Water requirements for the rehabilitation of Bolin Bolin Billabong

Remnant standing water in Bolin Bolin Billabong, February 2010

Report to Parks Victoria

Dodo Environmental 15 Yawla Street, McKinnon VIC 3204 [email protected] (03) 9557 3342

25 April 2010

Document control sheet Report title Water requirements for the rehabilitation of Bolin Bolin Billabong Version Final report Date 25 April 2010 Author Dr Paul Boon Client Parks Victoria (Garry French, Parks Victoria – Westerfolds Park)

Contact details

Dr Paul I Boon Dodo Environmental 15 Yawla Street McKinnon VIC 3204 AUSTRALIA

Phone/fax: (03) 9557 3342 e-mail: [email protected]

ABN: 12 365 734 616

Disclaimer

This report has been prepared on behalf of and for Parks Victoria. Dodo Environmental accepts no liability or responsibility for or in respect of any use of or reliance upon this report by any third party. The report was prepared in accordance with the scope of work and purposes outlined in the proposal. It is based on generally accepted practices, knowledge and standards at the time of preparation. No other warranty, expressed or implied, is made as to the professional advice included in this report. The report shows the approach taken and sources of information used by Dodo Environmental; I have not necessarily made an independent verification of the information used to prepare the report. The report is based on information available and conditions encountered at the time of preparation; Dodo Environmental disclaims any responsibility for any changes that may have occurred since then. Dodo Environmental does not warrant this document is definitive nor free from error and does not accept liability for any loss caused, or arising from, reliance upon the information provided herein.

Contents

Executive summary 4

1. Scope of the project 7

2. General approach, information sources and terminology

2.1 General approach 8 2.2 Information sources 10 2.3 Some notes on terminology 12

3. Historical perspective, current ecological condition, and threatening processes

3.1 Landscape context and neighbouring billabongs 14 3.2 Social significance 15 3.3 Ecological value 16 3.4 Hydrology and wetland type 16 3.5 Conceptual models of billabong structure and function 16 3.6 Ecological condition – findings of the literature review 18 3.7 Changes to Bolin Bolin Billabong since European colonization 24 3.8 Hydrological regimes at Bolin Bolin Billabong 31 3.9 Vegetation responses to chronic desiccation 33 3.10 Main threatening processes 38 3.11 Synthesis 41

4. Management objectives 43

5. Hydrological requirements for rehabilitation

5.1 Flows in the Yarra River required for billabong inundation 45 5.2 Hydrological regime required for rehabilitation – broad recommendations 49 5.3 Fine-tuning the water regime to maximise biodiversity 51 5.4 Sources and volumes of water required 54 5.5 Potential hazards with use of treated storm-water 62 5.6 Synthesis and conclusions 65 5.7 Assessment of likely rehabilitation success 65

6. Implementation and on-going management

6.1 Importance of adaptive management approach 68 6.2 Need for long-term monitoring program 69

7. References 72

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Executive summary

Dodo Environmental was contracted by Parks Victoria in February 2010 to undertake an analysis of the water requirements of Bolin Bolin Billabong, a high-value ox-bow lake on the floodplain of the Yarra River in metropolitan north-east Melbourne. The billabong is located in the Yarra Flats Parkland, which is managed by Parks Victoria. A number of agencies, however, have an interest in the rehabilitation and on-going management of the site, including Parks Victoria, Manningham Shire Council and Melbourne Water. Like many wetlands on the Yarra floodplain, Bolin Bolin Billabong has been subject to significant land-use changes in the catchment and, particularly over the recent ~12+ years of drought, marked changes to its wetting and drying cycles.

Bolin Bolin Billabong and its surrounding floodplain is one of the few wetlands in the metropolitan area of north-eastern Melbourne whose geomorphology has not been modified since European colonization. Moreover, with other nearby billabongs such Banyule Billabong, it has the potential to form a landscape mosaic of potentially very high ecological and social value. Given these factors, an appropriate management objective is that Bolin Bolin Billabong should be rehabilitated as far as possible to reflect its ecological condition before European colonization. Complete restoration is an unreasonable aim: the presence of weeds in the floodplain flora, the likely continuous infestation with exotic fish, and on-going presence of feral animals such as deer, cats and dogs, for example, make an aim of complete restoration unrealistic. The guiding principle behind rehabilitation should be to maximise the ecological resilience of the site. Resilience is this context means the ability of the billabong and floodplain to withstand external threats to their ecological condition and to return to the original condition when such threats pass.

A process was developed to identify a hydrological regime suitable for rehabilitating the billabong and adjacent floodplain, based on a consideration of the geomorphological setting, present and likely prior vegetation types, current ecological condition, threatening factors, and management objectives. Factors that could limit rehabilitation success were identified, as well as a brief outline of future work needed to advance rehabilitation of the site. The quality of treated storm-water required to maintain biodiversity and the provision of ecosystem services was investigated, and interim trigger levels proposed for Total Nitrogen and Total Phosphorus. In addition to longer-term issues with eutrophication, a risk could be posed by the creation of ‘blackwater’ events when the billabong is initially filled after the recent but prolonged period of dryness. Nevertheless, re-instating a more natural hydrological regime is crucial to the success of other rehabilitation efforts.

The recommended water regime should facilitate the ecological rehabilitation of Bolin Bolin Billabong. A deep pool in the eastern loop of the billabong should remain near-permanently inundated to a maximum depth of ~2 m. Water levels within this pool, however, should be allowed to fluctuate naturally with the seasons. The western ends of the northern and southern arms should be inundated to a depth of at least 30 cm for the first three years, in order to help drown out invading River Red Gum saplings and other terrestrial vegetation. After that time, a longitudinal and lateral gradient in wetting should be established so that a complex mosaic in vegetation (and animal habitats) is created along the east-west thalweg of the billabong. The surrounding floodplain should be inundated annually (to a depth of ~10 cm) in order to re- establish the pre-European pattern of wetting and drying. It may be necessary to implement a program of River Red Gum thinning to remove the abundant saplings on the billabong floor.

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After initial removal of unwanted saplings, subsequent wetting and drying should be sufficient to prevent their re-colonizing this part of the billabong.

There are three potential sources of water for the recommended water regime:  Yarra River water, achieved through natural bank-full (via flood runners and the two channels) or larger but rarer over-bank flows  Yarra River water, pumped from the river at appropriate times, and  Treated storm-water from the surrounding urban catchment.

Natural flooding of Bolin Bolin Billabong via bank-full or over-bank flows of the Yarra River is currently constrained by four factors: i) chronic extraction of water from the Yarra River for human use, which has lead to markedly depressed river levels; ii) historically low discharge as a result of dry conditions over the past ~12+ years; iii) presence of small raised areas at the western end of the billabong which alienate it and the floodplain from the river; and iv) dense growth of exotic plants on the floodplain and in the western channels, which increase hydrological roughness for over-bank and bank-full flows. Given these factors, it is recommended that the initial filling of the billabong should be done by pumping water from the Yarra River. This conclusion agrees with the earlier recommendation by Sinclair Knight Merz (2006). After the billabong has been filled and the subsoils thoroughly wetted, there are many advantages to then use treated storm-water to maintain more natural wetting and drying cycles in the billabong and floodplain.

Sinclair Knight Merz (2006) estimated the volume of water to fill Bolin Bolin Billabong at 74 ML. A more sophisticated analysis was undertaken in the current study, using a conceptual hydrological model to identify the various components of the overall water requirements of the site:

VTotal = VThalweg + VFloodplain + VSoil saturation + VEvaporation/transpiration – VRainfal/run-off

The table below summarises the likely volumes of water required in first and subsequent years of the rehabilitation of Bolin Bolin Billabong.

Estimates of water volumes required to rehabilitate Bolin Bolin Billabong. These estimates should be read in conjunction with the assumptions and caveats listed in the body of the text.

Component Volume per year (ML) Year 1 Year 2 Year 3 Billabong thalweg inundation 40 * * (pool only) Billabong thalweg inundation 10 10 10 (wet-dry arms)** Floodplain inundation** 8 8 8 Soil saturation (billabong ~20–55 ~20–55** ~20–55** thalweg only) Soil saturation (combined ~50–150 ~50–150*** ~50–150*** billabong-floodplain complex) Evaporative losses (billabong 24–48 24–48 24–48 only) * nil if evaporative losses of 24-48 ML year-1 are made up annually ** likely to be at lower end of range in 2nd and 3rd years, due to prior saturation of thalweg during 1st wetting *** assumes one inundation per year: approximately double this value for two inundations annually

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These volumes sum to a total of ~82–106 ML for the better-understood components of filling the billabong thalweg (pool and alternately wet-dry arms: 50 ML), inundating the floodplain (8 ML), and accounting for evaporative losses from the billabong only (24–48 ML). They do not include the water necessary beforehand to saturate the soil. An additional amount, probably between ~50–150 ML, is required to saturate the soils before water can start to accumulate in the billabong and inundate the floodplain. Once this extra water is considered, ~132–256 ML will be required (as a minimum) to initially fill the billabong and inundate the floodplain in the first year. A smaller volume, ~94–153 ML, would be required to initially fill the billabong only (and not affect the floodplain) and maintain water levels in the billabong in the face of surface- water evaporation.

It is critical to stress that all these calculations involve a number of assumptions and the critical caveats must be remembered when applying these estimates. The most important assumptions and caveats are:  Areas of the thalweg occupied by the ~2 m-deep permanent pool (50%) and the alternating wet-dry regions in the northern and southern arms of the more elevated parts of the thalweg (50%).  Groundwater interactions have been neglected, but water could flow into and out of the billabong via lateral and vertical seepage. Given the billabong’s raised location above the Yarra River, losses are more likely than gains and so the calculated volumes for initial filling and maintaining high water levels are likely to be under-estimates rather than over- estimates.  Evaporative and evapotranspirational losses from the floodplain have been neglected.  A floodplain inundation of 10 cm is assumed, and an inundation of 30 cm in the thalweg of the billabong is assumed to be sufficient to kill River Red Gum saplings (50 cm used in model calculations).  The estimate of the volume of water required to saturate the soil in the billabong thalweg and on the surrounding floodplain is very approximate. It will depend crucially on soil porosity, root penetrations, and depth to the impervious layer, none of which are well quantified at present.  This estimate also ignores the possibility that deep cracks have formed in the subsoil and that water could easily percolate into the soil profile until the cracks have clogged with transported clays etc.  The estimate of evaporative losses from the wet billabong assume an average annual rate of 5 mm day-1. The realized rate of evaporative loss will depend on factors such as air temperature, humidity, wind speed, and shelter afforded by plants. Moreover, the presence of abundant River Red Gum saplings in the billabong basin would likely increase rates of evapotranspiration over those from open water.  No allowance has been made for periodic flushing of the billabong pool. The initial fill with Yarra River water is likely to be of high quality, but subsequent ‘top-ups’ with treated stormwater may result in chronic eutrophication of the wetland, especially if the water is enriched with plant nutrients such as nitrogen and phosphorus.

The report concludes with a set of recommendations on the adoption of an adaptive management approach to rehabilitating the site (including the use of conceptual models) and on a minimum set of monitoring activities.

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1. Scope of the project

Dodo Environmental was contracted by Parks Victoria in February 2010 to undertake an analysis of the water requirements of Bolin Bolin Billabong, a high-value ox-bow lake on the floodplain of the Yarra River in metropolitan north-east Melbourne. The billabong is located in the Yarra Flats parkland, which is managed by Parks Victoria. A number of agencies, however, have an interest in the rehabilitation and on-going management of the site, including Parks Victoria, Manningham City Council and Melbourne Water.

Like many billabongs on the Yarra floodplain, Bolin Bolin Billabong has been subject to significant land-use changes in the catchment and, particularly over the recent ~12 years of drought, marked changes also to its wetting and drying cycles. In general terms, the present project seeks to identify the water regime required to rehabilitate Bolin Bolin Billabong nearest to its original (i.e. pre-European) condition. More specifically, Manningham City Council identified the basis of works for the project as:  Clarify the inundation regime (including volume of water) required to rehabilitate Bolin Bolin Billabong, particularly with reference to the report by Sinclair Knight Merz (2006).  Advise on the quality of water required to fill the billabong, preferably including a risk assessment of the various options to provide that water.  Advise on the location and operation of inflow and outflow structures.

The investigation grew out of an earlier report (Dodo Environmental 2009 a, b) into the ecological condition and rehabilitation potential of five wetlands in the Greater Melbourne region. The current report is based somewhat on that earlier study but with a focus on Bolin Bolin Billabong specifically. As outlined in the Proposal (28 January 2010), the report includes the following components:  A detailed overview, using published and grey literature, of Bolin Bolin Billabong, that includes information on its pre-European condition, changes since European colonization, and current ecological condition.  A qualitative analysis of the factors that affect the ecological condition of the billabong and, in particular, the relative importance of hydrological versus non-hydrological stressors.  Identification of the water regime most suited to rehabilitate Bolin Bolin Billabong, including a critique of prior recommendations (e.g. by Sinclair Knight Merz 2006).  A preliminary estimate (including range of likely values) of the volume of water required to implement the recommended water regime.  Overview of the range of water-quality issues likely to be confronted during the rehabilitation of the billabong.  General recommendations on the implementation and day-to-day operation of the recommended water regime, including an analysis of inlet and outlet structures.

Two additional components were included during the inception meeting and site inspection of mid February 2010:  Inclusion of a conceptual model of billabong ecology.  General recommendations on the monitoring required to assess the effectiveness of any hydrological interventions aimed to improve the ecological condition of the billabong.

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2. General approach, information sources and terminology

2.1 General approach

Ways to determine environmental water requirements of floodplains and wetlands

The range of approaches that can be used to determine the environmental water needs of floodplains and their wetlands were outlined in the earlier report to Melbourne Water (Dodo Environmental 2009 a). In brief, there are two main types of approaches:  Hydrology-driven approaches, and  Ecology-driven approaches.

Hydrology-driven approaches aim to describe the existing water regime, compare it with pre- disturbance (which usually means pre-European) regimes, and then re-instate the original hydrological conditions. The underlying assumption is that the biota are adapted to the pre- disturbance water regimes and that restoration of the original hydrological conditions will result in a rehabilitated ecosystem (Davis et al. 2001). In turn, that assumption is based on the notion that the ecological integrity of an aquatic system is dictated by the natural pattern of variation in flows; this dependency and its ecological ramifications has been called the natural flow-regime paradigm by Poff et al. (1997). The importance of periodic inundation and drying for floodplains and their wetlands has long been recognized by ecologists (Junk et al. 1989).

The hydrology-driven approaches are thus ‘top-down integrative’ approaches which take as given that anthropogenic-induced deviations from the natural pattern of flows will result in changes to the ecological integrity of lowland rivers, their floodplains and wetlands. Usually these changes are in the direction of undesirable changes to biodiversity and ecosystem function (Bunn & Arthington 2002), and result in degraded ecological condition and a reduced ability of the site to deliver the ecosystem services expected by the local community.

In contrast to hydrology-driven approaches to wetland rehabilitation, ecology-driven approaches seek to determine fundamental hydrological requirements of the biota that are currently present or desired to be present, and then to devise water regimes that best deliver those requirements. They are thus ‘bottom-up building black’ methods that must identify the water requirements of the individual biota and critical ecological processes. The problem of course is that very often this information is not available.

The method currently used to determine environmental flows in Victorian streams (the FLOWS method: see Department of Sustainability and Environment 2002) is largely an ecology-based, building-block method. Reviews of these and other methods to determine environmental flows (mainly to rivers rather than to wetlands) can be found in Arthington & Zalucki (1998) and Arthington et al. (2006).

In both general approaches, the implementation of environmental flows is a necessary, but not sufficient, intervention for the rehabilitation of degraded aquatic systems. Other management changes, such as replanting of desired-but-lost plant species and control of grazing by native animals (e.g. kangaroo), feral animals (e.g. deer, rabbits) and stock (e.g. cattle, sheep), are critical if the degraded wetlands are to be rehabilitated most effectively.

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Approach used in this investigation

The method used in this investigation marries elements of both the hydrological and ecological approaches. It is summarized in Figure 2.1.

Figure 2.1: Flow chart showing steps involved in general approach to determine ‘ideal’ hydrological regimes for Bolin Bolin Billabong. Source: Dodo Environmental (2009 b).

The site inspection identified the underlying geomorphology of the billabong and land use in the immediate vicinity; this information was supplemented by information available in existing reports and lengthy discussions with Parks Victoria staff. Particular attention was paid to the position of the billabong in the landscape and its current vegetation; position in the landscape (e.g. distance from river, elevation, existence of levees etc) is a fundamental controller of inundation regimes, and vegetation provides the essential habitat required for faunal communities, contributes much to the aesthetic appearance of a site, is likely to respond strongly to alterations in hydrology and land use, and can be readily monitored in order to gauge the effectiveness of the rehabilitation.

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Information was then collated on the main factors that were likely to affect the current ecological condition and could limit rehabilitation in the future. Information in prior reports on Bolin Bolin Billabong was supplemented with that available for other nearby billabongs on the Yarra River floodplain and from the discussions with Parks Victoria staff. A broad distinction was made between hydrological and non-hydrological factors, in recognition of the different roles played by water regime (i.e. hydrology) and other land-use factors (e.g. grazing by native and feral animals, weed infestations etc) in influencing wetland condition and the likely success of rehabilitation.

Collation of such background information then allowed for the identification of an ‘ideal’ hydrological regime to meet the identified environmental objectives. The ‘ideal’ hydrological regime is a broad description of the most appropriate wetting and drying cycles, rather than detailed hydrological analyses that include factors such as commence-to-flow values, spells analysis etc. It was beyond the scope of the project to undertake such detailed analyses but may be required in future studies and if rehabilitation actions are to proceed further.

The identification of an ‘ideal’ hydrological regime was undertaken in three steps. First, a specific management objective was outlined. Second, the landscape setting of the billabong and its likely pre-disturbance water regime was established as far as was possible with existing information. Third, the wetting and drying requirements of the main plant communities were predicted on the basis of available information. Ecological water requirements were described on the basis of information in the available literature; in some cases this information was quite robust and water requirements of individual plant taxa are well known (e.g. for River Red Gum Eucalyptus camaldulensis). In other cases, information was available only at the level of genus or higher. Accordingly, there is some variability in the robustness with which ‘ideal’ hydrological regimes could be identified for the various sites, depending mainly on the current state of knowledge of plant-water interactions for the various plant communities.

The factors likely to limit future rehabilitation and the variables that should be used to monitor the effectiveness of rehabilitation, especially from hydrological interventions, were then outlined in general terms. Given the variation in robustness with which ‘ideal’ hydrological regimes could be identified, an adaptive management approach is essential to monitor effectiveness and ‘fine- tune’ subsequent hydrological interventions. Such adaptive management requires a commitment to ecological monitoring using an appropriate set of environmental indicators or variables within an appropriate monitoring design.

2.2 Information sources

Written information

Three types of written literature were used. The first was any report that specifically mentioned Bolin Bolin Billabong or nearby ox-bow lakes on the Yarra River floodplain. A crucial study was the scientific paper by Leahy et al. (2005). Monographs on the Yarra River by Beardsell & Beardsell (1999), Lacey (2004) and Presland (2008) provided supplementary information. Valuable studies in the grey (i.e. unpublished) literature included the FLOWS study of the Yarra River by Sinclair Knight Merz (2005 a, b), analysis of watering options of various Yarra River billabongs by Sinclair Knight Merz (2006), and the two comprehensive studies of Yarra River billabongs by Mitchell et al. (1994, 1996).

A further check of possible information sources was made by using the bibliography of the Yarra River prepared by the Water Studies Centre at Monash University (Water Studies Centre

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2005). It revealed one conference paper (Eliniezer et al. 1995) and three student theses from Monash University (Fletcher 1976; Hodges 1979; Varrasso 2000). The compilation ceased at 2005 and it is not clear whether more recent studies have been undertaken since then. A check at Monash University for the three theses revealed that one was unobtainable (Varrasso 2000); Fletcher (1976) examined two billabongs on the Yarra River floodplain north-west of Lilydale and thus not directly relevant to the current investigation; Hodges (1979) investigated five billabongs near the study site (Burke Road billabong, Golf course billabong, Boulevard billabong, Banyule billabong and Templestowe Road billabong) but Bolin Bolin Billabong was not examined.

The second source of written information was literature on the known hydrological requirements of different plant and animal taxa. This information was used to identify ‘ideal’ wetting and drying regimes and was drawn from a number of documents, both primary sources and review articles (e.g. Ward 1994; Briggs 1998; Green et al. 1998; Nichol & Ganf 2000; Roberts & Marston 2000; Siebentritt et al. 2004; VEAC 2006). The Murray Flow Assessment Tool (MFAT: see Young et al. 2003) provided detailed information on the hydrological requirements of a limited number of plant taxa as well as for some bird groups. The information was supplemented by studies of water requirements of waterbirds, especially in inland wetlands dominated by River Red Gums by Briggs & Thornton (1995, 1999) and Briggs et al. (1997, 1998), and the comprehensive review undertaken by GHD (2006) of the water requirements of faunal different groups in the Murray-Darling Basin.

There are two difficulties with using information in the second category. First, is strongly based on wetland systems and biota in the Murray-Darling Basin, and the climate (and hence wetland hydrology) there is not the same as that of metropolitan Melbourne. Second, relatively few taxa have been studied in sufficient detail for firm recommendations to be made. For plants the information base is most robust for River Red Gum Eucalyptus camaldulensis, Black Box Eucalyptus largiflorens (not found in the study area), Giant Rush Juncus ingens, Lignum Muehlenbeckia florulenta (also not found in the study area), Common Reed Phragmites australis (earlier but no longer present at Bolin Bolin Billabong), Cumbungi Typha spp., and Eelgrass Vallisneria spp. For water birds, most information was available for waterfowl, grebes and colonial-nesting species.

The third source was literature on the outcomes of hydrological interventions that have been undertaken in south-eastern Australia over recent decades. Those studies were examined for three reasons: i) to provide information on likely water requirements of the taxa not covered in the detailed reviews discussed above; ii) to provide a ‘sanity check’ of the recommendations made later in the report; and iii) to identify the factors that have been shown to limit the effectiveness of hydrological interventions in the past. The earlier report to Melbourne Water (Dodo Environmental 2009 a) describes these other studies in detail.

Internet sources

The internet was searched for relevant information on any aspect relating to Bolin Bolin Billabong specifically or to Yarra River billabongs more generally. The search uncovered sporadic information on bird abundances around the billabong collected by the Bird Observers Club of Australia and Eremaea Birds, as well as information on the use of Bolin Bolin Billabong by a diverse range of community groups. The eMelbourne site that provided an overview of the post-European history of the lower and mid Yarra River (Beardsell et al. 2008) was also useful.

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Personal enquiries

Discussions were had with Garry French and Cam Beardsell (both Parks Victoria) to obtain information on the site and to check my draft report. Janet Holmes (Department of Sustainability and Environment) was contacted to determine whether Bolin Bolin Billabong had yet been subject to an assessment under the State-wide Index of Wetland Condition. Rob Dabal (Melbourne Water) was contacted to determine whether that organisation had undertaken any relevant studies. Damien Cook (Australian Ecosystems, Carrum Downs) was contacted to determine whether his consulting firm had undertaken an ecological analyses of the site, given that they had undertaken a wide range of investigations on other nearby wetlands and billabongs (e.g. see reports in Dodo Environmental 2009 b). Similarly, Geoff Carr and Andrew McMahon (both Ecology Australia, Fairfield) were contacted to see whether they had undertaken any relevant studies or had access to other information. Dr Andrew Sharpe (Sinclair Knight Merz) was contacted to see whether conceptual models for Victorian floodplain river systems had been developed as part of VEFMAP (Victorian Environmental Flows Monitoring and Assessment Program) studies. Revisions to the draft report (dated 12/03/2010) were made in the light of feedback from Garry French and Cam Beardsell, as well as from Lachlan Johnson and Andrew Allan (both Manningham City Council), during a final project meeting on 16/04/2010.

Field-site visit

The billabong was visited on 10/02/2010 with Garry French and Cam Beardsell.

2.3 Some notes on terminology

The discipline of ecological restoration/rehabilitation is rife with terminological confusion and inexactitude. The problem becomes even more worrying when aquatic ecosystems are considered, because many of the often-used words, such as ‘ephemeral’, ‘intermittent’ and ‘seasonal’, are used by different people to mean different things.

Rehabilitation and restoration

Much confusion in natural-resource management centres on the words ‘rehabilitation’ and ‘restoration’. Dictionaries tend not to differentiate between the two (e.g. see Shorter Oxford English Dictionary, page 1784) but there is a distinction worth making with degraded ecosystems (Bradshaw 1997). In accordance with the notions of Bradshaw (1997), the term ‘restoration’ in this report is taken to mean the reversion of a degraded ecosystem to its original condition. Lake (2001, page 110) referred to this process as ‘inducing and assisting abiotic and biotic components of an environment to recover to the state that they existed in the unimpaired or original state’. In most cases the original condition is equated with pre-European condition, but even that end-point has its conceptual and pragmatic problems. This is not the place to review the difficulties with bona fide ‘restoration’, but if the reader wants more information the texts by Elliot (1997), Seddon (1997), Horton (2000) and Ryder et al. (2008) are good places to start.

In contrast, ‘rehabilitation’ describes an acceptable improvement in ecological condition (e.g. see Lake 2001) and, in most cases, is a more realistic management objective. The term is used in the same way as it is in a rehabilitation hospital for the war-wounded: the intention of medical care and intervention in rehabilitation hospitals is not (indeed cannot) be the complete recovery of, say double amputees or paraplegics, but their treatment with prostheses, physiotherapy etc in order that they can become useful and functioning members of society. Much the same argument holds for the rehabilitation of damaged (i.e. degraded) natural systems, where management

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interventions are expected to markedly improve the ecological condition of the system and allow it to again deliver, in broad terms, the sorts of ecosystem services the community expects.

Hobbs (2005) developed the argument further, and distinguished among three types of management outcomes: i) maintenance; ii) improvement; and iii) reconstruction. In his scheme, reconstruction broadly equates with restoration, and improvement with rehabilitation.

Wetland hydrology

Boulton & Brock (1999) proposed a simplified classification of temporary wetlands to remove the ambiguity in using terms such as ephemeral, intermittent and seasonal. ‘Temporary wetland’ means, in the broad sense, any wetland that dries out, no matter how briefly, or recedes to small pools. As shown in Table 2.1, permanence and predictability of inundation are the two hydrological components used to structure the classification system proposed by Boulton & Brock (1999).

Table 2.1: Simplified classification of temporary wetlands. Adapted from Boulton & Brock (1999) and Paijmans et al. (1985).

Wetland type Predictability and duration of filling Ephemeral Filled only after unpredictable rainfall and runoff. Surface water dries within a couple of days of filling and seldom supports macroscopic aquatic life. Episodic Dry most of the time, with rare and very irregular wet phases that may persist for months. Annual inflow is less than minimum annual loss in 9 years out of 10. Intermittent Alternately wet and dry, but less frequently and less regularly than seasonal wetlands. Surface water persists for months to years after filling. Seasonal Alternately wet and dry every year, according to season. Usually fills in the wet part of the year and dries predictably every year during the dry season. Surface water persists for months, long enough to support macroscopic aquatic life. Biota adapted to desiccation. Permanent Predictably filled although water levels may vary across seasons and years. Annual inflow is greater than minimum annual loss in 9 years out of 10. May dry during extreme droughts. Biota generally cannot tolerate desiccation.

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3. Historical perspective, current ecological condition, and threatening processes

3.1 Landscape context and neighbouring billabongs

Bolin Bolin Billabong is one of a small number of remnant billabongs on the floodplain of the Yarra River in north-eastern metropolitan Melbourne. It is located in the Yarra Flats parkland, in the suburb of Bulleen. Nearby billabongs include, upstream and to the north and north-east, Banyule Flats Billabong (a site examined in Hodges 1979 and Dodo Environmental 2009 b) and Annulus Billabong. Downstream are Willsmere Billabong, Baileys Billabong, Reedy Billabong, Horseshoe Billabong and Burke Road Billabong (Burke Road Billabong webpage, http://home.vicnet.net-au/~fobrb/b-bongs.html, internet resources accessed 16/02/2010). Figure 3.1 shows the location of Bolin Bolin Billabong and its spatial relationship with the nearby Annulus and Banyule Flats Billabongs.

Banyule Flats Billabong

Yarra River

Annulus Billabong

Bolin Bolin Billabong

Figure 3.1: Location of Bolin Bolin Billabong with respect to two nearby billabongs on the floodplain of the Yarra River. Source: Google Earth, accessed 13/02/2010.

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Figure 3.2 shows a close-up aerial view of the billabong. Mitchell et al. (1996) identified two locations where water could flow into the billabong from the Yarra River: a south-western location with a commence-to-flow elevations of 10.4 m AHD; and a north-western location with a commence-to-flow elevation of 11.0 m. These two locations are shown with yellow arrows in Figure 3.2. Discussions during the final project meeting of 16/04/2010, however, indicated that a third location is now probably the site where most water enters the billabong when the Yarra River is in flood. This location, with a commence-to-flow elevation of ~12 m AHD (Garry French, pers. comm. 16/04/2010) is near the northern-most end of the billabong. It is indicated with the red arrow in Figure 3.2.

Yarra River

12 m AHD

Likely sites of water ingress and egress from Yarra River Bolin Bolin Billabong 11.0 m AHD

10.4 m AHD

Figure 3.2: Aerial view of Bolin Bolin Billabong, showing the two location and commence-to-flow elevations of likely inflows of flood waters from the Yarra River according to Mitchell et al. (1996, Figure 15). The red arrow shows the entry point with a commence-to-flow elevation of ~12 m. The image dates from February 2006. Source: Google Earth, accessed 13/02/2010.

3.2 Social significance

Mitchell et al. (1996) noted that Bolin Bolin Billabong was particularly important for the Wurundjeri people before, during and after European colonization. It provided large amounts of food, particularly eels, and early records show that about 300 people had gathered at the billabong in December 1843 (Mitchell et al. 1996, page 21). That was an important time for fish trapping in the mid-lower Yarra River, but just before eels had fattened and become ready for

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harvest (Gott 2010 a, b). The area remains highly significant for aboriginal people today (see presentation by Mick Woiwod at http://www.yarrahealing.catholic.edu.au/stories- voices/index.cfm?loadref=82, internet resources accessed 16/02/2010). During the time of Mitchell et al. (1996)’s investigation, Bolin Bolin Billabong was reported as a popular spot for anglers because of its depth and diversity of fish habitats (Mitchell et al. 1996, page 49). It is unlikely to have this recreational value at the moment. Nevertheless, the billabong and its surrounds are used heavily for excursions by community groups and by nature lovers. The search of the internet, for example, showed recent excursions to the billabong by the Indigenous Flora and Fauna Association and the Field Naturalists Club of Victoria. The area is subject also to a strong revegetation program by community groups such as Bullen Art & Garden (Bulleen Art & Garden 2009). As shown later in the section on fauna of the site, Bolin Bolin Billabong is frequently visited by bird watchers, and their records add considerably to what is known about the natural values of the area.

3.3 Ecological value

Cam Beardsell (pers. comm. 10/02/201) noted that historically there were ~70 floodplain wetlands in the Chandler Basin of north-eastern metropolitan Melbourne and that only ~10 remain today. The only one of the remaining 10 that has not suffered changes to its geomorphometry (e.g. due to earthworks etc) is Bolin Bolin Billabong. Beardsell & Beardsell (1999, page 28) concluded that Bolin Bolin Billabong and parts of the nearby lagoon on the opposite side of Bulleen Road were ‘..the best preserved [floodplain] communities in the region’. Banyule Flats Billabong, just upstream (see Figure 3.1), was described as having ‘The most intact stand of seasonal wetland remaining in the lower Yarra Valley’ (page 27). Seen together, therefore, Bolin Bolin Billabong and Banyule Flats Billabong form the crucial parts of a mosaic of remnant floodplain wetlands in this part of Melbourne.

3.4 Hydrology and wetland type

The following sections (Section 3.8 and 5.1) describe the hydrology of the site, both in regard to long-term changes arising from regulation of the Yarra River and more recent changes to wetting and drying cycles that have taken place over the past ~15 years. The point worth noting is that, under the typology used to classify wetlands in Victoria (see Table 3.4), Bolin Bolin Billabong is described as a Deep Freshwater Marsh (Department of Sustainability and Environment Interactive Map webpage, accessed 17/02/2010). Before hydrological matters are discussed, however, it is worth digressing into what is known more broadly about the ecological structure and function of billabongs and, in particular, whether there currently exist any broadly applicable conceptual models that could be used in this study.

3.5 Conceptual models of billabong structure and function

What are conceptual models? They are ‘non-quantitative planning tools that identify the major anthropogenic drivers and stressors on natural systems, the ecological effects of these stressors, and the best biological attributes or indicators of these ecological responses’ (Ogden et al. 2005 a). Section 6 notes the crucial position held by a conceptual models in the adaptive management approach to natural- resource management. Moreover, conceptual models are now recognized as a critical components of modern-day monitoring programs (Maddox et al. 1999; Gross 2003; Ogden et al. 2005 a, b). In fact, Gross (2003) argued that conceptual models can ‘integrate current understanding of system dynamics, identify important processes, facilitate communication of complex interactions, and illustrate connections between indicators and ecological states or processes. Well-constructed conceptual models provide a

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scientific framework for the monitoring program and justification for the choice of indicators’. Equally importantly, they are useful in fostering communication and debate among environmental scientists, staff in natural-resource management agencies, and the general public (Gentile et al. 2001). Plumb (1999) provided an excellent overview of the different ways conceptual models can be expressed, including written narratives, tables and schematic diagrams such as box-and- arrow models.

The diagrams of vegetation change with elevation (Figure 3.4 below) represent one type of conceptual model that has been developed, in that case probably not knowingly as a conceptual model, for the billabong. Similarly, the diagram that outlines the sources of water and loss mechanisms (Figure 5.6) also represent a conceptual model for the site. In a search for more generalized models that could guide management of the site, a reasonably exhaustive search of the internet with a range of key words failed to identify an existing conceptual model for billabongs or floodplains of south-eastern Australia. The closest was the work of Roberts & Ludwig (1991), who stated that their paper developed a simple conceptual model of riparian vegetation on floodplain wetlands of a regulated river, but my reading of the paper shows no such model. This was rather a surprising finding, as much effort has been put into researching and managing these types of ecosystems in Australia over the past ~20 years, particularly in the Murray-Darling Basin. To make sure that I had not missed any critical reports that included a conceptual model of billabong structure or function, a check with Dr Andrew Sharpe (Sinclair Knight Merz, Armidale: a member of the team undertaking the Victorian Environmental Flows and Assessment Program, or VEFMAP) also indicated the lack of suitable models for floodplain wetlands. The only conceptual model specifically developed for floodplain billabongs in south- eastern Australia that I could find is shown in Figure 3.3, an early model of food webs in billabongs.

Figure 3.3: Simplified food-web structure in a river (left-hand side) and billabong (right-hand side). Source: Boon et al. (1990, Fig 11.1).

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3.6 Ecological condition – findings of the literature review

The vegetation and ecological condition of a number of billabongs on the Yarra River floodplain have been described in recent investigations (e.g. Australian Ecosystems (2009) for Yering Backswamp; Australian Ecosystems (2007) for Banyule Flats Reserve; Burke Road Billabong Reserve Committee of Management (2009 a, b) for Burke Road Billabong; and Beardsell (2000) and Lorimer (2006) for a large range of sites, mostly terrestrial, in Banyule Shire and Boroondara Shire, respectively). The most recent detailed investigations into the ecology of Bolin Bolin Billabong are those of Mitchell et al. (1994, 1996) and they thus pre-date the current period of drought. The area has not been subject to an assessment under the State-wide Index of Wetland Condition (Janet Holmes, pers. comm.19/02/2010)

Vegetation

Mitchell et al. (1996) identified three vegetation communities and four habitat zones at Bolin Bolin Billabong. The four habitat zones were: i) deep open water (Lagoon Aquatic Floating and Submerged Herbfield); ii) shallow water (Lagoon Emergent Herbfield); iii) mudflats (Lagoon Mudflat Herbfield); and iv) woodland (Floodplain Woodland Billabong Banks, River Flats and Terraces). During the time of the vegetation surveys (November to December 1993 and December 1994 to March 1995), Bolin Bolin Billabong retained some water and so the results are difficult to apply to the present circumstances. Their map of vegetation types (pages 73 on of Mitchell et al. 1996), for example, show a central band of deep water in the middle of the billabong, with small patches of shallow water at spots along the perimeter and occasional mudflats near the eastern-most corner. There is a clear ecotone between the two vegetation types with increasing elevation from the water’s edge at the billabong. Figures 19 and 20 of Mitchell et al. (1996) show the lateral gradation of plant species, and they are reproduced below (Figure 3.4). Note that deep water is shown as being present in November 1992 and shallower water in March 1995.

The description of vegetation in Victoria, however, has changed since the surveys of Mitchell et al. (1996) and vegetation in Victoria is now described by a system of Ecological Vegetation Classes, or EVCs. An EVC is one or a number of vegetation communities, described in floristic and structural terms, that appears to be associated with a recognisable and coherent environmental niche. The field inspection, discussions with Parks Victoria staff, and interrogation of the Interactive Map webpage of the Department of Sustainability and Environment (see Figure 3.5) indicated that the two dominant Ecological Vegetation Classes in the study area were: i) EVC 56 Floodplain Riparian Woodland surrounding the billabong; and ii) EVC 172 Floodplain Wetland Aggregate in the billabong proper.

Figure 3.5 shows the current distribution of EVCs at the study site, taken from the Interactive Map webpage of the Department of Sustainability and Environment.

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Figure 3.4: Vegetation transects at two locations at Bolin Bolin Billabong. Mitchell et al. (1996, Figures 19 & 20). Figures reproduced with permission from Damien Cook.

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EVC 56

EVC 172

Figure 3.5: Current (2005) EVCs at the Bolin Bolin field site. Bolin Bolin Billabong is shown at the bottom and Annulus Billabong at the top. Source: Department of Sustainability and Environment Interactive Map webpage, accessed 17/02/2010.

EVC 56 Floodplain Riparian Woodland is characterized by an open eucalypt woodland up to ~20 m tall dominated by species such as River Red Gum Eucalyptus camaldulensis, Swamp Gum Eucalyptus ovata and Manna Gum Eucalyptus viminalis. The shrub layer is diverse and can consist of a wide range of species in the genera Acacia, Ozothamnus, Bursaria, Melicytus, Persicaria and Senecio. The shrub and ground layers, however, are highly susceptible to invasion by exotic plants, and common weeds in the EVC include Hawthorn Crataegus monogyna, Ash Fraxinus spp., Madiera Winter-cherry Solanum pseudocapsicum, Blackberry Rubus spp., various thistles and docks (e.g. Sonchus oleraceus and Rumex crispus, respectively), Wandering Tradescantia Tradescantia fluminensis and a suite of troublesome grasses such as Panic Veldt grass Ehrharta erecta var. erecta and Kikuyu Pennisetum clandestinum. Native grasses (e.g. Common Tussock-grass Poa labillardierei) may be present also. Detailed information on the EVC is available from the benchmark descriptions of EVCs in various bioregions from the website of the Department of Sustainability and Environment (http://www.dse.vic.gov.au).

The identification by the Department of Sustainability and Environment of the large numbers of potential weed species that can infest Floodplain Riparian Woodland is consistent with analyses of the threat posed by weeds in other floodplain and billabong environments in south-eastern Australia. Smith & Smith (1990), for example, concluded that the invasion of the floodplain by weeds has been one of the most pervasive effects of European colonization on floodplain ecosystems of the River Murray. They argued that about one-third of all plant species on the floodplain of the River Murray were exotic, compared with a value of about 10% for the Australian flora as a whole. Smith & Smith (1990) proposed that the lowest number of weeds occurred in the most frequently flooded parts of the River Red Gum zone of the floodplain, where an average of 20% of the flora was exotic.

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EVC 172 Floodplain Wetland Aggregate is a collective label for a range of vegetation types that occur in the dampest parts of floodplains and along riparian zones of Victorian streams. It describes a mosaic of reedbeds, sedgelands and rushlands. The main plant species commonly present in the EVC include the emergent Tall Sedge Carex appressa, various rushes Juncus app., various sedges Cyperus spp., and Common Spike-sedge Eleocharis acuta. Common Reed Phragmites australis would usually be present also, but has been lost from Bolin Bolin Billbong during the recent dry period (Cam Beardsell, pers. comm. 16/04/2010) Common weeds include Drain Flat-sedge Cyperus eragrostis and Water Couch Paspalum distichum. In Bolin Bolin Billabong, the main constituent EVC of the Floodplain Wetland Aggregate is likely to be Floodplain Pond Herbland (Cam Beardsell, pers. comm. 10/02/2010). EVC 810 Floodway Pond Herbland describes a ‘low herbland on the drying mud of floors of ponds on floodway systems (mainly riverine floodplains). The floristics (and diversity) can be quite variable (both spatially and temporally), according to the traits of the relevant individual pond. The floristics also vary in temporal cycles with the ‘unvegetated’ unit and probably between seasons at some locations’.

A full analysis of plant species at the field site was beyond the scope of the project. The only comprehensive data are in Mitchell et al. (1996), which shows full species lists obtained for the two vegetation surveys taken in November-December 1993 (a preliminary survey) and December 1994 to March 1995 (a more detailed investigation). Table 11 of Mitchell et al. (1996) shows that 55 indigenous and 79 introduced plant species were recorded at Bolin Bolin Billabong. In other words, of the 134 species recorded, nearly 60% were exotic. Tables 15-17 of Mitchell et al. (1996) show the total plant species recorded during the surveys, of which 19 species (see their Table 12) were identified as significant.

Often forgotten in vegetation descriptions are the non-vascular plants. Mitchell et al. (1996) reported on phytoplankton in the billabong and recorded a wide range of algal taxa spread across most of the main algal groups. In fact, ‘At each sampling, algae belonging to at least 9 orders were observed in each billabong’ (Mitchell et al. (1996, page 79). Algal concentrations (as chlorophyll a) were 5 g L-1 in November 1993 and 13 g L-1 in April 1995.

Water quality

As described later, the billabong has been almost dry for much of the past decade and the only water-quality data that are available come from the studies of Mitchell et al. (1994, 1996). Mitchell et al. (1996) analyzed water samples from the billabong for a full set of analytes in November-December 1993 and May 1994, and for a smaller set in December 1994 and April 1995. Samples were taken from five sites within the billabong, although not all sites were sampled at each time or for all analytes (see pages 31 and 34 of Mitchell et al. 1996 for a full description of the sampling design). Table 3.1 shows a summary of the water-quality data obtained by Mitchell et al. (1996). Some water-quality data for other Yarra floodplain billabongs are available in the student theses by Fletcher (1976) and Hodges (1979), and for billabongs more generally in Boon et al. (1990). Note that the 2002 ANZECC guidelines do not assign water-quality trigger values for wetlands in south-eastern Australia.

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Table 3.1: Summary of water-quality data, mostly averaged over five sites and four sampling times from 1993 to 1995. Adapted from Mitchell et al. (1996, Table 9). * = single measurement

Water-quality variable Median concentration Range Dissolved oxygen (mg L-1) 4.2 3.2 – 8.4 pH 6.7 6.0 – 7.1 Conductivity (S cm-1@25oC) 223 193 – 260 Turbidity (NTU) 42 23 – 40 Suspended solids (mg L-1) 23 14 25 Total nitrogen (mg L-1) 1.6 1.4 – 2.1 Total phosphorus (mg L-1) 0.21 0.016 – 0.22 Zinc (mg L-1)* 0.015 Cadmium (mg L-1)* 0.001 Copper (mg L-1)* < 0.001 Lead (mg L-1)* 0.014 Nickel (mg L-1)* 0.005 Chromium (mg L-1)* 0.005 Mercury (mg L-1)* <0.00005 Arsenic (mg L-1)* 0.004

Sediment chemistry

Mitchell et al. (1996) described also the results of a small amount of sediment sampling in Bolin Bolin Billabong. The results are shown in Table 3.2.

Table 3.2: Summary of mean concentrations of metals, nutrients, pesticides and PCBs (polychlorinated biphenyls) in surface sediments from Bolin Bolin Billabong. Samples were taken three sampling sites at three times from 1993 to 1995. Adapted from Mitchell et al. (1996, Table 10). ND = not detected

Sediment variable Mean Within Ontario concentration (1989) criteria? Copper (mg kg -1 dry weight) 16 Yes Lead (mg kg -1 dry weight) 26 Yes Zinc (mg kg -1 dry weight) 82 Yes Arsenic (mg kg -1 dry weight) 6.1 Not applicable Cadmium (mg kg -1 dry weight) <1 Yes Chromium (mg kg -1 dry weight) 23 Yes Nickel (mg kg -1 dry weight) 12 Yes Iron (mg kg -1 dry weight) 12,000 Not applicable Manganese (mg kg -1 dry weight) 130 Not applicable Total Kjeldahl nitrogen (mg kg -1 dry weight) 3,200 Not applicable Total phosphorus (mg kg -1 dry weight) 400 Not applicable Total PCBs (mg kg -1 dry weight) ND Not applicable Triazines (mg kg -1 dry weight) ND Not applicable Organochloride pesticide residues (mg kg -1 ND Not applicable dry weight)

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Macroinvertebrates

The most detailed description of aquatic macroinvertebrates is found in Mitchell et al. (1996). Sweep samples for aquatic macroinvertebrates were taken in November 1993 and May 1994, and returned 37 and 28 species, respectively. Airlift samples at the same dates returned 8 and 3 species, respectively. The most common taxa by far were chironomids (midge larvae) in the genera Chironomus, Dicrotendipes and Paratanytarsus. Chrinomids accounted for over one-half of all the total abundance. The next most abundant group was oligochaetes (aquatic worms). Gooderham & Tsyrlin (2002) provide an easily accessible overview of these taxa. Zooplankton (rotifers and microcrustaceans) were collected at the same dates as macroinvertebrates. Copepods (especially Calamoecia lucosi and Boeckella spp.) were abundant, and cladocerans (water fleas) and rotifers were present but generally less common.

Vertebrates

The most detailed reports on the wildlife of Bolin Bolin Billabong are found in Mitchell et al. (1996). Their data are summarized in Table 3.3.

Table 3.3: Summary of terrestrial vertebrate fauna at Bolin Bolin Billabong. Adapted from Mitchell et al. (1996, Table 18).

Group Number of species Native Introduced Mammals 9 4 Birds 105 12 Reptiles 5 Amphibians 7

A number of regionally significant species were recorded for the site, including Peregrine Falcon Falco peregrinus, Buff-tailed Rail Rallus philippensis, Latham’s Snipe Gallinago hardwickii and Rufous Songlark Cincloramphus mathewsi.

More recent data on bird communities is available from the information available on websites of the Bird Observers Club of Victoria and Eremaea Birds. The website of Eremaea Birds shows information on bird sightings at Bolin Bolin Billabong collated from 23 lists that span the period 1990–2010 (http:www.eremaea.com/sitespecieslist.aspx?context=sitespecieslist&site=1756, internet resources, accessed 16/02/2010). In total, 85 bird species were recorded, with the ‘best single-day list’ having 63 species (1/02/2006).

Almost all of the bird groups were represented in the database, including ducks, grebes, cormorants, darters, heron and egrets, ibis, goshawks, falcons, moorhens, lapwing, snipe, gulls, pigeons and doves, cockatoos, parrots, cuckoos, kingfishers, honeyeaters, cuckoo-shrikes, raven, swallow, thrushes, fantails, whistlers and white-eyes, fairywrens, thornbills, misteltoebirds, magpie-lark, butcherbirds, starlings and finches. The most commonly observed species of waterbird were Pacific Black Duck Anas superciliosa, Grey Teal Anas gracilis, Chestnut Teal Anas castanea, Australasian Grebe Tachybaptus novaehollandiae, White-faced Heron Egretta novaehollandiae and Dusky Moorhen Gallinula tenebrosa. The terrestrial fringes were also important sites for birds, and commonly observed species included Spotted Dove Streptopelia chinensis, Common Bronzewing Phaps chalcoptera, Yellow-tailed Black-Cockatoo Calyptorhynchus funereus and Sulphur- crested Cockatoo Cacatua galerita, Kookaburra Dacelo novaeguineae, White-plumed Honeyeater

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Lichenostomas peniccillatus and Grey Shrike-thrush Colluricincla harmonica. Powerful Owl Ninox strenua are present also (Garry French, pers. comm. 16/04/2010).

Additional recent information on birds is available from the website of the Bird Observers Club of Victoria. The records for Bulleen Park and Bolin Bolin Billabong include data for October and December 2009 and for these two dates 42 bird species were observed (http://www.melboca.org.au/outings/site-lists/bulleen&bolin.html, internet resources, accessed 16/02/2010).

The fish communities were described in Table 30 of Mitchell et al. (1996). The fish assemblages were depauperate, and only two native species (Short-finned Eel Anguilla australis and Flat Headed Gudgeon Philynodon grandiceps) were recorded in the billabong: in contrast, 12 native fish species were recorded for the lower Yarra River. More species of exotic than native fish species were recorded for the billabong, including Goldfish Carassius auratus, Carp Cyrpinus carpio, Mosquito Fish Gambusia holbrooki, Weatherloach Misgurnus anguillicaudatus and Redfin Perca fluviatilis. The investigation of five nearby billabongs by Hodges (1979) returned similar fish assemblages, with the common factor being dominance by exotic taxa.

3.7 Changes to Bolin Bolin Billabong since European colonization

Alterations to hydrology of the mid Yarra River and its floodplain wetlands

The high ecological value of Bolin Bolin Billabong and its surrounding woodland is especially apparent when compared with the scale of loss of floodplain wetlands following European colonization in the mid Yarra River.

It is likely that nearly two-thirds of the natural wetlands in the Greater Melbourne region were lost by draining, filling or other modifications after European settlement (Mark Smith, Port Phillip and Westernport Catchment Management Authority, pers. comm. 3/03/2009). In fact, Presland (2008, page 62) argued that ‘No feature of the original landscape of the Melbourne area has been so deliberately altered as the wetlands and drainage patterns’.

Lacey (2004) used a range of historical information to show that the large and diverse array of billabongs on the floodplain of the mid Yarra River, around Banyule and Bulleen, existed until about 1960. Few remain today, as is seen in Figure 3.1. Figure 3.6 reproduces the diagram of Lacey (2004) that compares the pre-European and current-day distribution of billabongs near the study site. It supports the conclusion by Cam Beardsell (see Section 3.3) that many, if not most, of the floodplain wetlands in metropolitan north-east Melbourne have been lost to drainage, or else highly modified geomorphologically.

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Figure 3.6: Comparison of pre-European and present distribution of floodplain wetlands in the Yarra Flats and Bulleen Flats region. Source: Lacey (2004, Figure 4). Reproduced with permission from Australian Scholarly Publishing.

The wetlands and billabongs that do remain on the Yarra floodplain have experienced marked changes to their wetting and drying cycles since European colonization (Sinclair Knight Merz 2005 a). The majority of floodplain wetlands that were present in the area before European colonization probably fell into the Deep Freshwater Marsh category, which means that they would have been flooded almost continuously, to a depth of up to 2 m, but may have occasionally dried out during drought. (Table 3.4 shows the typology that is currently used to classify wetlands in Victoria and the inundation regimes that typify each of the individual categories.)

The development of the Yarra River floodplain has seen extensive drainage and flood mitigation works which, combined with reductions in discharge because of regulation (e.g. Upper Yarra Reservoir) and extraction, caused most of these floodplain wetlands to become flooded for shorter durations, shallower depths and at a lower frequency than in pre-European times. The result of this development has been that many of the formerly Deep Freshwater Marshes that

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occurred along the Yarra River in pre-European times have shifted in hydrological regime to the Freshwater Meadow category, a class of wetlands that hold water for < 4 months per year and are < 0.3 m deep (Table 3.4). As concluded in Dodo Environmental (2009 a), the remnant wetlands that remain on the floodplain of the Yarra River have, as a result of river regulation, extraction of flows, and creation of alienating structures such as levees, generally become ‘drier’ following European colonization. The prolonged period of drought experienced in south- eastern Australia since ~1997 has exacerbated such long-term changes in wetland and floodplain hydrology along the entire Yarra River system.

Table 3.4: Summary of characteristics of different wetland category types in Victoria as described under the Norman & Corrick classification scheme.

Wetland category Water depth (m) Inundation Freshwater meadow < 0.3 < 4 months per year Shallow freshwater marsh < 0.5 < 8 months per year Deep freshwater marsh < 2 Permanent (but may dry out every 4-5 years) Permanent open freshwater < 2 (shallow) Permanent > 2 (deep) Permanent Semi-permanent saline < 2 < 8 months per year Permanent saline < 2 (shallow) Permanent > 2 (deep) Permanent

Changes to Bolin Bolin Billabong since European settlement

Beardsell et al. (2008) noted that a large complex of discrete wetlands, including the Bolin Bolin site, formerly ran alongside the Yarra River for about 1 km in pre-European times and contained some areas of deep water that may have been more-or-less permanently inundated. They posited that these lower sections of the Yarra River were likely vegetated with River Red Gum and Swamp Paperbark Melaleuca ericifolia, vegetation types which probably extended also onto fringing marshy areas and billabongs around Kew and Bulleen.

Lacey (2004) collated the historical information on the Yarra River around Banyule and Bulleen and noted that during the journey by the author Rolf Bolderwood (of Robbery Under Arms fame) in 1845, the floodplain was heavily timbered with deep, reed-fringed lagoons. This journey took place just after the arrival of the first European settlers to Bulleen in 1837, who initially ran sheep on the river flats. Dairy farming began in 1840 and loggers soon arrived to cut the abundant timber. Lacey (2004, page 88) noted that by the early 1840s the river flats were being used to grow wheat, barley and potatoes. Drought and flood discouraged the continued farming of these crops, but the river flats were still suitable for dairy grazing. Despite the clearing of trees and agricultural use to which the flats had been put, the Bolin Bolin area still had a ‘substantial growth of trees’ (page 88) at the end of the 19th Century. The floodplain and its wetlands changed markedly in the 1960s and 1970s, as billabongs were filled with rubbish and covered with soil. Many of the river flats were converted to playing fields, golf courses.

In pre-European times Floodplain Riparian Woodland (EVC 56), consisting of River Red Gum, Manna gum Eucalyptus viminalis and lesser amounts of Swamp Gum Eucalyptus ovata and Narrow- leafed Peppermint Eucalyptus radiata, probably covered the river flats and River Red Gums were common on the floodplain and around the billabong (Lacey 2004). The understory would have been diverse and include taxa such as Sweet Bursaria Bursaria spinosa and Golden Wattle Acacia pycnantha (Beardsell & Beardsell 1999). Because of the mosaic of geomorphological features,

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however, a number of plant communities would have been present, associated with the range of hydrological conditions brought about by variations in elevation and flooding regime (Department of Sustainability and Environment 2006). Beardsell & Beardsell (1999), for example, showed a 1860 map of the Bulleen area, with extensive floodplain wetlands around Bolin Bolin. Lacey (2004) refers also to an 1841 survey and 1860 geological map which show the billabong as ‘Lake Bulleen’. Figure 3.6 shows the modelled pre-European vegetation around Bolin Bolin Billabong. A comparison with Figure 3.5 (present-day vegetation) reveals the loss of EVC 56 from much of the area around the site.

EVC 56

EVC 172

Figure 3.6: Modelled pre-European (1788) EVCs at the Bolin Bolin field site. Bolin Bolin Billabong is shown at the bottom and Annulus Billabong at the top. Source: Department of Sustainability and Environment Interactive Map webpage, accessed 17/02/2010 .

Currently Bolin Bolin Billabong is a shallow, turbid, eutrophic, U-shaped wetland that floods infrequently and receives some groundwater inputs (Lacey 2004; Leahy et al. 2005). In pre- European times it had low nutrient concentrations and a reasonably stable phytoplankton assemblage dominated by the planktonic diatom Cyclotella stelligera (Leahy et al. 2005). Soon after European colonization of the area, erosion in the catchment increased at least 30-fold and there were marked changes to the phytoplankton communities. Nutrient increases were apparent after about 1920 (Leahy et al. 2005) and currently the billabong is dominated by Chlorophyceae or green algae (Mitchell et al. 1996).

In addition to the palaeolimnological work undertaken by Leahy et al. (2005), historical photographs can be used to glean additional information on the site. Although they can be extremely useful in rehabilitation ecology (Fensham & Fairfax 2002), a strong limitation with historical aerial photography is that high-quality images are often available only after about World War 2 (e.g. see Boon et al. 2008). Lacey (2004) noted that aerial and oblique photography dating from 1945 show River Red Gums lining the billabong’s fringes, as well as some Tree Violet Melicytus (formerly Hymenanthera) dentata and willows on the fringes, but few wattles or

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other shrubs. Aquatic plants at that time would have included sedges and rushes. The ground- layer was grassy and grazed by cattle. Ducks, swans and other waterbirds (e.g. Nankeen Night Heron Nycticorax caledonicus) would have made use of the open water that was then present: Lacey (2004, page 89) makes reference to over 100 Nankeen Night Heron flying out of the willows in 1937 that then fringed the billabong.

During the course of this investigation, Garry French (Parks Victoria) uncovered a number of historical aerial and oblique photographs of Bolin Bolin Billabong. The earliest one dates to 1931 and is shown in Figure 3.7. It shows the billabong filled with water and the surrounding floodplain and riparian zone almost devoid of native vegetation.

Figure 3.7: Bolin Bolin Billabong (arrowed) in 1931. This image is a section of an aerial photograph. Note that North in this image faces towards the right. Source: Garry French, Parks Victoria.

An oblique photograph of the billabong in the late 1970s is shown in Figure 3.8. Again the billabong is filled with water and the floodplain is mostly cleared. There are, however, some large trees (River Red Gums?) on the banks.

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Figure 3.8: Bolin Bolin Billabong (arrowed) in the late 1970s. Source: Garry French, Parks Victoria.

Two aerial photographs of the billabong are available for the mid-later 1970s: one in December 1975 and the other in March 1978. An image from the latter time is shown in Figure 3.9.

Figure 3.9: Bolin Bolin Billabong (arrowed) in March 1978. This image is a section of an aerial photograph. Source: Garry French, Parks Victoria.

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Figure 3.10 shows Bolin Bolin Billabong in March 1984 (i.e. after Melbourne Metropolitan Board of Works acquired the site in 1983). As before, the billabong is filled with water and the riparian vegetation is sparse.

Figure 3.10: Bolin Bolin Billabong (arrowed) in March 1984. This image is a section of an aerial photograph. Source: Garry French, Parks Victoria.

The final of the available aerial photographs of the site is shown in Figure 3.11. It shows the billabong in December 2004, after it had filled with water from the flooding Yarra River. (Section 3.8 addresses the historical pattern of wetting and drying in the billabong.) The billabong is filled completely with water and there is a large expanse of open water in the deepest part, the pool end at the eastern bow of the billabong. The markedly improved density of riparian and floodplain vegetation around the billabong is immediately apparent when this image is compared with those of the billabong in the 1970 and 1980s and, in particular, in the 1930s.

(Note that Figure 3.1 shows the billabong in February 2006 and this is the most recent aerial image we have of the site. It also shows permanent turbid water at the eastern end, as well as shallower water in the northern arm and what appears to be damp mud well vegetated with ground-cover, possibly Persicaria spp., on the southern arm.)

As outlined earlier, Bolin Bolin Billabong is the only wetland of ~70 that existed historically in the Chandler Basin of north-eastern metropolitan Melbourne that had not suffered from fundamental changes to its geomorphometry as a result of earthworks etc. The lack of change in the shape of the billabong’s basin is apparent in Figures 3.7 to 3.11.

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Figure 3.11: Bolin Bolin Billabong in December 2004, immediately after the first inundation with Yarra River floodwaters. Source: Garry French, Parks Victoria.

3.8 Hydrological regimes at Bolin Bolin Billabong

Hydrological modelling by Mitchell et al. (1996)

A hydrological analysis of Bolin Bolin Billabong by Mitchell et al. (1994, 1996) made use of a 1959 plan of the area and a 1978 base map, combined with gauging data on river height at Banksia Street (Heidelberg), to infer when water flowed into a number of billabongs in the region from the Yarra River. It was concluded that the Yarra River would need to reach a height of 10.4 m (AHD) before flood waters would flow into Bolin Bolin Billabong. As the normal Yarra River surface level (at the time of the study, and not necessarily today: see Section 5.1) at the billabong was 6.6 m AHD, water levels would need to rise 3.8 m over ‘normal’ levels for the billabong to start to be inundated with river water during high flows. The commence-to-fill height was considerably lower for Bolin Bolin Billabong than for Annulus or Banyule Billabongs, which would not start to fill until river heights of 11.5 and 13.4 m AHD, respectively. Figure 3.12 shows the lower commence-to-fill height for Bolin Bolin Billabong, taken from the report by Mitchell et al. (1996).

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Figure 3.12: Predicted commence-to-fill flows for three Yarra River billabongs, including Bolin Bolin Billabong. Source: Mitchell et al. (1996, Figure 12). An example period when Bolin Bolin Billabong was inferred to have commenced to fill with floodwaters is shown with an arrow.

The implication of this modelling is that Bolin Bolin Billabong was predicted to fill twice as often as the other two billabongs in the study. The median period between inflows for Bolin Bolin was calculated at ~9 months, which indicates that the billabong was inundated, on average, about three times every two years. Mitchell et al. (1996) concluded that, owing to the height difference between the billabongs and average water levels in the Yarra River, groundwater was unlikely to play a large role in their water balance. Moreover, the immediate catchment of Bolin Bolin Billabong is so small (9 ha: cf ~24 ha for Banyule Billabong) that drainage from the immediate catchment is also likely to be negligible for Bolin Bolin Billabong.

Mitchell et al. (1996) collated the known occasions when floodwaters from the Yarra River flowed into Bolin Bolin Billabong, and supplemented these empirical observations with modelled inundation patterns. Observed inflows occurred in April, June and July 1977 and August and November 1978. Modelled inflows were expected to have occurred in 1980, 1981, 1983, 1984 and 1985 (2 in 1985), 1987 and 1988, 1989 (2), 1990 (3), 1991, 1992 (2) and 1993 (4). These periods can be inferred partly also from Figure 3.12 Thus over the 16 year period spanned by the analysis, the billabong was expected to have received floodwaters from the Yarra River a total of 25 times. Not all years would result in inundation (e.g. inflows were not predicted for 1979, 1982 or 1986) and some years (e.g. 1985, 1989, 1990, 1992 and 1993) would have more than one wetting event.

On the basis of these investigations, Mitchell et al. (1996, page 97) concluded that ‘Bolin Bolin is likely to have permanent water, whereas Banyule and Annulus are likely to periodically dry out. It appears, therefore, that Bolin Bolin has a more predictable and stable water regime than Banyule and Annulus’. Interestingly, all the available historical aerial photographs until 1984 (i.e. Figures 3.7 to 3.10) show Bolin Bolin Billabong filled with water. Lacey (2004, Appendix Table 1) collated the known early flood dates for the Yarra River and that summation may throw additional light on that observation. The earliest wet period (1931, Figure 3.7), for example, may be a consequence of the floods in the Yarra River that occurred in December 1930. Similarly, the inundation observed for 1984 (Figure 3.10) may have resulted from the high water levels recorded in 1983 (see Figure 3.11). Moreover, Figures 19 and 20 of Mitchell et al. (1996), reproduced in Figure 3.4 of the present report, show that water was present in the billabong during November 1992 and March 1995.

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The direction of water flow into and out of the billabong during these periods of high river- water levels is somewhat controversial (see more detailed discussion in Section 5.1). Mitchell et al. (1996, Figure 15) shows that river water would commence to flow into Bolin Bolin Billabong from a north-western location at a river height of 11.0 m and from a south-western point at a river height of 10.4 m (see also Figure 3.2 of the current report). The difference would suggest that Yarra River water preferentially flows into the billabong along the southern arm from the south-western point. As noted in Section 3.1, however, it seems instead that now-a-days most water would enter the billabong from the Yarra River via a large channel where the northern arm is closest to the river, with a commence-to-flow elevation of ~12 m (see Figure 3.2). The field inspection of 10/02/2010 and project discussions during the meeting of 16/04/2010 indicate that the northern arm would be linked sooner with the river during high water levels, and it would allow a greater influx of flood water. Regardless of these considerations, the modelling undertaken by Mitchell et al. (1996) ceased at 1993 and anecdotal and unpublished information had to be used to describe wetting regimes since then.

Recent hydrological conditions

Discussions with Cam Beardsell (pers. comm.10/02/201) indicated that the billabong was filled from 1993 to 1996 inclusive. Lacey (2004) records that Bolin Bolin billabong dried in 1997 to the extent that only the large pool at the eastern end remained inundated. By early 2000 the bed of the billabong had become covered with juvenile River Red Gum and Silver Wattle Acacia dealbata in response to the non-aquatic conditions. According to Lacey (2004), it reflooded in October 2000, and ‘the inundated wattle saplings soon died and eventually many of the gum saplings too’ (Lacey 2004, page 91). Water levels fell again in 2001 and terrestrial trees (mostly River Red Gum and Silver Wattle) again started to colonise much of the wetland floor.

The billabong was dry until December 2004 and February 2005, when it was twice inundated by floods in the Yarra River (Cam Beardsell, pers. comm. 10/02/2010). These floods kept the billabong wet until August 2005 (Garry French, pers. comm. 16/04/2010) but, with the exception of a small pool at the deepest, eastern end (see Figure 3.2), it has been dry since then.

3.9 Vegetation responses to chronic desiccation

Responses to recent dry period

The analysis above indicates that Bolin Bolin Billabong was inundated, on average, at least yearly up until the middle of the 1990s. Hydrological modelling undertaken in the recent FLOWS study (Section 5.1) confirms this conclusion. Since 1997 it has been inundated with floodwaters only twice: in October 2000 and in a double-barrelled event in late 2004-early 2005. For the past four years it has not received any floodwaters and the only wet section is a small and stagnant pool in the most easterly end (see Figure 3.2). At the time of the site inspection in February 2010, the only part of the billabong that retained standing water was the deepest section at the extreme eastern end (Figure 3.13). Despite the generally dry conditions, however, the large mature River Red Gums around the billabong remain in excellent condition (Figure 3.14).

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Figure 3.13: Remnant pool in Bolin Bolin Billabong, February 2010. Note the fringe of Silver Wattle (arrowed) along the edges of the standing water. Photograph by Paul Boon.

Figure 3.14: Large mature River Red Gum in woodland around Bolin Bolin Billabong, February 2010. Photograph by Paul Boon.

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The vegetation response to the shift in hydrological conditions has been profound. River Red Gum saplings have colonized the bed of the billabong and Silver Wattle dominate the margins that were formerly periodically inundated but are now almost entirely terrestrial. Figure 3.15 shows the current state of vegetation in the billabong and along its margins

Figure 3.15: Terrestrialization of vegetation at the eastern end of Bolin Bolin Billabong, showing colonization of the bed with River Red Gum and of the edges by Silver Wattle, February 2010. Photograph by Paul Boon.

There is currently no widespread evidence of herbaceous vegetation typical of standing water, such as Water Ribbons Triglochin procera, Eelgrass Vallisneria australis or Pondweeds Potamogeton spp. There is also little evidence across the billabong as a whole of any of the larger emergent plants that commonly grow around the edges of billabongs, such as rushes, reeds or sedges.

Figure 3.4 showed the types of aquatic and fringing vegetation that would be expected to occur with a more natural water regime: Water Ribbons in the more permanent water, Tall Spike-sedge Eleocharis sphacelata and parrot-feathers (milfoils) Myriophyllum spp. along the shallower margins and, in the more ephemeral zones as the billabong grades into more fully terrestrial habitats, a range of plant taxa such as Lesser Joyweed Alternanthera denticulata group, Swamp Club-sedge Isolepis inundata, Knotweeds Persicaria spp., River Swamp Wallaby-grass Amphibromus fluitans and Austral Rush Juncus australis.

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Rather than taxa such as these that normally characterize billabongs with natural wetting and drying cycles, the common plant species around the periphery of the billabong today are terrestrial taxa. Figures 3.13 and 3.15 showed, as an example, the dominance of the canopy layer by River Red Gum and Silver Wattle, respectively. The understory also is currently dominantly by terrestrial taxa: Figure 3.16 show the abundant regrowth of Tree Violet in the formerly wet arms of the billabong.

Figure 3.16: Abundant regrowth of Tree Violet in the formerly wet arms of Bolin Bolin Billabong, February 2010. Photograph by Paul Boon

Weeds are currently also dominant in those parts of the billabong and its fringing woodlands that are not as wet as formerly. Figure 3.17 shows the abundant Wandering Tradescantia Tradescantia fluminensis in the westerly parts of the floodplain. Elsewhere on the now-dry floodplain are other creeping weeds such as Moth Plant Araujia sericifera, and exotic grasses such as Panic Veldt Grass and Kikuyu. Rambling Dock Acetosa sagittata was present too in this part of the billabong-floodplain.

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Figure 3.17: Abundant growth of Wandering Tradescantia in the western arms of the floodplain around Bolin Bolin Billabong, February 2010. Photograph by Paul Boon

Likely vegetation response to more natural wetting and drying cycles

A quite different set of vegetation communities would exist if water were returned to Bolin Bolin Billabong. The first response would be a cessation of the encroachment by terrestrial taxa, as exemplified by the juvenile River Red Gums growing in the bed of the billabong basin and the Silver Wattles down the margins into what would have formerly been episodically wet environments. Under current conditions, the increased abundance of ‘out-of-balance’ native taxa such as River Red Gums1 is a direct result of the altered hydrological conditions.

1 River Red Gums are ‘out-of-balance’ only in terms of a presumed or desired vegetation composition for the wetland. They are entirely ‘in balance’ in terms of their taking advantage of the new hydrological regime in the billabong, specifically the absence of prolonged wet periods that would drown-out the saplings that had recruited in the past 5-10 years following previous over-bank flows.

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The second response would a re-instatement of the aquatic and semi-aquatic taxa that would have occurred in prior EVC 172 Floodplain Wetland Aggregate and, more particularly, EVC 810 Floodway Pond Herbland in the wetter parts. The submerged and emergent plant taxa noted above would either return or need to be re-introduced in a revegetation program. Moreover, floating species such as the fern Azolla spp. and Duckweed Lemna disperma would grow on the surface of ponded water.

The third response is a likely reduction in the incidence of weed and other undesirable plant species on the floodplain. Prolonged dry phases in billabongs and floodplains often render them more susceptible to weed invasions (Smith & Smith 1990). Whilst there are a number of problematic weeds that can invade permanently wet billabongs (e.g. Brazilian Milfoil or Parrots Feather Myriophyllum aquaticum), weed problems are generally less of an issue when naturally fluctuating hydrological conditions are re-instated as the noxious taxa are generally intolerant of periodic or prolonged inundation. Moreover, the on-going presence of terrestrial weed species in the billabong thalweg, which would normally be vegetated with aquatic or semi-aquatic species, poses an on-going source of propagules and is thus a threat to the establishment of native terrestrial vegetation on the floodplain (Cam Beardsell, pers. comm. 16/04/2010).

3.10 Main threatening processes

The Scoping Review of Research and Management Needs of Australian Wetlands (Bunn et al. 1996) undertook an analysis of threats to wetlands on a nation-wide basis. It concluded that there were four main classes of ecological threat to wetlands:  Altered water regimes  Habitat modification  Pollutants, and  Weeds and pest fauna.

These four categories provide a useful basis for analysing the main threats to Bolin Bolin Billabong.

Altered hydrology

Bolin Bolin Billabong faces a number of threats to its ecological condition. Paramount among these is the altered hydrological regime which has seen the billabong go without inundation for much of the past 12+ years. As noted above, this fundamental shift in wetting and drying cycles has had manifold impacts on the vegetation of the site, with flow-on consequences for the provision of food and habitat to the animals that would normally depend on the billabong- floodplain environment for resources.

Not well considered in the review by Bunn et al. (1996) was the issue of climate change. The earlier reports on other billabongs in the Greater Melbourne region (Dodo Environmental 2009 a, b) reviewed the projected magnitude of climate change over the coming decades and likely ecological impacts. In brief, those reports identified projected increases in annual temperatures in Melbourne of 0.6–1.2oC by 2030 and up to 3.8oC by 2070. The number of days over 35oC could increase by 10–13 days per year by 2030, and by up to 26 days per year in 2070. Associated with the higher temperatures and (generally) decreased rainfall is a projected increase in evaporation of up to 5% by 2030 and possibly 16% by 2070.

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These changes will result in marked reductions in stream-flow and, even more so, for over-bank flow. Timbal & Jones (2008), for example, predicted that the percentage reduction in run-off was likely to be about twice as great as any reduction in rainfall in Melbourne’s water catchment areas. It cannot be stressed too highly that a reduction in rainfall will not result in a linearly corresponding reduction in stream-flow and, further, that a given reduction in stream-flow will not result in a linear reduction in over-bank flow. A reduction in river discharge of, say, 30% may result in a far greater reduction in the frequency of over-bank flows as commence-to-fill discharges are less rarely met.

Habitat modification

Habitat modification is often a ‘catch-all’ phrase that inevitably includes the specific impacts arising from changes to water regime, infestation by weeds and animal pests, and pollution. For billabongs, however, it is often useful to think of the term as including those more general, landscape-scale changes that effect ecological degradation. The most important types of habitat modification of billabongs include:  clearing, draining and damming  grazing  cropping  extraction and mining, especially of sand and gravel  commercial harvesting of wetland products  creation of fish barriers  fire  dumping of rubbish, and  recreation, including hunting, fishing and use of recreational vehicles.

Bolin Bolin Billabong is affected by only a few of these types of threat. It is perhaps the only billabong left in the Chandler Basin that has not been filled or otherwise affected by earthworks. It is not subject to harvesting of fish or plants, but being in a densely populated urban setting, may be affected by rubbish dumping. That threat, however, is probably low because of the poor road access and high frequency of visitation (and thus surveillance) by walkers, bird watchers etc.

Pollution

Floodplain wetlands can be threatened by five main classes of pollutants:  plant nutrients, especially nitrogen and phosphorus  salinity  suspended solids  toxic compounds, including heavy metals and pesticides, and  natural organic compounds, such as sewage.

The greatest risks arising from toxic pollutants probably relate to the proximity of the billabong to Bulleen Road. Road run-off could have high concentrations of oil and heavy metals. There is little or no evidence of contamination of the billabong by heavy metals in the analyses undertaken by Mitchell et al. (1996: see Table 3.2 of the present report) but those analyses were done ~15 years ago and traffic densities may have increased by one half during the intervening time.

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If stormwater were to be used as the water source to maintain water levels in the billabong in the future, issues with water quality will have to addressed. The two main issues are: i) nutrient enrichment (on the longer term, especially if treated storm-water is used); and ii) the creation of ‘blackwater’ after the billabong is first filled, even if it filled with low-nutrient river water. Both issues are discussed in length later in the report (see Section 5.7).

Weeds and undesirable plant species

Carr (1991) made the useful distinction between environmental weeds (species that aggressively invade natural environments and displace the native flora) and noxious weeds (species that affect agricultural production). Weed species could be exotic (species introduced from overseas), alien (species that occur naturally outside the region), indigenous (species native to, and evolved in, a particular region or country) or naturalised (species that have become established outside gardens, farms or plantations and are self-maintaining). Ecologically out-of-balance indigenous species often posed particular weed problems.

River Red Gums pose a significant weed issue at Bolin Bolin Billabong and can be considered an ecologically out-of-balance species in that context. The infestations of Silver Wattle (Figures 3.13 & 3.15) and Tree Violet (Figure 3.16) probably fall into the same category. A wide range of other plants could pose a weed problem to floodplains surrounding billabongs in the Greater Melbourne region. In addition to the species discussed earlier, potentially serious weeds on the floodplain include grasses (e.g. Serrated Tussock Nasella trichotoma), other perennial ground-cover plants (e.g. Angled Onion Allium triquetrum; Toowoomba Canary-grass Phalaris aquatica), shrubs (e.g. Hawthorn Crataegus monogyna; African Box-thorn Lycium ferocissimum) and trees (e.g. Sweet Pittosporum Pittosporum undulatum). Moreover, the on-going presence of terrestrial weeds in the basin of the billabong does represent an on-going source of propagules, which compromises attempts at weed control on the surrounding floodplain. Willows (Salix spp.) often form a particularly problematic group that may infest both the floodplain and the water body proper of billabongs (Blood 2001). Willows seem to be under control at the Bolin Bolin site and so are not discussed further.

A number of aquatic plant taxa pose threats to billabongs themselves and can be grouped according to habitat and life form (Sainty & Jacobs 1981). Many are aquarium or pond escapees, such as Water Hyacinth Eichornia crassipes and Water Lettuce Pistia stratiotes. Both are, or were until recently, commonly sold in the aquarium trade. Water lilies Nymphaea spp. are also commonly grown in ornamental ponds and have the potential to invade billabongs and other static bodies of water. Alligator Weed Alternanthera philoxeroides is a Weed of National Significance that has been reported around Melbourne in ornamental lakes. Canadian Pondweed Elodea canadensis and Dense Waterweed Egeria densa are both common aquarium plants that have proven invasive in many streams of south-eastern Australia. Egeria densa in particular is cold-tolerant, which allows it to colonise ornamental lakes around Canberra. Parrots Feather Myriophyllum aquaticum is an introduced freshwater macrophyte, native to South America and commonly sold as an aquarium plant. It has a propensity to escape cultivation and it is now naturalised in many wetlands and slow-moving aquatic systems outside of its native range. It was first recorded in Australia in 1908 and has since spread throughout wetlands, ponds and slow-moving streams in south-east Queensland, near-coastal areas of New South Wales, much of Victoria, Tasmania and parts of south-western Western Australia (Toomey & Boon 2007). Parrots Feather has been reported in some of Melbourne Water’s constructed wetlands. Some emergent taxa are also potential weed species, including Sharp Rush Juncus acutus and Soft Rush Juncus effusus, as well as Drain Flat-sedge Cyperus eragrostis.

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Pest animals

Bolin Bolin Billabong is located in a densely populated part of Melbourne. It can be expected to have resident or itinerant pests such as feral cats, foxes and rabbits. Cam Beardsell (pers. comm. 10/02/2010) noted that possums and snakes caught in the local area by pest controllers are often released into terrestrial vegetation around Bolin Bolin Billabong. Samba Deer also cause plant damage at the site.

Aquatic pests, however, probably pose a greater threat to the rehabilitation of the billabong proper. As described in Section 3.4, five species of introduced fish have been recorded for the billabong, including Goldfish Carassius auratus, Carp Cyrpinus carpio, Mosquito Fish Gambusia holbrooki, Weatherloach Misgurnus anguillicaudatus and Redfin Perca fluviatilis. Given that Roach Rutilis rutilis and Tench Tinca tinca occur in the Yarra River (Mitchell et al. 1996, Table 30) it is possible that these two taxa could also invade the billabong should it be more permanently inundated in the future.

Of these fish species, Carp probably represent the greatest threat. The impact of Carp on aquatic systems of south-eastern Australia has been reviewed in detail by Koehn et al. (2000). It is generally acknowledged that Carp damage submerged aquatic plants and increase water turbidity (Roberts 1998). They probably have a role also in increasing the availability of nutrients in the water column, largely by consuming benthic organisms and excreting the nitrogenous waste products as ammonia and phosphorus waste products as inorganic orthophosphate. These effects may translate into increased risks of algal blooms in the affected waters (Recknagel et al. 1998).

The direct impacts of Carp on native fauna are less well understood. Indeed, rather than being a direct cause of many environmental problems, an argument can be mounted that Carp are simply responding to the more fundamental changes that have taken place over the past ~50 years. Habitat disturbance and altered water regimes, for example, have favoured invasion by Carp (Fletcher et al. 1985; Gehrke et al. 1995). Similarly, the creation of vegetated, still-water habitats creates excellent conditions for other taxa of introduced fish, including Redfin and Tench. The lack of regular drying cycles also allows Carp to grow to large size, whereas a more natural regime of episodic (and more-or-less complete) drying of billabongs would kill the fish.

3.11 Synthesis

The detailed information presented in this section of the report highlights three important hydrological and ecological issues in the ecological condition and future management of Bolin Bolin Billabong.

First, it is likely that until the mid-late 1990s the billabong filled regularly with floodwaters from the Yarra River. Inundation events probably occurred at least once a year on average, and rarely would the billabong go for successive years without being wetted with floodwaters, at least to some extent. Since the mid-late 1990s, however, the billabong has been wetted only twice (2000 and 2004–2005). The reasons for the recent prolonged dry phase are mostly related to lack of rain since 1996 but, at a more fundamental level, are related also to river regulation and extraction and the alienating effects of the levee that separates the billabong from the Yarra River.

Second, the direct consequence of the progressive desiccation of the billabong has been the loss of those vegetation communities that are typically ‘wetland’ types and the invasion of the

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billabong floor and margins with terrestrial native but ‘out-of-balance’ species including River Red Gum and Silver Wattle.

Third, although Bolin Bolin Billabong is of extremely high ecological and social value, it is potentially affected by a wide range of threatening processes. The most pressing is the altered water regime, but others include (in an approximate order of importance), infestations by exotic plants (both aquatic and terrestrial), impacts arising from introduced fish (especially Carp), and poor water quality (especially if stormwater is used as the main water source). Poor water quality could arise from both excessive nutrients (in the longer term) and from the creation of low- oxygen ‘blackwater’ events in the short term soon after the billabong and floodplain are re- wetted.

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4. Management objectives

Parks Victoria (2008) set out general management objectives for the Yarra Valley Parklands, including recommendations for its billabongs. In general, the billabongs are to be managed ‘to maintain natural floodplain processes’ (Parks Victoria 2008, page 17). Management objectives for Bolin Bolin Billabong do not seem to have been explicitly stated to date. Parks Victoria (2008, Map 3), however, identified the site as being located in a conservation management zone surrounded by an area managed for conservation and recreation.

As described earlier, Bolin Bolin Billabong and its surrounding floodplain is one of the few in the region that has not been modified geomorphologically. Moreover, with other nearby billabongs such Banyule Billabong, it has the potential to form a wetland mosaic of potentially very high ecological and social value. Given these factors, a reasonable management objective is that Bolin Bolin Billabong should be rehabilitated as far as possible to reflect its ecological condition before European colonization. As discussed in Section 2.3, complete restoration is an unreasonable aim: the presence of weeds in the floodplain flora, the likely continuous infestation with exotic fish, and on-going presence of feral animals such as deer, cats and dogs, for example, make an aim of complete restoration unrealistic.

The guiding aim for rehabilitation should be to maximise the ecological resilience of the site. Resilience is this context means the ability of the billabong and floodplain to withstand external threats to their ecological condition and to return to the original condition when such threats pass. A useful analogy is with human health: the best protection for a middle-aged man against heart disease is to be good general health. Similarly, the best ‘protection’ for the billabong and floodplain against undesired outcomes is to be in good general ecological health. A good introduction to the topic is Walker & Salt (2006) and more technical reports, including a historical overview of how the idea of ecological resilience has developed, can be found in Gunderson et al. (2010).

Central to the management objective of rehabilitating the billabong and ensuring maximum ecological resilience is the re-introduction of more natural wetting and drying cycles. These will allow a greater range of wetland vegetation types to establish and will help control invasion of the wetland floor and adjacent floodplain by out-of-balance native taxa (especially River Red Gum and Silver Wattle ) and exotic weeds. The control of River Red Gum and Silver Wattle is important from two perspectives.

First, the dense thickets of saplings and small trees that have recruited during earlier wetter periods screen out many potential vistas of the billabong from walking paths around it. This is therefore largely an aesthetic and recreational issue.

Second, the dense thickets preclude the development of other wetland vegetation communities in the billabong and along its fringes, and thus decrease the floristic, structural and habitat value of the site. An argument could be mounted also that they pose a fire risk in a highly urbanised part of Melbourne. One benefit from the massive recruitment of River Red Gums, however, is that their density may preclude invasion of the wetland floor by exotic weeds that would otherwise find the terrestrial environment highly suitable.

The re-introduction of more natural hydrological cycles would have strong benefits for fauna as well as flora. Permanent deep-water zones would provide habitat for fish (albeit mostly exotic unless careful management were undertaken) and waterbirds such as swans, ducks, grebes and

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cormorants. The shallower fringing zones would provide feeding areas for herons, coots, moorhen, spoonbills and stilts. Fringing marshlands would provide excellent cover for birds such as bitterns and rails, swamp hens and warblers. The more terrestrial margins and floodplain woodlands would provide habitat for magpies, various parrots and cuckoos, kookaburras and honeyeaters.

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5. Hydrological requirements for rehabilitation

5.1 Flows in the Yarra River required for billabong inundation

Section 3.8 reviewed the hydrological investigations undertaken by Mitchell et al. (1996). That earlier report concluded that the flows into Bolin Bolin Billabong would commence once the Yarra River reached a height of 10.4 m (AHD). As the normal surface level of the river at the billabong at time of that study was 6.6 m AHD, river levels would need to rise ~3.8 m for the billabong to start to be inundated. Water could enter the billabong at either of the two westerly arms, and it was thought that the more southerly arm would commence to flow first (at 10.4 m), then the northern arm (at 11.0 m). Sections 3.1 and 3.8 also outlined the discussions held during the project meeting of 16/04/2010, which indicted a third pathway for the ingress of riverine water: a northerly channel with a commence-to-flow elevation of ~12 m AHD (see Figure 3.2).

Figure 5.1 shows the northern-most inlet channel with the putative commence-to-flow elevation of ~12 m (see red arrow in Figure 3.2), with Cam Beardsell standing at the highest level attained by the inflowing waters from the Yarra River during the 2004–2005 flood. According to his observations, river water then entered the billabong along this flow path and not via the two theoretically lower channels at the western end of the billabong.

Figure 5.1: Inlet channel at the northern arm of Bolin Bolin Billabong, showing the approximate point of the highest water level attained during the 2004–2005 flood. Photograph by Paul Boon

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According to the study by Mitchell et al. (1996), the billabong would commence to fill via two channels at the western end, the first at a river height of 10.4 m AHD, followed by ingress through a second more northerly channel with an elevation of 11 m AHD (see Sections 3.1 & 3.8). Figure 5.2 shows the second westerly channel at the time of the site inspection in February 2010.

Figure 5.2: Inlet channel at the western arms of Bolin Bolin Billabong, predicted by Mitchell et al. (1996) to commence to flow at a river height of 11 m AHD. The two people show the scale of the channel, and the magnitude of infestation with ground-cover weeds, which would contribute significantly to the hydrological roughness experienced by over-bank flows. Photograph by Paul Boon

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Since the time of the Mitchell et al. (1996) study, however, the ‘normal’ water level in the Yarra River has fallen considerably (Figure 5.3). Such a drop has important implications for decisions on how best to get water into the billabong at appropriate times of year, and to keep it in the billabong for long-enough periods.

Figure 5.3: Level of the Yarra River near Bolin Bolin Billabong during the site inspection, February 2010. The photograph was taken from the small levee at the top of the bank between the northerly and southerly arms of the billabong. Photograph by Paul Boon

The recent (2005) study of environmental flows required for the Yarra River provides more up- to-date information on river heights near Bolin Bolin Billabong. Bolin Bolin Billabong falls within Reach 6 of the Yarra River as described in the FLOWS study undertaken by Sinclair Knight Merz (2005 a, b). Under natural conditions, the Yarra River in Reach 6 (at Chandler Highway) had a mean annual discharge of 2,571 ML day-1. Extractions have reduced flows by 38% and the current mean annual discharge is now (i.e. at the time of the FLOWs analysis2) only 1,571 ML day-1. The greatest reduction has occurred during low-flow periods, but the frequency of floods also has been affected by regulation and extraction. For example, under natural conditions a flow of ~28,000 ML day-1 corresponded to a 1 in 5 year flood in this section of the river; currently a flood of this magnitude occurs only over every 50 years (Sinclair Knight Merz 2005 a, page 24). Because of inflows from unregulated tributaries upstream, the seasonality of discharge in Reach 6 has been maintained, and highest flows take place in August to November. The periods of lowest flow are February to May (Figure 5.4).

2 The Yarra River FLOWs analysis was completed by SKM in 2005. It is worth noting that the current flows (i.e. over the past ~5 years since the SKM report) show even further reductions due to on-going drought and the qualification of rights which has allowed further extraction over those shown in Figure 5.4 (Dr Simon Treadwell, Sinclair Knight Merz, pers. comm. 9/03/2010). Similar qualifications apply to the calculation of flood periodicity in the later parts of this paragraph and in the subsequent text.

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Figure 5.4: Average daily flows for the Yarra River at Chandler Highway gauging station. Source: Sinclair Knight Merz (2005 a, Figure 4.15). Reproduced with permission from Sinclair Knight Merz.

Sinclair Knight Merz (2005 a, b) concluded that the lower-lying billabongs in Reach 6 (therefore presumably including Bolin Bolin: see Figure 3.12) would be expected to commence to fill at flows of ~13,000 ML day-1 and, under natural conditions, that filling would occur about once a year and perhaps slightly more frequently. Under current conditions in a flow of this magnitude now occurs only once every 3–4 years3. All billabongs would be inundated by a moderate flood of ~26,000 ML day-1 and a flow of this magnitude would have occurred naturally about once every 3 years; it currently occurs only once every 30 years. Although such findings are broadly consistent with the earlier work of Mitchell et al. (1994, 1996) and the observations on recent wetting and drying regimes outlined in Section 3.8, it is stressed that the current project did not undertake independent hydrological modelling and so they cannot be verified at this stage. Moreover, however, the flow values estimated by Sinclair Knight Merz would have been based on the well-established FLOWS approach, in other words, using channel surveys and hydrological modelling using the HEC-RAS model. They are thus likely to be robust estimates of commence-to-flow values and not merely extrapolation from anecdotal observations.

Sinclair Knight Merz (2005 b) made a series of flow recommendations for the Yarra River, based on the State-wide FLOWS approach. They concluded that, of all the non-estuarine reaches of the Yarra River, over-bank and bank-full flows occurred least often in Reach 6. For example, under natural conditions, over-bank flows in Reach 6 would occur about every 2–3 years; currently they occur only every 10 years. To redress the lack of floodplain inundation, Sinclair Knight Merz (2005 b, page 115–125) recommended two types of high-flow event be maintained in the river.

First, bank-full flows of ~13,000 ML day-1 for 1–2 days every 2 years would be sufficient to reach the top of the bank at most sites along Reach 6 and would commence to inundate wetlands that had a direct connection with the river. Bolin Bolin Billabong, despite being among the lowest of the sites studied by Mitchell et al. (1996), is nevertheless alienated from the river by the chronically low river levels, a minor levee between the billabong and the river, and the dense

3 See also Footnote 2 on previous page for implications of past-2005 reductions in flow on flood frequency.

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growth of groundcover weeds such as Wandering Tradescantia that would impede over-bank flows. Although the recommendation was for one bank-full flow every 2 years, it was acknowledged that under natural conditions one or two bank-full flows would have occurred every year. These types of bank-full flow typically occur in September or October. Second, the FLOWS study recommended an over-bank flow of 21,500 ML day-1 once every four years for a duration of 1–2 days in Reach 6 (Sinclair Knight Merz 2005 b). A flow of this magnitude would over-top the river banks in at least some locations and would very likely spill into Bolin Bolin Billabong and probably fill it completely. Although over-bank flows typically occurred in winter or spring, they can occur at any time of year.

5.2 Hydrological regime required for rehabilitation – broad recommendations

Bolin Bolin Billabong is classified as a Deep Freshwater Marsh in the Norman and Corrick classification scheme (Section 3.4). This allocation is consistent with the geomorphology of the bed of the billabong, and the fundamental geomorphology of the billabong suggests strongly that its central parts would retain water for long periods after over-bank flows. Cam Beardsell (pers. comm. 10/02/2010) reported that the pool had been dry only once or twice in the past ~30 years. Moreover, that water was consistently present in pool in the suite of historical aerial photographs (Figures 3.7–3.11) and that some standing water is left in the remnant deepest pools at the eastern end of the billabong (Figure 3.13) nearly five years after the most recent inundation by river water further strengthens the idea that the billabong is a bona fide Deep Freshwater Marsh.

On this hydrological-geomorphological criterion, it would be expected that the thalweg (i.e. the deepest part of the ox-bow channel) of Bolin Bolin Billabong would be inundated with up to 2 m of water on a near-permanent basis, with the acknowledgement that it could dry out completely every 4–5 years during periods of extreme dryness (Table 3.4). Of course, the edges and more elevated sections of the thalweg would experience less frequent and shorter inundation. In order to maintain a range of vegetation communities and associated habitat diversity, as well as fundamental biogeochemical processes such as nutrient cycling, water levels in the billabong should fluctuate seasonally and it should be allowed to dry out completely at times. This general recommendation for the ‘ideal’ water regime contrasts markedly with the hydrological conditions that have been experienced since the mid-late 1990s (Section 3.8). The three main components of the ‘ideal’ regime – near-permanent inundation of up to 2 m in the deepest parts; seasonal fluctuations of water levels during the wet phase in the shallower parts and on the floodplain; and episodic drying-out of the billabong – are discussed next.

Need for near-permanent inundation

Bolin Bolin Billabong requires near-permanent inundation, especially for the first few years after filling, for a number of reasons. First, the billabong falls into the category of a Deep Permanent Marsh and, on these and related geomorphological criteria, it would be expected to be filled near permanently with water under pre-European hydrological conditions.

Second, a wet period of at least 3 years, with surface water at least 30 cm deep, is required to kill the excessive number of River Red Gum saplings that have recently established on the bed of the wetland. Note, however, that inundation alone may not be sufficient to kill the well- established saplings and more interventionist approaches (e.g. thinning) may be needed as well, at least in the first stages of billabong rehabilitation. In a recent report, Cunningham et al. (no date, but probably 2009 or 2010) concluded that ‘ecological thinning’ should be trialled on parts of the Victorian Murray River floodplain where River Red Gum had become established in

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billabongs as a result of chronic lack of inundation. It is beyond the scope of the present report to go into this topic in greater detail, other than to advise that such an approach may be required to remove the well-established River Red Gum saplings at Bolin Bolin Billabong.

Third, near-permanent inundation will drown-out the terrestrial plants, both native (e.g. Silver Wattle, Tree Violet) and exotic, that have invaded into the fringes of the billabong and lower parts of the floodplain from the surrounding Floodplain Riparian Woodland and perhaps even further a-field from nearby urban areas. Inundation is more likely to be successful in drowning out these terrestrial taxa, as they are not as well adapted to periodic inundation as is River Red Gum.

Fourth, near-permanent inundation, when combined with seasonally fluctuating water levels and rarer, episodic complete draw-down, will allow a diverse range of aquatic vegetation communities to develop in and along the fringes of the billabong. These plant communities will advance and retreat as water levels rise and fall seasonally.

Fifth, and linked closely with the fourth rationale, is that near-permanent water will provide a wide range of habitat types for animals. Large and diverse populations of waterbirds (swans, duck, teal, grebe, pelicans, heron, egret, spoonbill, cormorant, moorhen and coot), for example, could again make use of the billabong and its margins for shelter, roosting, foraging and breeding. Aquatic invertebrates too would benefit from having a permanent pool of water in the billabong. Under the current desiccated condition, there is little or no aquatic habitat that could be used by these species. Terrestrial animals species would likely to benefit indirectly from permanent water also, for example, via the provision of food for bats and riparian or fully terrestrial birds species.

Near-permanent inundation is especially important for fauna that live in wetlands in the Deep Permanent Marsh category. For such fauna periods of inundation that are too short may be very environmentally detrimental: colonial-nesting water birds, for example, require time to build up their fat reserves, build nests, lay and incubate eggs, then feed and fledge the young birds. Studies undertaken for the Living Murray (Murray Flow Assessment Tool, or MFAT) program indicate that successful breeding of these species of bird requires that wetlands be inundated for at least 4 months and preferably 6 months; shorter inundation periods could lead to the abandonment of nests and thus unsuccessful breeding. Ducks seem to be the quickest to breed and they may require only 3–4 months to raise their young (1–2 month lag time followed by 2 months to lay, incubate and raise fledglings); if they are already in good condition, ducks can breed successfully with only 2 months of wetland flooding.

Need for seasonal water-level fluctuations

Notwithstanding the primary recommendation for parts of the billabong to be kept mostly in a wet phase, there are strong reasons for allowing water levels to fluctuate seasonally. Fluctuations in water levels allow the development of heterogeneous plant communities around the edges of the pool. Amphibious types of aquatic plants, which tolerate alternating wetting and drying, can then colonise the margins and form structurally and ecologically diverse habitats which maximise foraging and refuge opportunities for a range of biota (Brock & Casanova 2000). The next section (Section 5.3) outlines some of the fine-scale modifications that will have to be implemented to maximise the ecological benefits of re-instating a more natural wetting and drying regime in the billabong.

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Need for episodic draw-down

A characteristic of almost all natural wetlands in south-eastern Australia is that even the largest can experience episodically almost complete draw-downs of water levels. The native biota are well adapted to surviving dry periods in wetlands and temporary waters (Boulton & Brock 1999) and complete desiccation of a wetland can be an effective tool to manage exotic fish such as Carp, especially if combined with electrofishing (Koehn et al. 2000). A wide range of exotic fish, including Carp, Mosquito Fish, and Redfin, are found in Bolin Bolin and nearby billabongs (e.g. see Hodges 1979) and they will need to be controlled. Periodic complete desiccation of near- permanently wet billabongs is one of the most effective carp-control mechanisms currently available and is not counter-indicated for many of the wetlands of south-eastern Australia, even those falling into the ‘near-permanent water’ category. Therefore if the billabong were to be drained episodically (say, no more frequently than every ~10 years), it is unlikely that long-term ecological harm would result.

5.3 Fine-tuning the water regime to maximise biodiversity

The broad recommendation is that Bolin Bolin Billabong be kept in a near-permanently wet condition but water levels allowed to fluctuate seasonally and the entire billabong allowed, if necessary, to dry-out no more than approximately once a decade. Fine-tuning, however, will be needed for the following aspects of the billabong’s ecology.

Control and health of River Red Gums

A balance must be struck at Bolin Bolin Billabong between controlling the unwanted growth of juvenile River Red Gums in the bed of the billabong and the robust growth of trees along the edges and into the floodplain. In particular, the health of the large adult trees along the banks needs to be maintained.

Across most of south-east Australia, River Red Gum woodlands and forests typically commence to flood in July to September, remain wet over spring, then dry out in December or January. Inundation is usually required for 2–3 months to allow River Red Gum seeds to germinate and young plants to establish (Roberts & Marston 2000). Floods in successive years create the best conditions for recruitment, as the young plants that establish in the first year become especially well established during the second. Although studies undertaken as part of the MFAT program and Victorian River Red Gum investigation (Young et al. 2003; VEAC 2006) have shown that the ability of River Red Gums to recover after periods of drought or (less commonly) prolonged inundation varies geographically across southern Australia, as a generalization it seems that a flood frequency of once every 1–3 years is sufficient to maintain good condition in adult plants and to assure frequent-enough recruitment of young plants to assure mixed-aged cohorts in the population. In many cases, the more frequent the flooding, the taller the adult trees: VEAC (2006, Table 5.6) concluded that River Red Gums >30 m tall were generally found in areas that were flooded for an average of 5 months and a minimum of 1 month, but trees grew to only <~20 m in areas flooded for an average of 1–2 months and a minimum of 0.5 month.

Although the adult trees are obviously flood tolerant, River Red Gum communities generally should not experience continuous flooding for more than 2 years nor periods without flooding of more than 2 years (Roberts & Marston 2000). It is this first part of the recommendation that can be used to advantage in Bolin Bolin Billabong to bring under control the rampant growth of River Red Gum saplings in the bed of the billabong. As noted above, as a preliminary recommendation it is likely that a wet period of at least 3 years, with surface water at least 30 cm

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deep, will be required to kill the excessive number of River Red Gum saplings that have recently established on the bed of the wetland. Thinning may be required as well, if prolonged inundation is not successful as a control technique. Indeed, it is recommended that further consideration be given to thinning the abundant River Red Gum saplings before hydrological interventions are implemented, as it would be simpler to thin the young plants while the billabong is largely dry than after it had been flooded and soils thus boggy and difficult of access.

The use of River Red Gums as a keystone species to identify the finer aspects of ‘ideal’ wetting and drying cycles in Bolin Bolin Billabong does not mean that water regimes have been selected purely on the basis of selectively advantaging this species. Instead, it is assumed that a hydrological regime suitable for the appropriate growth of River Red Gums, if combined with the maintenance of near-permanent water in the channel of the billabong, seasonally variable water levels around the edges and episodic draw-down of the floodplain, will provide the suite of wet, dry and variably inundated niches within and along the billabong required to create the hydrological conditions suitable for a wide suite of submerged and amphibious taxa to thrive.

Timing of inundation and drawdown

There is near universal agreement that floodplain wetlands in south-eastern Australia should usually be inundated in late winter or early spring, as this flooding regime generally mimics the seasonal pattern of over-bank flow for those sites still connected to their parent river and for filling from the immediate catchment for those that are not. In the case of River Red Gum, for example, floods in August–October are recommended in the MFAT studies. VEAC (2006) recommends that, ideally, flooding should commence in August for Victorian River Red Gum forests. Other taxa, however, are more tolerant of flooding at different times of year. Common Reed Phragmites australis, for example, favours inundation from late winter to late summer and the canopy then attains maximum density in mid–summer. In this species, winter floods are often ineffective in promoting vegetative growth because the plant enters a long period of dormancy over winter. Cumbungi Typha spp. behaves similarly, but is likely to be invasive if water levels are kept too high over summer. In the case of obligately submerged taxa (e.g. Eelgrass), permanent inundation is usually required (even if plants can survive some period of drawdown: see Salter et al. 2008).

If Bolin Bolin Billabong were to commence to fill by August–September and portions of it commence to dry-out by ~February–March the following year, the site will have experienced a wet period of about 6 months. The minimum duration usually recommended in the wetland literature (~3 months) is based mostly on the requirement for aquatic plants and invertebrates to complete their life cycle and lay down resistant egg and seed banks, and for selected species of waterbirds to breed successfully (e.g. see Kingsford & Auld 2005). Although wet periods of as short as 3 months may be sufficient for some taxa of aquatic plants to complete their life cycles and for ducks to breed, it is a bare minimum for successful breeding by other waterbird species and much longer periods may be required for these other taxa to successfully breed and young birds to fledge (Briggs et al. 1997; Briggs & Thornton 1999). A wet period of 5-6 months would seem, as a broad generalization, to be a suitable inundation period for many billabongs. As noted above, there are significant risks of wasting water without achieving appreciable ecological outcomes if billabongs are inundated for too short a period or at the ‘wrong’ time of year: water bird breeding may be unsuccessful and plants may be unable to lay down drought-resistant propagules.

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Water level fluctuations and hydrological mosaics

Considerable fine tuning will be required during the first years of experimental inundation to determine the precise levels of the water in the northern and southern arms of the billabong (Figure 5.5). The following recommendations are made as a general guide for that fine tuning.

Gradient in wet-dry conditions

Northern arm

Deepest pool Southern arm

Figure 5.5: Aerial view of Bolin Bolin Billabong, showing the northern and southern arms of the billabong and the easterly pool at the deepest end. The image dates from February 2006. Source: Google Earth, accessed 13/02/2010.

Water up to ~2 m deep should be maintained in the deep pool at the eastern loop, where a small body of remnant water remains today (see Figures 3.13 & 5.5). In the absence of accurate bathymetry, it is difficult to quantify the area that would be filled but it is assumed for subsequent calculations that a permanent pool would occupy ~50% of the area of thalweg. During the field inspection of February 2010, Cam Beardsell pointed out the more elevated parts of the northern arms of the thalweg that could be expected to be inundated less frequently and less deeply than the deep pool at the eastern loop. A recommended pattern of a longitudinal gradient in wet-dry conditions in the arms of the billabong, arising from those discussions, is shown in Figure 5.5 above.

Deep, permanent water in the eastern loop would allow the growth of truly aquatic plant species such as Water Ribbons, Eelgrass and various Pondweeds in the water and a floating plant community of Azolla and Duckweeds on its surface. Permanent water would permit also the re-

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colonization of the billabong with native fish such as Short-finned Eel and Flat Headed Gudgeon (with the high likelihood of exotic species being present too). As noted earlier, this body of permanent water will provide habitat and feeding grounds to a range of water birds.

Water levels in more elevated parts of the northern and southern arms (Figure 5.5) should be controlled carefully so that a gradient in waterlogging regimes is created up the arms and into their western reaches. The hydrological gradient along the arms will vary from permanent water near the eastern pool, to annual inundation in the intermediate sections, to flooding about once every 2–3 years as the arms extends to the west. Such a waterlogging gradient would create a mosaic of different aquatic and floodplain vegetation, and with it a rich mosaic of habitats for birds and animals. It is assumed for the sake of subsequent calculations that this ‘gradient region’ occupies the other 50% of the thalweg.

The northern and southern arms are now too dry to support what would have been their normal aquatic-wetland-floodplain vegetation and instead have been invaded by terrestrial species such as Tree Violet (see Figure 3.16). Under the proposed rehabilitation-orientated hydrological regime, the arms would not be characterized by the growth of truly aquatic plant species such as would occur in the deep pool at the eastern loop, but instead would become vegetated with taxa such as emergent rushes, reed and sedges (e.g. Tall Spike-sedge and Parrot-feathers (milfoils)) along the wetter margins and, in the more ephemeral zones as the billabong grades into more fully terrestrial habitats, a range of plant taxa such as Lesser Joyweed and River Swamp Wallaby- grass. Seasonal appearances of plants such as Knotweeds (Persicaria spp.) would be expected as water levels receded at the end of summer. The driest parts of the wetland sections of the arm would see re-colonization by native plant taxa such as Swamp Paperbark Melaleuca ericifolia. Figure 3.4 showed the types of variation in plant species that would be expected to occur along the northern arm in response to such a fine-tuned hydrological intervention.

The importance of maintaining a complex of hydrological mosaics in the billabong cannot be underestimated. Although it is common the describe the broad hydrology of a wetland with terms such as ‘Deep Permanent Marsh’, it is in fact the subtle variations in topography and inundation history that control the range of plants that grow in wetlands and the diversity of habitats that they create. Further information on the critical importance of seemingly minor variations in with-in wetland water regimes is available in Raulings et al. (2010), which describes the situation for a Ramsar-listed wetland fringing the Gippsland Lakes

5.4 Sources and volumes of water required

Water sources

There are three potential sources of water for the recommended water regime:  Yarra River water, achieved through natural bank-full (via flood runners and the two channels) or larger but rarer over-bank flows  Yarra River water, pumped from the river at appropriate times, and  Treated storm-water from the surrounding urban catchment.

Natural flooding of Bolin Bolin Billabong via bank-full or over-bank flows of the Yarra River is currently constrained by four factors: i) chronic extraction of water from the Yarra River for human use, which has lead to markedly depressed river levels (see Figure 5.3); ii) historically low discharge as a result of dry conditions over the past ~12 years; iii) the minor levees that seem to present at the western end of the billabong and which alienate the billabong and floodplain from the river; and iv) the dense growth of exotic plants on the floodplain and in the southern

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channel (Figures 3.17 & 5.2), which increase hydrological roughness for over-bank and bank-full flows (via flood runners), respectively.

In principle, natural bank-full or over-bank flows would provide the best, and cheapest, option for inundating the billabong. The trouble with this simple option is that the past 12+ years has demonstrated that natural flooding events are too unreliable to inundate the billabong adequately. One possibility would be to lower the entry level of the channels that feed into the northern and southern arms (see Figure 3.2), but even that may be insufficient to allow enough water to enter the billabong and at the appropriate time of year unless significant on-ground works were undertaken. Sinclair Knight Merz (2006) came to a similar conclusion, and argued that channel deepening would have to be extensive and that a large regulatory structure would be needed to maintain water levels within the billabong and prevent out-of-season inundation.

Even if such on-ground works were undertaken, the periodicity and magnitude of flooding in the foreseeable future (if recent hydrological patterns continue to apply) may well remain inadequate. As noted earlier in the earlier review of the Yarra River FLOWS study (Sinclair Knight Merz 2005 a, b), the flows of ~13,000 ML day-1 that are needed to commence to fill the lower-lying billabongs in Reach 6 of the Yarra River used to occur about once a year but now occur only once every 3–4 years. The larger flows (~26,000 ML day-1) that are needed for good over-bank inundation used to occur about once every 3 years but currently occur only once every 30 years. Thus whilst the FLOWS recommendations would result in a marked improvement in the flooding frequency of the billabong and its floodplain, the suggested periodicity of filling is still only about one-half it would have been under natural (i.e. pre- regulation) conditions and, for the initial phases of billabong restoration, more frequent inundation spells would be needed in the early years.

To conclude, it is not safe to rely on ‘natural’ floods to inundate the billabong with the frequency and timing required for its effective rehabilitation. Moreover, if some of the elements of likely climate-change scenarios do become apparent in coming years, especially a shift to summer rainfall as a consequence of more southerly monsoonal events, summer may become the season when the river floods rather than the current timing in winter-spring (Figure 5.4). The ecological outcomes of such a temporal shift are not clear at this stage, but it is clear that the existing biota have adapted to a wet winter-spring and dry summer-autumn hydrological cycle. Partial rehabilitation of the wetland, undertaken on the basis of wetting of only small sections or selectively letting water into either the northern or southern arm, is not likely to result in the rehabilitation of the site as a whole. Finally, in their review of water-supply options for Bolin Bolin Billabong, Sinclair Knight Merz (2006) concluded that lowering the inlet channels was unlikely to be feasible, because of the depth of excavation needed and the size of the regulator that would be needed to exclude water from the billabong when it was not needed. In my mind, those conclusions still hold.

The third option – use of treated storm-water – could be limited by the volumes of water available and their reliability, especially for the first 2–3 years of rehabilitation. This issue is discussed further in Section 5.5. A large volume of water is needed initially to fill the billabong and additional, but smaller, amounts will be required again for the next 2–3 years to control infestations by River Red Gum and Silver Wattle. It may be that there is insufficient stormwater to meet this initially large volumetric requirement, although it is likely that sufficient volumes would be available for routine maintenance of water levels in subsequent years, at least in the billabong proper.

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If these assumptions hold, three broad conclusions about the suitability the alternative water sources can be made.

First, it is likely that the best option for providing the large volume of water needed to initially fill the billabong is pumping from the Yarra River. Sinclair Knight Merz (2006) discussed pumping from the Yarra River as a feasible option for filling the billabong. They estimated that ~74 ML of water was required to fill Bolin Bolin Billabong to an average depth of 2 m. The next part of this report analyses that estimate.

Second, treated storm-water could be used as a ‘top-up’ in later years. Using treated storm-water in this way has a number of advantages. First, it would provide a long-term and reliable source of water for an environmental outcome that would probably not be otherwise available. Second, the timing of its availability is broadly consistent with the ecological needs of the billabong (wetting mostly in winter-autumn) when other demands (e.g. watering playing fields) are low. Third, the use of treated storm-water in this way could actually increase the overall availability of water for environmental purposes in other parts of north-east Melbourne as it lowers the demand on Yarra River supplies. Potential hazards in using treated storm-water in this way are discussed in Section 5.5.

Third, in general terms it is risky to rely on natural over-bank flows of the Yarra River to inundate high-value billabongs such as Bolin Bolin.

Volume of water required

Sinclair Knight Merz (2006) gave the area of Bolin Bolin Billabong as 3.7 ha. The basis of the estimate of ~74 ML is not made clear in the report, but it seems to be derived from the area and recommended depth of flooding: inundation of the entire 3.7 ha of the billabong to a depth of 2 m would require 74,000 m3 of water, or 74 ML.

The volume of 74 ML equates with that volume needed to fill the billabong thalweg only. A more refined calculation can be made by accounting for the most important sources and sinks of water in the billabong-floodplain system. Water is required to fill the main pool in the billabong thalweg and to inundate (to a lesser degree) the northern and southern arms of the billabong. In addition to that required to fill the thalweg, water will be required for the following hydrological components of the site:  Water to inundate the adjacent floodplain to a suitable depth.  Water to saturate the soil before water levels can rise in the thalweg or floodplain.  Water to make up losses arising from evaporation and plant transpiration.

Opposing these water requirements and routes for water loss is the input of water as rainfall and run-off from the immediate catchment. Note that groundwater interactions have not been considered, but are likely to lead to the estimated requirements made below being under- estimates.

The following equation outlines a hydrological balance for the site and can be used to estimate the volumetric water requirement:

VTotal = VBillabong thalweg + VFloodplain + VSil saturation + VEvaporation/transpiration – VRainfal/run-off

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Figure 5.6 develops a simple conceptual model of the water balance of the site and shows how the equation can be applied to Bolin Bolin Billabong and its floodplain as a whole.

It is understood that a survey of the billabong’s bathymetry will soon be undertaken by Melbourne Water for Manningham City Council; that information would allow greater confidence in estimates of the water required for initial inundation and subsequent ‘topping up’.

V Rainfall/run-off V Evaporation/transpiration

V Billabong pool V Billabong arms V Floodplain

Floodplain Billabong wet-dry arms 0.5 m 0.1 m

Billabong pool 2 m

Soil saturation zone V Soil saturation

Figure 5.6: Conceptual model of the hydrological balance of Bolin Bolin Billabong. Green-shaded boxes represent the important volumetric components.

Each component of the equation and conceptual model is now considered in turn.

VBillabong thalweg

The volume of water required to fill the billabong was apparently calculated by Sinclair Knight Merz (2006) on the basis of an area of 3.7 ha area X a depth of 2 m (3.7 ha X 2 m = 74,000 m3 = 74 ML). That is likely to be a maximum estimate, and smaller volumes can be calculated if shallower depths are implemented and not all the thalweg is filled. As outlined above, there are good ecological reasons for filling the deepest pool near-permanently to a depth of ~2 m but having the western ends of the northern and southern arms inundated less frequently and with shallower water.

If we assume that the deep pool will have an area of 50% of the total billabong thalweg (i.e. ~2 ha) and needs to be filled to a depth of 2 m, then 40 ML is needed to fill the deep pool at the

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eastern loop (2 ha X 2 m = 40,000 m3 = 40 ML). The remaining 1.7 ha of the thalweg should be inundated to a depth of at least 30 cm to kill the River Red Gum saplings that had invaded over the recent dry period. If an inundation depth of 50 cm is assumed and the area rounded to the nearest ha, the volume required to inundate the remainder of the billabong is 10 ML (2 ha X 0.5 m = 10,000 m3 = 10 ML). Thus a lower estimate of the volume to fill the thalweg to provide a pool of 2 m depth over 50% of the billabong and the remaining 50% of the area to be filled to a depth of 0.5 m to kill River Red Gum saplings is 40+10 ML = 50 ML. (Note that volumes have been rounded up to the nearest ML.)

VFloodplain

Additional water is required to inundate the adjacent floodplain. The area of the immediate catchment of Bolin Bolin Billabong is 9 ha (Mitchell et al. 1996) but not all is floodplain. If we assume that the area of floodplain to be inundated is twice that of the billabong (see Figure 5.5), then an area of 7.4 ha of floodplain requires wetting. An inundation depth of 10 cm is assumed. These assumptions lead to an estimate of the volume of water required for floodplain inundation of ~8 ML (7.4 ha X 0.1 m = 7,400 m3 = 7.4 ML). As before the volume has been rounded up to the nearest ML.

VSoil saturation

The estimates above on the volume of water required to fill the thalweg (permanent deep pool and alternately wet-dry zone) and inundate the surrounding floodplain refer only to volumetric dimensions of the surface features (some of which are assumed). They do not consider the water needed beforehand to saturate the underlying soils so that water can then pond in the billabong or on the floodplain.

Because the site has been inundated only rarely over the past 12+ years, the soils are extremely dry. Groundwater levels have probably dropped considerably too. Thus additional water is needed to fill the empty (air-filled) pore space in the soil so that it becomes saturated and then can allow water to start to accumulate in the pool, the remainder of the billabong thalweg, or the floodplain. Moreover, roots from River Red Gum saplings and adjacent trees may have penetrated the impervious clay bed of the billabong, and violation of the bed will lead to a downwards loss of water until cracks are filled by expanding clays or by sedimented particles brought in with flood waters. Local groundwater levels are likely to have dropped as a result not only of the prevailing dry conditions but also of the growth of terrestrial vegetation (e.g. Silver Wattle) along the edges of the billabong and in surrounding areas. The historical loss of other fringing wetland types, such as Swamp Paperbark wetlands, may have exacerbated the fall in groundwater levels.

This component of the water requirements is the hardest to calculate. It depends critically on knowledge of the porosity of the soil, the extent and severity of root penetration, and the depth of soil that needs to be saturated (i.e. the depth to the impervious layer in the subsoil). These factors are not well known at present, so the calculation will have to be very approximate and a range of values given. The porosity of soils can vary from ~25% for gravels, to 35% for sands and ~30–60% for clays (Hazelton & Murphy 2007, Table 2.15). Agricultural soils typically have a porosity of ~50% (Hazelton & Murphy 2007, Table 2.16) but sandy and well-structured soils may have values closer to 25% (Peverill et al. 1999)

If we assume a depth to the clay subsoil of 2 m and a soil porosity of 50%, with a total area that needed to be saturated of 11 ha (3.7 ha of thalweg and 7.4 ha of floodplain), then ~110 ML of

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water is needed to saturate the soil before water can start to accumulate (11 ha X 2 m X 0.5 pore space = 110,000 m3 = 110 ML). Smaller volumes will be needed if the depth to the impervious subsoil layer is only 1 m (i.e. 55 ML) but proportionally more if the clay subsoil is deeper (e.g. 165 ML to a depth of 3 m). Similarly, if the porosity is only 30% but 2 m of soil above the clay layer still needs to be saturated, then only 66 ML of additional water is needed for saturation. If the porosity is 30% and the impervious layer is shallow and only 1 m below the surface, only ~33 ML of additional water is needed for saturation. This last value, however, assumes the lowest porosity and shallowest clay layer and is therefore unlikely. In conclusion, the volume of water needed to saturate the soil probably lies between about 50 and 150 ML.

A similar calculation can be made for the volume of water required to saturate the smaller area of the billabong thalweg rather than the entire billabong-floodplain complex. With a billabong area of 3.7 ha, it is estimated that the volume of water required to pre-saturate the thalweg could be ~37 ML if we assume a porosity of 50% and a clay depth of 2 m (3.7 ha X 2 m X 0.5 pore space = 37,000 m3 = 37 ML). As with the calculation for the entire billabong-floodplain complex, this estimate could easily range ~50% in either direction, from ~20 ML to ~55 ML.

VEvaporation/transpiration and VRainfall/run-off

Additional water will be required also to make up evaporative and transpirational losses, but they will be offset to some extent by direct rainfall and run-off from the immediate catchment.

With a catchment area of 9 ha (Mitchell et al. 1996) and an assumed average annual rainfall of ~500 mm year-1 (http//www.bom.gov.au/climate/aveages/tables/cw_086282_all.shtml, internet resource accessed 02/03/2010), a volume of 45 ML could, in theory, be added annually to the billabong and floodplain from rainfall and run-off in the immediate catchment. Some, perhaps most, of the potential run-off will be ‘lost’ as it penetrates into the floodplain soils and only under the wetter years could appreciable volumes be expected to make it into the billabong. For this reason, I assume that the catchment-runoff component does not make an appreciable contribution to the overall water balance of the billabong

As with the volume of water needed to saturate the soils before water can start to pond in the billabong, the evaporative and transpirational losses are hard to estimate. Evaporative loses from open water and bare soil may be approximated with a Class A evaporation pan: long-term data for Melbourne airport indicate that the mean daily evaporation ranges from ~2–8 mm day-1 from winter–summer (http//www.bom.gov.au/climate/aveages/tables/cw_086282_all.shtml, internet resource accessed 02/03/2010). Note that an evaporative loss of 5 mm day-1 (approximately the average annual rate for Melbourne) equates to a loss of 5 L per m2 of surface per day.

Rainfall falling on the billabong thalweg will offset partially the evaporative losses from the water surface. An average annual rainfall of 500 mm year-1 and an average annual evaporative loss of 1,800 mm year-1 (5 mm day-1 X 365 days) yields a difference of ~1,300 mm year-1 of evaporative water loss to be made up annually. That equates to a volume of 48 ML over the 3.7 ha of the billabong (3.7 ha X 1.3 m = 48,100 m3 = 48 ML). If evaporative losses from only the ponded section of the billabong are considered, the volumes can be approximately halved, to ~24 ML.

The remaining 7.4 ha of floodplain also will lose water due to evaporation and, more importantly, via transpiration from plant surfaces. We can assume an annual average evaporative loss of 5 mm day-1 and an area of floodplain of 7.4 ha, then evaporative losses will sum to ~135 ML (7.4 ha X 5 mm day-1 X 365 days = 135,000 m3 = 135 ML). For the sake of these

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calculations, it is assumed that evaporative losses from the floodplain are made up for by rainfall, although it is clear that the balance is strongly negative as a result of evaporative losses markedly exceeding rainfall inputs. It is worth noting also that far greater losses of water are often incurred from areas covered with plants, owing the greater transpirational losses from leaves, than from open water. The ratio between simple evaporation from bare water and evapotranspiration from vegetated areas can be a factor of 2 or 3. This factor is also neglected in the calculation.

Table 5.1 summarises the likely volumes of water required in first and subsequent years of the rehabilitation of Bolin Bolin Billabong.

Table 5.1: Estimates of water volumes required to rehabilitate Bolin Bolin Billabong. These estimates should be read in conjunction with the assumptions and caveats listed in the body of the text.

Component Volume per year (ML) Year 1 Year 2 Year 3 Billabong thalweg inundation 40 * * (pool only) Billabong thalweg inundation 10 10 10 (wet-dry arms)** Floodplain inundation** 8 8 8 Soil saturation (billabong ~20–55 ~20–55** ~20–55** thalweg only) Soil saturation (combined ~50–150 ~50–150*** ~50–150*** billabong-floodplain complex) Evaporative losses (billabong 24–48 24–48 24–48 only) * nil if evaporative losses of 24-48 ML year-1 are made up annually ** likely to be at lower end of range in 2nd and 3rd years, due to prior saturation of thalweg during 1st wetting *** assumes one inundation per year: approximately double this value for two inundations annually

Summary and caveats

In summary, the total volume of water required to inundate the billabong and floodplain once in the first year are as follows:  Water to fill to billabong thalweg: ~40 ML for the deep pool and a further ~10 ML for the alternately wet-dry portions of the rest of billabong arms (total = ~50 ML).  Water to inundate the adjacent floodplain: ~8 ML.  Water to saturate the soil before water levels can rise and water start to pond: ~50–150 ML for entire billabong-floodplain complex; ~20–55 ML for billabong thalweg only.  Water to make up losses arising from evaporation and plant transpiration: ~48 ML for billabong thalweg alone, or 24 ML for the ponded section only. Evaporative and evapotranspirational losses from the floodplain have been ignored.

These volumes sum to a total of ~82–106 ML for the better-understood components of filling the billabong thalweg (pool and alternately wet-dry arms: 50 ML), inundating the floodplain (8 ML), and accounting for evaporative losses from the billabong (24–48 ML). They do not include the water necessary beforehand to saturate the soil. An additional amount, probably between ~50–150 ML, is required to saturate the soils before water can start to accumulate in the billabong and inundate the floodplain. Once this extra water is considered, ~132–256 ML

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will be required (as a minimum) to initially fill the billabong and inundate the floodplain in the first year. A smaller volume, ~94–153 ML, would be required to initially fill the billabong only (and not affect the floodplain) and maintain water levels in the billabong in the face of surface- water evaporation.

The calculated volumes are approximately two to three times greater than that estimated by Sinclair Knight Merz (2006). But they feel ‘intuitively right’, given the range of factors other than mere thalweg dimensions that need to be considered when inundating chronically dry floodplain wetlands. Unfortunately, an admittedly not exhaustive search of the literature did not uncover any reports which compared the estimated versus realised volumes of water required in earlier trials of floodplain inundation in other parts of Australia. Future work should seek to locate such reports, if they exist.

If adaptive management (see Section 6.1) shows that more than one inundation of the floodplain is required in order to control invasion by terrestrial plants and facilitate the growth and recruitment of native plants (e.g. as is likely to be the case for River Red Gums: see Section 5.3 below), an additional ~8 ML of water s required for each subsequent inundation. It is assumed that the soils will remain saturated to at least some extent between inundations, so all of the (large and poorly-defined) volume of ~50–150 ML required initially to saturate the floodplain soils will not be required for subsequent inundations each year. Some additional water, however, will probably be needed over the estimated 8 ML for floodplain inundation, as the assumption of permanent soil wetting is not likely to be met in practice. Thus subsequent inundations of the floodplain will require more in total than the calculated 8 ML of water for the first inundation, but considerably less than the ~50–150 ML calculated to be needed to initially saturate the floodplain soils. Similar calculations can be applied to the ~10 ML required to inundate and keep wet the rest of the billabong thalweg in an alternating wet-dry condition, as these soils too will have been pre-wetted to some extent by the first inundation. The volume of water required to saturate the soils will necessarily be considerably lower in the second and third years, as it is assumed that half the billabong thalweg will be kept permanently inundated as a deep pool. Thus the values of ~20–55 ML noted for subsequent years are likely to be towards the lower end of the range.

It is critical to stress that all these calculations involve a number of assumptions and that the critical caveats must be considered when attempting to apply these estimates. The most important assumptions and caveats are:  Areas of the thalweg occupied by the 2 m-deep permanent pool (50%) and the alternating wet-dry regions of the more elevated parts of the thalweg (50%).  Groundwater interactions have been neglected, but water could flow into and out of the billabong via lateral and vertical seepage. Given the billabong’s raised location above the Yarra River, losses are more likely than gains and so the calculated volumes for initial filling and maintaining high water levels are likely to be under-estimates rather than over- estimates.  Evaporative and evapotranspirational losses from the floodplain have been neglected.  A floodplain inundation of 10 cm is assumed, and an inundation of 30 cm in the thalweg of the billabong is assumed to be sufficient to kill River Red Gum saplings (50 cm used in model calculations).  The estimate of the volume of water required to saturate the soil in the billabong thalweg and on the surrounding floodplain is very approximate. It will depend crucially on soil porosity, root penetrations, and depth to the impervious layer, none of which are well quantified at present.

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 This estimate also ignores the possibility that deep cracks have formed in the subsoil and that water could easily percolate into the soil profile until the cracks have clogged with transported clays etc.  The estimate of evaporative losses from the wet billabong assume an average annual rate of 5 mm day-1. The realized rate of evaporative loss will depend on factors such as air temperature, humidity, wind speed, and shelter afforded by plants. Moreover, the presence of abundant River Red Gum saplings in the billabong basin would likely increase rates of evapotranspiration over those from open water.  No allowance has been made for periodic flushing of the billabong pool. The initial fill with Yarra River water is likely to be of high quality, but subsequent ‘top-ups’ with treated stormwater may result in chronic eutrophication of the wetland, especially if the water is enriched with plant nutrients such as nitrogen and phosphorus.

Future work should refine my preliminary calculations, and in particular quantify the porosity of the soils and the depth to impervious layer. These two values play a critical role in the least well- understood component in the hydrological equation: the amount of water needed initially to wet the soils and allow water to pond in the billabong and on the floodplain.

5.5 Potential hazards with use of treated storm-water

There are a number of hazards associated with the use of treated storm-water to top-up Bolin Bolin Billabong. The hazards are both hydrological and physico-chemical.

Will enough water be available?

Two memoranda from GHD to Manningham City Council (from David Howard to Lachlan Johnson, dated 06/11/2009 and 18/11/2009) take as their assumption that 74 ML of water will be needed to provide environmental water to rehabilitate Bolin Bolin Billabong. The assumption was based on the best available knowledge at the time, the estimate by Sinclair Knight Merz (2006) of the volume to fill the thalweg. The calculations above, however, suggest that ~50 ML are needed initially to fill the billabong thalweg to create a deep pool at the eastern loop and create alternating wet-dry regions along the northern and southern arms.

It is proposed that water for the initial fill should come from the Yarra River. Unless there is a fortuitous flood within the next few years, it is likely that water will have to be pumped from the river during the initial stages of billabong rehabilitation. After the early years, however, treated storm-water could provide a suitable long-term supply. Thus where treated storm-water could be used most effectively is in subsequent ‘top-ups’ to replace evaporative losses (estimated at ~24–48 ML annually for the billabong only) and possibly for inundation of the floodplain (estimated at ~ 8 ML for already-wet soils). The largest volumetric requirement, however, is likely to be for water to saturate the chronically dry thalweg and floodplain soils. That could take up to ~150 ML but could be as low as ~20 ML if only the billabong thalweg were wetted and the most optimistic assumptions are made about soil properties.

Further work, including meetings of the relevant groups (e.g. Parks Victoria, Manningham City Council, Melbourne Water, GHD, Dodo Environmental, etc), will probably be needed to clarify this matter.

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Flashy nature of discharges

Because of the highly impervious nature of the catchment, storm-water discharges are highly flashy. In other words, they rise quickly and fall quickly. Rise and fall times within the billabong will need to be controlled, otherwise adverse ecological outcomes are highly likely to accrue; excessively fast rise and fall times would, for example, disrupt the breeding of frogs and waterbirds, and the recruitment of aquatic and riparian plants. Moreover, storm-water inputs can occur at any time of year and their timing may not always co-incide with what is ecologically optimal. For example, large storm-water inputs during summer would not allow for the critical seasonal draw-down that is needed over the summer-autumn period. Such ecologically inappropriate timing of inundation could lead to invasion by undesirable taxa such as Typha spp. and other aggressive aquatic weeds. A bypass to the river, as occurs at present, therefore should be implemented to avoid un-seasonal inputs of large amounts of treated storm-water to the billabong. This topic is addressed in detail next.

Risk of using Bolin Bolin Billabong as a water storage

A potential risk that needs to be raised explicitly is the temptation to use Bolin Bolin Billabong as a temporary water storage, especially during summer, for other irrigation activities in the nearby areas. That temptation should be resisted, since the maintenance of seasonal fluctuations in water levels in the billabong is a critical element of its rehabilitation (see Sections 5.2 & 5.3).

Large inflows of storm water may be expected to occur over summer, as a result of summer storms. Whilst a portion of the discharge may be diverted into the billabong according to the ecological needs at the time, it is imperative that Bolin Bolin Billabong not be used routinely as a simple receiving basin for such episodic high flows. If the entire thalweg is kept near- permanently inundated, especially if storm water constitutes a large proportion of inflows and water levels are kept high over summer, a simplified ecological community is likely to develop, dominated by large areas of open-water (perhaps highly susceptible to algal blooms) and fringed by areas of dense River Red Gum. In contrast, the maintenance of fluctuating water levels, with a summer draw-down and perhaps even episodic complete desiccation of the wetland once or twice per decade, would allow a diverse range of wetland EVCs to maintain themselves. Associated with this floristic diversity would be a diversity in vegetation structure and thus habitat and foraging sites for wetland fauna.

Water-quality issues and long-term eutrophication

The physiochemical hazards relate mostly to water quality and the topic was addressed in detail in Dodo Environmental (2009 a). The available water-quality data, although dated and limited (Table 3.1) show that water in Bolin Bolin Billabong was often turbid, but of low salinity and low nutrient concentrations. The challenge when utilizing storm water as a supplementary source of water will be to maintain low concentrations of nitrogen and phosphorus in the water column of the billabong. If this is not done, there is risk of algal blooms and invasion of the wetland by exotic and out-of-balance indigenous plant taxa that do well under nutrient-rich conditions: water milfoils Myriophyllum spp. and Typha spp. are examples of two taxa that would be expected to thrive under such conditions. The introduced Reedmace Typha latifolia is present in the nearby Banyule Billabong and it could colonize Bolin Bolin Billabong under wetter conditions. Constant high water would probably encourage also the expansion of the native Cumbungi species, Typha domingensis and T. orientalis. Considerable pre-treatment will be needed to ensure water of sufficient quality is used as a supplementary source of environmental water for the billabong.

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The topic of appropriate water quality for stormwater intended for use to ‘top-up’ high-value billabongs was discussed in detail in Dodo Environmental (2009 a). The reader is referred to that report for a detailed analysis of the topic, but Table 5.2 summarizes the most important water-quality recommendations of that earlier study. Table 5.2 compares the proposed trigger values against existing guideline concentrations for Total Nitrogen and Total Phosphorus in shallow aquatic systems.

The proposed range of critical concentrations for Total Phosphorus and Total Nitrogen may seem unrealistically low when compared with what can be achieved by current best practice for stormwater treatment. Against this criticism, it can be argued that it is inappropriate to set nutrient trigger levels aimed to protect natural systems on the basis of what is technologically achievable at present; the correct assessment can be made only in terms of what is required to protect the biodiversity and ecosystem services provided by the billabong or wetland. If the current best practice cannot meet a water quality sufficient to protect wetlands and billabongs, the appropriate strategy is not to relax the existing guidelines so that compliance can be seen to be achieved, but to improve the technology. The caveats outlined in Section 8.8 of Dodo Environmental (2009 a) need to be read in conjunction with these recommendations. Note that the ANZECC 2000 guidelines do not propose trigger values for water quality in wetlands, so values for lakes and reservoirs have been used instead to provide a comparative background.

Table 5.2: Comparison of proposed trigger concentrations for Total Phosphorus and Total Nitrogen with existing nutrient guidelines. Modified from Dodo Environmental (2009 a, Table 8.7).

Guideline or protocol Total Phosphorus Total Nitrogen (g P L-1) (g N L-1) Dodo Environmental (2009)  Maintain existing macrophyte dominance and ecological 50 500 structure and function in billabongs  Rehabilitate algal-dominated 25 350 billabongs subject to historic nutrient enrichment ANZECC-ARMCANZ (2000) for lakes 10 – 25 350 – 1000 and reservoirs in south-eastern Australia Ecological Engineering (2005) for Marshes on the central coast of New 10 1000 South Wales Melbourne Water (2003) for shallow 100 700 constructed lakes Melbourne Water (2005) for shallow 10 – 100 350 – 700 constructed lakes European Water Framework Directive for good water quality in shallow lakes  Søndergaard et al. (2005) < 50 < 1000  Moss et al. (2003) < 30 not identified

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5.6 Synthesis and conclusions

Whilst the simplest and, at first examination, best choice would be to allow natural bank-full and over-bank flows from the Yarra River to provide inundation water for Bolin Bolin Billabong, the recognition that we may have possibly entered (since ~1996) a new phase of climate and river discharge in south-eastern Australia has to be a paramount consideration when planning for the rehabilitation of the site. These hydrological changes, for example, make it unrealistic to assume that natural high flows will be sufficient to inundate the billabong, even if the existing channels at the northern and western parts were deepened (see Figure 3.2). This conclusion concurs with that reached earlier by Sinclair Knight Merz (2006).

A water regime has been outlined which, if implemented, should facilitate the rehabilitation of Bolin Bolin Billabong. The central features of the recommended water regime are that the deep pool in the eastern pool should remain near-permanently inundated to a depth of ~2 m. Water levels, however, should be allowed to fluctuate naturally with the seasons. The northern and southern arms should be inundated to a depth of at least 30 cm for the first three years (assessed at 50 cm for volumetric requirements), in order to drown out the invading River Red Gum saplings. After that, a gradient in wetting should be established so that a complex mosaic in vegetation (and animal habitats) is created along the thalweg. The surrounding floodplain should be inundated annually in order to re-establish the pre-European pattern of wetting and drying.

The volume of water required for rehabilitation was initially estimated at 74 ML by Sinclair Knight Merz (2006). That estimate has been refined by considering in more detail the range of hydrological components that have to be analysed in a hydrological budget for the site. Table 5.1 shows the volumes required for each component over a three-year cycle. In brief, ~82–106 ML is required to account for the better-understood components of filling the billabong thalweg, inundating the floodplain, and accounting for evaporative losses from the billabong. An additional amount, probably between ~50–150 ML, is required to saturate the soils before water can start to accumulate in the billabong and inundate the floodplain. In other words, ~132–256 ML will be required (as a minimum) to initially fill the billabong and inundate the floodplain in the first year. A smaller volume, ~94–153 ML, would be required to initially fill the billabong alone and maintain water levels in the face of surface-water evaporation.

5.7 Assessment of likely rehabilitation success

Hydrological factors

Volume of water required for initial fill

Large volumes of water will be needed to initially fill the billabong and wet the floodplain after their long period of desiccation. It is likely that water will have to be pumped from the Yarra River, and a licence obtained for its extraction. Subsequent ‘top-ups’, however, should be able to be obtained from treated storm-water, and indeed there are a number of benefits (specifically to the billabong and in general terms more regionally) of this approach.

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Water quality

The two main water-quality hazards are related to:  Nutrients and eutrophication (on the longer term)  Blackwater events and fish kills (immediately after the initial inundation).

The limited amount of available data show that the water column of Bolin Bolin Billabong was low in nutrients and salinity. Although sometimes turbid, the water seemed to be of generally high quality. The high quality of the water is probably attributed to its source being over-bank flows from the Yarra River. In contrast, storm water is of notoriously low (and variable) quality and considerable pre-treatment will be needed to ensure nutrient concentrations are low enough not to prompt algal blooms of encourage the growth of exotic taxa well suited to eutrophic conditions. Storm water is likely also to be enriched in toxicants such as heavy metals and pesticides: it will need to be shown that pre-treatment will reduce the concentrations (and loads) of these pollutants as well.

A potential hazard exists also with the risk of blackwater events and fish kills. Research on floodplain woodlands dominated by River Red Gums in south-eastern Australia has shown that floodplain inundation following heavy rain often results in the creation of so-called ‘blackwater events’ in billabongs and rivers (e.g. see Howitt et al. 2007). Wetting the surface soils allows large amounts of dissolved organic carbon (tannin and lignin) to be leached from leaf litter and the soil; these organic materials are quickly metabolised by aquatic micro-organisms; oxygen is consumed more quickly by the bacteria than it can be replenished by diffusion from the air; and hypoxia or even anoxia results. Fish (and other animals) then die because of the low concentrations of dissolved oxygen.

Conditions in Bolin Bolin Billabong would seem, on a first inspection, to be suitable for hypoxia or anoxia to develop in the water column, especially soon after the first inundation. The risk is significant because the bed and margins of the billabong are heavily treed and the surface soils contain large amounts of plant debris. This organic detritus can act as a source of dissolved organic carbon for bacterial growth, and the quiescent billabongs waters are not readily re- aerated by diffusion of oxygen from the atmosphere. Very careful monitoring would be needed during the first fill to check for the creation of blackwater.

Non-hydrological factors

Like all remnant wetlands in an urban setting, Bolin Bolin Billabong faces a number of management challenges.

Weeds and out-of-balance native species

Parts of the site are heavily infested with weeds, especially in the western sections. A rigorous program of weed control in the eastern sections, however, has had remarkable success in removing exotic plants. The excessive growth of some native taxa, especially River Red Gum and Silver Wattle, represents plants growing in an ‘out-of-balance’ regime. The re-instatement of a more natural wetting ands drying regime will be necessary to control native species that have taken advantage of the terrestrialization of the billabong and floodplain. In the case of the abundant River Red Gum saplings on the billabong floor and lower parts of the floodplain, ecological thinning may be required before a more natural water regime is implemented. Excessive growth of native and introduced taxa, such as Typha spp. and Brazilian Milfoil, also will need to be monitored and controlled. Moreover, rehabilitation of the billabong should not

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be seen in isolation from processes taking place in the floodplain areas more generally, and weed control will remain a core activity in these other areas.

Pest animals

Vermin such as foxes and feral cats are present in the area and have adverse effects on billabong biota (e.g. predation on birds and turtles, disruption of breeding success etc). These also will require control if the greatest benefits from implementing a more natural hydrological regime are to accrue. At leats five species of exotic fish – Carp, Goldfish, Mosquito Fish, Redfin and Weatherloach – have been found in the billabong during wetter periods, and they will need to be monitored and controlled.

Public expectations and recreational use

Bolin Bolin Billabong is located in a highly urbanised part of metropolitan Melbourne and is a heavily used recreational area for the local community. Both factors pose opportunities and constraints on the billabong’s rehabilitation. On the positive side, friends’ groups can help greatly in controlling weeds and monitoring the effectiveness of any rehabilitation efforts. On the other side, public expectations about killing excessive number of River Red Gum saplings or Silver Wattle and in using storm water to re-introduce more natural wetting and drying regimes will need to be managed. Access to the site for recreation increases the risk of the spread of aquatic weeds and the introduction of new weed species.

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6. Implementation and on-going management

6.1 Importance of adaptive management approach

The implication of our limited, and generally only recently gained, understanding of the hydrology and ecology of wetlands and floodplains is that interventions aimed to rehabilitate degraded sites are necessarily experimental and always involve some element of risk and of uncertain outcomes. Given uncertainty in ecological responses and the importance of ‘learning by doing, it is recommended highly that the entire wetland-floodplain rehabilitation project be considered within an adaptive management framework. An adaptive management framework is required not only because the precise ecological responses are not well known (even if they are understood in broad principles) but also because there is considerable uncertainty as to the volumes of water required to saturate the soils and inundate the billabong and floodplain. Especially during the filling stages, close attention will have to be paid to monitoring water levels in the billabong and the loss of water due to seepage and evaporation.

For nearly 20 years, adaptive management has been recognised as the single most effective approach to natural-resource management and, in particular, to the integration of monitoring and research and the addressing of large-scale, complex ecological questions (Walters 1986; Gentile et al. 2001). Allan & Stankey (2009) recently edited a book that outlines the approach and its application to natural systems.

The adaptive-management approach is shown diagrammatically in Figure 6.1. This figure shows that the adaptive management approach has a number of inter-related steps:  Develop a conceptual understanding of how the wetland functions, taking into account the importance of the surrounding landscape;  Develop management objectives consistent with the fundamental characteristics of the wetland ecosystem;  Predict the likely direction and, where possible, magnitude of environmental changes under different types of management interventions;  Undertake the chosen management interventions. In the case of wetlands in semi-arid regions, these will centre on hydrological manipulations, stock access and associated land-management actions;  Monitor the outcomes of these interventions at all relevant scales and will all relevant indicators (e.g. soils, groundwater, vegetation, fauna, community-scale ecological processes, etc);  Compare the monitoring results with the outcomes predicted on the basis of the conceptual models;  Revise the earlier conceptual understanding of the ecosystem; and  Modify the management plans and future interventions as necessary in the light of this improved understanding.

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Conceptual model of Environmental Bolin Bolin objectives for the Billabong site

Implementation of Revisions to hydrological regime management and associated land- objectives, management actions conceptual model and management Adaptive practices management feedback loop

Environmental changes in Feedback from response to monitoring interventions program

Environmental changes detected in monitoring program

Figure 6.1: Summary of the adaptive management approach as applied to the rehabilitation of Bolin Bolin Billabong.

6.2 Need for long-term monitoring program

Monitoring and reporting is a central component of the adaptive management approach. A number of manuals have been prepared to outline the most effective methods for monitoring wetlands and floodplains (e.g. Baldwin et al. 2005). The implementation of a monitoring program can be expensive, both in terms of financial resources and human involvement. At the minimum, it is recommended that monitoring at Bolin Bolin Billabong include three types of activities:

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 Regular monitoring of surface-water levels in the deep pool at the eastern loop of the billabong thalweg. Water depths could be monitored weekly or fortnightly (less frequent is not recommended) if a gauge (calibrated to AHD) were placed in the deepest part of the billabong. Alternatively, a continuous datalogger could be placed in the pool. The advantage of the data logger is that water depths are measured continuously (every 15 minutes or so, depending on settings) and the data require to be downloaded only every month or so. The disadvantage is cost and the possibility of vandalism. We, however, have installed data loggers in a prominent wetland in the Gippsland Lakes and in the heavily used Nagambie Lakes without vandalism.

Whilst water levels were being checked, careful monitoring would be needed also of pH and Dissolved Oxygen concentrations to check for the creation of blackwater (see Section 5.7). This type of monitoring would be needed mostly at the commencement of inundation, as that is the time blackwater and low oxygen tensions are most likely to be generated.

 Regular (probably seasonal) monitoring of the response of plants to the altered water regime. Monitoring should include a floristic description and an estimate of cover/abundance for the aquatic, fringing riparian and terrestrial taxa. If information on the frequency of native and exotic species is obtained simultaneously, valuable information can be collated on the threat posed by weeds and the effectiveness of the weed-control program.

 Groundwater levels during the initial inundation and subsequent draw-down periods. For the purposes of calculating the water requirements, it was assumed that there are now billabong-groundwater interactions and that groundwater does not contribute to inflows of water into the billabong. With an inundated billabong, however, there may be loses to the groundwater through lateral and vertical seepage: the installation and monitoring of piezometers would allow groundwater levels to be tracked over time. The installation of piezometers could throw light also on the type of soils under the billabong and depth to the impervious clay layer.

Depending on resources, a large number of other attributes of the billabong could be monitored on a routine or less regular basis. Although many of these activities are expensive (e.g. plant and animal surveys, regular assays of water quality), the involvement of community groups may alleviate some of the resourcing issues. Sections 3.3 & 3.6 reported on the available ecological information, for example that generated by bird-watching groups, and a mechanism should be created to collate that type of data into the monitoring program. If resources permit, fish monitoring should be considered highly because of the likelihood of exotic fish species re- entering the billabong when it is flooded (even after pumping) and the adverse ecological impacts that they can create.

In addition to monitoring, a number of ‘once-off’ activities should be instigated too. The vegetation description by Mitchell et al. (1996) is now over 15 years old and should be repeated. There would be significant benefits in surveying also the billabong and floodplain to obtain a better understanding of the bathymetry of the thalweg and surrounding floodplain; the benefits of simple elevational surveys in better understanding patterns of water flow and areas of land inundated with particular flows was demonstrated clearly in studies I undertook of billabongs along the Goulburn River floodplain near Shepparton in the early 1990s. As noted earlier, it is understood that Melbourne Water will soon undertake an bathymetric survey of the billabong and floodplain.

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Regardless of the type of monitoring undertaken, it is essential that monitoring results are interpreted, written up and made available for public dissemination. In fact, given the high value of the Bolin Bolin site, the regular production of a monitoring summary could readily showcase the rehabilitation efforts and bring considerable credit to the agencies participating in the rehabilitation program.

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7. References

Allan C & Stankey GH (2009). Adaptive management. A practitioner’s guide. CSIRO, Collingwood & Springer, Dordrecht.

Arthington AH et al. (2006). The challenge of providing environmental flow rules to sustain river ecosystems. Ecological Applications 16: 1311-1318.

Arthington AH & Zalucki JM (1998). Comparative evaluation of environmental flow assessment techniques: review of methods. Occasional Paper 27/98. Land & Water Resources Research and Development Corporation, Canberra.

Australian Ecosystems (2007). Ecological assessment and management recommendations for Banyule Flats Reserve. Report to Banyule City Council, Melbourne.

Australian Ecosystems (2009). Yering Backswamp vegetation management plan. (Draft) Report to Melbourne Water, Melbourne.

Baldwin DS et al. (2005). Recommended methods for monitoring floodplains and wetlands. Murray-Darling Basin Commission, Canberra.

Beardsell C (2000). Vegetation communities of the City of Banyule. Report to Banyule City Council.

Beardsell D & Beardsell C (1999). The Yarra: a natural treasure. Royal Society of Victoria, Melbourne.

Beardsell D et al. (2008). eMelbourne; the city past and present. Internet resource http://www.emelbourne.net.au/biogs/EM00589b.htm, accessed 28/08/2008.

Blood K (2001). Environmental weeds. A field guide for SE Australia. Bloomings Books, Melbourne.

Boon PI et al. (1990). Billabongs. In Understanding the River Murray. (Edited by N Mackay & D Eastburn). Pages 182-198. Murray-Darling Basin Commission, Canberra.

Boon et al. (2008). Vegetation changes over a four-decade period in Dowd Morass, a brackish- water wetland of the Gippsland Lakes, south-eastern Australia. Proceedings of the Royal Society of Victoria 120: 403-418.

Boulton AJ & Brock MA (1999). Australian freshwater ecology. Gleneagles Publishing, Adelaide.

Bradshaw AD (1997). What do we mean by restoration? In Restoration ecology and sustainable development. (Edited by KM Urbanska et al.) Pages 8-14. Cambridge University Press, Cambridge.

Briggs SV (1988). Guidelines for management of inland wetlands in southern New South Wales. Wetlands (Australia) 8: 1-2.

Briggs SV & Thornton SA (1995). Management of River Red Gums for waterbird nesting. Corella 19: 132-138.

72

Briggs SV & Thornton SA (1999). Management of water regime in River Red Gum Eucalyptus camaldulensis wetlands for waterbird breeding. Australian Zoologist 31: 187-197.

Briggs SV et al. (1997). Relationships between hydrological control of River Red Gums wetlands and waterbird breeding. Emu 97: 31-42.

Briggs SV et al. (1998). Relationships between control of water regimes in River Red Gum wetlands and abundance of waterbirds. Corella 22: 47-55.

Bulleen Art & Garden (2009). Submission to Victorian Environmental Assessment Council metropolitan Melbourne investigation. Submission 1/119.

Bunn SE et al. (1996). Scoping review of research and management needs of Australian wetlands. Occasional Paper 01/97. Land and Water Resources Research and Development Corporation, Canberra.

Bunn SE & Arthington AH (2002). Basic principles and ecological consequences of altered flow regimes for aquatic biodiversity. Environmental Management 30: 492-507.

Burke Road Billabong Reserve Committee of Management (2009 a). Burke Road Billabong: resources and background. Department of Sustainability and Environment, Melbourne. Internet resource http://home.vicnet.net.au/~fobrb/dbackgd.html, accessed 4/08/2009.

Burke Road Billabong Reserve Committee of Management (2009 b). Burke Road Billabong: management plan. Department of Sustainability and Environment, Melbourne. Internet resource http://home.vicnet.net.au/~fobrb/dmanplan.html, accessed 4/08/2009.

Carr GW (1991). Environmental weed invasions and their conservation implications. In Flora of Melbourne. Edited by Gray M. Pages 26-33. Society for Growing Australian Plants, Melbourne.

Cunningham SC, Baker PJ & Horner GJ (no date: presumably 2009 or 2010). Proposed ecological thinning trials for River Red Gum forests of the middle Murray River floodplain in Victoria. Report by Australian Centre for Biodiversity, School of Biological Sciences, Monash University, Clayton.

Davis JA et al. (2001). Environmental water requirements to maintain wetlands of national and international importance. Environmental Flows Initiative Technical Report Number 1. Environment Australia, Canberra.

Department of Natural Resources and Environment (2002). FLOWS – a method for determining environmental water requirements in Victoria. Department of Sustainability and Environment, East Melbourne.

Dodo Environmental (2009 a). Environmental management practices and general use of billabongs. Phases 1 & 2: Billabongs of the Greater Melbourne region. Report to Melbourne Water, East Melbourne.

Dodo Environmental (2009 b). Environmental management practices and general use of billabongs. Phase 3: Specific case studies. Report to Melbourne Water, East Melbourne

Eliniezer K et al. (1995). Understanding natural processes in degraded urban billabongs and associated wetlands. In 'National Conference on Wetlands for Water Quality Control' p. 455. (Townsville, Qld.)

73

Elliot R (1997). Faking nature. The ethics of environmental restoration. Routledge, London.

Fensham RJ & Fairfax RJ (2002). Aerial photography for assessing vegetation change: a review of applications and the relevance of findings for Australian vegetation history. Australian Journal of Botany 50: 415-429.

Fletcher AR (1976). Interspecific interactions and relationships in a billabong community. BSc (Hons) thesis. Monash University, Clayton.

Fletcher AR et al. (1985). Effects of carp (Cyprinus carpio L.) on aquatic vegetation and turbidity of waterbodies in the lower Goulburn River Basin. Australian Journal of Marine and Freshwater Research 36: 311-327.

Gehrke PC et al. (1995). River regulation and fish communities in the Murray-Darling system, Australia. Regulated Rivers: Research and Management 11: 363-375.

Gentile JH et al. (2001). Ecological conceptual models: a framework and case study on ecosystem management for South Florida sustainability. Science of the Total Environment 274: 231- 253.

Gooderham J & Tsyrlin E (2002). The waterbug book. CSIRO Publishing, Collingwood.

Gott B (2010 a). Native bush seasons – middle Yarra time lines calendar. Internet resource http://www.livingwithnature.com.au/inspiration_middle.html, accessed 16/02/2010

Gott B (2010 b). Seasonal calendars for the Melbourne area. Internet resource http://www.pvms.vic.edu.au/seasonal_calendar.html, accessed 16/02/2010.

Green D et al. (1998). Assessment of environmental flow needs for the lower Darling River. Report to Murray-Darling Basin Commission by the Department of Land and Water Conservation Centre for Natural Resources.

Gross JE (2003). Developing conceptual models for monitoring programs. Internet resource http:///science.nature.nps.gov/im/monitor/conceptualmodels.cfm, accessed 26/10/2009.

Gunderson LH et al. (2010). Foundations of ecological resilience. Island Press, Washington.

Hazelton P & Murphy B (2007). Interpreting soil test results. What do all the numbers mean? CSIRO Publishing, Collingwood.

Hobbs RJ (2005). Restoration ecology and landscape ecology. In JA Weins & MR Moss (eds) Issues and perspectives in landscape ecology. Cambridge University Press, Cambridge.

Hodges DN (1979). Billabongs of the Yarra Valley Metropolitan Park - some ecological and recreational considerations. M Env Sc thesis. Monash University, Clayton.

Horton D (2000). The pure state of nature. Sacred cows, destructive myths and the environment. Allen and Unwin, St Leonards.

Howitt JA et al. (2007). Modelling blackwater: predicting water quality during flooding of lowland river forests. Ecological Modelling 203: 229-242.

74

Junk et al. WJ (1989). The flood pulse concept in river-floodplain systems. Canadian Journal of Fisheries and Aquatic Sciences Special Publication 106: 110-127

Kingsford RT & Auld KM (2005). Waterbird breeding and environmental flow management in the Macquarie Marshes, arid Australia. River Research and Applications 21: 187-200.

Koehn J, Brumely A & Gehrke P (2000). Managing the impacts of carp. Bureau of Rural Sciences, Canberra.

Lacey G (2004). Still glides the stream. The natural history of the Yarra from Heidelberg to Yarra Bend. Australian Scholarly Publishing, Melbourne.

Lake PS (2001). On the maturing of restoration: linking ecological research and restoration. Ecological Management and Restoration 2: 110-115.

Leahy PJ et al. (2005). The impact of European settlement on Bolin billabong, a Yarra River floodplain lake, Melbourne, Australia. River Research and Applications 21: 131-149.

Lorimer GS (2006). Inventory and assessment of indigenous flora and fauna in Boroondara. Report to Boroondara City Council.

Maddox D, Poiani K & Unnasch R (1999). Evaluating management success: using ecological models to ask the right monitoring questions. Internet resource http:///science.nature.nps.gov/im/monitor/docs/Maddox_etal_1999_Ecol_Models.pdf, accessed 27/10/2009.

Mitchell P et al. (1994). Yarra billabongs study. Descriptive report. Report to Melbourne Parks and waterways.

Mitchell P et al. (1996). A study of three billabongs in Yarra Valley parklands. Report to Melbourne Parks and Waterways.

Nichol JM & Ganf GG (2000). Water regime, seedling recruitment and establishment in three wetland plant species. Marine and Freshwater Research 51: 305-309.

Ogden JC et al. (2005 a). The use of conceptual ecological models to guide ecosystem restoration in South Florida. Wetlands 25: 795-809.

Ogden JC et al. (2005 b). Total system conceptual ecological model. Wetlands 25: 955-979.

Paijmans K et al. (1985). Aspects of Australian wetlands. CSIRO Water and Land Resources Technical Paper 44: 1-71.

Parks Victoria (2008). Yarra Valley Parklands. Management Plan. Parks Victoria, Melbourne.

Peverill KI et al. (1999). Soil analysis. An interpretation manual. CSIRO Publishing, Collingwood.

Plumb G (2003). Really useful conceptual models: metaphors, censorship and negotiated knowledge. Internet resource http:///science.nature.nps.gov/im/monitor/docs/Plumb_2003_Reallyusefulmodels.pdf, accessed 27/10/2009.

75

Poff N et al. (1997). The natural flow regime. BioScience 47: 769-784.

Presland G (2008). The place for a village. How nature has shaped the city of Melbourne. Museum Victoria, Carlton.

Raulings E et al. (2010). The importance of water regimes operating at small spatial scales for the diversity and structure of wetland vegetation. Freshwater Biology 55: 701-715.

Recknagel F et al. (1998). Common carp in natural wetlands: impacts and management. In Wetlands in a dry land; understanding for management. Edited by Williams WD. Pages 259-266. Environment Australia, Canberra.

Roberts J (1998). Carp: sifting through the issues. In Wetlands in a dry land; understanding for management. Edited by Williams WD. Pages 253-257. Environment Australia, Canberra.

Roberts J & Ludwig JA (1991). Riparian vegetation along current-exposure gradients in floodplain wetlands of the River Murray, Australia. Journal of Ecology 79: 117-127.

Roberts J & Marston F (2000). Water regime of wetland and floodplain plants in the Murray-Darling Basin. CSIRO Land & Water Technical Report 30-00.

Ryder D et al. (2008). Vision generation: what do we seek to achieve I river rehabilitation? In River futures. An integrative scientific approach to river repair. (Edited by GJ Brierley & KA Fryiers). Pages 16-27. Island Press, Washington.

Sainty G & Jacobs S (1981). Waterplants of New South Wales. Water Resources Commission, Sydney.

Salter J et al. (2008). Does salinity reduce the tolerance of two contrasting wetland plants, the submerged monocot Vallisneria australis and the woody shrub Melaleuca ericifolia, to wetting and drying. Marine & Freshwater Research 59: 291-303.

Seddon G (1997). Landprints. Reflections on place and landscape. Cambridge University press, Cambridge.

Siebentritt MA et al. (2004). Effects of an enhanced flood on riparian plants of the River Murray, South Australia. River Research and Applications 20: 765-774.

Sinclair Knight Merz (2005 a). Determination of the minimum environmental water requirement for the Yarra River. Flow and non-flow related environmental and social issues associated with the Yarra River. Report to Melbourne Water, Melbourne.

Sinclair Knight Merz (2005 b). Determination of the minimum environmental water requirement for the Yarra River. Minimum environmental water requirements and complementary works recommendations. Report to Melbourne Water, Melbourne.

Sinclair Knight Merz (2006). Watering options for Yarra River billabongs. Report to Melbourne Water, Melbourne.

Smith P & Smith J (1990). Floodplain vegetation. In Understanding the River Murray. (Edited by N Mackay & D Eastburn). Pages 214-228. Murray-Darling Basin Commission, Canberra.

76

Timbal B & Jones DA (2008). Future projections of winter rainfall in southeast Australia using a statistical downscaling technique. Climatic Change 86: 165-187.

Toomey M & Boon PI (2007). Response of Brazilian Milfoil (Myriophyllum aquaticum) to salinity and water-level fluctuations and its potential to invade wetlands of the Gippsland Lakes, south- east Victoria. Proceedings of the 3rd Victoria Weed Conference 3-4 October 2007, Bendigo.

Varrasso N (2000). The floristic transition of a Yarra Valley parkland billabong: A historical account from 1838 to present. MA Qualifying thesis. Monash University, Clayton.

VEAC (2006). River red gum forests investigation. Discussion paper. Victorian Environmental Assessment Council, Melbourne.

Walker B & Salt D (2006). Resilience thinking. Sustaining ecosystems and people in a changing world. Island Press, Washington.

Walters C (1986). Adaptive management of renewable resources. MacMillan, New York.

Ward K (1994). Flood requirements of the wetland flora in Barmah Forest, Victoria. Department of Conservation and Natural Resources, Benalla.

Young WJ, Scott AC, Cuddy SM & Rennie BA (2003). Murray flow assessment tool. A technical description (based on version 1.4). CSIRO Land and Water, Canberra.

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