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3 Stage Three, Step Two: Assess river recovery potential: Place reaches in their catchment context and assess limiting factors to recovery

3 Stage Three, Step Two: Assess river recovery potential: Place reaches in their catchment context and assess limiting factors to recovery

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Stage Three of the River Styles framework



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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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Figure 11.6 Trajectories of change for reaches of the partly-confined valley with bedrock controlled discontinuous

floodplain pockets River Style in Bega catchment. Capital letters equate to the evolutionary stage in Figure 10.6.

Modified from Brierley et al. (2002). Reproduced with permission from Elsevier, 2003

River Styles that have experienced reversible geomorphic change have the potential to adjust along either a

restoration or creation pathway. Reaches of the partly-confined valley with bedrock controlled discontinuous

floodplain pockets River Style fall into this category. The trajectory taken depends largely on the condition of the

reach and the degree to which human disturbance has altered the flux boundary conditions. The poorer the condition

of the reach, the lower it sits on the degradation pathway of the recovery diagram, and the less likely it is that

restoration will occur. In these cases creation is underway.



Stage Three of the River Styles framework



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Figure 11.7 Trajectories of change for reaches of the low sinuosity sand-bed River Style in Bega catchment (from Fryirs,

2001). Capital letters equate to the evolutionary stage in Figure 10.7

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 a low sinuosity

fine-grained River Style to a low sinuosity sand-bed 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, this recovery pathway has been eliminated for this River Style. Creation of a new condition is

underway along these reaches.



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recovery potential, requires analysis of limiting

factors to recovery and the connectivity of geomorphic processes, placing each reach within its

catchment context. This foresighting process

entails analysis of river character and behavior,

sensitivity to change, proximity to a threshold,

geomorphic condition, position in the catchment,

and limiting factors and pressures, building on

analyses completed in Stages One and Two of the

River Styles framework. Procedures used to assess

river recovery potential are shown in Figure 11.8.

11.3.1 Determine reach sensitivity and

geomorphic condition

Analyses completed in Stage Two of the River

Styles framework are used to determine the sensitivity of a reach to change and its geomorphic con-



dition. Sensitivity to change reflects the capacity

for adjustment of each River Style. Reaches with

significant adjustment potential are sensitive to

change, while those with limited or localized adjustment potential are resilient to change. In Stage

Two, Step Three, reaches are differentiated into

good, moderate, and poor condition categories. In

general, adjustments to sensitive reaches drive

propagation of geomorphic change through a

system.

11.3.2 Assess limiting factors and pressures

in the catchment

The capacity of a reach to realize its full recovery

potential is dependent on limiting factors and pressures that operate within a catchment, and their

lagged and off-site impacts. In the River Styles



Figure 11.8 Stage Three, Step Two:

Assessing river recovery potential:

Place reaches in their catchment

context and assess limiting factors to

recovery





Figure 11.9 Post-European settlement alluvial sediment budget for Bega catchment. Reproduced from Fryirs and

Brierley (2001) with permission from Elsevier, 2003

At the time of European settlement, around 55 million m3 of alluvial material was stored along river courses in the

Bega system. Of this, around 23 million m3 of material has been released since the 1850s. Almost half of the sediment

released (around 10 million m3) has been sourced from incised valley fills. Channel expansion along tributary and

trunk streams has yielded an additional 4 million m3. Just over 6 million m3 has been restored as readily reworked

instream storage units (e.g., sand sheets and channel bars) in the contributing catchment. This material is

progressively being transferred through the system as the tail of the sediment slug. Most sediment yielded from the

contributing catchment (around 14 million m3) has been efficiently flushed to the lowland plain through bedrockcontrolled midcatchment reaches, with a sediment delivery ratio of around 70%. The contributing catchment is

effectively exhausted of sediment.

Along the lowland plain, significant channel expansion immediately following European settlement yielded around

2 million m3 of material. In subsequent decades, however, the role of this reach was altered. Of the 16 million m3 of

material supplied to the lowland plain, the majority (around 12 million m3) is stored in within-channel sand sheets,

ridges, bars, and benches, and on the floodplain as sand sheets. Most of the instream sediments have now been

stabilized by exotic vegetation. The lowland plain acts as a large sediment sink. Low slope and a wide valley combine

to ensure that the sediment slug remains in this part of the landscape. Of all the material released from Bega

catchment, just over 3.5 million m3 has been transferred to the estuary, with a catchment sediment delivery ratio of

around 16%. This sort of analysis provides the baseline upon which the linkage and cascading of sediment can be

assessed, allowing the recovery potential of each reach to be assessed.



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Stage Three of the River Styles framework



10m3

10m



3



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framework, limiting factors to recovery are defined as catchment-specific physical considerations that constrain the potential of a reach to

move along a recovery trajectory. Limiting factors

include sediment availability and transport capacities, discharge considerations, and vegetation distribution, character, and composition. Pressures

are human-induced practices that can be either internal or external to the catchment. They include

land-use change, vegetation, and water management, climate change, social and political perspectives, etc. Their consequences may be either

negative or positive in environmental terms. The

importance of any limiting factor or pressure differs not only between catchments, but between

reaches within a catchment. Because of the changing nature and rate of (bio)physical linkages in a

catchment, different reaches are subjected to

markedly variable off-site and/or lagged impacts.

These factors are also important when deriving

catchment-framed visions (Stage Four of the River

Styles framework). Examples of various limiting

factors and pressures, and procedures used to

assess them, are outlined below.

11.3.2.1 Deriving a catchment

sediment budget

Given the emphasis on geomorphic structure and

function, and the movement of sediment through

catchments, sediment availability presents a critical constraint on the river recovery potential.

Spatial predictability of disturbance response is influenced primarily by the patterns of sediment

stores within a catchment, and controls on the

degree to which they are likely to be reworked.

Ultimately, success in predicting patterns and

rates of sediment flux throughout a catchment,

and associated river changes, is constrained by

knowledge of the residence time of different sediment storage units. Identification of sites that are

most sensitive to change is critical in assessment

of the frequency with which perturbations are

likely to occur. Such notions determine, for example, whether reaches are likely to be subjected to

sediment starvation, sediment slugs will be generated and conveyed, or fluxes will remain roughly

constant over time.

In the River Styles framework, particular emphasis is placed on analysis of the bedload compo-



nent, as this is the primary determinant of river

morphology. Specific consideration is given to the

relationship between bedload transport capacity

and sediment availability in each reach. This

determines the cascade and lagged effects of

sediment release following disturbance events,

shaping the prospects for geomorphic recovery

(Fryirs and Brierley, 2001). The downstream pattern of River Styles provides a useful basis for these

analyses.

Deriving a catchment-scale sediment budget is a

significant exercise in its own right. Products from

this sort of work are presented in (Figures 11.9 and

11.10; Fryirs and Brierley, 2001). At the very least,

analysis should examine the distribution and

blocking effect of buffers, barriers, and blankets

within a catchment. This provides a basis for determining from where in the catchment sediments

of varying caliber will be derived, and over what

timeframe it will be delivered to different parts of

the system.

11.3.2.2 Hydrological analyses

Whether brought about by flow regulation or as

secondary responses to altered ground cover,

changes to flow conditions may modify river

morphology. As channel geometry adjusts, the

stream power conditions under which rivers operate are modified. If threshold conditions are

breached in sensitive reaches, the river becomes

more prone to adjustment, possibly compromising

the potential for geomorphic river recovery.

Determining the stream power conditions under

which each type of river operates and the threshold

conditions under which change is likely to occur,

gives some indication as to whether reaches will be

event sensitive or event resistant. This provides a

basis to predict where in the catchment change is

likely to occur, and where off-site impacts will be

manifest.

11.3.2.3 Vegetation analyses

Different River Styles have differing geomorphic

responses to vegetation removal, with different

consequences for river recovery. In some instances, the removal of resisting factors along the

valley floor may enhance erosion, potentially

pushing the channel beyond critical threshold



Stage Three of the River Styles framework



Figure 11.10 Redistribution of alluvial

sediment stores following European

settlement of Bega catchment

Areas of high sediment yield are

located where extensive volumes of

material were stored in locally wider

sections of valleys (a). These features

are concentrated at the base of the

escarpment and along the lowland

plain. Although significant volumes of

material remain stored along channel

networks, they are located primarily

along the lowland plain, where the

sediment slug presently sits (b).

Tributaries and upper parts of the

catchment are experiencing the tail of

the sediment slug and store very little

sediment. These reaches are adjusting

to decreased rates of sediment supply.

Upper parts of the catchment are

sediment starved. This has serious

implications for the geomorphic

recovery potential of these rivers where

channel contraction via instream

sedimentation is the primary recovery

mechanism. Reproduced from Fryirs

and Brierley (2001) with permission

from Elsevier, 2003.



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Table 11.3 Examples of internal and external pressures on

river systems.

Internal to the catchment

• Direct alterations to the sediment and water regimes of

rivers and catchments, such as water and sediment

extraction (e.g., irrigation) and artificial storage

• Water allocation and water reform initiatives

• Land management practices (e.g., intensity of cropping or

stock densities)

• Alterations to land cover (either afforestation or

deforestation)

• Indirect changes associated with riparian vegetation and

woody debris removal or placement

External to the catchment

• Management responses to natural occurrences such as

droughts, flood events, salinity

• Management structures and government/politicalinduced pressures

• Global environmental change

• Population structure change/trends

• Climate change (e.g., greenhouse or El Nino Southern

Oscillation)



conditions. This may compromise the potential of

the system to recover (e.g., Brooks et al., 2003). The

expansion of exotic vegetation, and associated

patterns of seed dispersion, may also affect recovery potential (e.g., Brooks and Brierley, 2000).

Alternatively, if native seed sources remain intact,

recovery may be enhanced decades after initial

clearance. Mapping of the distribution of vegetation and seed sources provides valuable insight

into geomorphic recovery potential.

11.3.2.4 Assessment of pressures

Any assessment of geomorphic river recovery potential must be framed in context of humaninduced pressures that will impact on the

operation of processes in the future, and their interactions in the physical environment. Table 11.3

outlines some of these pressures. The range of pressures varies from system to system. In the River

Styles framework, particular consideration is

given to pressures that threaten the conservation

of intact reaches. If excess pressures are applied to

a system, management responses aim to reduce or

negate the influence of these pressures. An assess-



ment is made of whether the nature and distribution of point and nonpoint impacts are likely to

change over a given timeframe, and whether the

extent of pressures will increase or decrease. In determining the recovery potential of systems and

deriving appropriate catchment-based visions,

strategic measures must be put in place to address

future negative impacts. The key is to identify

where social and economic policies can minimize

environmental degradation and enhance river

recovery.

11.3.3 Place each reach in its

catchment context

The linkage of geomorphic processes throughout a

catchment is dictated largely by the spatial configuration and downstream pattern of River Styles.

This in turn determines the propagation of disturbance responses and off-site impacts experienced

elsewhere in a catchment. For example, a reach in

good geomorphic condition may sit immediately

downstream of a reach in poor condition. The recovery potential of the former will be dictated by

the degree of upstream degradation and downstream transmission of degrading processes.

Hence, while the downstream reach is in good geomorphic condition, it may only have moderate

recovery potential.

Reach position is easily identified by revisiting

analyses completed in Stage One, Step Three of the

River Styles framework where downstream patterns of River Styles were assessed. To simplify

analyses, a “catchment tree” is constructed that

depicts the linkage of geomorphic processes

(Figure 11.11). The branches of the tree represent

examples of each pattern of River Styles found

along tributary networks. A number of boxes can

be added to represent different segments along the

river. In Figure 11.11, this is based primarily on the

distribution of valley-settings. These boxes feed

into the trunk stream, which is also broken into

different segments. Statements on river sensitivity, geomorphic condition, and limiting factors

operating on each segment are placed in each box.

An analysis is then made of the connectivity and

off-site impacts of each segment in both upstream

and downstream terms. This forms the basis for

determining the recovery potential of each reach

in the catchment.



Stage Three of the River Styles framework



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Figure 11.11 Catchment scale linkages and their impact on the recovery potential of rivers along different river courses

that display different downstream patterns of River Styles. Symbols refer to the downstream pattern of River Styles in

Bega catchment

The recovery potential of rivers in Bega catchment is dependent on the position of the reach in the catchment, its

condition and resilience to change, and the upstream availability of sediment that enhances (or diminishes) prospects

for recovery. Reproduced from Fryirs and Brierley (2001) with permission from Elsevier, 2003.



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Chapter 11



Figure 11.12 Decision-making tree for determining the recovery potential of a reach



11.3.4 Determine the recovery potential

of each reach

Using a range of previously compiled information,

a decision-making tree is developed to determine

the recovery potential of each reach (Figure 11.12).

This analysis combines assessments of geomorphic condition, river sensitivity, and position in



catchment. The latter factor summarizes the

effect of limiting factors and the connectivity of

geomorphic processes throughout the system.

In general terms, reaches in moderate to good

condition that sit high in the catchment, close to

intact and good condition reaches, are assigned a

high recovery potential rating. Impacts of disturbance are likely to be limited, providing an oppor-



Stage Three of the River Styles framework

tunity to recover relatively quickly. Reaches that

remain in good condition but are isolated in the

catchment are generally resilient to change and

will absorb off-site impacts. They are given a high

or moderate recovery potential rating. The position of poor condition reaches dictates the recovery potential of remaining reaches. Poor condition

reaches are often sensitive to change. Off-site impacts are propagated at differing rates, with differing consequences through the system. Moderate

and low recovery potential ratings are assigned according to the sequencing of reaches and their resilience or sensitivity to change. By assessing the

recovery potential of each reach of each River Style

in the catchment, a map is produced that shows

river recovery potential (Plate 11.1).



11.4 Products of Stage Three of the River

Styles framework

Four main products are produced in Stage Three of

the River Styles framework:



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• recovery diagrams for each River Style noting

the trajectory of change of each reach;

• tables, maps, or flow diagrams that describe and

quantify limiting factors and pressures operating

in the catchment;

• a “catchment tree” noting the linkage of geomorphic processes and off-site impacts;

• a catchment map showing the recovery potential of each reach.

These products, together with those produced in

Stages One and Two, provide a physical template

with which to create a catchment vision, identify

target conditions for river rehabilitation, and prioritize strategies for river management in Stage Four

of the River Styles framework.



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