Good and bad news for MDB

A series of reports reviewing the predictions of the MDB Guide to the Draft Plan was released by the South Australian Government at the end of July.

The MDB Guide proposed extra flow volumes of 3000, 3500 or 4000 gigalitres/year Basin-wide.  Hydrological modelling by South Australia’s Goyder Institute predicted that those augmented flows would be down to an 1880–2500 extra GL per year by the time the River Murray reaches the SA border, after meeting water demands upstream.

The extra flow will clearly benefit ecological assets in the Lower Murray and at the Murray Mouth. However, actual benefits will ultimately depend heavily on how (timing, size, etc) environmental flows are delivered, and on how the upstream storages and other environmental assets are managed.

Theoretically, the extra flow coming into SA could mean more frequent wetting for floodplain wetlands and Red Gum (Eucalyptus camaldulensis) communities. The bad news is that even with 4000 GL extra annually (equivalent to 2507 GL/year at the border), the timing, size and duration of river flows are unlikely to let these flows meet all environmental water requirements (EWRs).

Further, with environmental flows being released more often upstream, the reviews suggest there is less chance of storages being able to fill to the point where they could release really big flows that could inundate floodplain wetlands and plant communities associated with Black Box (E. largiflorens).  Black Box mostly occupies the highest points on the floodplains of Murray-Darling Basin rivers, and the trees hence need big floods to reach them –  floods  that these days may be seen once every 10-20 years or less.

Near the Murray Mouth, Coorong and Lower Lakes (CLLMM), the reviews conclude there will potentially be enough extra water volume to meet EWRs sometimes, depending on how the flows are delivered.

The good news is that all three of the proposed scenarios, however, are likely to improve water quality in the River Murray and the Lower Lakes. Risks of blue-green algal blooms, acidification of the Lower Lakes and high (1400 EC) salinity in the river are all reduced (in the modelling) by the potential extra flows. However, salt is still likely to accumulate in wetlands and on floodplains.

Back in the real world, sampling data from the SA Murray and Lower Lakes over recent weeks shows that salinity has dropped considerably because of flushing by high flows between mid-2010 and early 2011. Salinity in the river, measured in 57 profiles sampling at 1 m intervals from surface to bed, is reported as having halved since the height of the drought. The water quality is reported in July as ‘excellent’ overall.  Near the mouth, at the Lower Lakes in April, Lake Albert’s salinity was at 5000 EC compared to 20,000 EC in January 2010 and may fall further, and salinity in L. Alexandrina was below 500 EC after reaching 8000 EC in early 2010.

The April report from the SA EPA and Dept of Environment and Natural Resources states:

“Salinity remains stable and at low levels across Lake Alexandrina* due to dilution from river inflows and export of salt through the barrages. Salinity levels still remain elevated in Lake Albert* compared to historical values. pH and alkalinity continue to remain satisfactory at all sites in the main lake water bodies. Low levels of acidity (in the form of soluble metals) have been found on several areas on the lake margins that turned acidic during the 2007–2009 drought. This is not currently cause for concern as alkalinity and pH are at satisfactory levels in these areas.”

Read the Goyder reports here.

*  Lake Alexandrina and Lake Albert are two large, regulated lakes which form part of the historic estuarine lake system at the oceanic outlet of the Murray-Darling Basin in South Australia.  The region also includes the Ramsar-listed Coorong wetlands.

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