What were the effects of the Millennium Drought on the Lower Lakes and Murray Mouth?

From late 1996 to 2010 the region experienced a significant drought known as the Millennium Drought.

River Murray flows reached historically low levels due to the combined impacts of extreme drought and decades of over-allocation across the Murray–Darling Basin.

As a result, inflows into the Lower Lakes were not able to replenish evaporative losses and average lake levels dropped to unprecedented lows.

Connectivity (both hydrological and ecological) was lost between Lake Alexandrina, the Goolwa Channel and Lake Albert.

The region teetered on the verge of complete environmental collapse.

Disconnection of the wetlands, exposure of up to 20,000 hectares of acid sulfate soils and increased salinity levels occurred with parts of the Coorong becoming five times saltier than the sea.

Wetlands dried out and they, along with exposed lakebed areas, were colonised by introduced terrestrial species.

Much of the region’s submerged aquatic habitat, particularly in local channels, was significantly reduced or eliminated.

Diadromous fish species, the life-cycles of which require movement between freshwater, estuarine and marine habitats, were unable to pass through the barrages to complete their life cycle due to the low water levels in Lake Alexandrina.

What measures did South Australia undertake to protect the environment?

  • Purchased water to help maintain water levels in the lakes.
  • Permanent wetlands were temporarily closed to prevent high salinity and nutrient rich water from draining back into the river.
  • Undertook additional monitoring for salinity, acid sulfate soils and riverine ecology.
  • A range of projects around the Lower Lakes resulted in:
  • o temporary barriers being built at Narrung, Goolwa and Currency Creek to manage the acid sulfate soil risk and provide an ecological refuge
  • o rescued a population of Murray-Hardy Head and kept them on land until the conditions improved.
    o 2029 tonnes of limestone applied by air
    o 1978 tonnes of limestone applied as barriers
    o approximately 6,580 ha of land revegetated either by machine seeding (air or land) and hand planting
    o 130,000 seedlings being planted on higher ground around lakes.
  • Contracted the University of Western Australia to undertake acidification prediction studies, using bio-geochemical modelling to determine acidification triggers in the Lower Lakes.
  • Commenced the Coorong, Lower Lakes and Murray Mouth Program and the Riverine Recovery Program.

What measures did South Australia undertake to support critical human water needs?

  • Implemented water conservation through restrictions, rebates on low flow devices and rainwater tanks.
  • Implemented contingency options by the temporary closure of wetlands to save evaporative losses.
  • Developed a State Water Security Plan that diversified Adelaide’s water supplies through construction of the Adelaide Desalination Plant, waste water recycling and stormwater harvesting schemes.
  • Modified the major River Murray pumping offtakes for Adelaide’s water supply, constructed new pipelines around the Lower Lakes and fast tracked the country water quality improvement program.
  • Pumped water into storages in the Adelaide Hills, where the evaporation is less to provide a buffer against potential water quality issues.
  • Secured a water reserve each year to provide for critical human water needs and built additional water reserves through market purchase.
  • Provided assistance to the Riverland horticultural industry by purchasing water to keep permanent plantings alive.
  • Extended footings on ferries to allow them to operate at lower river heights and fast tracked environmental approvals for dredging of the Murray Mouth to enable navigation and to provide water for the Coorong.
  • Undertook investigations into levee bank cracking and failure.
  • Undertook an Environmental Impact Statement for a temporary weir near Pomanda Island (Wellington Weir).

Why don't we open the barrages during dry times?

Acidification in the Lower Lakes was a significant risk during the Millennium Drought, but letting in seawater would have had an even more serious impact and would irreversibly damage this Ramsar site.

Opening the barrages and introducing seawater was considered and investigations were conducted to fulfill statutory requirements. It was found that the introduction of seawater would likely lead to extreme increases in salinity and heavy metals, creating a hypersaline toxic ecosystems. A healthy, estuarine/marine community would not establish.

From the knowledge gained through the Millennium Drought, the “Drought Emergency Framework for Lakes Alexandrina and Albert” was developed and agreed by Ministerial Council. The Emergency Framework provides decision-making steps and potential management actions. The consideration of selected options needs to be made against a range of environmental, social and economic factors that would be applicable at that time.

Why doesn’t South Australia construct a permanent weir at Wellington?

A weir at Pomanda Island was investigated during the Millennium Drought as an option for maintaining water levels between Wellington and Lock 1. The proposal was for a temporary structure and was designed for a nominal three year life. An EIS assessment process was undertaken in the event that it was needed, but the weir was not pursued as water levels were reinstated before the criteria for its construction were met.

It is not possible to construct a permanent weir. The sediments are so soft and bedrock so deep that even a temporary weir at the site would continuously sink and would require regular reinstatement at significant cost.

With a weir in place, a minimum through flow would be required to avoid major water quality issues upstream of the weir. It was found that the volume required would actually be sufficient to maintain suitable water levels in the Lower Lakes, removing any necessity for a weir (or for the introduction of seawater).

Can SA Water’s offtakes be moved above Lock 1?

Apart from being a lengthy and costly process, it would be expected to require additional water from New South Wales and Victoria to ensure there was sufficient water in the weirpool above Lock 1 at peak times so the required quantities could be extracted from multiple extraction points at once.

There are also stock and domestic, irrigation and industrial water users below Lock 1 that would be required to be supplied.