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3 Stage Two, Step Two: Interpret river evolution to assess whether irreversible geomorphic change has occurred and identify an appropriate reference condition

3 Stage Two, Step Two: Interpret river evolution to assess whether irreversible geomorphic change has occurred and identify an appropriate reference condition

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Table 10.4 Measurement procedures for each geoindicator. Reprinted from Fryirs (2003) with permission from Elsevier, 2003.

Degree of freedom /

geoindicator



Definition



Examples of geomorphic tools or techniques used to assess each geoindicator



CHANNEL PLANFORM – The outline of a river from above is a function of material texture, valley slope, valley setting, and vegetation structure

Number of channels

Count of the number of channels

• Identification of absent, discontinuous, single, or multichannel variants

Sinuosity of channels The degree of channel curvature along the

• The ratio between channel length along the thalweg and valley length along its axis

length of a river

Lateral stability

The degree to which the channel can move

• Identification of channel expansion, bank erosion, migration, and avulsion processes

on the valley floor

Geomorphic unit

The building blocks of rivers. Each

• Analysis of form and sedimentology to interpret processes responsible for formation of

assemblage

geomorphic unit has a distinct form–

geomorphic unit

process association

• Assessment of the juxtaposition and assemblage of units

• Assessment of channel–floodplain connectivity and unit condition (e.g., signs of

reworking, dissection, etc.)

Riparian vegetation

The character and density of vegetation

• Qualitative rating of the composition (native versus exotic), continuity, and structure of

in the riparian zone, linked to the

vegetation assemblages in the riparian zone

geomorphic structure and flow regime



303



BED CHARACTER – Is a function of flow regime, sediment availability, and the capacity of the reach to transfer materials

Grain size and sorting The size, distribution, and arrangement of

• Visual estimates of the percent of the bed that comprises different grain size fractions

materials stored and transported on

• Analysis of sediment distributions on different geomorphic units

the bed

Bed stability

Capacity of channel bed to adjust vertically

• Interpretation of vertical bed activity via incision

Hydraulic diversity

The character of flow as it passes over the bed • Visual water surface flow estimates (see Thomson et al., 2001)

Sediment regime

The storage, transfer, and delivery capacity

• Identifying sediment process zone (i.e., source, transfer, accumulation; Schumm, 1977)

of the reach. Measures the capacity and/

• Quantitative measure of sediment transport capacity versus sediment availability to

or competence of the reach to transport

interpret supply vs. transport limited reaches

sediment



Stage Two of the River Styles framework



CHANNEL ATTRIBUTES – Channel structure is a function of bed and bank material texture, vegetation cover, bed slope, and discharge

Size

The width and depth of the channel

• Width : depth ratio and cross-sectional area of the channel relative to the catchment area

it drains

Shape

The cross-sectional form of the channel

• Identification of irregular, compound, symmetrical, or asymmetrical channels

Bank morphology

The shape and character of each bank

• Identification of uniform vertical, uniform graded, faceted, undercut banks

• Characterization of bank texture using grain size analyses

Instream vegetation

The character and density of aquatic and

• Qualitative rating of the composition (native versus exotic) and coverage of vegetation on

structure

terrestrial vegetation. Linked to the

instream geomorphic surfaces

geomorphic structure and flow regime

Woody debris loading The character and density of woody debris

• Qualitative rating of the type, alignment, and abundance of woody debris in the channel

and its relationships to the geomorphic

structure and flow regime



304



Chapter 10



Table 10.5 Geoindicators used to measure the geomorphic condition of River Styles in Bega catchment (from Fryirs,

2001).

Geoindicator / River Style



Partly-confined valley with bedrockcontrolled discontinuous floodplain



Channelized fill



Low sinuosity sand bed



Channel attributes

Size

Shape

Bank morphology

Instream vegetation structure

Woody debris loading



YES

YES

YES

YES

YES



YES

YES

YES

YES

NO



YES

YES

YES

YES

YES



Channel planform

Number of channels

Sinuosity of channels

Lateral stability

Geomorphic unit assemblage

Riparian vegetation



NO

NO

YES

YES

YES



NO

NO

YES

YES

YES



YES

YES

YES

YES

YES



Bed character

Grain size and sorting

Bed stability

Hydraulic diversity

Sediment regime



YES

YES

YES

YES



YES

YES

YES

YES



YES

YES

YES

YES



Figure 10.3 Stage Two, Step Two:

Interpret river evolution as a basis

for identifying irreversible

geomorphic change and a reference

condition



305



Stage Two of the River Styles framework

Escarpment

Sites used for

ergodic reasoning

10



0

Br



20



km



og



o



er

Riv



Bemb



oka

Ri

ve

r



Oce

ific

Pac



W

olu

m



la



Cree

k



Cre

ek



dy

Ree



Riv

er



an



Bega

Beg

a



Figure 10.4 Use of ergodic reasoning

to determine evolutionary sequences

Using procedures outlined in Chapter

3, evolutionary sequences for the

intact valley fill and channelized fill

River Styles in Bega catchment were

constructed using ergodic reasoning.

Each of the four reaches selected for

analysis experienced the same sets of

geomorphic changes at different

times. This analysis has been used to

assess how these rivers have adjusted

in the past and to predict how reaches

that have not experienced the full

suite of changes may adjust in the

future.



(m)



306



Chapter 10

10.3.1 Identify the timeframe over

which environmental conditions in

the catchment/region have been

relatively uniform



The evolutionary sequence of a reach provides an

appreciation of how flux boundary conditions

have adjusted in response to factors such as human

disturbance (and subsequent recovery) over a

period where environmental conditions have been

relatively uniform. Lag effects and threshold

breaches, whether triggered by natural events or

human impacts, are identified and interpreted. For

example, in coastal valleys of New South Wales,

stable environmental conditions extend from the

mid–late Holocene to present. Within this period,

sea level stabilized and climatic conditions have

remained relatively constant. Since 1788, human

disturbance associated with colonial settlement



has induced significant geomorphic changes along

river courses. In other settings, many landscapes

continue to be shaped by lagged responses to

glaciation/deglaciation or tectonic uplift. Hence,

the timeframe over which the evolutionary sequence required to isolate impacts of human

disturbance is specific to the setting under

investigation.

10.3.2 Construct an evolutionary sequence for

each River Style in the catchment

An experienced and skilled fluvial geomorphologist is required to construct an evolutionary sequence for each River Style in the catchment. Field

evidence is used to provide detailed knowledge of

the temporal sequence of changes along one reach

of each River Style. If chronostratigraphic dating

is available, time constraints are placed on the





Figure 10.5 Evolution of the intact valley fill and channelized fill River Styles in Bega catchment. Modified from Fryirs

(2003). Reprinted with permission from Elsevier, 2003

The stratigraphy of valley fills in Wolumla subcatchment reflects recurrent phases of cutting and filling over the last

6,000 years. However, the present incisional phase is considered to be the largest and most extensive of any that has

occurred over this timeframe. Hence in Wolumla subcatchment, and in most other base of escarpment valleys,

irreversible geomorphic change has been recorded over the last 200 years.

At the time of European settlement, most base of escarpment valleys in Bega catchment contained the intact valley

fill River Style, characterized by unincised swamps with discontinuous drainage lines (a). The valley floor comprised

mud and sand with a distinct vegetation pattern dominated by Melaleuca ericifolia. Only two analogous intact valley

fill River Style reaches remain in Bega catchment (i.e., along Frogs Hollow and Towridgee Creeks (b)).

Stratigraphic and historical portion plans indicate that analogous features occurred along Wolumla Creek in the late

1860s, as “Wolumla Big Flat” is noted on the portion plans. Following anthropogenic disturbance to swamp surfaces,

knickpoints retreated through the valley fill of upper Wolumla Creek by 1900 (c). A fundamental shift in the

behavioral regime of this river occurred, irreversibly transforming this reach into a channelized fill River Style.

Incision was quickly followed by channel expansion, producing a channel that was locally more than 10 m deep and

100 m wide (d). This wide, deep channel comprised continuous sand sheets with occasional bench features, and a

poorly defined low flow channel. Riparian vegetation cover was poor. Given low channel roughness, there was limited

capacity to retain finer grained materials within the channel. A contemporary version of this phase is evident in upper

Numbugga catchment (d).

Air photographs from 1944 indicate that along Wolumla Creek, the incised trench was beginning to infill (e). In

some places, over 3 m of material has accumulated on the channel bed while benches have continued to build along

channel margins. These act to reduce channel width and depth, and modify channel alignment. Subsequent air

photograph runs in 1962, 1971, 1989, and 1994 show little change in geomorphic structure. This reach is now

characterized by increased heterogeneity in its geomorphic unit assemblage (f). Channel infilling and narrowing

continue to occur, producing a compound channel with a vegetated inset floodplain. Initially, a well-defined low flow

channel develops. With time, the low flow channel will become locally swampy and mud will be retained on the

channel bed analogous to processes occurring along Reedy Creek (g). There are signs that the channel bed is becoming

discontinuous. Eventually this will instigate redevelopment of an intact swamp within the incised trench, inducing

greater water retention (increasing base flow) (h). Given the irreversible nature of geomorphic change since European

settlement, this latter condition (h) is identified as the expected reference condition under contemporary flux

boundary conditions as indicated by the box.



z



308



Chapter 10



nature of change, allowing the timeframe and rate

of change to be quantified. Field insights are tied to

historical information such as portion plans, old

photographs, explorers’ notes, bridge surveys, sequential sets of air photographs, and historical

maps. Where field evidence is poor or has not been

preserved, ergodic reasoning is used to fill the gaps

in the evolutionary sequence. Selected reference

reaches used for ergodic reasoning analysis must

be of the same River Style, occupy a similar position in the catchment with near-equivalent channel gradient, and operate under the same set of

imposed boundary conditions (e.g., Kondolf and

Downs, 1996; Fryirs and Brierley, 2000). An example of how ergodic reasoning has been used to determine the evolutionary sequence for the intact

valley fill and channelized fill River Styles in Bega

catchment is presented in Figure 10.4.

Timeslices are often constrained by available

data. In an Australian context, the following timeslices are helpful: pre-European settlement (field

sedimentology, dating techniques, and analysis of

portion plans), 1860s–1900s (timing of first photographs), 1940s (military air photograph set), and

1960s–present (subsequent air photograph series,

photographs, maps, and contemporary field analy-



ses). Planform and cross-sectional views are constructed for each timeslice. Planform maps are

drafted directly from maps and air photographs,

while cross-sections present a schematic summary

of the geometry, geomorphic unit structure, sedimentology, and vegetation character of the reach.

These schematic diagrams summarize the range of

river character and behavior along the reach.

Once an evolutionary sequence has been constructed, reaches are placed within the evolutionary context of its River Style. This is used to:

• assess river character and behavior prior to

human disturbance and determine changes in the

post-human disturbance phase;

• identify how flux boundary conditions have impacted upon the timing and causes of river change;

• determine whether human disturbance has resulted in irreversible geomorphic change over

management timeframes;

• identify a reference condition;

• predict future adjustments and the trajectory of

change (in Stage 3);

• determine potential creation and restoration

conditions for each reach (in Stage 3).

Evolutionary sequences for the channelized

fill, the partly-confined valley with bedrock-





Figure 10.6 Evolution of the partly-confined valley with bedrock-controlled discontinuous floodplain River Style in

Bega catchment (from Fryirs, 2001)

Analysis of the evolution of the partly-confined valley with bedrock-controlled floodplain River Style was conducted

along Middle Tantawangalo Creek. Along these river courses the pre-European settlement river was characterized by a

narrow, deep channel with significant hydraulic diversity induced by large woody debris and a heterogeneous

assemblage of geomorphic units including pools, riffles, bar complexes, islands, etc. (a). Following removal of riparian

vegetation, channels widened and the channel bed became increasingly homogeneous (b). As the channel widened,

significant volumes of sediment were released. With the additional large inputs of bedload material from upstream,

sand sheets covered the channel bed infilling pools and smothering riffles (c). Contemporary versions of this condition

are evident along Sandy Creek. The low flow channel is poorly defined and braids atop large sand sheets. No pools are

evident. Given the poor cover of riparian vegetation and lack of instream roughness, the reach has limited capacity to

retain fine grained sediments. Devegetated banks and erodible bank materials accelerate channel expansion and the

rate of lateral channel movement. Convex banks are characterized by point bars and discontinuous pockets of

floodplain.

Over time, the low flow channel becomes better defined. A contemporary example of this stage occurs along Middle

Tantawangalo Creek (d). Sediment inputs and outputs eventually become roughly balanced, with point bar and point

bench storage on the inside of bends and maintenance of sediment throughput on the channel bed. As the channel

becomes narrower and deeper, it adopts a more sinuous course. Point bars and point benches store significant volumes

of material, as do within-channel ridges which form as a result of vegetation colonization. Pools reemerge as sediment

is flushed through the reach. An example of this stage occurs today along Upper Tantawangalo Creek (e). Given that

geomorphic change since European settlement has been reversible, but the flux boundary conditions under which this

river operates have been altered, the latter condition (e) is identified as the expected reference condition under

contemporary flux boundary conditions as indicated by the box.



310



Chapter 10



controlled discontinuous floodplain pockets, and

the low sinuosity sand bed River Styles in Bega

catchment are presented in Figures 10.5–10.7.

10.3.3 Has geomorphic change been

reversible or irreversible?

Determination of an appropriate reference condition must reflect a realistically attainable river

structure and function given the prevailing boundary conditions expressed over management timeframes of 50–100 years. This requires assessment

of whether system responses to human disturbance have brought about irreversible changes to

river character and behavior. The evolutionary sequence of each reach is used to identify if, how, and

when irreversible geomorphic change occurred.

Irreversibility is defined as a wholesale shift in the

behavioral or process regime of a river that induces

a shift to a new River Style. If a shift in River Style



has occurred, the assemblage of geomorphic units,

channel planform, and bed character have changed

to such a degree that the river operates in a fundamentally different manner to its former state. A return to the predisturbance state will not occur

without significant physical intervention or manipulation. The contemporary capacity for adjustment of the River Style requires redefinition for

the new River Style and a solid line is used on the

evolution diagram to note the change in River

Style (e.g., Figures 10.5 and 10.7). Reversible geomorphic change occurs when adjustments occur

within the contemporary capacity for adjustment

of the River Style under investigation (e.g., Figure

10.6).

In many cases, identification of irreversible

change to river character and behavior is a straightforward exercise. For example, regulated flow or

urban development result in irreversible alteration to the geomorphic structure of a river. These





Figure 10.7 Evolution of the low sinuosity sand-bed River Style in Bega catchment (from Fryirs, 2001)

The evolutionary development of the lowland plain river around Bega township is documented in Brooks and Brierley

(1997, 2000). Portion plans dating from the 1850s and paleochannel indicators (i.e., Casuarina lined channel margins)

show that the pre-European settlement lowland plain of the Bega and Brogo Rivers was characterized by a deep, narrow

channel with a series of pools and riffles (a) (Brooks and Brierley, 1997, 2000). It was a mixed load system with fine

grained suspended load material deposited on the floodplain in overbank events. The loading of large woody debris

was likely high, and riparian vegetation was dominated by Casuarina cunninghami and Lomandra spp. The

floodplain consisted of an open woodland association and the backswamps were dominated by Melaleuca spp. Given

the relatively low channel capacity, transfer of water and organic matter to the floodplain was readily maintained. A

low sinuosity fine grained River Style occurred along this lower section of the catchment.

The lower course of the Bega River expanded from around 40 m wide to 140 m wide within a few decades of

European settlement (c), essentially as a consequence of the removal of riparian vegetation. Photographs from the

1890s show a wide, shallow channel with a homogeneous sand sheet that is free of vegetation. The river has been

transformed from a mixed load to a bedload dominated system. Pools have been infilled, and up to 2 m of sand has

accumulated on floodplains previously dominated by silt (Brooks and Brierley, 1997). An irreversible change to a low

sinuosity sand bed River Style had occurred.

Detailed field investigations indicate that relatively little change to river structure occurred along the lower Bega

River between 1920 and 1960 (Brooks and Brierley, 2000) (d). However, since the 1960s, willows and other forms of

vegetation (native colonizers and exotics) have choked the channel. This has produced a complex pattern of channelmarginal benches, bars and islands. Channel contraction and the reworking of instream sediments (i.e., the formation

of islands) have returned some structural heterogeneity to the channel (f).

Over time the geomorphic function of the lowland plain has changed. In its predisturbance state, this reach acted as

a transfer zone, with limited rates of sediment flux. As the channel expanded, significant volumes of sediment were

released. Subsequently, however, the lowland plain has stored large volumes of material derived from the upstream

sediment slug (Fryirs and Brierley, 2001). As the tail of the sediment slug passes, it is considered likely that the

lowland course of Bega River will be characterized by numerous narrow, deep channels in something akin to an

anabranching pattern (g). As these channels deepen, sand sheet inundation on the floodplain will be alleviated and the

habitat potential and transfer of flow, organics, and fine grained sediment to backswamps will be improved. Given

that geomorphic change since European settlement has been irreversible, this latter condition (g) is considered the

expected reference condition under contemporary flux boundary conditions (as indicated by the box).



Stage Two of the River Styles framework



-



311



312



Chapter 10



changes are often accompanied by changes to the

sediment and flow regimes. Equivalent changes

may result from inadvertent but systematic

changes to other flux boundary conditions, such as

riparian vegetation structure and cover. In these

cases, irreversible change is less easily defined and

requires looking into the past to identify shifts in

river character and behavior.

Assessing whether irreversible geomorphic

change has occurred provides the basis to define an

appropriate reference reach. If a reach has experienced irreversible geomorphic change, the condition of the reach must be assessed in light of the

newly adopted River Style. In this instance, comparing the contemporary reach with a reference

condition of the predisturbance river type is irrelevant in setting realistic management goals. If a

river still operates as the River Style that existed in

the predisturbance period, such that humaninduced adjustments are reversible, reach condition is assessed against a reference condition of

this River Style. In these cases, an expected reference condition is designed, reflecting the altered

flux boundary conditions under which the river

now operates.

The key to this analysis is identifying how disturbance may modify the threshold levels at which

irreversible change may occur. For example, in

some cases, reduced resistance following the removal of riparian vegetation lowers the threshold

levels for geomorphic adjustment via incision and

channel expansion. In these cases, small triggers

(such as small–moderate flood events) can breach

fundamental resistance thresholds, triggering

significant geomorphic change. Identification of

threshold conditions that trigger fundamental

changes to river forms and processes represents a

critical consideration for management applications. These insights guide designation of strategic

reaches, aiding the identification of reaches where

manipulation of river character and behavior will

have the most positive impacts, enhancing river

recovery potential (Chapter 12).

10.3.4 Derive desirability criteria for river

character and behavior based on relevant

geoindicators for each River Style

Assessments of relevant contemporary river attributes are used to identify good, moderate, and

poor conditions for each River Style. This is framed



in terms of the degrees of freedom that are used to

assess the capacity for river adjustment for each

River Style. Relevant geoindicators identified in

Stage Two, Step One are measured to give a reliable

and relevant signal about the condition of a reach

(cf., Elliott, 1996; Osterkamp and Schumm, 1996).

The range of selected geoindicators is River Style

specific. Although this procedure entails elements

of subjectivity, it is based on fundamental geomorphic principles and process-based understanding

of how each River Style works. A table is constructed that includes questions about the “desirability” of each relevant geoindicator for each

River Style. One question is asked for each relevant geoindicator (Tables 10.6–10.8).

10.3.5 Identify and select a reference reach for

each River Style

Any assessment of river condition must be

framed relative to some benchmark or reference

reach, thereby providing a determination of the

extent to which human-induced changes to river

character and behavior fall outside the long-term

pattern. In an Australian context, two options

seem appropriate:

1 an intact, pristine condition. For example, putting aside the impacts of Aboriginal practices (e.g.,

use of fire), the initiation of colonization in 1788

provides a suitable reference point. If patterns and

rates of river geomorphic change fall outside their

“natural” capacity for adjustment, a reach is

considered to have deviated from its “intact”

condition;

2 an assessment of the “best” condition that can

be attained by a river that has been altered by

human disturbance, given the prevailing catchment boundary conditions. As noted previously,

this “expected” reference condition must separate

reaches that have been subjected to reversible

and irreversible changes in response to human

disturbance.

Although the first option may be preferred from

a preservationist perspective, the second option

provides a far more practical and realistic perspective with which to appraise geomorphic river condition (cf., Cairns, 1989, 1991; Brookes, 1995; Gore

and Shields, 1995; Sparks, 1995). Given the extent

of human disturbance to river systems, whether

directly induced or an indirect response, comparison with an intact condition may seem little more



Stage Two of the River Styles framework



313



Table 10.6 Measures used to assess good condition reaches of the channelized fill River Style in the laterally-unconfined

valley setting in Bega catchment. Reproduced from Fryirs (2003) with permission from Elsevier, 2003.

Degrees of freedom

and relevant

geoindicators

Channel attributes

• Size



• Shape

• Bank morphology

• Instream vegetation

structure

Channel planform

• Lateral channel

stability

• Assemblage of

geomorphic units



• Riparian vegetation

Bed character

• Grain size and

sorting

• Bed stability

• Hydraulic diversity

• Sediment regime



Questions to ask for each reach of the River Style



Questions that

must be answered

YES



• Is channel size appropriate given the catchment area, the prevailing sediment

regime, and the vegetation character? (i.e., is the channel overwidened,

overdeepened, or does it have an appropriate width : depth ratio?)

• Is channel shape appropriate along the reach? (i.e., does the channel

have a compound shape, with inset surfaces within a symmetrical trench?)

• Are banks eroding in the right places and at the right rate? (signs of

deterioration include vertical or undercut banks along the reach)

• Is the instream vegetation structure appropriate? (i.e., is aquatic

vegetation colonizing the bed of the incised channel?)



3 out of 4



• Is the lateral stability of the channel appropriate given the texture and slope

of the reach? (signs of deterioration include channel expansion and

low flow channel reworking of bed materials)

• Is the assemblage, pattern, and condition of instream and floodplain

geomorphic units appropriate? Are key units present? (i.e., does the

reach have a series of insets and a swampy channel bed with no

signs of reworking such as dissection, stripping, or undercutting?)

• Is the continuity and composition of riparian vegetation near-natural with

few exotics?



2 out of 3



• Is the grain size, sorting, and organization of materials in different

geomorphic units appropriate? (i.e., are sands stored in insets,

and mud and organic matter stored on the channel bed?)

• Is bed stability appropriate? (signs of bed instability or disturbance

will include incision into sands or to bedrock)

• Is the sediment storage/transport function of the reach appropriate?

(i.e., is it acting as a sediment accumulation zone?)

• Are roughness characteristics and the pattern of hydraulic diversity

along the reach appropriate? (i.e., does the reach have a swampy

channel bed with a series of inset bench features?)



3 out of 4



than an academic exercise. This is NOT to say that

insights from near-intact reaches do not provide

fundamental guidelines to the “natural” structure

and function of rivers, and associated implications for geodiversity, biodiversity, conservation,

aquatic ecosystem functioning, etc. Unfortunately, however, relatively few remnants remain,

and these typically form an unattainable target

condition for river management practices.

The objective in identifying a reference reach is

to determine a morphological configuration that is

compatible with the prevailing flux boundary

conditions. Viewed in this way, channel attributes, channel planform (including the assemblage



of geomorphic units), and bed character must be

appropriate for the River Style under investigation

(cf., Hughes et al., 1986; Brookes and Shields, 1996;

Rhoads and Herrick, 1996). The approach used to

identify a reference condition in the River Styles

framework is framed in terms of whether irreversible geomorphic change has occurred following human disturbance. This is determined by the

sensitivity of the River Style to change (i.e., its capacity to adjust), and how it responds to alterations

in flux boundary conditions (i.e., flow and sediment transfer, and vegetation associations).

The nature and extent of human disturbance are

unlikely to be uniform across any particular catch-



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