Lower Lakes, Coorong and Murray Mouth



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4. WATER DELIVERY

4.1 Prioritisation of water requirements


The Living Murray Annual Watering Plan, developed by the EWG includes a flexible decision framework to guide prioritisation of environmental watering actions, icon site environmental watering proposals developed by the icon site managers with jurisdictional agencies, water availability forecasts and management objectives for water resource scenarios (see Table ). Further detail on these scenarios is provided in Appendix E).

Throughout the year the EWG recommends environmental watering actions, based on The Living Murray Annual Environmental Watering Plan and the volume available in The Living Murray environmental water portfolio, to the MDBA office for approval. Annual water planning and implementation is responsive to changing water resource conditions, opportunities and environmental priorities throughout the season and from year to year. Consequently the water requirements identified for the LLCMM icon site will be prioritised along with the requirements for all other TLM icon sites. The Living Murray portfolio alone is not expected to be sufficient to meet the full LLCMM icon site environmental water requirements and there will be opportunities for other environmental water holders to contribute water to the site as well.


‘Catastrophic extreme dry’


The Murray–Darling Basin recently experienced the worst drought in recorded history (2002–10) and combined with the over allocation and extraction of water from the system, resulted in reduced freshwater inflows to the Lakes and Coorong. With limited end-of-system flows, lake levels receded to below -1.0 m AHD, exposing large areas of previously saturated sulfidic sediments that formed acid sulfate soils upon drying.

The situation that the Lower Lakes and Coorong faced during this drought was unprecedented and is not considered in the icon site plan to be ‘extreme dry’ rather ‘catastrophic extreme dry’. If conditions are to return to what was experienced during this period then large volumes of water for direct delivery to the Lower Lakes will be sought from environmental water holders to maintain lake levels above 0.0 m AHD, noting that other icon sites will also require water and that a prioritisation process will occur. Lake levels need to be maintained to prevent the formation of acid sulfate soils and subsequent environmental collapse. Exact volumes required will depend on modelling, condition forecasting and water availability, but could be several hundred gigalitres. Refer to Appendix B (Table : Required volume (GL) to raise water levels in the weir pool from Lock 1 to the barrages by 10 cm from the corresponding starting water levelfor a description of volumes required to raise the weir pool from Lock 1 to the barrages (including the Lower Lakes) in 10 cm increments).

If this situation is repeated, programs to prevent large-scale acidification, potentially including the (re)construction of blocking banks at Narrung and Clayton; bioremediation programs and large-scale limestone dosing, may be necessary as an alternative to flows. These actions cost millions of dollars, and are less effective at mitigating acid sulphate soils compared to re-inundation through fresh water delivery. The Drought Emergency Framework for Lakes Alexandrina and Albert (MDBA unpublished3) outlines acidification-related hazards and trigger-levels associated with receding lake levels as well as potential management actions.

However, recent modelling to inform the Basin Plan (MDBA 2012) indicates that the initial volume to be recovered (2,750 GL) would be sufficient to keep lake levels above the 0.0 m AHD threshold if the same drought conditions were repeated, subject to water delivery as per the modelling assumptions. Once the Basin Plan is fully implemented, the likelihood of a return to a ‘catastrophic extreme dry’ event is limited.

Refer to Appendix E for a description of required management activities in the LLCMM icon site under each water availability scenario.

Table : Ecological objectives under different water availability scenarios






Extreme dry

Dry

Median

Wet

Ecological watering objectives

Avoid irretrievable loss of key environmental assets

Ensure priority river reaches and wetlands have maintained their basic functions

Ecological health of priority river reaches and wetlands have been protected or improved

Improve the health and resilience of aquatic ecosystems

Management objectives

  • Avoid critical loss of species,
    communities and ecosystems

  • Maintain key refuges

  • Avoid irretrievable damage or catastrophic events

  • Avoid exposure of acid sulfate soils

  • Maintain river-Lower Lakes-Murray Mouth-Coorong functioning though likely with reduced reproductive capacity

  • Maintain key functions of high priority wetlands

  • Manage within dry -spell tolerances

  • Support connectivity between sites

  • Enable growth, reproduction and large-scale recruitment for a diverse range of flora and fauna

  • Promote higher floodplain-river Lower Lakes-Murray Mouth-Coorong connectivity

  • Support high flow Lower Lakes, Murray Mouth, Coorong river and floodplain functional processes

Volumes

  • Up to 9 GL through barrage fishways (over 2 months)

  • <0.5 GL for threatened fish sites

  • 1-2 GL for fringing wetlands

  • Water to maintain levels above 0 m AHD

  • ~1,800 ML/day through barrages and fishways (~650 GL/yr)

  • >1,800ML/day through barrages and fishways (>650GL/yr)

  • >2,800 ML/day through barrages and fishways (>1,000 GL/yr)

Example priority actions for LLCMM icon site

  • Pumping to threatened fish refugia

  • Pumping to fringing lakes wetlands

  • Barrage fishways operational for minimum 2 months per year, alternatively use Goolwa boat lock if lake levels too low

  • Divert water from SE drainage system into Coorong

  • Murray Mouth dredging

  • Maintain lake levels >0.0 mAHD via delivery of environmental water

  • Barrage fishways operational for minimum 6 months

  • Small attractant flows from Goolwa and Tauwitchere barrages next to fishways

  • Small release from Boundary Creek for estuarine conditions

  • Larger volumes to the Lakes to operate water levels > 0.35 m AHD and to prime the site for recovery

  • Murray Mouth dredging if required

  • Barrage fishways operational year-round

  • Medium attractant flows from Goolwa and Tauwitchere barrages

  • Release from Boundary Creek for estuarine conditions

  • Each barrage has some bays open

  • Larger volumes to the Lakes allow for fluctuating levels

  • Maintain water levels and salinity in Coorong to promote Ruppia

  • Maintain estuarine habitats to facilitate fish recruitment

  • Barrage fishways operational year-round

  • Large flows through all barrages

  • Each barrage has most bays open

  • Flows directed to keep Murray Mouth open, to restore large extent of estuarine habitats in the Coorong and to reduce salinities in the South Lagoon

  • Managed fringing lakes wetlands fitted with flow control structures operated to create mudflat habitat



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