Lower Lakes, Coorong and Murray Mouth



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3.2 Water requirements


Water level is a key environmental attribute that determines the availability of physical habitat for fish, birds, plants and other aquatic life. Salinity levels, their degree of variation and rate of change, affect the aquatic species present and their “health”. Barrage outflows control the water level and influence salinity of Lakes Alexandrina and Albert and provide connectivity to the Coorong. Barrage outflows are the major “driver” of Murray Mouth openness and these, in combination, affect estuarine habitats, water levels and water quality of the entire Coorong.

The River Murray system is the major source of water for barrage outflows, although the Eastern Mount Lofty Ranges tributaries (i.e. Finniss, Currency, Angas and Bremer) also contribute. Additional flows from the South East drainage system, through Morella Basin/Salt Creek into the South Lagoon of the Coorong, are complementary to barrage outflows in controlling water level and salinity (though are much smaller volumes, are of varying salinity and have only a localised effect).


3.2.1 Lakes Alexandrina and Albert


In order to meet the icon site ecological targets, the Lower Lakes require environmental flows to achieve the following (Lester et al. 2011b; Muller 2010; Heneker 2010; Pollino 2011):

  • sufficient volumes to maintain lake levels above 0.0 m AHD to prevent exposure of sulfidic sediments (refer to Pollino 2011; MDBC 2008a)

  • sufficient volumes to vary lake levels seasonally between 0.35 m AHD and 0.75 m AHD annually (Figure ), with higher lake levels of between 0.5 m AHD and 0.83 m AHD every third year (Figure )

  • barrage outflows sufficient to maintain electrical conductivity in Lake Alexandrina below 1,000 µS/cm 95% of the time and below 1,500 µS/cm 100% of the time

  • a total average annual barrage outflow of 2,000 GL per three year rolling period (i.e. not less than 6,000 GL over three years) and not less than 650 GL in any one of the three years will ensure the above target salinities are met in the Lower Lakes

  • the river flows to the sea annually – higher security to low flow regime.

Fluctuating lake levels across the suggested range (Muller 2010) will maintain habitats for threatened fish (refer Target F2), and maintain and enhance aquatic vegetation (refer Target V3). Under a varied lake operating regime, shoreline mudflats are exposed and re-inundated (Target M1), providing suitable conditions for benthic invertebrates (Target I1), an important food source for migratory waders (Target B1). This range of lake levels also ensures operation of most fishways is achievable year-round, as well as providing greater connectivity between the Lakes and the Coorong via barrage releases (W3). If inflow conditions only allow fishways to operate for part of the year, timing should be prioritised for mid-winter and mid-summer, to optimise diadromous fish movement (Target F1). Barrage releases will promote an export of salt from the Lakes and the River Murray system as a whole, freshening in the Coorong (Target W1), and allowing the maintenance of an open Murray Mouth (Targets W2 and W4) (Table ).
this figure shows the target ‘operating envelope’ for lakes alexandrina and albert at an annual return interval of 1 (every year), showing upper and lower limits.

Figure : Proposed ideal operating envelope for Lakes Alexandrina and Albert at an Annual Return Interval of 1 (every year), showing upper and lower limits

The operating ranges described in Figure and Figure are primarily for the benefit of fringing vegetation and the prevention of lakeshore erosion. However, at the lower lake levels (e.g. 0.35 m AHD) consideration must be made of the impact of reduced access to water on the operation of barrage fishways, reverse barrage flows, saltwater intrusion and irrigator offtakes. Lowering the lakes to the minimum recommended level should only be implemented if higher flows are forecast and can reinstate lake levels. Therefore consideration of these factors must be made before intentionally reducing lake levels to the minimum proposed operating level. It is important to consider this an operating range, hence lake levels may only rarely achieve the extremes of the range.

this figure shows the target ‘operating envelope’ for lakes alexandrina and albert at an annual return interval of 3 (every 3 years), showing upper and lower limits.

Figure : Proposed ideal operating envelope for the Lower Lakes at an Annual Return Interval of 3 (once every three years), showing upper and lower limits.


3.2.2 Coorong and Murray Mouth


In order to meet the icon site ecological targets, the Coorong and Murray Mouth require environmental flows to achieve the following (as per Lester et al. 2011b):

  • There should be no years in which no flow passes over the barrages. The absolute minimum annual barrage flow should be between 50 and 120 GL.

  • Over any two-year period, at least 600 GL should be released to the Coorong to prevent certainty that South Lagoon salinity thresholds (maximum of 117 g L-1 (ppt) as described in Lester et al. 2011b) being exceeded.

  • At least 2,500 GL over two years as a minimum target (95% of the time) to prevent the Coorong from existing in a degraded state across the entire region.

Preferred flow requirements (Lester et al. 2011b):

  • Flows of at least 6,000 GL/y are recommended at least every 5 years (preferably every three years) and flows of at least 10,000 GL/y recommended every 17 years (preferably every 7 years).

Achieving these levels, flow and salinity targets are required to maximise the likelihood of achievement of icon site objectives. Greater description of these flow targets is provided in Appendix B.

Delivery of adequate freshwater through barrages releases will ensure an open Murray Mouth (Targets W2, W4) which in turn will maintain variable salinities in the Coorong North Lagoon and Murray Mouth Estuary (W1), improving condition for Ruppia tuberosa (Target V2), and deliver food sources to the Goolwa Cockle (Target I2). Maintaining estuarine habitats and improving the estuarine nature of the North Lagoon will support small-mouthed hardyhead (Target F3), important commercial fish species (Target F4), Ruppia megacarpa (Target V1) which will maintain or improve bird populations (Target B1).

A target salinity of 117 g L-1 (ppt) for the Coorong is described above, and optimum salinities for many species of 60-100 ppt are described below in Table : Environmental water requirements for the Murray Mouth and Coorong to achieve icon site ecological targets. These values reflect what modelling (Webster 2007, Lester et al. 2011a) suggests is likely to be achieved by returning flows. It is acknowledged that the optimum salinity for many species (e.g. small-mouthed hardyhead) lies at the lower end of the 60-100 ppt salinity range described in Table : Environmental water requirements for the Murray Mouth and Coorong to achieve icon site ecological targets. It is highly desirable that salinity in the Coorong, particularly the South Lagoon is reduced to support the regions key fauna and flora. However, it needs to be recognised that the salinity in the Southern Lagoon cannot be reduced to preEuropean conditions (Krull et al. 2009) over a short (i.e. 5 year) timescale, taking into account current water diversion rates and the relatively small volume of environmental water available. These limitations highlight the importance of investigating the ‘South Lagoon Pumping’ and ‘Upper South East Flows Restoration to the Coorong’ projects, however, Lester et al. (2011b) stress that the River Murray must continue to be the primary source of fresh water to the Coorong to maintain ecological condition. Refer to section 4.3.2 for further detail on these proposed environmental water delivery works.
Table : Environmental water requirements for Lakes Alexandrina and Albert to achieve icon site ecological targets

TLM Objective (M/F/B)

Target

Biota

Optimum salinity*

Level (m AHD)

Frequency (yrs)

Annual Volume (GL)

Duration and Timing

B

Maintain or improve bird populations in the Lower Lakes

Waders

fresh - brackish

0.35 - 0.7 0.5 - 0.85

1 in 1

1 in 3


min 650 GL/yr barrage outflow#

As per hydrographs

(Figs 5 &6)



Fish-eating birds, Water fowl (herbivorous)

< 1,000 EC

0.35 - 0.7 0.5 - 0.8

1 in 1

1 in 3


min 650 GL/yr barrage outflow#

As per hydrographs

(Figs 5 &6)



M & F

Maintain or improve recruitment success of diadromous fish in the Lower Lakes (and Coorong)

Fish requiring both marine & freshwater habitats

fresh - brackish

>0.5 m AHD for fishway releases

1 in 1

minimum outflow of 52 GL/y fishway operation +attractant flows

Fishways run for 12 months

F

Maintain or improve recruitment success of endangered fish species in the Lower Lakes

Murray hardyhead

brackish

0.35 - 0.7 0.5 - 0.8

1 in 1

1 in 3


min 650 GL/yr barrage outflow#

As per hydrographs

(Figs 5 &6)



Southern pygmy perch, Yarra pygmy perch

< 1,000 EC

0.35 - 0.7 0.5 - 0.8

1 in 1

1 in 3


min 650 GL/yr barrage outflow#

As per hydrographs

(Figs 5 &6)



O, F & B

Maintain or improve invertebrate populations in mudflats (Lakes)

Mudflat invertebrates

fresh - brackish

0.35 - 0.7 0.5 - 0.8

1 in 1

1 in 3


min 650 GL/yr barrage outflow#

As per hydrographs

(Figs 5 &6)



Aquatic invertebrates

< 1,000 EC

0.35 - 0.7 0.5 - 0.8

1 in 1

1 in 3


min 650 GL/yr barrage outflow#

As per hydrographs

(Figs 5 &6)



B

Facilitate frequent changes in exposure and submergence of mudflats (Lakes)

Mudflat invertebrates

N/A

0.35 - 0.7 0.5 - 0.8

1 in 1

1 in 3


N/A

As per hydrographs

(Figs 5 &6)



B

Maintain habitable sediment conditions

Mudflat invertebrates

N/A

0.35 - 0.7 0.5 - 0.8

1 in 1

1 in 3


min 650 GL/yr barrage outflow#

As per hydrographs

(Figs 5 &6)



F & B

Maintain or improve aquatic and littoral vegetation in the Lower Lakes

Freshwater aquatic vegetation

< 1,000 EC

0.35 - 0.7 0.5 - 0.8

1 in 1

1 in 3


min 650 GL/yr barrage outflow#

As per hydrographs

(Figs 5 &6)



F

Maximise fish passage connectivity between the Lower Lakes and Coorong

Fish requiring both marine & freshwater habitats

N/A

>0.5 m AHD for fishway releases

1 in 1

minimum outflow of 52 GL/y fishway operation + attractant flows

Fishways run for 12 months

TLM objectives: M refers to open Murray Mouth, B refers to enhanced migratory bird habitat and F refers to more frequent estuarine fish recruitment.

*Salinity ranges: Fresh 0 – 800 µS/cm, Brackish (Oligohaline) 800 – 8000 µS/cm. (USGS 2011).

#A minimum barrage outflow of 650 GL is dependent on larger outflows in subsequent years (averaging 2,000GL over 3 years), as described above.



Table : Environmental water requirements for the Murray Mouth and Coorong to achieve icon site ecological targets


TLM Objective (M/F/B)

Target

Biota

Optimum salinity (ppt)*

Preferred Annual Volume (GL)

Frequency

Duration & Timing

B

Maintain or improve bird populations in the Coorong

Waders, Fish-eating birds, Water fowl (herbivorous)

60 - 100 ppt South Lagoon

6,000

10,000


1 in 3

1 in 7


As per hydrographs

(Fig 7)


M, F

Maintain or improve recruitment success of diadromous fish in the Coorong

Fish requiring both marine and freshwater habitats

fresh - marine

52 GL fishway operation (min) + attractant flows

1 in 1

Year-round fishway operation

F

Provide optimum conditions to improve recruitment success of small-mouthed hardyhead in the South Lagoon

Small-mouthed hardyhead

60 - 100 ppt South Lagoon

6,000

10,000


1 in 3 1 in 7

As per hydrographs

(Fig 7)


F

Maintain or improve populations of black bream, greenback flounder and mulloway in the Coorong

Black bream, greenback flounder, mulloway

fresh – marine: maintenance of a salinity gradient is the key requirement

6,000

10,000


1 in 3

1 in 7 


As per hydrographs

(Fig 7)


M, F, B

Maintain or improve invertebrate populations in mudflats in the Coorong

Mudflat and subtidal invertebrates, waders

brackish - marine

6,000

10,000


1 in 3

1 in 7 


As per hydrographs

(Fig 7)


M, B

Facilitate frequent changes in exposure and submergence of mudflats in the Coorong

Mudflat invertebrates, waders

brackish - marine

Murray Mouth open (730-1,090 GL)

1 in 1

Diurnal tide ratios met

B

Maintain habitable sediment conditions

Mudflat invertebrates, waders

N/A

N/A

N/A 

N/A

F

Maximise fish passage connectivity between the Lower Lakes and Coorong

Fish requiring both marine and freshwater habitats, e.g. congolli

N/A

52 GL fishway operation (min) + attractant flows

1 in 1

Year-round fishway operation

F, M

Maximise fish passage connectivity between the Coorong and the sea

Fish that move between the Coorong & ocean, e.g. diadromous fish, mulloway, yelloweye mullet, black bream & greenback flounder.

N/A

Murray Mouth open (730-1,090 GL)

1 in 1

Diurnal tide ratios met

M

Maintain a permanent Murray Mouth opening through freshwater outflows with adequate tidal variations to improve water quality and maximise connectivity

Fish that move between the Coorong & ocean, e.g. diadromous fish, mulloway, yelloweye mullet, black bream & greenback flounder.

N/A

Murray Mouth open (730-1,090GL)

1 in 1

Diurnal tide ratios met

F, B, M

Maintain or improve Ruppia megacarpa colonisation and reproduction

Ruppia megacarpa

0 - 19 ppt North Lagoon for recruitment. Further investigation of optimum recruitment conditions and transplantation options also to be investigated

6,000

10,000


1 in 3

1 in 7


As per hydrographs

(Fig 7)


F, B, M

Maintain or improve Ruppia tuberosa colonisation and reproduction

Ruppia tuberosa

60 - 100 ppt South Lagoon

6,000

10,000


1 in 3

1 in 7


As per hydrographs

(Fig 7)


F, B, M

Establish and maintain a variable salinity regime with >30% of area below sea water salinity concentrations in estuary and North Lagoon

Ruppia megacarpa, diadromous fish, black bream, mulloway, yelloweye mullet and greenback flounder, benthic invertebrates.

< 35 ppt in 30% of area of estuary and North Lagoon

 modelling required

 modelling required

 modelling required

TLM objectives: M refers to open Murray Mouth, B refers to enhanced migratory bird habitat and F refers to more frequent estuarine fish recruitment.

3.2.3 Barrage flow delivery patterns and operations


While the annual volume delivered through the barrages is paramount to achieving the icon site’s objectives and targets, the pattern or seasonality of water delivery is even more critical for keystone species of the Coorong. Modelling has indicated that longer, sustained barrage outflows have greater benefits to Coorong salinity than short, pulsed releases (Lester et al. 2011a). Barrage flows should increase from early spring to peak over the early summer months to alleviate evaporative losses in the Coorong. This will maintain water levels in the Coorong during this period, inundating mudflats for enough time to allow Ruppia tuberosa to reproduce (Paton & Bailey 2010b). This ‘spring pulse’ of outflows will also provide recruitment cues for a number of other species in the estuary and Coorong. Following this, a gradual reduction in barrage flows from mid-summer will then provide more exposure of mudflats, and provide feeding habitat for migratory waders. This seasonal barrage release pattern, developed by J Higham (DEWNR) also provides for water exchange and salinity reduction in the Coorong South Lagoon. This pattern of delivery can be achieved over various annual water availability scenarios (Figure ). It should be noted that none of the volumes associated with Figure can be achieved from entitlement flow alone; considerable inflows in the form of environmental water (TLM and/or Commonwealth Environmental Water Holder (CEWH)) and/or unregulated flow is needed.
this figure shows the optimal barrage release rates and seasonal timing for various inflow scenarios

Figure : Hypothetical optimal timing of barrage releases for various annual flow scenarios

The difficulties in maintaining lake levels and barrage releases within a certain operating envelope per month must be highlighted. SA Water, in collaboration with the MDBA and South Australian government (DEWNR) engage in weekly barrage operating teleconferences to determine barrage opening and closing strategies to achieve the required lake level and flow targets. Water availability and tidal conditions (refer Appendix C) play a major role in the ability to operate the system, and day-to-day management decisions are often required to optimally manage barrage operations. Factors determining short-term (daily-weekly) barrage operations typically include salinity, weather, tides, fishway operations and knowledge of flow. SA Water, who manage day-to-day barrage operations will continue to liaise and consult on best operation practice for the barrages and strive to be proactive in their operation of the barrages to achieve maximal ecological outcomes and minimise inconvenience for the local community. This collaborative cross-agency management approach must continue if the proposed operations are to be achieved.

Barrage releases will occur in a strategic fashion depending on management objectives. For example, the bulk of flow would be directed through Goolwa Barrage to maintain an open Murray Mouth, whereas opening Tauwitchere Barrage, particularly at the southern end, would be primarily used for managing salinity in the Coorong.


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