Scenario ‘Dry’: assumes Lower Lakes water levels >0.35 m AHD with minor barrage outflows.
During dry conditions, there will be re-connection of fringing Lower Lakes wetlands (if the preceding year was in the ‘extreme dry’ category). Any earthworks installed through pumping and water delivery projects during the ‘extreme dry’ scenario that are preventing wetlands from natural re-connection will need to be removed.
The fishways at Tauwitchere Barrage (small vertical slot, large vertical slot, rock-ramp), Goolwa Barrage (vertical slot) and Hunters Creek (vertical slot) will be opened to allow releases through the fishways for a minimum of 6 months between August— February. This corresponds to a volume of around 30,000 ML/year for fishways alone. Attractant flows (i.e. prioritised at Tauwitchere and Goolwa barrages adjacent to a fishway) will be instated to direct fish to the fishways at Tauwitchere and Goolwa barrages.
Additionally, small releases through the Boundary Creek Barrage will be opened as a last priority if water is available to create additional estuarine conditions within the creek for invertebrate and aquatic plant responses. Releases through the Goolwa, Tauwitchere and Boundary Creek barrages should be calculated to maintain water and salinity levels within the Coorong to promote the growth and reproduction of Ruppia tuberosa. Larger volumes for direct delivery to the Lower Lakes and Coorong will be sought from Environmental Water Holders to maintain lake levels above 0.35 m AHD and to maintain salinity levels in the Lakes and Coorong. A target minimum annual barrage release of 650 GL is the preferred volume for this scenario, a minimum requirement to maintain target Lake Alexandrina salinity levels.
If water levels below Lock 1 remain low during this scenario, it may be necessary to continue pumping to maintain refuge habitats, as described in the ‘extreme dry’ scenario. Larger volumes of environmental water delivered to the site however should negate this need.
The ‘median’ water availability scenario assumes South Australia is receiving at least entitlement flows across the border (i.e. 1,850 GL/year), Additional Dilution Flow and potentially unregulated flow. This corresponds to a lake level of >0.35 m AHD, with all fringing wetlands inundated, and barrage outflows of more than 650 GL/year. All previously-managed threatened fish and fringing wetland sites have reconnected and pumping is no-longer required, and the threatened species that they hosted are now free to recolonise to other parts of the lower lakes.
Fishways should be operated year round to provide connection with the lakes and Coorong. Additionally, median flows through Boundary Creek barrage will be maintained to create estuarine conditions within the creek for invertebrate and aquatic plant outcomes.
Large volumes of environmental water will be requested from Commonwealth water-holders to enhance the small releases already possible under entitlement, Additional Dilution Flows (and potentially unregulated flow) conditions, especially during spring and summer. Larger volumes will enhance and prolong estuarine conditions in the Murray Mouth and North Lagoon regions. Larger volumes will also help to maintain an open Murray Mouth.
The following table describes the priority order of operation as available outflows increase (DFW 2010) (Table ).