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5 (Ir)reversibility and the river evolution diagram revisited

5 (Ir)reversibility and the river evolution diagram revisited

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Geomorphic responses of rivers to human disturbance

disturbance is inverse to the inherent resilience of

the system.

In general terms, when rivers are subjected to

relatively low levels of impact spread over a considerable period of time, they progressively adjust

while maintaining a roughly equivalent state. In

systems in which the natural behavioral regime

and morphological character of rivers fluctuate

among multiple states, gradual and low impact

forms of human disturbance may increase the periodicity with which changes among these various

states take place and the capacity for adjustment

expands. However, these adjustments are unlikely

to push the system to a new state that falls outside

the contemporary capacity for adjustment. In this

instance, although rates of change are modified,

adjustments tend to be localized and reversible.

These sorts of adjustments tend to occur in relatively resilient systems.

Systems that are resilient to geomorphic change

under natural conditions are unlikely to demonstrate significant geomorphic responses, regardless of the nature and extent of human-induced

disturbance. In these instances, the capacity for

system adjustment is so limited that profound

human disturbance may only bring about negligible landscape responses. These reaches absorb the

impacts of disturbance, typically through negative

feedback mechanisms that enable rapid adjustment after disturbance (Thomas, 2001; Werritty

and Leys, 2001). Stability thresholds ensure that a

resilient system will only fail under exceptional

stress. As a consequence, these reaches tend to experience reversible geomorphic change, with only

modest or localized adjustments to their geomorphic configuration.

When reversible geomorphic change occurs, a

fundamental shift in the type of river does not

occur. This is represented by a shift from Zone A to

Zone B in Figure 7.10. During these changes, the

key defining attributes of the type of river do not

change (i.e., the key geomorphic units remain unaltered). However, other structural and functional

attributes of the river are considered to be out-ofbalance. For example, a sand-bed meandering river

with floodplain ridges and swales, instream point

bars, pools and riffles, may naturally be expected to

have a sinuosity of between 1.7 and 2.0 with occasional cutoff formation occurring every 100 years

when operating under a certain set of flux bound-



233



ary conditions. With human disturbance, the sinuosity of the channel may span a range between 1.5

and 2.2, and cutoffs may occur every 30 years. The

key geomorphic structure of this type of river has

not been altered, but the rate of adjustment has

been accelerated, and the range of adjustment

has been widened. Hence, the potential exists for

the river to operate outside its natural capacity for

adjustment. Ongoing adjustments are reversible.

In contrast, if profound human disturbance is

instigated over a short period of time, threshold

conditions may be breached, pushing the system

outside its long-term range of behavior, and metamorphosis may ensue. This transition may take

the form of a relatively simple, one-step transformation such that the system oscillates around

a new state, or disturbance may set in train

progressive adjustments around multiple states.

Regardless of these latter scenarios, changes from

the predisturbance condition are likely to be irreversible over management timeframes. These

types of responses tend to occur along sensitive

reaches that are vulnerable to disturbance and

exhibit internal instability (see Chapter 6).

Depending on the nature and severity of human

disturbance, sensitive reaches can undergo a

fundamental and persistent change in their morphology and associated process domain, thereby

becoming a different type of river (Werritty and

Leys, 2001).

When irreversible change occurs, a wholesale

shift in the type of river occurs. This is represented

on Figure 7.10 by a shift from Zone A to Zone C if

change is induced from a natural state, or a shift

from Zone B to Zone C if change is induced by continued/sustained human disturbance. In this case,

the key defining attributes of the type of river have

changed (i.e., the key geomorphic units have

changed) and other structural and functional attributes have been altered, forming a different type

of river. Using the same example presented above,

if the sand-bed meandering river (represented by

Zone B in Figure 7.10) is affected by indirect

human disturbance, such that a sediment slug or a

headcut passes through the reach, the channel may

straighten to a sinuosity of 1.1. Instream geomorphic units may change from bank-attached point

bars to midchannel longitudinal bars. The floodplain may change from a lateral accretion system

to one that is dominated by vertical accretion of



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Chapter 7



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sand sheets. A low sinuosity sand-bed river has

been formed. The geomorphic character and behavior of this river has been fundamentally and irreversibly altered over management timeframes.

Various examples of river evolution diagrams that

integrate the impacts of human disturbance are

presented in Figures 7.11–7.14.

Figure 7.11 presents changes to the geomorphic

character and behavior of the Hunter River, in

New South Wales, Australia, following construction of Glenbawn Dam. Prior to 1958, this system

operated as a partly-confined river with bedrockcontrolled floodplain pockets. The channel comprised an array of gravel point bars, bedrock pools,



Figure 7.11 Geomorphic responses of

the Hunter River, New South Wales,

Australia to dam construction

Dam construction along the upper

Hunter River did not bring about a

change in river type, but the capacity

for adjustment of the river was

narrowed (see text). This reflects

the limited range of possible

adjustments that could be

experienced by this type of river.

Although the flow regime has been

markedly altered, the geomorphic

structure of the river remains

unchanged. Photograph (a) shows the

river downstream of the dam, while

photograph (b) demonstrates the

maintenance of base flow conditions

following dam construction.

Photographs were kindly provided

by Mark Elsley.



and gravel riffles, with discontinuous pockets of

floodplain on the insides of bends. Dam construction and the modified flow regime exerted relatively minor changes to the geomorphic structure of

this relatively resilient bedrock-controlled river

(Erskine, 1985). The river retains the key geomorphic attributes of the same type of river that was

evident prior to dam construction (i.e., changes

have been reversible in geomorphic terms, as

noted on the right-hand side of Figure 7.11).

However, the dam has had a range of secondary

geomorphic impacts. The dam traps bedload

material supplied from the upper catchment.

Downstream of the dam, degradation and armor-



Geomorphic responses of rivers to human disturbance



235



Figure 7.12 Impacts of clearance of

riparian vegetation and removal

of woody debris on geomorphic

changes to the Cann River, Victoria,

Australia

Prior to direct forms of human

disturbance, Cann River operated as

a slowly migrating, slowly accreting

river that was subjected to

occasional avulsion (see Figure 7.4).

Following clearance of riparian

vegetation and removal of woody

debris, the system became so

sensitive to change that the next

formative event breached threshold

conditions (see text). As the channel

incised and expanded, it operated as

a high energy system with much

greater capacity to transport

materials than its predisturbance

state. Hence its capacity for

adjustment was expanded.

Irreversible changes occurred over

timeframes of hundreds or

thousands of years. The photographs

show (a) the adjacent Thurra River

which remains in an intact

condition, (b) the Cann River as it is

today.



ing have occurred. The flow regime has been altered, as water releases from the dam maintain

base flow conditions for irrigation purposes. Peak

flows have been reduced, and the seasonality of

flow has been altered. In many ways the capacity

for adjustment of the river has been suppressed, as

the range of flux boundary conditions has been reduced. This is represented on the river evolution

diagram by the narrowing of the inner band on the



right hand side of Figure 7.11. Similarly, the amplitude and frequency of the pathway of adjustment

has been reduced, reflecting the lower geomorphic

effectiveness of flood events.

Figure 7.12 conveys changes to the Cann River

in East Gippsland, Australia following removal of

riparian vegetation and desnagging operations in

the mid-twentieth century (Brooks et al., 2003;

Brooks and Brierley, 2004). Prior to European set-



236



Chapter 7



tlement of the area, the river operated as a low capacity, slowly meandering sand-bed channel with

a high loading of woody debris and rainforest vegetation association on the floodplain (Figure 7.4).

Every few thousand years the river was subjected

to avulsion, as indicated by the natural capacity for

adjustment on the left-hand side of Figure 7.12.

Following human disturbance, and the associated

passage of a sediment slug, river character and behavior were fundamentally altered, as the system

was transformed into a low sinuosity sand-bed

river (depicted on the right-hand side of Figure

7.12). The system has been irreversibly altered

over management timeframes. Channel incision

and lateral expansion have created a low sinuosity

trench that is largely decoupled from the floodplain. Rates of sediment transfer are several orders

of magnitude higher than prior to disturbance. The

capacity for adjustment of the new river system is

far greater than its predecessor. The pathway of adjustment has been altered to reflect the change in

river behavior. As the energy of the system has increased significantly, the new river type sits higher

within the potential range of variability. Based on

sediment supply and transport rates in the contemporary system, it is estimated that it would

take many thousands of years for the system to

recover to its predisturbance state (Brooks and

Brierley, 2004).

Impacts of channelization on the Ishikari River

in Hokkaido, Japan are conveyed in Figure 7.13.

Prior to channelization, this river system was a

low energy, fine-grained meandering river with

a marshland floodplain (left-hand side of Figure

7.13). Large wetlands and cutoffs occurred on the

floodplains. After the Second World War, the city

of Sapporo expanded significantly, and additional

land along the Ishikari River was required for development. The marshlands were drained and

resurfaced with fill, and an extensive channelization scheme was undertaken along the lower

Ishikari River. A canal was dredged and lined with

concrete bricks. Meander bends were cutoff and

plugged, significantly shortening the river, to convey flood flows as efficiently as possible to the sea.

An extensive network of flood control structures

and canals was emplaced, some utilizing the old

channel network. The Ishikari River was irreversibly altered and retains little in the way of its

inherent geomorphic diversity or ecological value.



The energy of the system has likely increased, but

the capacity for adjustment has been severely constrained (right-hand side of Figure 7.13). Water

quantity and sediment supply are stringently controlled through the use of reservoirs and weirs, producing a regularly fluctuating, artificial pathway

of adjustment. Other than localized bank erosion,

little geomorphic adjustment is allowed to occur.

Changes to rivers in Rhone Basin of the French

Pre-Alps in the last 400 years are presented in

Figure 7.14 (based on Bravard et al., 1999; Piégay et

al., 2000). Multiple responses to various forms of

human disturbance have been recorded in these

catchments. Most rivers experienced channel

metamorphosis following deforestation of the

upper catchment and a resulting increase in bedload transport that induced the development of a

braided planform. The capacity for adjustment of

these rivers was high, as indicated on the left-hand

side of Figure 7.14. Wide and shallow channels

transported and stored significant volumes of

gravel, and floodplains were subjected to regular

flooding. In response, channel embankments were

built and gravel extraction became common.

Subsequently, in the mid–late nineteenth century,

afforestation and erosion control management

strategies were applied in the upper catchment, including construction of artificial reservoirs. This

altered the yearly water fluxes, reduced peak discharges, and decreased seasonal flows. As a result

of these management practices, sediment supply

from upstream decreased and incision occurred

downstream. In the case of the Drome River, channel incision averaged around 3 m and extended to

bedrock or an armored layer (Piégay and Schumm,

2003). Channel incision led to the formation of a

single channel within the previous braid plain.

Channel dynamism decreased and artificial levees

were undermined as bends moved and became

cutoff (Piégay et al., 2000). The river had been

transformed to a meandering gravel bed river

(right-hand side of Figure 7.14). Subsequent

encroachment of vegetation into this alluvial

corridor has led to channel constriction and the

formation of inset floodplain surfaces (Bravard et

al., 1999; Piégay and Schumm, 2003). Management

strategies now aim to reinstigate a braided river

system in parts of these catchments through artificial injection of gravel and removal of artificial

sediment storage reservoirs from the upper catch-



Geomorphic responses of rivers to human disturbance



237



Figure 7.13 Impacts of channelization

and floodplain drainage on the Ishikari

River, Hokkaido, Japan

Channelization of the Ishikari River in

Hokkaido, Japan dramatically altered

the geomorphic structure and function

of the river. Irreversible geomorphic

changes marked the transition from

a meandering fine grained system to

a local sinuosity canal (see text).

Although the capacity for adjustment

is shown to have expanded on the river

evolution diagram, the river oscillates

within a relatively narrow zone most of

the time (i.e., the capacity for

adjustment has been suppressed and

the natural capacity for adjustment has

been eliminated). Infrequent high

magnitude events flood areas beyond

the channel zone. Photographs were

kindly provided by Tomomi Marutani.



ment (Bravard et al., 1999). As the energy of the system has decreased over time, the contemporary

sinuous single-thread pattern sits at a lower position within the potential range of variability on the

river evolution diagram, and has a different pathway of adjustment than the former braided river

configuration (Figure 7.14).

The transformation of river courses, whether induced by a thousand cuts, or near-instantaneous

changes to boundary conditions, presents an

important context with which to interpret likely

future pathways and rates of geomorphic adjust-



ments. The contextual information outlined

above presents critical insights with which to

guide management efforts that strive to work with

the variable and dynamic nature of river forms and

processes. Appraisal of contemporary river character and behavior in context of former conditions

can be used to determine whether human-induced

changes to catchment boundary conditions have

resulted in irreversible geomorphic changes over

management timeframes. Such assessments

have major implications for identifying reference

conditions against which to assess geomorphic



238



Chapter 7



Figure 7.14 The effect of human

disturbance on rivers in the French

Pre-Alps

Rivers of the French Pre-Alps were

in a disturbed state with significant

capacity for adjustment in the late

nineteenth century (see text). Rural

depopulation, along with

government reafforestation

programs, facilitated the recovery of

these systems. Many braided gravelbed systems returned to a

meandering configuration, such

that rivers now operate as lower

energy system that are subjected

to different forms of geomorphic

activity (and associated pathways of

adjustment).



condition and recovery potential (see Chapters

10 and 11). These considerations determine what

character and behavior is considered to provide

an appropriate benchmark against which to appraise the contemporary condition of any given

reach under prevailing boundary conditions (see

Part C).

In the River Styles framework, contemporary attributes of rivers are related to the capacity for

change under current conditions, whether that

represents an irreversibly altered human-induced

set of conditions or otherwise. In making these assessments, the range and rate of contemporary

processes along a reach are related to a “natural”

condition (i.e., how the reach is expected to look



and behave in the absence of human disturbance).

Assessment of whether river response to human

disturbance is reversible or permanent is appraised

in terms of the assemblage of river forms and

processes along the reach (i.e., the assemblage of

geomorphic units).



7.6 Synopsis

Regardless of underlying causes, whether natural,

purposeful, or unintended/accidental, all river systems are subject to disturbance events that promote adjustments to their behavioral regime. In

some instances, change occurs. Most rivers have



Geomorphic responses of rivers to human disturbance

suffered detrimental effects of human disturbance.

These impacts have gathered momentum over

time, especially since the nineteenth century. The

construction of structures such as dams, levees,

and concrete-lined trapezoidal channels, and

activities such as sand/gravel extraction, have

induced enormous damage to river structure and

function. Many channels have been homogenized

and effectively separated from their floodplains.

Intended modifications have resulted in a range of

unintentional consequences, such as changes to

flow and sediment transfer regimes, patterns and

rates of erosion and sedimentation, hydraulic resistance, and flow velocity. Changes to geomorphic river character and behavior have brought

about significant adjustments to habitat availability, species diversity, and aquatic ecosystem

functioning.

In many parts of the world, management strategies now strive to work with natural processes and

enhance river recovery, aiming to undo the consequences of past actions. In many areas, notable improvements in river condition mark a reversal in

the trend of environmental degradation. Learning

lessons from past experiences in this age of repair



239



requires detailed documentation, interpretation,

and explanation of how past and present human

impacts shape the catchment-specific nature and

rate of river forms and processes, unraveling these

impacts in the light of natural system variability.

Human impacts on river systems vary in type

and extent, ranging from site-specific works along

a particular reach (e.g., bridge construction or emplacement of a stormwater outlet) to catchmentwide changes in ground cover. Catchment-specific

attributes, and variability in the character, extent,

history, and rate of human-induced disturbance,

ensure that cumulative changes induced by

human impacts are system-specific. In many settings it is now impossible for rehabilitation programs to regain some form of predisturbance

condition. As such, management efforts must

work towards the best-achievable river structure

and function given the prevailing boundary conditions under which any given reach operates. The

River Styles framework provides a catchmentbased physical platform with which to guide management activities that respect the inherent

diversity and ever-changing nature of river systems, as outlined in the next part of this book.



PA RT C

The River Styles framework

(D)evelopment of sustainable management strategies for aquatic ecosystems requires an

intimidatingly sophisticated level of knowledge of the spatial context and causal linkages

among human actions, watershed processes, channel conditions, and ecosystem response. . . .

(W)e should be cautious about our ability to predict ecosystem response based on simplified

models of complex systems. . . . (L)andscape management strategies founded upon documented

linkages between geomorphological processes and ecological systems should be developed based

on sound data and relationships supported by appropriately scaled models, rather than

predicated on the predictions of complicated, overparameterized models. . . . (N)o simple

cookbooks or manuals . . . can capture the inherent regional complexity or interactions

between geomorphic processes, riverine habitat and ecological systems. We can translate

understanding based on the general physics that underpins fluvial geomorphology to any

region; however, it is much more difficult to generalize how regional differences interact with that

physics to structure the manner in which river processes influence ecological systems

(and vice-versa). Consequently, we need to pursue regional research programs to develop

a sound empirical basis for understanding system behavior and for developing models to

usefully extrapolate system behavior into the management arena.

Dave Montgomery, 2001, pp. 247–52



Overview of Part C

The River Styles framework provides a coherent,

catchment-wide template for river management

activities. Key considerations that underpin the

framework include:

• emphasis is placed on linkages between river

forms and processes and their capacity to adjust in

any given setting. Various attributes of river character are tied directly to interpretations of river behavior. Appreciation of river dynamics lies at the

heart of the framework;

• procedures are applied at the catchment

scale, focusing on controls on river character and

behavior, and their linkages, within any given

system;

• appraisals of geomorphic river condition and recovery potential form separate layers of analysis

that build on evolutionary trajectories of each

reach in the catchment;

• collectively, these insights provide an information base with which foresighting exercises are ap-



plied to predict likely river futures, providing a

future-focus for management applications.

The first five chapters in this part document the

various components of the River Styles framework. In Chapter 8 an overview of the framework

is presented, highlighting how the approach breaks

down the diversity and changing nature of river

forms and processes. Issues considered include an

overview of the principles required for an effective

river classification scheme, how the River Styles

framework addresses these issues, and practical

considerations in application of the framework.

Chapter 9 documents Stage One of the framework.

This entails catchment-scale mapping of river

character and behavior, and explanation of downstream patterns of river types. In Chapters 10 and

11 approaches used to analyze geomorphic river

condition (Stage Two) and recovery potential

(Stage Three) are presented. Human-induced

changes to river forms and processes are analyzed

in light of natural (ongoing) evolutionary tendencies to frame the (ir)reversibility of river adjust-



242



Part C



ments. From this, the trajectory of likely future

geomorphic river condition and/or recovery

potential is appraised. Chapter 12 outlines how information on river character and behavior, condition, and recovery potential provides a geomorphic

template for river management practice. This

forms Stage Four of the River Styles framework.

Throughout these chapters, flow diagrams are



used to depict the sequence of steps through which

information is collated. Various boxes are presented to provide examples of the types of products derived from each stage of the framework. These

examples are drawn from the Bega catchment, on

the south coast of New South Wales, Australia,

where the framework has been applied in its

entirety.



C H A PT E R 8

Overview of the River Styles framework and

practical considerations for its application

There is nothing more basic than categorization to our thought, perception, action, and speech.

Every time we see something as a kind of thing, for example, a tree, we are categorizing.

Whenever we reason about kinds of things – chairs, nations, illnesses, emotions, any kind of thing

at all – we are employing categories. . . . Without the ability to categorize, we could not function

at all, either in the physical world or in our social and intellectual lives. An understanding of

how we categorize is central to understanding of how we think and how we function, and

therefore central to an understanding of what makes us human.

George Lakoff, 1987, pp. 5–6



8.1 Moves towards a more integrative river

classification scheme

The inherent complexity of the natural world

presents many problems in the development of a

workable and comprehensive approach to river

classification. Rather than endeavor to create a

universal scheme with which to frame management efforts in a prescriptive sense, the approach

to breaking down reality adopted in this book provides a learning tool that can be applied to determine the geomorphic components of any given

landscape. In no sense, however, do these considerations represent an endpoint for management

applications – quite the opposite, in fact!

Part B of this book outlined the conceptual underpinnings of approaches to analyze river character (Chapter 4), behavior (Chapter 5), and change

(Chapter 6). Key principles that emerge from these

chapters are outlined in Table 8.1. These principles

form a series of filters of information upon which

the four stages of the River Styles framework are

built, as indicated in Figures 8.1 and 8.2. Analyses

of geomorphic river character and behavior,

viewed from cross-sectional and planform perspectives, provide a platform upon which separate

sets of procedures are used to appraise geomorphic

river condition (Figure 8.1). Interpretation of river



evolution is applied to determine whether the condition of any given reach of a particular River Style

has deteriorated or improved over time. From this

catchment-wide appraisal, and using insights into

biophysical fluxes and associated linkages, future

scenarios are constructed to appraise geomorphic

river recovery potential (Figure 8.2). In the examples shown, this is framed in terms of future sediment availability and off-site impacts of change.

These types of analysis provide a platform for

informed and geoecologically sound river

management.

Various practical issues that underpin the approach to geomorphic river classification applied

in the River Styles framework are summarized in

Table 8.2. Just as important as design attributes of a

classification scheme, however, is the way in

which it is used. Procedures must be applied in a

consistent and nonprescriptive manner. The timeframe of reference and resources used in making

the classification must be stated explicitly.

Although rigor in application is essential, there are

inherent dangers in overly rigid, prescriptive, and

inflexible classification schemes. Any classification scheme must be used with caution, guiding

observations rather than structuring what is

seen (see Miller and Ritter, 1996; Thorne, 1997;

Kondolf, 1998c; Newson et al., 1998; Goodwin,



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