Lower Lakes, Coorong and Murray Mouth


Scenario ‘wet’: assumes South Australia entitlement flow + Additional Dilution Flow with lake levels >0.5 m AHD



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Scenario ‘wet’: assumes South Australia entitlement flow + Additional Dilution Flow with lake levels >0.5 m AHD.


The ‘wet’ water availability scenario assumes South Australia is receiving entitlement flows across the border (i.e. 1,850 GL/yr), plus Additional Dilution Flow and unregulated flows. This corresponds to a lake level of >0.5 m AHD, and a ‘1 in 3 year lake level operating regime’ (refer to Figure 10) with all fringing Lower Lakes wetlands inundated. The major difference to the ‘median’ scenario is the addition of considerable unregulated flows on top of entitlement flows, which allows for sustained lake levels over summer and large volumes of water released through the barrages (>1,000 GL/yr). While also having the ability to provide extra ‘attractant flows’ to guide fish to the barrage fishways, large release volumes also provide the opportunity to export salt from the Lower Lakes, and can provide extended estuarine conditions into the Coorong and Murray Mouth region.

Targeted releases should be underpinned by the following principles (DFW 2010):

• a flexible, adaptive and consultative response to actual impacts of specific decisions

• as per the ‘Median’ water availability scenario, all fishways to be opened year-round

• releases to be informed by both real-time monitoring and observation and previous monitoring outputs, with direction from the barrage operations committee

• slower releases initially to ensure the ecological system adjacent to the barrages is not ‘shocked’

• release of ‘trickle flows’ for longer periods, which are likely to be more beneficial ecologically that larger flows for shorter periods

• the release hydrograph should ideally mimic the natural hydrograph to the best extent possible

• potential impacts on mouth openness should be considered keeping in mind the ‘rule of thumb’ that releases should be split in a ratio of around one to two (1:2) between Tauwitchere and Goolwa barrages

• when operating the barrages to draw salt out of the Coorong South Lagoon, large outflows should be directed through Tauwitchere Barrage to push freshwater into the Coorong, followed by a recession of outflows with a corresponding drop in Coorong levels. Barrage outflows during flow recession should be targeted through Goolwa barrage, to assist receding water from the Coorong to be drawn into the estuary, and to maintain an open Murray Mouth.

After it has been ascertained that fishways can remain open for a year, and additional flows are available for release, the following staged approach described in Table should be followed. This process allows for a gradual ramping up and down of barrage release volumes consistent with the shape of the hydrograph, and will need to be further developed as confidence increases regarding timing of inflows and release volumes. Additional barrage bays should be opened adjacent to fishways, so that fish can follow the plume of water and be attracted towards the fishway entrance. However, under relatively high volumes of outflow, the Murray Mouth estuary region is often ‘saturated’ with fresh water, i.e. there is very little head difference between the fresh and estuarine sides of the barrages. Under these conditions it is assumed that most species of fish can pass through barrage bays with relative ease, and hence aren’t reliant on using the barrage fishways (B. Zampatti, pers. comm., 31 January 2011). The fishways are most effective under lower release conditions.

Large volumes of environmental water sourced from Commonwealth water-holders during ‘wet’ water scenarios are still extremely beneficial to the ecology of the system. Additional flows are critical to maintaining salinities in the Coorong South Lagoon below 100 ppt, the threshold for the small-mouthed hardyhead, a current keystone species (Aldridge et al. 2008). Recent modelling (Lester et al. 2011a) has determined that large volumes in any one year not only have an immediate benefit to salinities during that year, but also in future years. The optimal flow scenarios of 6,000 GL once in every three years and 10,000 GL once in every seven years will be typified by a ‘wet’ scenario.

The high flows experienced in 2010–11 have resulted in a rapid change of conditions coming out of the drought, and a sudden change to very high water levels in both the Lower Lakes and Coorong. A significant drop in wader numbers, even in the context of the declines in the last five years, has been recorded in the summer of 2010–11 (D. Paton, pers. comm., 28 March 2011), likely due to the saturation of the Coorong and inundation of mudflat habitat. Under a wet scenario, mudflat habitat may be created in managed fringing wetlands of the Lower Lakes (i.e. those wetlands with flow regulators) to drop water levels and temporarily expose mudflats for waders. However, very few fringing lakes wetlands are fitted with flow regulators (see Table : Description of managed fringing wetlands of Lakes Alexandrina and Albert) and many require upgrades and maintenance.

Priority_list_of_larger_barrage_releases,_likely_impacts_for_fish_and_all_ecological_benefits,_following_all_fishways_being_opened'>Table : Priority list of larger barrage releases, likely impacts for fish and all ecological benefits, following all fishways being opened



Priority

Structure

Fish

First Step Ecological Objective

1

Adjacent bays (2) next to Goolwa vertical slot fishway

Extra flow adjacent to fishway will attract a greater number of fish from downstream

FR, OM, BH

2

Adjacent bays (2) next to Tauwitchere Rock Ramp and Large Vertical Slot

Extra flow will attract a greater number of fish from downstream

FR, OM, BH

3

Boundary Creek flows 1 bay

Open barrage gates may provide passage for larger fish species

FR, OM, BH

4

Mundoo Barrage flows 1 bay

Large bodied fish may be able to find passage through barrage bay.

FR, OM, BH

5

Ewe Island Barrage flows 1 bay

Large bodied fish may be able to find passage through barrage bay.

FR, OM, BH

6

Goolwa Barrage flows (2-8 bays)

Large bodied fish may be able to find passage through barrage bay. Attractant flow towards fishway.

FR, OM, BH

7

Tauwitchere Barrage flows (2-6 bays)

Large bodied fish may be able to find passage through barrage bay. Attractant flow towards fishway.

FR, OM, BH

8

Boundary Creek (all gates open)

Large bodied fish may be able to find passage through barrage bay. Attractant flow towards fishway.

FR, OM

9

Progressively open Goolwa, Tauwitchere and Ewe Island Barrages – split flow releases depending on management objective

Large bodied fish may be able to find passage through barrage bay. Attractant flow toward fishway.

FR, OM

10

Mundoo Barrage (all gates open)

Large bodied fish may be able to find passage through barrage bay.

FR, OM






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