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2 Stage Three, Step One: Determine the trajectory of change

2 Stage Three, Step One: Determine the trajectory of change

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328



Chapter 11



Figure 11.3 Stage Three, Step One:

Determining the trajectory of

change



Reaches on the creation trajectory are adjusting to

altered flux boundary conditions.

The future recovery trajectory is dependent on

the history of change (i.e., whether irreversible

geomorphic change has occurred), present reach

conditions, and prevailing flux boundary conditions. The further down the degradation scale a

reach sits, the less likely it is that it will recover

along the restoration pathway, and creation is the

most likely future scenario. Reaches that have experienced reversible geomorphic change have the

potential to recover along the restoration pathway.

These reaches are generally in good or moderate

geomorphic condition and sit higher on the degradation pathway. However, if flux boundary conditions have been severely altered, creation is

underway. These reaches tend to be in poor condition and sit low on the degradation pathway.

Ultimately, both the restoration and creation pathways reflect the operation of self-healing processes

that improve river condition. By definition, reaches that have experienced irreversible geomorphic

change are unable to recover along the restoration

pathway. As such, they recover along a creation

pathway irrespective of their contemporary geomorphic condition.

Since effective rehabilitation strategies work

with the contemporary condition and trajectory of

river changes, the position of each reach on the recovery diagram must be determined (Figure 11.1).

The procedure used to assess the trajectory of

change in the River Styles framework is outlined

in Figure 11.3. With the evolutionary framework

of river changes in-hand (Stage Two, Step Two), the

position of each reach on the recovery diagram is

determined using a decision tree. Based on this



analysis, stages of adjustment for each reach and

trajectory of change are interpreted.

11.2.1 Position each reach on the evolutionary

sequence of the River Style

The first step in determining the trajectory of

change is to apply ergodic reasoning and position

the reach under investigation on the evolutionary

sequence for each River Style constructed in Stage

Two, Step Two.

11.2.2 Translate each evolutionary timeslice

onto the recovery diagram using the

decision tree

The second step towards assessing the trajectory of

change involves translating the entire evolutionary sequence of the River Style onto the recovery

diagram. The position of each timeslice relates directly to its geomorphic condition and past evolutionary adjustments. Figure 11.4 summarizes the

decision-making process that is applied for each

reach in the catchment.

11.2.2.1 Is the reach intact?

The first decision involves evaluating whether

the reach has developed a range of attributes such

that contemporary river condition does not

differ from an intact (predisturbance) state. Intact

reaches operate within the natural capacity for

adjustment of a River Style in the absence of

human disturbance. A comparison is made between the character and behavior of the intact version of a River Style and the contemporary reach.



Stage Three of the River Styles framework



329



Figure 11.4 Decision tree for

determining the trajectory of

change of a reach. Modified from

Fryirs and Brierley (2000) with

permission from © V.H. Winston &

Son, Inc., All rights reserved



If the geomorphic character and behavior of the

reach are not significantly different from the predisturbance condition, the reach is considered to

be intact.

11.2.2.2 Has the reach started to recover?

A reach that is recovering has moved to the righthand side of Figure 11.1. Assessment of whether

geomorphic recovery is underway is determined

through interpretation of historical information

and knowledge about system structure and function derived in Stages One and Two of the River

Styles framework. In particular, analysis of

changes to the assemblage of geomorphic units

within a reach provides a key indicator of whether

recovery is underway. For example, the formation

of benches that narrow enlarged channels are

an initial recovery mechanism. Similarly, the

reemergence of pools and the redefinition of a low

flow channel following the passage of a sediment

slug are indicative of geomorphic recovery.

Reaches that show initial signs of recovery are considered to be at the turning point (Figure 11.1).

These reaches are generally in moderate geomorphic condition.

If the reach has been altered, and is experiencing

progressive deterioration in its geomorphic condition, it is placed in the degraded category. These

reaches are generally in poor or moderate geomorphic condition. In degrading reaches, the range of

geomorphic units is inappropriate for the environ-



mental setting. For example, sand sheets that have

infilled pools or cover the floodplain indicate that

there is an oversupply of sediment to that reach.

Such reaches are adjusting their character and behavior to this change in sediment regime. Specific

indicators of degradation or modified rates of geomorphic change may be detected, without any indication that processes of geomorphic recovery are

underway.

11.2.2.3 Is restoration possible?

The final two steps in the decision tree shown

in Figure 11.4 determine whether a reach is adjusting along a restoration or creation pathway. The

recovery trajectory is dependent on the history of

change (i.e., whether irreversible geomorphic

change has occurred), the present reach condition,

and the flux boundary conditions under which the

reach operates (i.e., altered or unaltered). If flux

boundary conditions have not been significantly

altered along reaches that have experienced reversible geomorphic change, there is potential for

restoration to occur. These reaches are generally in

good geomorphic condition and sit high on the

degradation pathway. Restoration is underway

when the geomorphic unit assemblage of a reach

approximates the intact, predisturbance character

and behavior. For example, the reinstigation of

swamps and discontinuous watercourses are indicative of geomorphic restoration along cut-andfill rivers.



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Chapter 11



11.2.2.4 Is the reach adopting a new condition?

Reaches that have experienced irreversible geomorphic change recover along the creation

pathway irrespective of their contemporary geomorphic condition. These reaches do not have the

capacity to return to a predisturbance character

and behavior under prevailing boundary conditions, and restoration is no longer a viable option.

In this case, a “new” condition is defined, and

reaches recover towards the best attainable condition for the contemporary River Style. Reaches

that have experienced irreversible geomorphic

change were identified in Stage Two, Step Two of

the River Styles framework. If irreversible change

led to a shift in River Style such that the reach now

displays geomorphic structures that were not present at any stage in the evolutionary sequence, a

solid line is placed across the degradation pathway

of the recovery diagram. This depicts the irreversible nature of change and adjustment along the

creation trajectory (Figures 11.5 and 11.7).

Reaches that have experienced reversible geomorphic change can recover along either a restoration or creation pathway, depending on the

geomorphic condition of the reach and the degree

to which human disturbance has altered the flux

boundary conditions under which the reach operates (Figure 11.6). Reaches that are in poor geomorphic condition, and which operate under altered

catchment boundary conditions, sit lower on the

degradation scale and are less likely to recover

along a restoration pathway. Under these conditions, creation is the most likely future scenario.

The further down the degradation scale a reach

sits, the less likely it is that it will recover along a

restoration pathway.

11.2.3 Determine the trajectory of change

of each reach in the catchment (i.e., either

degradation, restoration, or creation)

Using the recovery diagram for each River Style, a

direct assessment is made of the trajectory of

change of each reach in the catchment. The position of each reach in the evolutionary sequence for

that River Style determines where it sits on the recovery diagram. In turn, the position of the reach

on the recovery diagram determines its trajectory

of change. If the reach sits on the degradation path-



way it shows signs of continued deterioration. If

the reach sits at the turning point, the nature of

future flow and sediment regimes will dictate

whether the reach continues to deteriorate or

moves onto a recovery pathway. If the reach shows

signs of recovery, it sits on either a creation or

restoration pathway. Examples of the resultant

diagrams are presented in Figures 11.5–11.7.



11.3 Stage Three, Step Two: Assess river recovery

potential: Place reaches in their catchment context

and assess limiting factors to recovery

The effectiveness and extent of river recovery

processes reflect a wide range of catchmentspecific considerations. Reaches of different River

Styles are at different phases of degradation and recovery. The character and mechanisms of recovery

differ from system to system conditioned by the

type of river, the extent of disturbance, reach position relative to off-site or lagged disturbance responses, and a range of physical issues related to

limiting factors operating within the catchment.

Assessment of limiting factors and pressures that

constrain the recovery of landscapes and their associated ecosystems enables the adoption of management strategies that minimize the impacts of

these constraints. Such practices address underlying causes of change, rather than their symptoms.

These factors must be assessed on a catchment-bycatchment and reach-by-reach basis. In the River

Styles framework, the changing nature of withincatchment linkages of physical processes is

examined, relating disturbances in one part of a

catchment to off-site impacts and lagged responses

elsewhere.

Given that rivers are evolving, adjusting entities, a reach can sit at any position on the degradation pathway and, given favorable conditions, may

move onto a recovery pathway at any point.

However, even though a reach may sit on a recovery trajectory, the river may not have the “potential” to move along that pathway. Recovery

potential is defined as the capacity for improvement in the geomorphic condition of a reach in the

next 50–100 years. Under the same set of environmental conditions, a reach may potentially adjust

along multiple recovery trajectories. Analysis

of the pathway of adjustment, and prediction of



Stage Three of the River Styles framework



331



z



Figure 11.5 Trajectories of change for reaches of the channelized fill River Style in Bega catchment (from Fryirs, 2001).

The letters equate to the evolutionary stage in Figure 10.5

In Bega catchment, reaches that are sensitive to change have experienced irreversible change in River Style

(represented by the black lines on the degradation pathway). In this case, the river has changed from an intact valley

fill River Style to a channelized fill River Style. Reaches of the latter now operate under altered catchment boundary

conditions. These reaches are unlikely to recover along a restoration pathway over the next 50–100 years. Hence,

creation of a new condition is underway along these reaches.



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