSTANDING

STREAM EVOLUTION

Review of existing channel evolution models

Morphological response to disturbance that involves

channel incision may be considered in two dimensions: verti-

cal adjustment involving degradation and aggradation of the

bed and lateral adjustment involving retreat and advance of

the banks (Little et al., 1981; Thorne et al., 1981). Vertical

adjustments dominate initial responses driven by erosion

and lowering of the bed until the banks become unstable,

whereas lateral adjustment dominates as geotechnical bank

failures and toe scour result in widening. Eventually, the

width of the unstable channel becomes sufﬁciently large

that near-bank ﬂows lose their competence to entrain and

remove failed bank material, so that channel width ﬁrst

stabilizes and then decreases as slumped bank material builds

bank toe benches and berms at one or both margins. Provided

B. CLUER AND C. THORNE136

DOI: 10.1002/rra

that no further disturbance occurs, the channel recovers a

dynamically meta-stable form when its banks and berms

stabilize and the energy slope adjusts to match local sediment

transport capacity to the supply of sediment from upstream

(Simon and Thorne, 1996).

During the 1980s, identiﬁcation that morphological

response is usually characterized by bed degradation followed

by bank collapse, widening and eventual stabilization led to

the formulation of a generalized CEM by Schumm et al.

(1984). The ﬁve-stage model of Schumm et al.isbasedon

ﬁeld monitoring of unstable streams in North Mississippi,

and a space-for-time substitution that uses observations made

simultaneously along the stream to indicate how channel

changes at a given cross-section would occur through time if

the reach were considered systematically (Figure 1). Simon

and Hupp’s (1986) six-stage model adaptation (Figure 2)

was based on post-disturbance evolution of channelized

streams in West Tennessee, although it has subsequently been

shown also to apply in a wide variety of physiographic

settings (Simon and Thorne, 1996).

The most obvious difference between the ﬁve-stage and

six-stage CEMs is that Simon and Hupp include a

Figure 1. Schumm et al. (1984) Channel Evolution Model with typical width–depth ratios (F). The size of each arrow indicates the relative

importance and direction of the dominant processes of degradation, aggradation and lateral bank erosion. (Redrawn with permission from

Water Resources Publications)

Figure 2. Simon and Hupp’s (1986) Channel Evolution Model. [Adapted from Simon and Hupp (1986).]

SEM INCORPORATING HABITAT AND ECOSYSTEM BENEFITS 137

DOI: 10.1002/rra

‘constructed’stage between the ‘pre-modiﬁed’and ‘degrad-

ation’stages of Schumm et al. This stems from the common

channelization, straightening and re-sectioning of streams in

their study area. Hence, Stage III in Simon and Hupp’s

model corresponds to Stage II in the model of Schumm

et al. in representing a condition where the channel is

degrading, but bed lowering has not yet increased bank

height sufﬁciently to trigger instability (Little et al., 1981;

Thorne et al., 1981). A second difference is that bed scour

continues in Stage IV of Simon and Hupp’s model even

though the banks are retreating because of geotechnical

failure, simultaneously producing channel degradation

and widening. This contrasts with the equivalent Stage III

in the model of Schumm et al., which indicates that the

bed elevation starts to aggrade once widening commences.

The third difference between the CEMs is the greater

emphasis placed on the inﬂuence of bank and riparian vege-

tation processes in Simon and Hupp’s model; an emphasis

subsequently validated by ﬁeld research that established

the effectiveness of vegetation as a ‘riparian engineer’

(Gurnell and Petts, 2006).

In the years following formulation of these CEMs, many

of the incised channels from which the models were derived

tended to stabilize as a result of natural recovery, assisted in

many places by engineering stabilization (Simon and Darby,

2002). Despite this, evidence from long-term monitoring of

late-stage evolution in these streams has revealed that

signiﬁcant changes to channel morphology continue beyond

the end-stages in the original CEMs, through increases in

sinuosity and roughness coupled with reductions in sedi-

ment load and mobility. Thorne (1999) reported late-stage

morphological evolution featuring closing of back channels,

invasion of sloughs and bar tops by vegetation, adoption of

a sinuous path by the regime channel established during

Stages V/VI and renewed bank retreat along the outer

margins of developing meander bends in the evolving

channel planform (Figure 3). This may partly explain why

sediment concentrations have remained stubbornly high in

many streams deemed, according to the established CEMs,

to have recovered from incision (Shields, 2009). These

late-stage evolutionary changes are widely observed, and

Thorne (1999, page 118) proposed that ‘an additional stage

(Stage VI/VII) be added to existing CEMs to account for

late-stage incised channel evolution from straight or braided

to meandering’.

It is timely to further revise CEMs in two important

respects. The ﬁrst stems from consideration of extended

histories of channel adjustment unavailable in the 1980s,

which indicate that late-stage evolution may involve adjust-

ments to channel planform not included in the existing

models. The second arises because recent reconstruction of

past ﬂuvial environments based on the age and stratigraphy

of valley-ﬁll deposits in Europe and the Eastern USA

challenges the general assumption that alluvial streams

were predominately single threaded in their ‘natural’,pre-

disturbance condition.

Physical evidence for precursor and successor stages

Walter and Merrits (2008) and Merritts et al. (2011)

established that from the late 17th to early 20th centuries,

settlement by Europeans altered streams throughout the

Eastern USA through forest clearance (that increased ﬂow

and sediment yields) and the widespread construction of

low (3–5 m high) but valley-wide mill dams, each of which

created shallow reservoirs that inundated wetlands and

deposited sediment, obscuring the pre-existing anastomosed

channel networks. Once timber resources were depleted,

agriculture dominated and other power sources had been

developed, these mill dams were abandoned. They subse-

quently failed, and channel incision into the post-settlement,

valley-ﬁll deposits created single-threaded channels.

Walter and Merrits (2008) concluded that ‘The current

condition of single gravel-bedded channels with high, ﬁne-

grained banks and relatively dry valley-ﬂat surfaces

disconnected from groundwater is in stark contrast to the

pre-settlement condition of swampy meadows (shrub-scrub)

and shallow anabranching streams’(p.303), leading them to

propose that seminal geomorphic studies including those

performed by Leopold and Maddock (1953), Wolman

Figure 3. Late-stage morphological evolution involving development of cross-sectional asymmetry and planform sinuosity through closing of

back channels, invasion of sloughs and bar tops by vegetation, adoption of a sinuous path by the regime channel established in Stages V/VI

and renewed bank retreat along the outer margins of developing bends in the channel planform (modiﬁed from Thorne, 1999)

B. CLUER AND C. THORNE138

DOI: 10.1002/rra

(1955) and Wolman and Leopold (1957), which established

relationships between the dominant discharge, channel form

and ﬂoodplain building processes, were in fact based on

channel and ﬂoodplain morphologies that were the products

of prior anthropogenic disturbance.

Although controversial (e.g. Hupp et al., 2013), these

ﬁndings are neither particularly new nor unique to the East-

ern USA. Paleohydrological studies in Europe have estab-

lished that channel–ﬂoodplain associations once thought to

be ‘natural’actually represent the outcomes of accelerated

sediment production and deposition that buried multi-

threaded woodland stream systems (Harwood and Brown,

1993; Sear and Arnell, 2006; Brown and Sear, 2008). In

the Paciﬁc Northwest (Collins et al., 2003; Pollock et al.,

2003; Montgomery, 2004) and Intermountain (Woelﬂe-Ers-

kine et al., 2012) regions of the Western USA, several

authors demonstrate that multi-threaded networks of branch-

ing streams and connected wetlands were common prior to

European settlement, where single-thread channels and rela-

tively dry ﬂoodplains currently occupy ‘intact’alluvial

valleys.

Similarly, historical reconstructions of valley sediments

throughout the California Coastal Range show that branching

stream channels (Tompkins, 2006) and wetlands (Grossinger

et al., 2007) were commonplace in basins prior to European

settlement and land management for drainage and ﬂood

control. It is important to note that the Mediterranean

climate of this region resulted in seasonal drying of some

pre-disturbance branching channel systems, but Kondolf and

Tompkins (2008) ascertained that these still provided richer

aquatic habitat than the post-disturbance regulated and

contracted perennial single-thread channels that replaced them.

The remaining global extent of network channels is concen-

trated in less developed countries and to stream systems where

the scale is too large (e.g. Okavango Delta, Sudd Swamp in

the Nile basin and Florida Everglades) or the location too

remote (e.g. glacial outwash plains and mountain meadows)

for river and ﬂoodplain management to be effective in elimin-

ating them. Recent ﬁeld research in light of conceptual CEMs

prompts the consideration that the single-thread conﬁguration

taken to represent the initial, undisturbed morphology of an

alluvial stream may have actually evolved from earlier, truly

pre-disturbance, multi-channel morphologies that were not

only more extensive and complex but also provided greater

diversity and richer habitats and ecosystem functions. There

is, thus, a case for adding a precursor branching stage to the

existing CEMs and for integrating habitat and ecosystem

beneﬁts into the model framework.

The stream evolution model

In light of the issues and arguments set out above, we devel-

oped a SEM by combining the stages featured in the original

CEM models (Schumm et al. (1984); Simon and Hupp

(1986)), inserting a precursor stage to better represent pre-

disturbance conditions and adding two successor stages to

cover late-stage evolutionary changes missing from the ori-

ginal models (Table I). We also represent channel evolution

in the SEM as a cyclical rather than a linear sequence. This

modiﬁcation stems from the fact that early models represent

channel response as progressing linearly through a sequence

of stages (see Figures 1 and 2) whereas evidence from the

stratigraphy of Holocene valley-ﬁll deposits and ﬁeld moni-

toring of changes in contemporary, incised channels indi-

cates that evolution in disturbed ﬂuvial systems is cyclical

(e.g. Hawley et al., 2011). A common criticism of the ori-

ginal CEMs is that it is rare for a stream to exhibit all of

the stages in the model and even rarer for them to occur in

the indicated sequence. For example, a reach may experi-

ence repeated episodes of incision and rapid widening

(Stages II and III of Schumm et al. or Simon and Hupp’s

Stages III and IV) without recovering any of the lost bed ele-

vation through intervening episodes of aggradation (Bledsoe

et al., 2007). We suggest that these and other behaviours

could be better represented as non-linear responses and

‘short-circuits’in a cyclical SEM, as illustrated in Figure 4.

STREAM EVOLUTION MODEL STAGE LINKED TO

HABITAT AND ECOSYSTEM BENEFITS

Background and approach

Stream morphology interacts with the ﬂow and sediment

regimes (discharge, seasonality and variability), channel

boundary characteristics (bed sediments, bank materials and

vegetation) and water quality (temperature, turbidity, nutrients

and pollutants) to produce, maintain and renew habitat at a

range of spatial and temporal scales. The potential for a stream

to support resilient and diverse ecosystems generally increases

with its morphological diversity, although restoration of lost

diversity does not guarantee recovery of any particular target

or iconic species, which may be limited by factors unrelated

to stream morphology (Palmer et al., 2005).

It follows that the morphological adjustments experienced

by unstable, incising streams have serious implications for

the diversity and richness of habitat and ecosystem services

it can provide. Despite this, no attempt has been made, to

date, to identify and evaluate the habitat and ecosystem

beneﬁts associated with evolutionary stage. To address this

gap in knowledge, we performed a systematic exploration

of links between the physical and vegetative attributes of

the stream and the habitat and ecosystem beneﬁts it provides

for the eight stages in the SEM. Streams were assessed per

stage on the basis of the authors’interpretations of processes

and physical attributes coupled with assessment of infor-

mation compiled from published relationships between

SEM INCORPORATING HABITAT AND ECOSYSTEM BENEFITS 139

DOI: 10.1002/rra

Table I. Previous Channel Evolution Models and the proposed Stream Evolution Model with description of reach-average characteristics, or stages

B. CLUER AND C. THORNE140

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stream attributes, functional habitats and freshwater ecology

(e.g. Harper et al., 1995; Padmore, 1997; Newson and

Newson, 2000), and synthesis of newly available knowledge

gained from recent publications that have established

linkages between ecosystem functions and common stream

types (e.g. Thorp et al., 2010).

In this evaluation, the physical attributes considered for

the stream were hydrologic, hydraulic and geomorphic.

Such is the importance and inﬂuence of vegetation that it

was dealt with as a separate attribute of the stream environ-

ment. On the basis of the physical and vegetative attributes

of the stream, habitat and ecosystem beneﬁts were evaluated

in terms of habitat, biota, resilience and persistence, and

water quality. The bases on which the evaluations were

performed are described in the following sections, with details

of each stage’s unique attributes listed in Tables II and III.

Hydrologic regime

The hydrological regime is crucial to creating and maintain-

ing morphological diversity and supporting ecological

integrity, underscoring its signiﬁcance to channel change.

All elements of the regime are important, ranging from base

ﬂows (and periods of zero discharge in ephemeral streams)

to ﬂood events that provide the ‘ﬂood pulse advantage’

(Junk et al., 1989; Poff et al., 1997). Timing and seasonality

are also signiﬁcant with, for example, secondary production

and selection for ﬂood-linked life history characteristics

depending on the ﬂood pulse occurring during late spring

or summer (Thorpe et al., 2006).

From the perspective of the SEM, stages involving

channelization, dredging or incision that concentrate ﬂows

within the channel to accentuate ﬂood peaks may damage

or wash out physical and habitat features and diminish

ﬂoodplain interactions. Conversely, the attenuating effects

of ﬂoodplain and multi-channel morphologies and enhanced

capacity to store sediment associated with other SEM stages

tend to enhance ﬂood-related morphological features and

ecological beneﬁts.

Floodplain connectivity also inﬂuences the types,

quantities and qualities of hydrological beneﬁts provided

by ﬂoods: beneﬁts that are central to the productivity of

aquatic, riparian and ﬂoodplain ecosystems. Floodplains

absorb, retain and then release ﬂoodwater, increasing the

Figure 4. Stream Evolution Model based on combining the Channel Evolution Models in Figures 1–3, inserting a precursor stage to better

represent pre-disturbance conditions, adding two successor stages to cover late-stage evolution and representing incised channel evolution

as a cyclical rather than a linear phenomenon. Dashed arrows indicate ‘short-circuits’in the normal progression, indicating for example that

a Stage 0 stream can evolve to Stage 1 and recover to Stage 0, a Stage 4-3-4 short-circuit, which occurs when multiple head cuts migrate

through a reach and which may be particularly destructive. Arrows outside the circle represent ‘dead end’stages, constructed and maintained

(2) and arrested (3s) where an erosion-resistant layer in the local lithology stabilizes incised channel banks

SEM INCORPORATING HABITAT AND ECOSYSTEM BENEFITS 141

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Table II. Physical and vegetative attributes for each stage in the Stream Evolution Model

B. CLUER AND C. THORNE142

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hydroperiod, slowing times to concentration, attenuating

downstream ﬂood peaks and recharging aquifers that keep

ﬂoodplains moist and contribute to base ﬂows. Base ﬂow

is an important hydrologic attribute because it governs

habitability and biodiversity for aquatic species (Bêche

et al., 2009).

Stream hydrology interacts with groundwater via the

hyporheic zone (the part of the subsurface hydrological

system beneath and adjacent to the channel that is closely

coupled to the stream and with which water is exchanged

freely) to increase the capability of the watercourse to

support a diverse range of valuable habitats, especially dur-

ing low ﬂows (Boulton et al., 1998). The dimensions and

contribution of the hyporheic zone may be large. For

example, the hyporheic zone of the Flathead River, Montana

extends laterally for 2 km and supplies more than half of the

nutrients available to the aquatic ecosystem (Stanford and

Ward, 1988). Connectivity between the stream and the

Table III. Habitat and ecosystem beneﬁts for each stage in the Stream Evolution Model

SEM INCORPORATING HABITAT AND ECOSYSTEM BENEFITS 143

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hyporheic zone may be limited and even severed during

some stages in the SEM by channelization, incision and/or

the ingress of ﬁnes into coarse channel substrates.

Hydraulics

Research in the ﬁeld of ecohydraulics has for several years

focused on the inﬂuences of velocity and depth on channel

habitats for ﬁsh and other aquatic species (Gordon et al.,

1992), whereas the importance of the stream providing a

range of velocity–depth combinations to support a wide

range of species through all their life stages has been

demonstrated through numerous applications of models

such as IFIM (Bovee et al., 1998). Hydraulic diversity not

only supports quality and variety in aquatic habitats but also

interacts with bedforms and drives bed material and sub-

strate sorting processes that contribute to diversity in benthic

habitat.

Newson and Newson (2000) showed how the hydraulic

characteristics of channels could be categorized using

physical biotopes and functional habitats identiﬁed from the

conﬁguration of the water surface (Figure 5). The ecological

beneﬁts provided by the stream depend not only on the extent

and variety of biotopes but also on their pattern, positioning

and patchiness, as well as the extent to which hydraulic diver-

sity is maintained across a range of discharges. For example,

deep pools are vital in providing aquatic habitats linked to

cool, hyporheic ﬂows during hot, dry periods (Baxter and

Hauer, 2000), whereas marginal deadwaters concentrate

nutrients, provide rearing habitats during normal ﬂows and

act as refugia during ﬂoods (Lancaster and Hildrew, 1993;

Schwartz and Herricks, 2005).

From the perspective of the SEM, the extent and persist-

ence of key physical attributes such as hydraulic diversity

and the existence of marginal deadwaters are likely to be

evolutionary stage dependent.

Geomorphic attributes

Although debate continues between geomorphologists and

ecologists regarding the relative contributions to habitat

quality and diversity made by different channel forms and

features (e.g. King and Tharme, 1993; Williams, 2010), it

is generally agreed that supporting ecosystems with habitats

that are rich and resilient, that range from micro-scale to

meso-scale, to macro-scale, and that persist across a wide

range of hydrologic conditions is vital. Consequently,

signiﬁcant geomorphic attributes include the dimensions,

geometry, substrate characteristics and sediment features of

the channel, as well as the equivalent attributes of those

portions of the hydrologically and hydraulically connected

ﬂoodplain.

Channel dimensions and geometry. Metrics selected to

represent the physical size and channel shape are wetted

area and the length and complexity of the shoreline.

The utility of these attributes may be illustrated by

considering that at all in-bank ﬂow depths, a stream

provides a larger wetted area and a longer, relatively

more complex shoreline when it has a varied cross-

section. It follows that for a given ﬂow capacity, streams

with multi-channel morphologies provide more shoal and

edge habitat than equivalent streams with single-threaded

channel conﬁgurations.

Figure 5. Physical biotopes and associated functional habitats suggested by Newson and Newson (2000). Used with permission from Sage

Publications

B. CLUER AND C. THORNE144

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Channel and ﬂoodplain features. Channel features that

contribute signiﬁcantly to habitat quality and diversity

include bedforms, bars, islands, banks, riparian margins,

conﬂuences and difﬂuences. For example, conﬂuences are

sites of energy concentration where large-scale turbulence

is generated, areas of local acceleration and deceleration

are found, sediment sorting is vigorous and large wood tends

to accumulate. Unsurprisingly, conﬂuences have been found

to be both ecological hotspots (Benda et al., 2004) and places

where ecological communities shift (Rice et al., 2001). Multi-

channel streams (i.e. braided, anastomosed or meandering

streams with wide point bars and chute channels) have

numerous conﬂuences capable of contributing habitat and

ecosystem beneﬁts similar to those found relatively

infrequently at tributary junctions in single-thread streams.

Other particularly important geomorphic attributes

include in-stream sediment storage and the proportion of

the bankline that is accreting, stable or unstable, which have

major implications for avifauna. These attributes, and their

contributions to habitat and ecosystem beneﬁts, are altered

by the morphological adjustments associated with channel

response to disturbance.

Signiﬁcant ﬂoodplain attributes include the extent and con-

nectivity of inundation surfaces, side channels and wetlands.

It is difﬁcult to overstate the importance of ﬂoodplain extent

and connectivity to sediment storage, carbon sequestration

and nutrient processing (particularly denitriﬁcation). Flood-

plains have also been demonstrated to increase biocomplexity

(Amoros and Bornette, 2002), and ﬁsh particularly beneﬁt

from ﬂoodplain rearing (Henning et al., 2006; Jeffres et al.,

2008) in connected channel–ﬂoodplain systems.

The signiﬁcance of access to off-channel aquatic and

wetland habitats has been further illustrated with reference

to ephemeral ﬂoodplain tributaries (Hartman and Brown,

1987), periodically ﬂooded morphological features such as

alcoves and backwaters (Bell et al., 2001), seasonally closed

estuary lagoons (Hayes et al., 2008) and even artiﬁcial water

bodies such as gravel pits (e.g. Roni et al., 2006) where, for

example, salmon have been shown to grow faster than in

even the best ‘in-channel’habitats. This led Bond et al.

(2008) to conclude that access to off-line habitats, although

available only seasonally, provides population-scale beneﬁts

to salmon by increasing the numbers of juvenile ﬁsh that

reach the size threshold for marine entry, ocean growth

and survival, in time elevating the numbers of ﬁsh that

return as adults.

Substrate. Both the size and spatial distributions of

substrate are important aspects of the channel that are

controlled by erosion, transport and deposition processes

of sediment. Sediment sorting is particularly signiﬁcant in

coarse-bedded streams, for two reasons. First, selective

entrainment and hiding alter the mobility of different size

fractions to generate the bed armouring that is vital to

macro-invertebrate and spawning habitats. Second, self-

organization of moving grains by size creates clusters and

patches of differentially sized substrate (Brayshaw et al.,

1983), providing homes to a range of benthic organisms

with different habitat requirements. Substrate size and

sorting also interact with broader-scale hydraulic diversity

and sediment dynamics, with the result that bed sediment

sizes vary widely between, for example, the head and the

tail of bars and the upstream and lee sides of log jams.

Substrate characteristics have been shown not only to be

highly responsive to changes in the balance between sedi-

ment supply and local transport capacity but also to strongly

inﬂuence morphological evolution through the impacts of

ﬁning and coarsening on ﬂow resistance and bed mobility

(Simon and Thorne, 1996). Thus, changes in substrate

sorting and patchiness are associated with the evolutionary

stages in disturbed channels in ways that are highly signiﬁ-

cant to habitat and ecosystem beneﬁts.

Vegetation

Multiple attributes of aquatic, emergent, riparian and ﬂood-

plain vegetation inﬂuence ﬂuvial processes, channel morph-

ology, stream functions and hence the quality and diversity

of habitat. Vegetation that provides cover from predators,

moderates water temperature by shading and stabilizes

banks through root reinforcement may be removed during

channelization or operational maintenance, or undermined

by incision or widening. It follows that loss of vegetation,

particularly during Stages 2–6 in the SEM because of scale,

has the potential to degrade, compromise or eliminate a

signiﬁcant proportion of the pre-disturbance habitat and

ecosystem beneﬁts provided by vegetation.

Metrics used to represent the contribution of vegetation to

habitat and ecosystem beneﬁts include the presence of plants

(aquatic, emergent, riparian and ﬂoodplain) together with

two further vegetative attributes: leaf litter production and

tree trunk recruitment to the ﬂuvial system. The former sup-

ports primary production and hence the trophic status of the

watercourse, whereas the latter contributes indirect beneﬁts

through cycling nutrients and carbon, generating hydraulic

and morphological diversity, promoting channel stability

and sediment storage capacity, enhancing substrate sorting

and patchiness, and driving shallow hyporheic ﬂow.

Riparian succession is an important attribute whose

processes depend on channel migration and/or evolution

that topples climax communities and provides opportunities

for pioneer species and developing assemblages to create

new habitats that contribute fresh ecological beneﬁts. How-

ever, realization of the beneﬁts of plant successions depends

on the rate of colonization being able to match the pace at

which existing assemblages are removed (Shafroth et al.,

SEM INCORPORATING HABITAT AND ECOSYSTEM BENEFITS 145

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2002), which is diminished during the more destructive

stages of disturbed channel evolution.

Habitat and ecosystem beneﬁts

The natural ecological functioning of rivers is related to

hydromorphological complexity through provision of habi-

tat (Newson and Newson, 2000), and Thorp et al. (2010)

identiﬁed that most natural beneﬁts increase with physical

complexity, peaking in streams featuring network channels

(i.e. with anabranching or anastomosing network plan-

forms). The attributes selected to represent habitat and

ecosystem beneﬁts are described in the following sections

under the general headings of habitat, water quality, biota

and resilience. Table III details the unique habitat and

ecosystem beneﬁts attributed to each stage of the SEM.

Habitat. Refugia from hydrologic extremes (ﬂood and

drought) are important to the persistence of habitat and

ecosystem beneﬁts. For example, ﬁsh will not persist in a

reach without refuge from high velocities, intense

turbulence and elevated turbidity during ﬂoods. Typical

refugia include marginal deadwaters, back channels and

off-line habitats such as side channels, oxbows and

wetlands that are accessible during ﬂoods. Flood refugia

may form at a variety of scales ranging from the lee of a

piece of in-stream wood (small) to hydraulically rough bar

tops (medium), to hydraulically connected ﬂoodplains

(large). Conﬂuences and difﬂuences also provide ﬂood

refugia because one channel usually carries the majority of

the ﬂow and sediment, whereas fauna can easily access

calmer and clearer water in the other.

Drought refugia depend on the existence of morpho-

logical features such as deep pools and scour holes that are

hydrologically connected to the hyporheic and groundwater

zones. Typically, drought refugia are provided by free pools

found at bends and branch conﬂuences, in streams with

well-developed pool–rifﬂe sequences, and by forced pools

downstream from rock outcrops, log jams and tributary

junctions. It follows that the existence of ﬂood and drought

refugia, and the ease with which aquatic animals can access

them, depend on the hydrologic, hydraulic, morphologic

and vegetative attributes of the stream, which are strongly

inﬂuenced by the stage of evolution.

The presence of exposed tree roots was also identiﬁed

as providing signiﬁcant habitat and ecosystem beneﬁts

because they fulﬁl multiple needs, for a range of animals,

during various life stages (Raven et al., 1998). For ex-

ample, exposed roots slow velocities and dampen turbu-

lence while providing cover from predators and shade

from direct sunlight, attributes functioning over a wide

range of ﬂows.

Water Quality. Water quality is a fundamental attribute of

habitat and ecosystem beneﬁts. The metrics selected to

represent it are clarity, temperature and nutrient cycling.

Water clarity is decreased by turbidity due to high

concentrations of total suspended solids. High total sus-

pended solid concentrations are associated with reach-

scale channel instability that generates elevated loads of

ﬁne sediment derived from local and upstream bed scour

and bank retreat, together with the excessive concentra-

tions of ﬁne organic matter that result from the wide-

spread destruction of vegetation in and around unstable

reaches, in addition to loading from upstream lakes and

wetlands.

The ranges of many aquatic species are limited by water

temperature, especially during droughts and summer dry

periods (Poole and Berman, 2001) when their survival

depends on base ﬂows fed by seepage of relatively cool,

clear water from groundwater and/or spring-fed tributaries,

coupled with shade that limits direct sunlight from warming

stream water during daylight hours (Nielson et al., 1994;

Baxter and Hauer, 2000).

Whereas external factors control the net ﬂux of heat to

the stream, the presence or absence of deep pools connected

to the hyporheic zone affect how water temperatures

respond (Triska et al., 1989; Poole and Berman, 2001). As

the exchange of water between the hyporheic zone and the

stream is inﬂuenced by substrate, bed topography and chan-

nel pattern (Poole and Berman, 2001), temperature response

is heightened during the early and middle stages of stream

evolution.

Nutrient cycling is vital to the stream environment and the

ecosystem it supports (Hynes, 1983). Nutrient processing is

heavily inﬂuenced by stream velocity, exchange between

the stream and the hyporheic zone, and the capacity of

aquatic and riparian sediment bodies and vegetation to store

and release nutrients. It follows that the capacity of the

stream to cycle nutrients effectively increases with the

extent of the wetted area relative to the ﬂow and the degree

to which it is hydrologically connected to the ﬂoodplain,

hyporheic zone and groundwater, all of which are evolution-

ary stage dependent.

Biota. According to Newson and Newson (2000), ‘it is a

valid working principle in ecology that diversity of habitat,

if it can be described, paves the way for predictions of the

potential diversity of biota.’This principle is complimented

by the Riverine Ecosystem Synthesis model developed by

Thorpe et al. (2006), which predicts that biodiversity,

system metabolism and many other functional ecosystem

processes are enhanced by habitat complexity at the valley-

to-reach scale and which proposes that biocomplexity should

be related to hydrogeomorphic complexity. This is the case

because habitat diversity and niche availability increase with

B. CLUER AND C. THORNE146

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the diversity of channel and ﬂow conditions. Ward and

Tockner (2001) suggested that it is hydrological connectivity

that controls biodiversity at the ﬂoodplain scale, and they

concluded that overall biodiversity peaks at intermediate

levels of connectivity. Thorp et al. (2010) capture this

succinctly, noting that ‘Biodiversity, as measured by species

richness and trophic feeding diversity, is usually greater in

physically complex [reaches] (Roach et al., 2009) because

habitat diversity is greater and opportunities for both ﬂuvial

and ﬂoodplain specialists abound (Galat and Zweimüller,

2001).’In interpreting how habitat and ecosystem beneﬁts

vary between stages in the SEM, these ﬁndings suggest

that biodiversity (expressed through species richness and

trophic diversity) is a representative biotic attribute that

should vary in relation both to the morphological diversity

of the channel and the extent and frequency of ﬂoodplain

connectivity.

The proportion of native plant species is another biotic

attribute relevant to evaluating the ecosystem beneﬁts

provided by the stream and how beneﬁts vary as the stream

evolves. Stages that involve channelization, incision or rapid

widening destroy established assemblages and provide

opportunities for invasive species to colonize eroding stream

banks, retreating terrace edges and accreting berms during

the middle stages of channel evolution. Theoretically, native

species should be better adapted to the more natural condi-

tions recovered during the latter stages of the SEM (Rey

Benayas et al., 2009), and they should, therefore, have a

competitive advantage over invasive species, although this

is by no means certain.

The foundation for a rich and robust ecosystem lies in

ﬁrst-order and second-order productivities, selected as the

third biotic attribute in this evaluation. Thorpe et al. (2010;

70) note that ‘[Reaches] with a greater range of current

velocities and substrate types offer habitat niches for a greater

diversity and potential productivity of algae and vascular

plants.’It follows that productivity is in proportion to the

hydrological, hydraulic, morphological and vegetative diver-

sity of the stream.

Resilience. It is vital that the habitat and ecosystem beneﬁts

provided by the stream persist over the periods necessary

for ﬂora and fauna to become fully established, and

this depends on their life cycle and resilience. To be

considered resilient, habitat and ecosystem beneﬁts must

be able to withstand disturbance in general, and ﬂoods and

droughts in particular. Hence, in evaluating habitat and

ecosystem beneﬁts, resilience is represented by these

attributes.

Disturbance to the ﬂuvial system may occur at the catch-

ment, reach or local scales and may result from a wide range

of events and activities that affect the ﬂow regime, the sedi-

ment regime or the boundary characteristics of the channel

(Thorne et al., 2010). Drivers of catchment-scale disturb-

ance include climate change (temperature, precipitation,

rain–snow partitioning), land-use change (urbanization,

deforestation, afforestation, agricultural intensiﬁcation, farm

abandonment), wild ﬁres, volcanic eruptions and seismic

events. Reach-scale disturbances may result from natural

events (e.g. base level and valley slope changes due to

neotectonics, beaver introduction, or vegetation changes

due to infestation and die back) or anthropogenic impacts

associated with capital works and/or operational mainten-

ance for a variety of purposes (including ﬂood control, land

drainage and navigation).

Generally, disturbance to the habitat and ecosystem

beneﬁts provided by the affected reaches depends on the

type and extent of morphological response. In reaches that

have adjusted naturally to the prevailing ﬂow and sediment

regimes, responses are distributed between the nine degrees

of freedom that an alluvial channel can change (Hey, 1978).

This allows dynamically adjusted streams to remain in

meta-stable equilibrium, so that they absorb disturbances

while continuing to provide pre-disturbance habitat and

ecosystem beneﬁts. Conversely, in reaches that are

unstable or which are constrained artiﬁcially, responses are

focused in fewer of the degrees of freedom (Hey, 1978), which

focuses and ampliﬁes the morphological responses to disturb-

ance so that habitat and ecosystem beneﬁts are degraded or

destroyed. These characteristics of response to disturbance

have been considered in evaluating resilience as a function of

channel evolution.

It has long been recognized that disturbance by ﬂood

events is essential to support biocomplexity (Junk et al.,

1989) and natural hydrological patterns feature prominently

in the RAS model of Thorpe et al. (2006). However, ﬂoods

that are ampliﬁed by catchment changes, mistimed due to

manipulation of the hydrological regime or constrained

within constructed or incised channels may not beneﬁt biota,

especially native species that are adapted to the frequency,

duration and seasonality of natural events.

With this background, resilience to ﬂoods becomes

important to the on-going delivery of habitat and ecosystem

beneﬁts. The severity of ﬂood impacts is largely related to

the availability of ﬂoodplain space where the stream can

diffuse and store ﬂood ﬂows. Consequently, channelization

and incision reduce resilience during the middle Stages 2–6

of the SEM because the channel is isolated from its

ﬂoodplain and ﬂoods are conﬁnedtoareducedarea,exag-

gerating negative impacts on channel morphology and

sediment dynamics and reducing the extent and accessibil-

ity of refugia. These effects are partially reversed during

late-stage evolution when a new (proto) ﬂoodplain devel-

ops within the incised canyon, although resilience cannot

fully recover to its pre-disturbance level unless the channel

aggrades sufﬁciently that it reconnects with an original

SEM INCORPORATING HABITAT AND ECOSYSTEM BENEFITS 147

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ﬂoodplain that is, itself, still capable of functioning hydro-

logically and geomorphologically.

Drought resilience is primarily governed by the existence

of deep pools fed by perennial ﬂow from groundwater and/

or spring-fed tributaries, coupled with the ameliorating

effects of shading and connectivity with an extensive hypor-

heic zone. That said, ephemeral channels can still support

rich and diverse ecosystems provided that aquatic and

amphibious fauna are suitably adapted and have access

to proximal subsurface drought refugia, although deeply

desiccated reaches will require recolonization by primary

and secondary biota, which lengthens recovery times. It

follows that instability that scours or clogs bed sedi-

ments, reduces morphological diversity and destroys in-

channel and riparian vegetation will reduce drought

resilience.

EVALUATION OF STREAM ATTRIBUTES AND

HABITAT AND ECOSYSTEM BENEFITS

The physical and vegetative attributes associated with each

of the eight stages in the SEM are evaluated in Table II,

whereas the habitat and ecological beneﬁts are evaluated in

Table III. On the basis of the evaluations set out in Tables II

and III, scores were assigned to the attributes and beneﬁts

associated with each SEM stage according to an ordinal scale

where 3 = abundant and fully functional, 2 = present and func-

tional, 1 = scarce and partly functional, and 0 = absent or

dysfunctional.

The scores for each stage in the SEM are listed in

Tables IV and V, together with the sums for each SEM stage

compared with the maximum possible score. The results are

illustrated in Figure 6.

Table IV. Scores for the physical and vegetative attributes for each stage in the Stream Evolution Model. Scores are based on an ordinal scale

where 3 = abundant and fully functional, 2= present and functional, 1 =scarce and partly functional and 0 = absent or dysfunctional

B. CLUER AND C. THORNE148

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Physical and vegetation attribute scores are highest (92%)

for Stage 0 streams (pre-disturbance, anastomosing network)

but fall to a low of just 8% for streams in the Stage 4-3

short-circuit (degraded with renewed incision). Stage 8

streams provide the second highest scores, reﬂecting stream

recovery to something near its pre-disturbance (Stage 0)

conﬁguration. However, whereas in Stage 8 the stream

possesses all the physical and vegetative attributes and pro-

vides the full range of the ecosystem beneﬁts present in Stage

0 (i.e. its pre-disturbance condition), Stage 8 scores are some-

what lower because the stream system is smaller; it is inset into

a narrower functional ﬂoodplain.

Scores for the early and late stream Stages (0, 1 and 7, 8),

where ﬂoodplain attributes and processes are prominent, are

distinctly different from those Stages in-between (2 to 6).

The most abrupt change in stream attributes and beneﬁts is

the precipitous decline in both that occurs between Stages

1 and 2 because of the direct effects of channelization and

Stage 3 when incision disconnects the channel from its

ﬂoodplain.

A hysteresis loop is revealed when beneﬁts scores are

plotted as a function of the stream’s hydrogeomorphic attri-

butes (Figure 7). It is apparent that the habitat and ecosystem

beneﬁts provided by streams recover less quickly, and less

completely, following disturbance than do the correspond-

ing hydrogeomorphic attributes. Over short time scales, the

loop is likely broader because of delays in colonization

and the cumulative effects in physical and ecological

processes common to disturbed catchments.

DISCUSSION AND IMPLICATIONS OF THE STREAM

EVOLUTION MODEL FOR RIVER MANAGEMENT

AND RESTORATION

The SEM advances the lasting value of the CEMs originated

by Schumm et al. (1984) and Simon and Hupp (1986).

It builds on these models, taking advantage of advances

in knowledge and improved understanding of process-

response mechanisms and links between morphology, habi-

tat and ecosystem beneﬁts made during the quarter century

since they were conceived. This is done by redeﬁning the

pre-disturbance and post-recovery morphologies, replacing

linear progression with an evolutionary cycle, broadening

the scale to consider streams in their catchment rather than

simply as incised channels and linking habitat and ecosys-

tem beneﬁts to physical attributes and system responses to

disturbance.

In common with the original CEMs, the SEM offers users

interested in system-wide processes rather than reach-

speciﬁc morphological characteristics the opportunity to

undertake space-for-time substitution. In essence, this

Table V. Scores for the habitat and ecosystem beneﬁts for each stage of the Stream Evolution Model. Scores are based on an ordinal scale

where 3 = abundant and fully functional, 2= present and functional, 1 =scarce and partly functional and 0 = absent or dysfunctional

SEM INCORPORATING HABITAT AND ECOSYSTEM BENEFITS 149

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involves replacing the time dimension in the model with

distance from disturbance. When the SEM is considered in

the catchment context (Figure 8), what emerges immediately

is the value of conserving or restoring the processes

that characterize sediment transport at the reach scale. For

example, mid-catchment basins moderate sediment delivery

events or elevate prolonged loads and therefore buffer

sediment impacts to more responsive downstream reaches.

If sediment exchange reaches are turned into sediment

transfer reaches by channelization, ﬂood control projects,

or widespread bank revetment, downstream sediment loads

and calibres will increase, transmitting disturbances down-

stream that could have been moderated upstream and risking

overwhelming the capacities of lowland channels, riparian

zones, ﬂoodplains and associated wetlands to assimilate

sediment without damaging those relatively more valuable

reaches. Sediment accumulation in mid-catchment fans or

alluvial basins is the natural process by which sediment

pulses are processed in naturally functioning catchments.

This process not only ameliorate the impacts of coarse

sediment delivery in lowland reaches downstream but also

provides the mechanism by which Stage 1 or 7 streams

may attain the networks representative of Stage 0 or 8 that

result in comparatively greater ecological beneﬁts. More

generally, Figure 8 reminds end-users that stream evolution

pathways and habitat outcomes depend on the position

within the catchment as well as the type and severity of

the disturbance.

The SEM provides a lens for viewing reach-scale inter-

ventions (such as widespread bank stabilization intended

to manage sediment, sometimes considered a pollutant in

regulatory policies) when catchment-scale problems are the

root cause of elevated sediment loads (Doyle and Shields,

2012; Hupp et al., 2013). The SEM not only helps to iden-

tify the possible unintended consequences of invoking these

actions but also indicates the potential value of re-activating

sediment exchange and storage functions in mid-catchment

alluvial reaches that can buffer the more sediment-sensitive

reaches downstream, while transforming single-thread,

meandering channels into more ecologically valuable chan-

nel networks. This re-emphasizes the importance of longitu-

dinal and lateral connectivity in the sediment system and the

disproportionate risks of disconnecting alluvial streams

from sediment sink and source processes (i.e. ﬂoodplain).

Figure 6. Habitat and ecosystem beneﬁts provided in each stage of the Revised Channel Evolution Model. Each stage is represented by two

pie charts whose diameters signify the relative percentage of maximum beneﬁts as tabulated in Tables IV and V. For each stage, the pie chart

on the left summarizes the richness and diversity of the hydromorphic attributes, whereas the pie chart on the right summarizes the associated

habitat and ecosystem beneﬁts

B. CLUER AND C. THORNE150

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The SEM differs from its CEM predecessors in being

expressed as a cycle rather than a linear succession of

morphological states and adjustments. This recognizes that

vertical adjustments, lateral changes, and channel instability

and recovery often occur cyclically, with multiple episodes of

channel degradation/aggradation, widening/narrowing and en-

largement/shrinkage being generated through complex response

to a single external disturbance or the crossing of one or more

internal, geomorphic thresholds. The result is for late-stage

morphologies to be nested within the boundaries of channels

produced by earlier evolutionary stages, although a valley-ﬁll

cycle may result in a larger and richer end-stage network.

The SEM also recognizes that local and site-speciﬁc

conditions may cause short-circuits in the cycle with, for

example, repeated cycles of degradation. Similarly, the

expected sequence of post-disturbance evolution may be

arrested by natural controls (e.g. geologic or vegetation),

or reversed by new disturbances (e.g. sediment pulses or

afforestation), or perpetuated by management interventions.

These processes turn what would otherwise be transitional

stages into longer term conﬁgurations and limit recovery

of habitat and ecosystem beneﬁts.

Making strategic and cost-effective river management

decisions has never been more important, as stresses on

aquatic systems will increase as human demands for land

and water rise. It is now recognized that ﬂuvial functions

are fundamental to the generation of natural capital and

providing the ecosystem services upon which civilization

depends. It is also accepted that past efforts to channelize

and minimize rivers have depleted natural capital and

Figure 8. Process domains in the ﬂuvial system associated with the Revised Channel Evolution Model. Domains in a river basin can be

generally characterized as governed by supply, exchange/transfer or deposition of sediments. Channelization or embankments in the Alluvial

fan and Transfer zones diminish beneﬁcial sediment deposition processes and artiﬁcially promote downstream transfer of coarse sediment and

compromise habitats less resilient to increased sediment loads

Figure 7. Plot of habitat and ecosystem beneﬁts as a function of

hydrogeomorphic attributes, from Tables IV and V. There are gen-

erally two ﬁelds, streams that have greater than 50% of the hydro-

geomorphic attributes and habitat and ecosystem beneﬁts, and

streams with less than 30%, while Stage 6 streams are intermediate.

The most abrupt difference between adjacent stages is from 1 to 2,

where scores drop from nearly 75% to less than 25% in constructed

channels, primarily because of ﬂoodplain disconnection. A hyster-

esis loop reveals that habitat and ecosystem beneﬁts recover less

quickly and less completely than do the corresponding hydrogeo-

morphic attributes over long time scales, and likely, the loop is

broader over short times scales

SEM INCORPORATING HABITAT AND ECOSYSTEM BENEFITS 151

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severely damaged ecosystem services overall. Globally, many

alluvial systems that formerly exchanged sediment freely

with their ﬂoodplains are now levee conﬁned, channelized

and incised. In their earlier condition, these rivers could

accept, store and exchange periodic heavy ﬂoods and

inputs of sediment and nutrients generated by disturbances

upstream, such as widespread land sliding triggered by

extreme rainfall, wildﬁres and even volcanic eruptions; and

buffering downstream depositional reaches from excessive,

coarse sedimentation and other impacts. The result of the past

and continuing emphasis placed on ﬂood control, land

drainage and property stability has been to sensitize ﬂuvial

systems to sediment disturbances and systemic imbalances.

Elimination of sediment deposition and exchange processes

from basins makes the downstream environments vulnerable

to damage by singular events and less resilient to chronic

impacts of climate and land-use changes.

River restoration efforts typically focus on the geometry

of channels with the goals of reducing and then balancing

sediment loads at the reach scale, effectively attempting to

turn every reach into a sediment transfer zone. This perpetu-

ates an erroneous approach to management of the alluvial

channel system and may partially explain why the regener-

ation of high-quality habitat remains limited (Doyle and

Shields, 2012) and restoration of freshwater ecosystems

remains elusive (Bernhardt and Palmer, 2011): the channels

in most alluvial reaches are restored to forms equivalent to

Stages 3–6 in the SEM. These relatively low value forms

are then preserved through stabilization measures. Even

though using soft engineering and natural materials such

as biotechnical revetments and large wood has become com-

mon, stabilization impedes the ﬂuvial processes that could

drive continued evolution to the substantially more resilient

and valuable Stages 7 and 8.

The implications for river management that stem from the

SEM are that stabilizing channels in Stages 2 to 5 is not only

costly (requiring maintenance) and risky (places people and

property in hazardous locations) but also ecologically

counterproductive. Restoring streams to Stage 6 is also a

relatively ineffective strategy because without ﬂoodplain,

they are largely non-deformable and less resilient to future

catchment disturbances. Acceleration of natural evolution

by intervening to move the channel forward through the

cycle to Stage 7 or even 8 is suggested, or strategically

planning for a deformable Stage 6 that can suitably respond

to a future catchment disturbance event such as dam

removal. Where Stage 0-1 or 7-8 channels exist, maintaining

existing or enhancing degraded sediment deposition zones

upstream would be a valuable long-term conservation

strategy. While the advantages of a channel network in

terms of moderating sediment transfer, promoting sediment

exchange and enhancing the sediment processing functions

offered by the active ﬂoodplain of an alluvial valley are well

established and ecologically superior, constraints imposed

by development and/or infrastructure may complicate, delay

or prevent this as a restoration goal.

The decision to aim for single-thread, active meandering

(Stage 7) or prompted recovery of an anastomosing channel

network (Stage 8) will be constrained by space that can be

made available for lateral migration and stream evolution.

‘Passive restoration’through ceasing periodic de-silting

and maintenance of Stage 2 channels can rapidly transform

to a Stage 7 (active meandering) or 8 (an anastomosed

network), provided ﬂoodplain space is available or obtained

through strategic retreat.

The arguments advanced here concerning sediment

processes in the middle reaches of incised alluvial basins

are just one example of how the SEM could be used to

provide a framework for improved catchment management

and decision making in river restoration. Aligning manage-

ment and restoration objectives with SEM stages and evolu-

tionary trends can promote rather than counteract natural

processes. Adopting stream management approaches that

enhance natural evolution require more space than those

set on impeding it, but they hold the promises of increasing

biodiversity, promoting recovery of endangered species,

improving the resilience and sustainability of ecosystem

services both in the restored reach and those downstream,

optimizing climate resilience, minimizing risks to property

and human safety, and maximizing returns on restoration

investments.

ACKNOWLEDGEMENTS

The advice given by two anonymous reviewers led to

substantial improvements to the original manuscript, and we

are deeply indebted to Professor Stanley A. Schumm for a life-

time of original thinking and scientiﬁc progress in river science

and management, and for his willingness to share his insights.

REFERENCES

Amoros C, Bornette G. 2002. Connectivity and biocomplexity in water-

bodies of riverine ﬂoodplains. Freshwater Biology 47: 761.

Baxter CV, Hauer FR. 2000. Geomorphology, hyporheic exchange, and

selection of spawning habitat by bull trout. Canadian Journal of Fisher-

ies and Aquatic Sciences 57: 1470–1481.

Bêche LA, Connors PG, Resh VH, Merenlender AM. 2009. Resilience of

ﬁshes and invertebrates to prolonged drought in two California streams.

Ecography 32:778–788, 2009. DOI: 10.1111 /j.1600-0587.2009.05612.x

Beechie T, Pess G, Roni P, Giannico G. 2008. Setting river restoration

priorities: a review of approaches and a general protocol for identifying

and prioritizing actions. North American Journal of Fisheries Management

28:891–905.

Bell E, Duffy WG, Roelofs TD. 2001. Fidelity and survival of juvenile

coho salmon in response to a ﬂood. Transactions of the American

Fisheries Society 130: 450.

Benda L, Poff NL, Miller D, Dunne T, Reeves G, Pess G, Pollock M. 2004.

The network dynamics hypothesis: how channel networks structure river-

ine habitats. BioScience 54(5): 413. DOI: 10.1641/0006-3568(2004)054

[0413:TNDHHC]2.0.CO;2)

B. CLUER AND C. THORNE

152

DOI: 10.1002/rra

Bernhardt ES, Palmer MA. 2011. River restoration: the fuzzy logic of

repairing reaches to reverse catchment scale degradation. Ecological

Applications 21(6): 1926–1931.

Bledsoe BP, Brown MC, Raff DA. 2007. GeoTools: a toolkit for ﬂuvial

systems analysis. Journal American Water Resources 43(3): 757–772.

Bond MH, Hayes SA, Hanson CV, MacFarlane RB. 2008. Marine survival

of steelhead (Oncorhynchus mykiss) enhanced by a seasonally closed

estuary. Canadian Journal of Fisheries and Aquatic Sciences 65: 2242.

DOI: 10.1139/F08-131.

Boulton AJ, Findlay S, Marmonier P, Stanley EH, Valett HM. 1998. The

function signiﬁcance of the hyporheic zone in streams and rivers. Annual

Review of Ecology and Systematics 29:59–81.

Bovee KD, Lamb BL, Bartholow JM, Stalnaker CB, Taylor J, Henriksen J.

1998. Stream habitat analysis using the instream ﬂow incremental method-

ology. Information and Technology Report USGS/BRD/ITR-1998-0004.

U.S. Geological Survey Biological Resources Division, Fort Collins, CO.

Brayshaw AC, Frostick LE, Reid I. 1983. The hydrodynamics of particle

clusters and sediment entrapment in coarse alluvial channels. Sedimentol-

ogy 30(1): 137–143.

Brookes A. 1988. Channelized rivers: perspectives for environmental

management. John Wiley and Sons: Chichester, UK; 326.

Brown AG, Sear DA. 2008. ‘Natural’streams in Europe: their form and role

in carbon sequestration. Eos Transactions.AGU, Fall Meet. Supplement.

Collins BD, Montgomery DR, Sheikh AJ. 2003. Reconstructing the historic river-

ine landscape of the Puget Lowland. In Restoration of Puget Sound Rivers,

Montgomery DR, et al. (eds). University of Washington Press: Seattle; 79–128.

Dahl TE, Allord GJ. 1996. History of wetlands in the conterminous United

States. U.S. Geol. Survey Water-Supply Paper 2425.

Downs PW, Gregory KJ. 2004. River Channel Management: Towards Sus-

tainable Catchment Hydrosystems. Arnold: London; 395.

Doyle MW, Shields FD. 2000. Incorporation of bed texture into a channel

evolution model. Geomorphology 34: 291–309.

Doyle MW, Shields FD. 2012. Compensatory mitigation for streams under

the Clean Water Act: reassessing science and redirecting policy. Journal

American Water Resources Association:1–16. DOI: 10.1111/j.1752-

1688.2011.00631.x

Doyle MW, Shields D, BoydCF, Skidmore PB, Dominick D. 2007. Channel-

forming discharge selection in river restoration design. Journal Hydraulic

Engineering: 831. DOI: 10.1061/_ASCE_0733-9429_2007_133:7_831_

Galat DL, Zweimüller I. 2001. Conserving large-river ﬁshes: is the highway

analogy an appropriate paradigm? Journal of the North American

Benthological Society 20: 266–279.

Gordon ND, McMahon TA, Finlayson BL. 1992. Stream Hydrology: An

Introduction for Ecologists. Wiley: Chichester, UK.

Grossinger RM, Striplen CJ, Askevold R, Brewster E, Beller EE. 2007. His-

torical landscape ecology of an urbanized California valley: wetlands and

woodlands in the Santa Clara Valley. Landscape Ecology 22: 103–120.

Gurnell A, Petts G. 2006. Trees as riparian engineers: the Tagliamento

River, Italy. Earth Surface Processes and Landforms 31: 1558–1574.

Habersack H, Piégay H. 2008. River restoration in the Alps and their sur-

rounding: past experience and future challenges. In Gravel-Bed Rivers

VI: From Process Understanding to River Restoration, Habersack H,

Piégay H, Rinaldi M (eds). Elsevier: Amsterdam; 703–737.

Harper DM, Smith CD, Barham PJ. 1995. The ecological basis for management

of the natural river environment. In The Ecological Basis for River Manage-

ment, Harper DM, Ferguson ACD (eds). Wiley: Chichester; 219–38.

Hartman GF, Brown TG. 1987. Use of small, temporary, ﬂoodplain

tributaries by juvenile salmonids in a west coast rain-forest drainage

basin, Carnation Creek, British Columbia. Canadian Journal Fisheries

Aquatic Science 44: 262–270.

Harwood K, Brown AG. 1993. Fluvial processes in a forested anastomosing

river: ﬂood partitioning and changing ﬂow patterns. Earth Surface

Processes and Landforms 18(8): 741–748.

Hawley RJ, Bledsoe BP, Stein ED, Haines BE. 2011. Channel evolution

model of semiarid stream response to urban-induced hydromodiﬁcation.

Journal American Water Resources Association 48(4): 722–744.

Hayes SA, Bond MH, Hanson CV, Freund EV, Smith JJ, Anderson EC,

Ammann, A, MacFarlane RB. 2008. Steelhead growth in a small central

California watershed: upstream and estuarine rearing patterns. Transactions

of the American Fisheries Society 137: 114. DOI: 10.1577/T07-043.1.

Henning JA, Gresswell RE, Fleming IA. 2006. Juvenile salmonid use

of freshwater emergent wetlands in the ﬂoodplain and its implications

for conservation management. North American Journal Fisheries

Management 26: 367–376. DOI: 10.1577/M05-057.1

Hey RD. 1978. Determinate hydraulic geometry of river channels. Journal

of the Hydraulics Division, ASCE. 104(6): 869–885.

Hynes HBN. 1983. Groundwater and stream ecology. Hydrobiologia 100:

93–99.

Hupp CR, Noe GB, Schenk ER, Benthem AJ. 2013. Recent and historic

sediment dynamics along Difﬁculty Run, a suburban Virginia Piedmont

stream. Geomorphology 180–181: 156–169.

Jeffres CA, Opperman JJ, Moyle PB. 2008. Ephemeral ﬂoodplain

habitats provide best growth conditions for juvenile Chinook salmon in

a California river. Environ Biol Fish 83: 449–458. DOI: 10.1007/

s10641-008-9367-1.

Junk WJ, Bayley PB, Sparks RE. 1989. The ﬂood pulse concept in river-

ﬂoodplain systems. Canadian Special Publication of Fisheries and

Aquatic Sciences 106: 110–127.

King JM, Tharme RE. 1993. Assessment of the instream ﬂow incremental

methodology and initial development of alternative instream ﬂow

methodologies for South Africa. Report 295/1/94. Pretoria: Water

Research Commission,South Africa.

Kondolf GM. 2009. Fluvial geomorphology and river restoration: uneasy

allies. Eos Trans.AGU, 90(52), Fall Meet.Suppl., Abstract H52D-01.

Kondolf GM, Tompkins MR. 2008. Floodplain disconnection, changes in

river corridor complexity, and implications for river restoration along

lower Deer Creek, Tehama County, CA. AGU Fall Meeting Abstracts.

Lancaster J, Hildrew AG. 1993. Characterizing instream ﬂow refugia. Can.

Jour. Fish. Aqua. Sci. 50: 1663–75.

Leopold LB, Maddock TJJr. 1953.The hydraulic geometry of stream channels

and some physiographic implications. USGS Reports, Paper 252.

Lewis WMJr. 2001. Wetlands Explained: Wetland Science, Policy, and

Politics in America. Oxford Press: New York.

Little WC, Thorne CR, Murphey JB. 1981. Mass bank failure analysis of

selected Yazoo basin streams. Transactions of the American Society of

Agricultural Engineers 25(5): 1321–1328.

Marston RA, Girel J, Pautou G, Piegay H, Bravard JP, Arneson C. 1995.

Channel metamorphosis, ﬂoodplain disturbance, and vegetation develop-

ment: Ain River, France. Geomorphology 13(1–4): 121–131.

Merritts D, Walter R, Rahnis M, Hartranft J, Cox S, Gellis A, Potter N, Hilgartner

W, Langland M, Manion L, Lippincott C, Siddiqui S, Rehman Z, Scheid C,

Kratz L, Shilling A, Jenscheke M, Datin K, Cranmer E, Reed A,

Matuszewski D, Voli M, Ohlson E, Neugebauer A, Ahamad A, Neal C,

Winter A, Becker S. 2011. Anthropocene streams and base-level controls from

historic dams in the unglaciated mid-Atlantic region, USA. Philosophical

Transactions Royal Society 369:976–1009. DOI: 10.1098/rsta.2010.0335.

Montgomery DR. 2004. Geology, geomorphology, and the restoration

ecology of salmon. GSA Today:14–11. DOI: 10.1130/1052-5173.

Montgomery DR. 2008. Dreams of natural streams. Science 319: 291.

DOI: 10.1126/science.1153480

Newson MD, Newson CL. 2000. Geomorphology, ecology and river

channel habitat: mesoscale approaches to basin-scale challenges. Progress

in Physical Geography 24(2): 195–217.

Nielson JL, Lisle TE, Ozaki V. 1994. Thermally stratiﬁed pools and their

use by steelhead in northern California streams. Transactions of the

American Fisheries Society 123: 613–626.

SEM INCORPORATING HABITAT AND ECOSYSTEM BENEFITS 153

DOI: 10.1002/rra

Padmore CL. 1997. Biotopes and their hydraulics: a method for determining

the physical component of freshwater habitat quality. In Freshwater

Quality: Deﬁning the Indeﬁnable, Boon PJ, Howell DL (eds). HMSO:

Edinburgh; 251–57.

Palmer MA, Bernhardt ES, Allan JD, Lake PS, Alexander G, Brooks S,

Carr J, Clayton S, Dahm CN, Follstad Shah J, Galat DL, Loss SG,

Goodwin P, Hart DD, Hassett B,Jenkinson R, Kondolf GM, Lave R, Meyer

JL, O’Donnel TK, Pagano L, Sudduth E. 2005. Standards for ecologically

successful river restoration. Journal of Applied Ecology 42:208–217.

DOI: 10.1111/j.1365-2664.2005.01004.x

Phillips JD. 1992. The end of equilibrium? Geomorphology 5(3–5): 195–201.

Phillips JD. 1999. Divergence, convergence, and self-organization in landscapes.

Annals of the Association of American Geographers 89(3): 466–488.

Phillips JD. 2009. Changes, perturbations, and responses in geomorphic

systems. Progress in Physical Geography 33(1): 17–30.

Poff NL, Allan JD, Bain MB, Karr JR, Prestegaard KL, Richter B, Sparks R,

Stromberg J. 1997. The natural ﬂow regime: a new paradigm for riverine

conservation and restoration. BioScience 47:769–784.

Pollock MM, Heim M, Werner D. 2003. Hydrologic and geomorphic effects

of beaver dams and their inﬂuence on ﬁshes. AFS Symposium 37:213–233.

Poole GC, Berman CH. 2001. Pathways of human inﬂuence on water

temperature dynamics in stream channels. Environmental Management

27: 787–802.

Raven PJ, Holmes NTH, Fox PA, Dawson FH, Everard M, Fozzard IR, Rouen

KJ. 1998. River Habitat Quality: The Physical Character of Rivers and

Streams in the UK and the Isle of Man. Environment Agency: Bristol.

Rey Benayas JM, Newton AC, Díaz A, Bullock JM. 2009. Enhancement of

biodiversity and ecosystem services by ecological restoration: a meta-

analysis. Science 325: 1121–1124.

Rice SP, Greenwood MT, Joyce CB. 2001. Tributaries, sediment sources,

and the longitudinal organization of macrointertebrate fauna along river

systems. Canadian Journal Fisheries Aquatic Science 58: 824–840.

Roach KA, Thorp JH, Delong MD. 2009. Inﬂuence of lateral gradients of

hydrologic connectivity on trophic position of ﬁshes in the upper Missis-

sippi River. Freshwater Biology 54: 607–620.

Roni P, Morley SA, Garcia P, Detrick C, King D, Beamer E. 2006. Coho

salmon smolt production from constructed and natural ﬂoodplain

habitats. Transactions of the American Fisheries Society 135:1398–1408.

DOI: 10.1577/T05-296.1

Schoof R. 1980. Environmental impact of channel modiﬁcation. Water

Resources Bulletin 16(4): 697–701.

Schumm SA, Harvey MD, WatsonCC. 1984. Incised Channels: Morphology,

Dynamics, and Control. Water Resources Publications: Littleton, CO.

Schwartz JS, Herricks EE. 2005. Fish use of stage-speciﬁcﬂuvial habitats

as refuge patches during a ﬂood in a low-gradient Illinois stream. Journal

Fisheries Aquatic Science 62: 1540–1552.

Sear DA, Arnell NW. 2006. The application of palaeohydrology in river

management. Catena 66(1-2): 169–183.

Shafroth PB, Stromberg JC, Patten DT. 2002. Riparian vegetation response

to altered disturbance and stress regimes. Ecological Applications 12(1):

107–123.

Shields FD. 2009. Do we know enough about controlling sediment to mitigate

damage to stream ecosystems? Ecological Engineering 35:1727–1733.

Simon A, Darby SE. 2002. Effectiveness of grade-control structures in

reducing erosion along incised river channels: the case of Hotophia

Creek, Mississippi. Geomorphology 42(3): 229–254.

Simon A, Hupp CR. 1986. Geomorphic and vegetative recovery processes

along modiﬁed Tennessee streams: an interdisciplinary approach to

disturbed ﬂuvial systems. Forest Hydrology and Watershed Management.

IAHS-AISH Publ.167.

Simon A, Thorne CR. 1996. Channel adjustment of an unstable coarse-

grained stream: opposing trends of boundary and critical shear stress,

and the applicability of extremal hypotheses. Earth Surface Processes

and Landforms 21(2): 155–180.

Stanford JA, Ward JV. 1988. The hyporheic habitat of river ecosystems.

Nature 335:64–66.

Surian N, Rinaldi M. 2003. Morphological response to river engineering

and management in alluvial channels in Italy. Geomorphology 50(4):

307–326.

Thorne CR. 1999. Bank processes and channel evolution in the incised riv-

ers of North–Central Mississippi. In Incised River Channels, Darby SE,

Simon A (eds). J Wiley: Chichester, UK, ISBN 0-471-98446-9; 97–122.

Thorne CR, Murphey JB, Little WC. 1981. Bank stability and bank material

properties in the blufﬂine streams of North–West Mississippi. Appendix

D, Report to the Corps of Engineers, Vicksburg District under Section 32

Program, Work Unit 7 prepared by USDA-ARS Sedimentation Laboratory,

Box 1157, Oxford, Mississippi, DeCoursey DG (ed), April 1981, 258 p.

Thorne CR, Soar PJ, Skinner KS, Sear DA, Newson MD. 2010. Investigat-

ing, characterising and managing river sediment dynamics. In Guidebook

of Applied Fluvial Geomorphology, Sear DA, Newson MD, Thorne CR

(eds). Thomas Telford: London; 120–195.

Thorp JH, Thoms MC, Delong MD. 2006. The riverine ecosystem synthe-

sis: biocomplextity in river networks across space and time. River Re-

search and Applications 22: 123–147.

Thorp JH, Flotemersch JE, Delong MD, Casper AF, Thoms MC,

Ballantyne F, Williams BS, O’Neill BJ, Haase S. 2010. Linking ecosys-

tem services, rehabilitation, and river hydrogeomorphology. BioScience

60:67–74. DOI: 10.1525/bio.2010.60.1.11

Tompkins M. 2006. Floodplain connectivity and river corridor complexity:

implications for river restoration and planning for ﬂoodplain manage-

ment. Dissertation University California Berkeley,CA. 392 p.

Triska FJ, Kennedy VC, Avanzino RJ, Zellweger GW, Bencala KE. 1989.

Retention and transport of nutrients in a third-order stream in northwestern

California: hyporheic processes. Ecology 70:1893–1905.

Walter RC, Merrits DJ. 2008. Natural streams and the legacy of water-

powered mills. Science 319: 299. DOI: 10.1126/science.1151716

Ward JV, Tockner K. 2001. Biodiversity: towards a unifying theme for

river ecology. Freshwater Biology 46: 807–819.

Williams JG. 2010. Lost in space, the sequel: spatial sampling issues with

1-D PHABSIM. River Research and Applications 26: 341–352. DOI:

10.1002/rra.1258.

Woelﬂe-Erskine C, Wilcox AC, Moore JN. 2012. Combining historical and

process perspectives to infer ranges of geomorphic variability and inform

river restoration in a wandering gravel-bed river. Earth Surface Processes

and Landforms. DOI: 10.1002/esp.3276

Wohl E, Angermeier PL, Bledsoe B, Kondolf GM, MacDonnell L, Merritt

DL, Palmer MA, Poff NL, Tarboton D. 2005. River restoration. Water

Resources Research 41: W10301. DOI: 10.1029/ 2005WR003985.

Wolman MG. 1955. the natural channel of Brandywine Creek, Pennsylvania.

USGS Professional Paper 271.

Wolman MG, Leopold LB. 1957. River ﬂood plains: some observations on

their formation. USGS Professional Paper 282-C. 87–107.

B. CLUER AND C. THORNE

154

DOI: 10.1002/rra

Rivers as Natural Capital Assets: A Quick Scoping Review to Assess the Evidence Linking River Asset Condition to Changes in the Flow of Ecosystem Services

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RESTOR ECOL

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Mar 2025
EARTH SURF PROC LAND

The term ‘legacy’ usually refers to past human activities that have long since ceased, but contemporary human activities can potentially leave future legacies. While legacy sediment is often defined as having upstream anthropogenic origins, such as from land clearing, infrastructure such as flood control structures and dams can trap sediment, whether naturally or anthropogenically derived, that can persist for centuries or more, thus leaving a legacy of past sedimentation. Dam removal is becoming more common, and accounting of sediment caused by the impoundment is an important component of planning a removal and its impacts. We provide four examples to explore mechanisms of legacy sediment generation: (1) land use changes with no infrastructure or dams, (2) a review of non‐dam infrastructure that abstracts water and substantially reduces sediment transport, (3) a case study from the longest‐duration geomorphic monitoring (2007 to 2023) of a short (<10 m) dam removed in 2008 and (4) a review of a well‐studied tall (>10 m) dam removal. When dams are removed, fluvial processes typically erode sediment in and directly adjacent to newly formed or opened stream channels over short timescales, but inaccessible deposits remain as legacy sediment. We demonstrate that overbank flooding and sedimentation upstream of run‐of‐river dams can create unrecognised deposits outside of the modern reservoir pool. We further outline how certain dam and other infrastructure characteristics, dam removal style and sediment character can generate persistent legacy sediment and sometimes legacy landforms outside active stream channels where it is unlikely to be excavated by natural geomorphic processes except in the most extreme events. With the increase in dam building and dam removal in various parts of the world, legacy sediment deposits are becoming ubiquitous, with important consequences for river and land management.

The Flood Pulse Concept in River-Floodplain Systems

Article

Full-text available

Sep 1989
CAN J FISH AQUAT SCI

Abstract JUNK, W. J., P. B. BAYLEY, AND R. E. SPARKS, 1989. The flood pulse concept in river-floodplain systems, p. 110-127. In D. P. Dodge [ed.] Proceedings of the International Large River Symposium. Can. Spec. Publ. Fish. Aquat. Sci. 106. The principal driving force responsible for the existence, productivity, and interactions of the major biota in river—floodplain systems is the flood pulse. A spectrum of geomorphological and hydrological conditions produces flood pulses, which range from unpredictable to predictable and from short to long duration. Short and generally unpredictable pulses occur in low-order streams or heavily modified systems with floodplains that have been leveed and drained by man. Because low-order stream pulses are brief and unpredictable, organisms have limited adaptations for directly utilizing the aquatic/terrestrial transition zone (ATTZ), although aquatic organisms benefit indirectly from transport of resources into the lotic environment. Conversely, a predictable pulse of long duration engenders organismic • adaptations and strategies that efficiently utilize attributes of the ATTZ. This pulse is coupled with a dynamic edge effect, which extends a "moving littoral" throughout the ATTZ. The moving littoral prevents prolonged stagnation and allows rapid recycling of organic matter and nutrients, thereby resulting in high productivity. Primary production associated with the ATTZ is much higher than that of permanent water bodies in unmodified systems. Fish yields and production are strongly related to the extent of accessible floodplain, whereas the main river is used as a migration route by most of the fishes. In temperate regions, light and/or temperature variations may modify the effects of the pulse, and anthropogenic influences on the flood pulse or floodplain frequently limit production. A local floodplain, however, can develop by sedimentation in a river stretch modified by a low head dam. Borders of slowly flowing rivers turn into floodplain habitats, becoming separated from the main channel by levées. The flood pulse is a "batch" process and is distinct from concepts that emphasize the continuous processes in flowing water environments, such as the river continuum concept. Flooclplains are distinct because they do not depend on upstream processing inefficiencies of organic matter, although their nutrient pool is influenced by periodic lateral exchange of water and sediments with the main channel. The pulse concept is distinct because the position of a floodplain within the river network is not a primary determinant of the processes that occur. The pulse concept requires an approach other than the traditional limnological paradigms used in lotic or lentic systems. Résumé JUNK, W. J., P. B. BAYLEY, AND R. E. SPARKS. 1989. The flood pulse concept in river-floodplain systems, p. 110-127. In D. P. Dodge [cd.] Proceedings of the International Large River Symposium. Can. Spec. Publ. Fish. Aquat. Sci . 106. Les inondations occasionnées par la crue des eaux dans les systèmes cours d'eau-plaines inondables constituent le principal facteur qui détermine la nature et la productivité du biote dominant de même que les interactions existant entre les organismes biotiques et entre ceux-ci et leur environnement. Ces crues passagères, dont la durée et la prévisibilité sont variables, sont produites par un ensemble de facteurs géomorphologiques et hydrologiques. Les crues de courte durée, généralement imprévisibles, surviennent dans les réseaux hydrographiques peu ramifiées ou dans les réseaux qui ont connu des transformations importantes suite à l'endiguement et au drainage des plaines inondables par l'homme. Comme les crues survenant dans les réseaux hydrographiques d'ordre inférieur sont brèves et imprévisibles, les adaptations des organismes vivants sont limitées en ce qui a trait à l'exploitation des ressources de la zone de transition existant entre le milieu aquatique et le milieu terrestre (ATTZ), bien que les organismes aquatiques profitent indirectement des éléments transportés dans le milieu lotique. Inversement, une crue prévisible de longue durée favorise le développement d'adaptations et de stratégies qui permettent aux organismes d'exploiter efficacement 1 'ATTZ. Une telle crue s'accompagne d'un effet de bordure dynamique qui fait en sorte que l'ATTZ devient un « littoral mobile'<. Dans ces circonstances, il n'y a pas de stagnation prolongée et le recyclage de la matière organique et des substances nutritives se fait rapidement, ce qui donne lieu à une productivité élevée. La production primaire dans l'ATTZ est beaucoup plus élevée que celle des masses d'eau permanentes dans les réseaux hydrographiques non modifiés. Le rendement et la production de poissons sont étroitement reliés à l'étendue de la plaine inondable, tandis que le cours normal de la rivière est utilisé comme voie de migration par la plupart des poissons.

Wetlands Explained: Wetland Science, Policy, and Politics in AmericaWetland Science, Policy, and Politics in America

Book

Nov 2001

William M. Lewis

This book brings together in compact form a broad scientific and sociopolitical view of US wetlands. This primer lays out the science and policy considerations to help in navigating this branch of science that is so central to conservation policy, ecosystem science and wetland regulation. It gives explanations of the attributes, functions and values of our wetlands and shows how and why public attitudes toward wetlands have changed, and the political, legal, and social conflicts that have developed from legislation intended to stem the rapid losses of wetlands. The book describes the role of wetland science in facilitating the evolution of a rational and defensible system for regulating wetlands and will shed light on many of the problems and possibilities facing those who quest to protect and conserve our wetlands.

River restoration in the Alps and their surrounding: past experiences and future challenges

Article

Jan 2008

Geomorphology, ecology and river channel habitat: mesoscale approaches to basin-scale challenges

Article

Jun 2000

Technical Aspects of Wetlands History of Wetlands in the Conterminous United States

Article

Jan 1996

The ecological basis for the management of the natural river environment

Article

Jan 1995

River Channel Management: Towards Sustainable Catchment Hydrosystems

Article

Jan 2014

River Channel Management is the first book to deal comprehensively with recent revolutions in river channel management. It explores the multi-disciplinary nature of river channel management in relation to modern management techniques that bear the background of the entire drainage basin in mind, use channel restoration where appropriate, and are designed to be sustainable.

Gravel-Bed Rivers VI – From Process Understanding to River Restoration

Article

Jan 2007

978-0-444-52861-2 Book, Edited

Determinate Hydraulic Geometry of River Channels

Article

Jun 1978

Richard David Hey

Natural channels have the ability to adjust their flow and bankfull hydraulic geometry in response to flow conditions. Nine governing equations are necessary to define the complete adjustment process and relate channel response to discharge, sediment load, bed, and bank sediment, and the valley slope. A preliminary analysis enables the nature of this process-response model to be identified although insufficient information is known about all the processes for it to be used for design purposes. The necessary dependent and independent variables for the development of empirical design equations are identified by the model and it also confirms the determinate nature of alluvial channels.

Riparian vegetation response to altered disturbance and stress regimes

Article

Feb 2002

River damming and flow regulation can alter disturbance and stress regimes that structure riparian ecosystems. We studied the Bill Williams River in western Arizona, USA, to understand dam-induced changes in channel width and in the areal extent, structure, species composition, and dynamics of woody riparian vegetation. We conducted parallel studies along a reference system, the Santa Maria River, an unregulated major tributary of the Bill Williams River. Flood magnitude on the Bill Williams River has been dramatically reduced since the closure of Alamo Dam in 1968: the 10-yr recurrence interval flood in the pre-dam era was 1397 m(3)/s vs. 148 m(3)/s post-dam. Post-dam average annual flows were higher due to increased precipitation in a few years, but increases in post-dam May-September flows are largely attributable to dam operation. An analysis of a time series of aerial photographs showed that channels along the Bill Williams River narrowed an average of 111 m (71%) between 1953 and 1987, with most narrowing occurring after dam closure. Multiple regression analysis revealed significant relationships among flood power, summer flows, intermittency (independent variables), and channel width (dependent variable). The pattern of channel width change along the unregulated Santa Maria River was different, with less narrowing between 1953 and 1987 and considerable widening between 1987 and 1992. Woody vegetation along the Bill Williams River was denser than that along the Santa Maria River (27 737 stems/ha vs. 7559 stems/ha, P = 0.005), though basal areas were similar (14.3 M(2)/ha vs. 10.7 M(2)/ha, P = 0.42). Patches dominated by the exotic Tamarix ramosissima were marginally (P = 0.05) more abundant along the Bill Williams River than along the Santa Maria River, whereas the abundance of patches dominated by the native Populus fremontii or Salix gooddingii was similar across rivers (P = 0.30). Relative to Populus and Salix, Tamarix dominates floodplain vegetation along the Bill Williams River (P < 0.0001). Most stands of the dominant pioneer trees on both rivers became established in the 1970s and 1980s. Recent seedling establishment occurred in wider bands along the Santa Maria River (15.3 m wide vs. 5.4 m wide on the Bill Williams River, P = 0.0009), likely due to larger floods and associated seedbed formation along the Santa Maria River. Seedling survival rates were generally higher along the Bill Williams River, perhaps due to higher summer flows.

A Stream Evolution Model Integrating Habitat and Ecosystem Benefits

Abstract

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