RESTORE LAKE PEDDER REVIEW

Review of Potential

Responses to Restoration:

Organic soils and peatlands

Dr Christina Birnbaum

Dr Anita Wild

RESTORE LAKE PEDDER

REVIEW

This review provides a summary of the likely current state of soil and peat substrate inundated in the

Huon-Serpentine Impoundment including existing information and knowledge gaps, the major

opportunities and risk of the restoration of Lake Pedder to the substrates and a prioritised list of

studies and scope of work required to provide the knowledge to proceed.

Status at the Pedder 2000 Enquiry

Geomorphology

Field studies in 1995 by Professor Peter Tyler and Dr Kevin Kiernan indicate that the significant

geomorphological features of Lake Pedder (i.e. the sand bed, the bar and mega-ripples, small scale

ripple bedforms, Pedder pennies, the beach and the dunes) sustained only very minor damage

occurred during filling of the Huon-Serpentine Impoundment. The dunes and the sand bed were

shown to be largely intact and are expected to have remained submerged and largely protected

from wave erosion (Kiernan 2001). Evidence from work by Tyler (1993,1996) also suggests that the

Pedder Beach, bar and mega-ripples can also be expected to have remained intact. Alternatively, if

the mega-ripples were damaged or destroyed, Kiernan (2001) expected that they can re-form within

a year or two once the original lake levels are re-established. It is expected that there would be

some changes to the small-scale ripple bedforms since filling of the dams, however, they are rather

transient features of the landscape and will reform again once the conditions are met. The unique

Pedder Pennies, a geological specimen formed like a pebble in the shape of a penny and has

quartzite centre and mixture of iron and manganese coated rim, have been demonstrated by Tyler

et al. (1993, 1996) to remain abundant on the original lake bed floor.

Substrates (peats/organosols)

Although no fine-scale, systematic review of soils in the catchment was conducted prior to the

flooding event, it is known that soils were predominately organosols which high levels of organic

matter in various stages of decomposition and form in response to very wet climatic conditions and

a high watertable (water in the soil). When organosols dominate the landscape, these areas are

termed ‘peatlands’; south-west Tasmanian peatlands are similar (but generally shallower) to other

terrestrial peatlands around the world and exhibit the same processes of carbon capture and organic

matter retention. These peatlands have been termed ‘blanket bogs’ in Tasmania where they are

widespread in the south-west, they have formed under the buttongrass moorlands, which

dominated the Lake Pedder plains prior to flooding (Macphail and Shepherd 1973). Pemberton

(2001) also reported that the organosols at Lake Pedder are predominantly muck peat less than one

metre in depth and the surface horizon is likely to consist of fibrous peat that may be absent in other

areas.

In 1994, the Australian Broadcasting Commission (ABC) sent scuba divers to inspect the condition of

the sediment of the original lake bed and found only about 3 mm of sediment (Tyler 2001). This

means that the sediment accumulation is even less than predicted by Tyler et al. (1993,1996) where

they suggested that the sediment accumulation over Lake Pedder could be 0.5-1.0 cm/decade as

evident from other similar oligotrophic lakes around the world (Kiernan 2001; Tyler et al. 1993,

1996).

Erosion is currently believed to be confined to the perimeter of the Huon-Serpentine Impoundment

due to the limited area of wave action impacts from a legislated minimum operating water level.

Whilst not investigated or validated in this study, it is likely that the areas of noted erosion and

presumed peat desiccation on the southern perimeter of the impoundment noted by Kiernan (2001)

remain present.

Current status and improved knowledge

Ecological models of vegetation and soil formation

In southwest Tasmania, two ecological models have been proposed to explain the fire-vegetation-

soil feedbacks (Jackson 1968; Mount 1979). The two models assess interactions between vegetation,

fire and soil to explain the dynamics of fire-promoting and fire-sensitive vegetation in southwest

Tasmania (Wood and Bowman 2012). Briefly, Jackson’s (1968) “ecological drift” or alternative stable

states model argued that each vegetation promotes its own growth, while hindering the

establishment of other vegetation types (Wood and Bowman 2012). However, fire frequencies can

push the vegetation over its “resilience threshold” and cause a transition to a different vegetation

community (Wood and Bowman 2012). The other model “stable fire cycles”, proposed by Mount

(1979), suggests that site characteristics (i.e. geology, drainage and topography) are more important

than fire alone in determining vegetation patterns (Wood, Hua, and Bowman 2011). Recent study by

Wood and Bowman (2012) assessed the applicability of these two models in southwest Tasmania

across a number of vegetation types, including buttongrass moorlands; they found support to the

Jackson (1968) alternative stable states model as the most suitable vegetation dynamics model on

nutrient poor substrates in southwest Tasmania. Taken together, these authors concluded that

vegetation communities in southwest Tasmania represent a number of alternative stable states

which has important implications for the conservation and management of these areas under

projected future climate conditions, especially as rapid vegetation transitions (e.g. from drying out of

wet and humid vegetation) may contribute to an expansion of more flammable communities (Wood

and Bowman 2012).

Organosols

Following on from these ecological models described above, it is highly likely that submerged

organosols in Lake Pedder are in an alternative stable state and decomposing rather slowly. Tyler et

al. (1993,1996) suggested that the acidic conditions at the lake floor are likely to prevent extensive

organic matter decomposition. Indeed, some of the remains of the original vegetation was

immediately recognisable during Lake Pedder underwater expedition by Tyler (1993, 1996) and the

more-localised submersible imagery of 2020 (Restore Pedder 2020). Therefore, it is likely that most

of current Lake Pedder floor substrate is anoxic and acidic which impedes plant and organic matter

decomposition. The conservation of intact organic substrates and peatland landscapes has been

found elsewhere in areas inundated by freshwater. Subsequent international studies have also

shown evidence for preservation and little degradation of organic matter in anoxic temperate

environments, e.g. Black Sea (Sun and Wakeham 1994) and Lake Grosse Fuchskuhle (acidic bog lake

in Germany) (Conrad, Claus, and Casperb 2010).

Carbon Cycling and GHG emissions

Low organic matter decomposition rates in Lake Pedder then also suggest likely low CO2 (carbon

dioxide) and CH4 (methane) emissions. Indeed, reservoirs of the temperate West Coast Range (WCR)

of Tasmania have been described to have overall low diffusive greenhouse gases emissions (GHG)

due to good oxygenation, temperature <22 °C and low dissolved organic carbon content (Bastien

and Demarty 2013; Bastien, Tremblay, and Scanlon 2009). For example, Bastien and Demarty (2013)

assessed CO2 and CH4 fluxes in 2006, 2008 and 2010 in several lakes and reservoirs in Tasmania,

including WCR sites such as Lake Pedder, and the Central Plateau (Figure 1 and Figure 2). They found

that CO2 and CH4 diffusive fluxes in Lake Pedder were < 1000 mg CO2 per m2 per day and < 5 mg CH4

per m2 per day, respectively (Bastien and Demarty 2013). Emissions from Lake Pedder were low

compared with the relatively high rates of GHG emissions from some tropical storages tested with

the same methodology for this study. Further analysis also showed that age of reservoir and climate

factors such as water temperature, reservoir depth and rainfall also influence the diffusive emissions

from reservoirs (Bastien and Demarty 2013). However, subsequent analysis and a global review of

data (Deeming et. al 2016) for storages world-wide has that water storages are significant

contributors to GHG emissions and there is a high degree of variability of reservoir GHG emissions;

the 2016 review also concludes that factors related to reservoir productivity are better predictors of

emissions compared with latitude and age.

Figure 1 Carbon dioxide (CO2) diffusive flux from natural aquatic systems and reservoirs sampled in 2006, 2008 and 2010 in Queensland and Tasmania (Figure from Bastien and Demarty 2013).

Figure 2 Methane (CH4) diffusive flux from natural aquatic systems and reservoirs sampled in 2006, 2008

and 2010 in Queensland and Tasmania (Figure from Bastien and Demarty 2013).

Sedimentation and erosion

If the rate of sediment accumulation in Lake Pedder has remained in a similar range since 1994 (see

above), then it may be extrapolated that the sediment amount in 2020 remains low to moderate.

The lack of widespread deep accumulations of sediment throughout the basin is supported by

recent, February 2020 footage (Restore Pedder 2020), particularly over the intact Lake Pedder dune

system and the beach that is under very thin silt layer. However, sediment accumulation rates and

spatial deposition patterns will vary throughout the impoundment and studies to sample larger

areas and modelled accumulation areas would be required prior to restoration.

The potential status of sediment volume and distribution is important, because, unlike dam

removals on run-of-river schemes where dam removal will allow all the accumulated sediments to

be released downstream, the process of dewatering the Huon-Serpentine basins would only flush a

proportion of the accumulated sediment into the receiving rivers and some sediments (below the

offtake levels) will remain in the restored Lake Pedder and associated lake systems.

The videography study (Restore Pedder 2020) also found that the Edgar basin contained large

amounts of coarse woody debris such as branches, wood and debris at the bottom, some root

systems still intact and fine layer of silt covering everything, while the original sandy soil was still

present underneath the silt layer.

Conceptual model of the anticipated responses and potential impacts on organic soils during and immediately after drawdown of the Huon-Serpentine Impoundment to restore Lake Pedder.

Anticipated changes after restoration

Once the Huon-Serpentine Impoundment is drained and the original Lake Pedder restored

a large area of substrate will be exposed. Air photos, pre-flooding maps, scientific studies

and personal accounts of the Lake Pedder area all suggest that the area around the lake

was largely dominated by peatlands (Macphail and Shepherd 1973; Pemberton 2001). It

has also been established that large areas that dominated the plains around Lake Pedder

before it was flooded were buttongrass moorlands (i.e. blanket bogs) growing on

organosols. Research by Tyler et al. 1993 and Tyler 2001 suggests that the inundated soils

are still largely intact. Thus, once these areas will be dewatered over time they may start

resembling ‘drained peatlands’ which is a term to describe peatlands that have reduced

water tables and altered hydrology. Whilst this is not strictly the case for the newly-

dewatered peatlands of the Huon-Serpentine Impoundment which will still be receiving

recharge from precipitation, surface and groundwater, international studies have shown

that areas within restored impoundments tend to initially be dry and suffer some

desiccation (Auble et. al 2007) and is further exacerbated by the fact that dark peat soils

devoid of vegetation in Australia have been measured up to 80 degrees Celsius. In the

case of south-west Tasmania, the degree of drying will depend on the rate of drainage,

groundwater patterns, recharge from precipitation, the current environmental conditions

in the area (e.g. solar radiation, evaporation rates, humidity) and the degree of shading

associated with the rate of vegetation colonisation.

It is reasonable to presume that whilst in the initial stages following dewatering, the

catchment will display the characteristics of drained peatlands; these include:

• Change in soil microclimate (i.e. high surface temperatures, low conductivity,

temporary aridity) (Sliva 1998).

• Erosion and substrate instability as potential barriers to recolonization (Rochefort

and Lode 2006).

• Changes in vertical distribution of major plant nutrients [e.g. reduction in Ca and

Mg, southern Finland example (Laiho, Sallantaus, and Laine 1999)].

• Increased greenhouse gas emissions (Leifeld, Wüst-Galley, and Page 2019;

Tubiello et al. 2016).

• Loss of soil carbon storage function (Wüst-Galley, Mössinger, and Leifeld 2016).

• Changes in peat structure (i.e. increased aggregation and density) (Holden,

Chapman, and Labadz 2004).

• Harmful changes in peatland hydrology, loss of water (Cao et al. 2017).

Restoration of peatland function

Wetter conditions can initiate new bog formation on the top of the dried peat (Grover,

Baldock, and Jacobsen 2011). Overall, for peatlands to function there needs to be a net

balance of influx and efflux that maintains waterlogged conditions within the peat.

Developing peatlands are characterised by the production of organic matter in excess of

decomposition that leads to a net accumulation of plant‐derived organic matter (Watters

and Stanley 2007). A consistent water supply that provides a net positive water balance in

the beds is a prerequisite for the accumulation of peat, as decomposition processes are

accelerated when peat beds become exposed (Charman 2002). Organosols require a wet

environment and high-water table for their development thus if the precipitation is

sufficient and water table appropriate new peatland formation is plausible. Blanket bog

favors cool maritime conditions with a heavy reliable rainfall of 3500 mm per year

(Pemberton 1989). Furthermore, peat accumulation is facilitated by high relative humidity

(70- 85 %) and considerable annual rainfall; 2525 mm is the annual average from

Strathgordon Village weather station over the 1971-2020 period (Bureau of Meteorology

2020).

Whether or not peatlands will continue to form at the rate they have in the past in the

Lake Pedder area depends largely on the climatic conditions (predominantly temperature

and moisture) that the area will experience in the future. Future possible climatic

conditions are assessed in the Special Report on Emissions Scenario (SRES) by the

Intergovernmental Panel on Climate Change (IPCC). The report proposes several possible

scenarios, ranging from A1FI (a future world of rapid economic growth with fossil-

intensive technologies) to B1 (rapid transformation to a service and information economy

with clean and resource-efficient technologies).According to Flannigan et al. (2013) much

of Australian climate, especially Tasmania, is currently tracking according to A2 scenario

(Flannigan et al. 2013). According to current predictions of the A2 scenario, Tasmanian

temperatures are predicted to increase uniformly across the state on average by 2.6-3.3°

C late this century (Fox-Hughes et al. 2014). However, annual rainfall is predicted to be

quite variable across the state and annual rainfall is predicted to increase over coastal

regions (Fox-Hughes et al. 2014). Furthermore, after 2050, winter rainfall on the west

coast is predicted to increase while summer rainfall to decrease (Fox-Hughes et al. 2014).

Taken together, increasing temperatures and drier summers in the Lake Pedder area may

slow down or hinder peatland formation in the future and increase overall fire risk.

However, Fox-Hughes et al. 2014 have demonstrated that the accumulated annual fire

risk (1960-2090) in western Tasmania is the lowest compared to the rest of Tasmania.

If peatlands do restore and commence accumulation of large amounts of organic matter,

they are then more likely to act as a carbon sink.

Potential risks from restoration

Erosion of organosol surfaces

For over two decades since the flooding of Lake Pedder the lake water level has been 15

m higher than the natural lake level, and importantly, has been maintained at a highly

regulated, narrow operating range between 306.93mASL and 308.46mASL which has

minimized the wave erosion of the main shoreline landforms around the original Lake

Pedder. It is likely, that similarly to when the lake was filled, the drainage of the lake will

cause accelerated wave erosion on the sand dunes along the margins of Lake Pedder

according to the wind direction, wind fetch distance and the bathymetry of the lake floor

(refer to the restoration report for information on wind fetch modelling). Other risks

associated with the drawdown include the potential that wave action may uproot existing

trees that have been reported to be largely undecomposed and rooted in the protective

peat mat (Tyler et al. 1993, 1996).

Once the Impoundment is drained the exposed peat surface will be subjected to erosive

forces by wind, desiccation water and waves. Organosols are especially vulnerable to

physical disturbance once vegetation is removed and roots are disturbed and once

organosols dry out up to 2 cm crust forms that protects the soil surface.

Wildfire

As mentioned above, the dominant vegetation type in the Lake Pedder area is moorland

which is very flammable but also requires fire to help maintain it and protect the nearby

fire-sensitive vegetation from wildfires (di Folco and Kirkpatrick 2011; Jackson 1968). Fire

frequency in buttongrass moorlands is correlated with vegetation type and decreases in

soil carbon and nitrogen content as well as soil depth (Di Folco 2007). Whilst buttongrass

moorlands are adapted to fires and indeed require fire to prevent succession to other

vegetation types, it is also very flammable and thus poses a management dilemma as

management effort should balance between hazard reduction burning while preserving

ecological values of this unique ecosystem (Storey and Betts 2011). The organic soils are

also at extreme risk once initially exposed and peatlands are considered a high-biomass

ecosystem where fires are controlled by heat transfer and water content (Turetsky et al.

2015).

The flammability of these soils appears to be strongly correlated with topographic

position which appears to be a consequence of relative drying of soils in addition to fire

severity (di Folco and Kirkpatrick 2011). There are certainly examples from overseas and

Australian Alps showing that drained peat that was left to dry became highly flammable

emitting carbon into the atmosphere and thus contributing to carbon emissions (Page et

al. 2004; Wahren and Papst 1999). This appears to be the case as the extent of bushfires is

large (Figure 3) and there is evidence to suggest that the size of fires and area burnt in

moorlands in Tasmania has steadily increased since 1970 (from ~ 10 000 ha in 1970 to ~

30 000 ha in 2010) (French et al. 2016).

Figure 3 Unplanned bushfires and planned fires in the area surrounding the Huon-Serpentine Impoundment from 2003-04 to 2012-13. 2014 (PWS 2016)

The cause of these fires is also changing as climatic conditions warm and atmospheric

conditions become more ‘unstable’. Lightning-caused fire was rare in the TWWHA before

2000. However, since this time there has been an increase in both the number of fires

following lightning storms and the area burnt by these fires (Figures 4 and 5) (Styger,

Marsden-Smedley and Kirkpatrick 2018). This study also found that the increase in the

proportion of lightning strikes that occur in dry conditions has increased ignition

efficiency. These changes have important implications for the potential restoration of

Lake Pedder as higher projected fuel loads and drier climates could result in a further

increase in the number of fires associated with lightning.

Figure 4 Average number of lightning fires per fire season for five-year periods between 1980/81, 2012/15 and 2015/16. Lowess (segmented regression) line is shown. Graphic: Styger, Marsden-Smedley and Kirkpatrick 2018.

Figure 5 Average area burnt per season (ha) within the TWWHA. Graphic: Styger, Marsden-Smedley and Kirkpatrick 2018.

In addition to the increase in potential ignition sources from lightning, a very recent,

comprehensive study assessing the effect of soil moisture of organic soils in concert with

meteorological data from 63 sites across Tasmania confirmed a potential increase in soil

dryness. The study quantified the very strong correlation between moisture content and

soil combustibility and that Tasmanian organic soils are likely to be combustible in most

areas in most summers (Prior et al. 2020). These findings must be considered especially as

fire danger is predicted to increase during 21st century

In addition to the fire risk, peatlands in Tasmania are suggested to be near their climatic

limits and the future drier and warmer climate is very likely to make these ecosystems

more vulnerable to hydrologically stress and shrinking (Prior et al. 2020) and at risk of

reinforcing positive feedback loops of fire, drying and fire.

Desiccation and carbon cycling

Following Lake Pedder restoration, a large area of drying peat surface will be exposed.

Peatland ecosystems play a central role in global carbon (C) cycling as they contain more

organic C than any other terrestrial ecosystems (Dinel et al. 1988; Joosten, Hans 2016),

while also releasing carbon dioxide (CO2) and methane (CH4) to the atmosphere via

decomposition (Grover and Baldock 2012; Turetsky et al. 2002). Thus, large drying peat

surfaces will contribute to considerable greenhouse gas emissions. For example, a study

from Australian alps found maximum CO2 emissions from the surface of peatlands was

1.97 grams per square meter per day and annual CO2 emissions from dried peat predicted

to be anywhere between 772 to 8267 grams per square meter (depending on assessment

method used) (Grover and Baldock 2010). For comparison, a Swedish study assessed CO2

emission rates from eight peat topsoils and reported mean CO2 emission rates based on

soil dry mass at near-water-saturated conditions to range from 7-78 mg g-1 min-1 (Norberg,

Berglund, and Berglund 2018). Methane fluxes in peatlands vary from slight uptake to

efflux of more than 65,000 mol m−2 day−1 (Klinger et al. 1994). A recent study from

Quebec, Canada reported CH4 from bare peat of −87.5 mol m−2 day−1 to 6.25 mol m−2 day−1

which was similar to earlier studies from elsewhere in Canada (Mahmood and Strack

2011; Waddington and Day 2007).

Slow vegetation re-establishment

It is long acknowledged that the revegetation or assisted rehabilitation would have to

occur in areas of the drained Impoundment because natural unassisted revegetation

would be too slow to avoid peat soils drying out and eroding (House of Representatives

Standing Committee on Environment 1995). This should be an important consideration

from revegetation and restoration perspectives.

Potential mitigating factors and actions

Initially, once drained, the Lake Pedder catchment would have no to very little protective

vegetative cover to intercept rainfall which is likely to create significant flood peaks

followed by erosion that may disturb the peat mats and not intercept sediment

movement (Livingston 2001).

Further studies are needed to establish the effect of draining the dams on peat soils and

the most effective scenario of dam draining to preserve the peat soils. It is very probable

that drainage would impose both chemical and physical changes on peat soils that would

collectively have implications for soil abiotic and biotic properties, follow on effects on re-

vegetation efforts and micro- and macrofauna (Hart et al. 2002). To reliable assess the

effects of dam removal and drainage a conceptual model that takes into account seasonal

and yearly influences over upstream channel evolution following dam break would be

useful (Cannatelli and Curran 2012). For example, Channel evolution models (CEMs) are a

useful tool to predict perturbations to the system and channel changes because of

complex hydraulic and sediment transport processes that occur during drainage

(Cannatelli and Curran 2012). For instance, Lafrenz et al. 2011 assessed the effects of dam

dewatering and dewatered soils on downstream upland soils to assess physical and

chemical changes occurring in these soils in North America (Lafrenz, Bean, and Uthman

2011). The results from this study two years since dam removal showed that inundated

soils have higher organic matter percentage, cation exchange capacity, and nitrogen levels

than downstream soils that were not inundated (Lafrenz et al. 2011). Importantly, these

authors note that high levels of nitrogen in the upstream dewatered soils could lead to

colonization of non-native plant species (Lafrenz et al. 2011). Any increase in nutrients

may be significant in the generally oligotrophic systems of southwest Tasmania.

Taken together, dam drainage may release downstream accumulated nutrients from the

Huon-Serpentine Impoundment that would change the chemical soil properties of

peatlands and thus would affect restoration and specifically re-vegetation efforts. One

way to address this is to incorporate transient storage zones for retention of dissolved

nutrients, for example in channel locations where the water flow is naturally slow anyway

(Stanley and Doyle 2002).

Given the exposure of large amounts of bare organic soils, a fire-management plan –

particularly a rapid response to fire suppression would be required to assess the fuel loads

and address the vulnerability of the moorlands and exposed peat to fire risk. An important

factor to consider is the moisture content in the soil which has been recognized as the

main controlling driver of organic soil combustibility (Garlough and Keyes 2011; Schulte et

al. 2019). therefore, wildfire impacts are a serious risk consideration for potential

restoration especially in the context of warming climate (Miller et al. 2019).

Use of peat shading in priority areas could be also considered, especially in north facing

exposed slopes and dry areas. Shading “mulching” material is used to decrease thermal

variation of soil to reduce evaporation (Rochefort and Lode 2006). For example, in boreal

peatland restoration polyethylene plastic cover, greenhouse shading screen and straw

mulch has been widely applied (Rochefort 2001; Rochefort and Lode 2006).

Overwhelming evidence suggests, however, that straw mulching proved to be most

economical and effective in reducing temperature, evaporation loss and improving soil

moisture (Price, Heathwaite, and Baird 2003; Price, Rochefort, and Quinty 1998).

To stabilize and protect large exposed peatlands from erosion following draining rapid

vegetation cover should be established. Additionally, if economically feasible, a

consideration could be given to inoculating soils or/and seed material with microbes to

facilitate establishment of revegetated species (Wild n.d.), especially Acacia and

Eucalyptus that have been shown to grow better/faster when suitable microbes (e.g.

nitrogen fixing bacteria or mycorrhizal fungi) are present (Birnbaum et al. 2017; Pacovsky

et al. 1986; Sprent and Parsons 2000). For example, a recent study aimed at improving the

recovery of litter decomposition and microbial activity in New Zealand peat bogs

compared four different restoration treatments, i.e. 1) direct transfer of intact habitat

“islands”, 2) the addition of peat containing seed vs (3) no seed and 4) a recently mined

peat surface on litter decomposition and microbial activity as compared to the

undisturbed bog (Watts et al. 2008). These authors found that method #1) direct transfer

of intact habitat islands contributed to fastest recovery of litter decomposition and

microbial activity (Watts et al. 2008). However, these authors also stressed that even after

one year, the decomposition and microbial activity were still nowhere near to the levels

recorded in undisturbed bog (Watts et al. 2008).

Effective restoration of peatlands

To restore fully functional peat-accumulating ecosystem three steps need to be

undertaken (in priority order) (Schumann and Joosten 2008):

1. Peatland form and deposit;

2. Hydrological regime;

3. Vegetation/fauna.

In terms of peatland soils, peatland hydrology is central to effective restoration – keeping

peatlands intact or surfaces wetted and vegetation will re-establish. A recent review

article on peatland restoration in Australia and New Zealand highlights the importance of

restoring the hydrological function of peatlands by rehydrating dry and exposed peats via

retaining water, raising water tables and slowing surface and subsurface flows (Clarkson

et al. 2017).

Specifically, hydrological conditions of peatlands are considered one of the fundamental

driving forces in both the formation and degradation of peatlands. Water flow and

storage characteristics change as peatlands develop (Price et al. 2003). In developed

peatlands the accumulating dead peat beds become the predominant substrate for water

flow through wetland. Similarly, the living peat plants form an open structure that allows

the passage of excess water. A positive water balance is the primary requirement for

peatland growth and maintenance. Influx or recharge of water can come from

precipitation, surface runoff from the catchment or groundwater sources. Efflux, or

discharge of water, is usually through runoff downstream, seepage to groundwater or

evapotranspiration (Charman 2002).

Recent investigations have found that the protective peat mat is broken at the perimeter

of the Huon-Serpentine Impoundment by wave action erosion, particularly on the

southern and eastern shores. Erosion is also extensive in the natural drainage paths that

have a higher proportion of sand and are free draining, confirming observations by

Duckett (2001). Thus, targeted stabilization of these peat mats is required to prevent

erosion which would increase sediment loads before vegetation has been established via

revegetation or natural processes. For example, rip rap (also rip-rap, shot rock, rock armor

or rubble) from granite or concrete blocks could be used at the Huon-Serpentine

Impoundment perimeter and identified priority erosion control areas to protect the peat

mat from further erosion. Other options could include rock gabions or sack revetments

(Keown and Oswalt 1984) on prioritised areas deemed at high risk of erosion.

There are many examples of organic soil and peatland restoration in Australia, and

techniques are well established. The pictures below show effective restoration of organic

soils after fire disturbance on the Bogong High Plains in Victoria. Whilst such resource-

intensive treatments are not practical for widespread use, they could be implemented at

targeted risk areas.

Weirs spreading overland flow and capturing sediment and nutrients (2000 and 2012) on past

peatland community; wetland species are returning.

Edge of a peatland burnt to mineral soil – insulation and shading to reduce evapotranspiration

(2012 & 2016) on the Bogong High Plains, Victoria. This community has altered in state to a

grassland/heathland community supporting more shrubs and dryland species. It is unlikely to

recover to a peatland community however, the soils are now stabilised. Photographs: Elaine

Thomas and Anita Wild

Preference for speed and seasonality of draining

The Huon-Serpentine Impoundment has three outlets: McPartlan Pass Canal, Serpentine

Dam and Edgar Dam outlets (Livingston 2001). However, the usefulness of McPartlan Pass

Canal and Edgar Dam outlet is limited, and the major drainage outlet is through

Serpentine Dam. Hydrological models draining the Huon Serpentine Impoundment

indicate that it can be drained in approximately 12 months, however it is not possible to

keep the level behind serpentine Dam below the level of the original Lake Pedder unless

the dams are breached with features such as siphons or spillways (Livingston 2001).

Whilst there are still a lot of uncertainties about how the system would respond to

restoration, it is useful to consider how the rate of dewatering would likely affect the

different values of the lake system. Trying to determine the likely optimum rate of

dewatering for organic soils is complex. Impacts vary greatly depending on the erosion,

geomorphological and physical processes and it would be necessary to model and predict

the cumulative impacts in order to obtain a best prediction to limit impacts. Based on

current knowledge, the following diagram shows the expected optimum rate of dewater

for the relevant processes. The potency and intensity of these processes will also vary

over the timeframe of dewatering as the bathymetry of the impoundment varies,

hydrostatic pressures and gradients fluctuate and geomorphological processes adapt.

Following modeling and further investigations, it could also be possible that the rate could

also be varied at different stages in the process to manage specific risks based on the

shape and slope of the impoundment’s shoreline and geomorphic features or, say, the

pending emergence of the iconic Lake Pedder dune system.

For simplicity, the following table assumes a relatively constant rate of drainage and

shows the optimum rate of dewatering from the minimum practical time of 100 days to a

nominal 24-month period. The table lists the main environmental components, the risk

mitigation aims and the likelihood of achieving them at the different dewatering rates.

The likelihood is colour coded as follows: green represents the optimal or preferred rate

with the highest chance of meeting aim, orange represents sub-optimal rate with less

certainty of meeting the aim and red represents the highest risk rate with the lowest

likelihood of meeting the aim.

Component Risk mitigation aim 100

days

6

mths

12

mths

24

mths

Natural regeneration Maximise natural vegetation

regeneration and expansion

Erosion Minimise wave and wind

erosion risks to shorelines

Minimise slumping and

solifluction erosion of glacial

features

Minimise slumping and

delamination of peat surfaces

Minimise desiccation and

subsequent oxidation of peat

Minimise seepage erosion of

Lake Pedder dune system

Wildfire risk Maximise wetted area from

groundwater and capillary

action

Minimise oxidation of peat

surfaces from desiccation

Seasonality of dewatering

An assessment of the seasonality of dewatering is presented below and represents the

time when the maximum surface area exposed over the dewatering period and assumes

that vegetation cover will establish quickly on exposed surfaces. For example, dark peat

surfaces will absorb heat and dry out more quickly in summer, so it would be best to

reduce the amounts of bare peat exposed in summer if possible; spring is also a time of

high winds, so large areas of shoreline exposed would increase erosion risk.

Risk mitigation aim

Summer Autumn Winter Spring

Minimising wildfire risk

Minimising wind action

erosion risks to shoreline

Minimising desiccation of

peat surface

Minimising extreme peat

surface temperatures

Knowledge gaps, essential knowledge and necessary studies

Fire risk

A recent study in Tasmania has confirmed that the main factor controlling the

combustibility of organic soils is their moisture content and that Tasmanian organic soils

are likely to be combustible in most areas in most summers. This is a key consideration for

the restoration project and would need to be incorporated into the current TWWHA fire

planning resources prepared, reviewed and implemented by the Tasmanian Parks and

Wildlife Service.

This plan would specifically need to address the changes of flammable areas in the

landscape, assess fuel loads and address the vulnerability of the moorlands and exposed

peat to fire. Any plan and the responses would also need to acknowledge the increased

importance of maintaining the peat soils for restoration and the greater vulnerability in

the initial recovery phases when vegetation cover is sparse and soil temperatures high

and moisture content relatively low.

Geomorphological responses to draining

Dam removal methods will profoundly influence geomorphology of the impoundment as

well as the amount of sediment and its effect on downstream channels (Pizzuto 2002). It

will be important to consider strategies to address the sediment load, the timing of dam

removal and engineering considerations upstream and downstream from the dam

(Pizzuto 2002). For example, geomorphological effects upstream from the dam are usually

dominated by the channel formation as it incises into the sediment trapped in the

impoundment (Pizzuto 2002). However, since there is evidence that the sediment levels in

Lake Pedder are rather negligible then the dominating processes upstream from the dam

will be related to deposition and floodplain construction rather than erosion and incision

(Egan 2001; Pizzuto 2002). Whereas downstream from the dam sediment flow is likely to

be temporally variable and considering the large area of Lake Pedder then over time high

sediment accumulation may destroy some geomorphological features in the landscape

such as pools, riffles, alternate bars and armored beds.

GHG emissions risks

Thus far, it is well established from the literature that peatland drainage reduces water

content but increases CO2 production, thus it is important to find optimum drainage level

that minimises CO2 emissions (Norberg et al. 2018). For example, a recent study from UK

showed that increasing the water table from -50 cm to -30 cm lowered CO2 emissions by

31%, while not having any considerable effect on CH4 emissions, however, it did reduce

plant productivity (Matysek et al. 2019). Therefore, for revegetation purposes, water table

manipulations may be needed before and after restoration to facilitate the growing

season and curb the GHG emissions (Matysek et al. 2019). A study from North America

also found that CH4 emissions in peatland were strongly correlated with the water table

and vegetation volume (Mahmood and Strack 2011).

Field studies and modelling to assess the potential GHG (flux of CO2 and CH4) of oxidation

of newly-exposed, long-inundated organosols under the Huon-Serpentine Impoundment

should be undertaken to quantify the potential emissions associated with peat exposure,

subsequent desiccation and potential wildfire impacts.

It is uncertain if use of synthetic fertilisers in restoration treatments would increase

oxidation and subsequent GHG emissions (flux of CO2 and CH4) of peat and organosols as

has been shown in some studies. For example, a study from Finland found using a

greenhouse experiment that the highest N2O fluxes (255 µg per square meter per hour)

occurred after fertilization at high water content and at the beginning of the growing

season (Kettunen et al. 2005). Furthermore, this study also found that N2O fluxes were

higher under doubled CO2 concentrations, which suggests that warming climate and

increasing CO2 in the atmosphere may affect N and C dynamics in peatlands in the future

(Kettunen et al. 2005).

Literature review and glasshouse studies should be undertaken to assess the potential

responses of GHG emissions (flux of CO2 and CH4) of using synthetic fertilisers in

restoration treatments on organosols under the Huon-Serpentine Impoundment quantify

the potential emissions.

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Acknowledgements:

Dr Henrik Wahren kindly provided comment on a draft of this review.

This review is one of a series commissioned by Lake Pedder Restoration Inc. to better understand the

impacts of the full ecological restoration of the original Lake Pedder and surrounding ecosystems in

the Tasmanian Wilderness World Heritage Area. Released October 2020.

For more information go to www. lakepedder.org