Lower Lakes barrages
The Lower Lakes barrages are located between Lake Alexandrina and the Coorong, where the River Murray completes its 2,530 km journey and flows into the Great Australian Bight.
From the earliest days of settlement along the lower reaches of the river, there were strong representations from landowners for the construction of barrages. The primary reason for their construction was to keep the water fresh in the lower reaches of the River Murray, as well as Lake Albert and Lake Alexandrina.
Before the Lower Lakes barrages were built, tidal effects and the intrusion of seawater occurred during periods of low flow into the Lower Lakes and in the River Murray, up to 250 km upstream from its mouth. Although this effect was a normal part of the variation of the lower river system, the impacts were further intensified by increased regulation and upstream diversion of water that decreased periods of high flow along the lower Murray, and reduced overall flows to the Lower Lakes.
In 1931, the then River Murray Commission decided — after extensive investigation — to construct 5 barrages to help manage lake levels and improve water quality in the lower Murray and Lower Lakes system. Work on the Barrages commenced in 1935 and was completed in 1940. South Australia's Engineering and Water Supply Department undertook the work, with the costs shared equally by the governments of South Australia, Victoria, New South Wales and the Commonwealth.
Major tributaries of the River Murray join upstream of the barrages and, with only 8 km of the River Murray downstream of the Goolwa Barrage, the catchment of the barrages is almost the entire Murray–Darling Basin. Water from the upper Murray and Darling catchments takes many weeks or months to flow all the way to the Lower Lakes.
Local inflow can occur to the Lower Lakes following rain on the eastern Mt Lofty Ranges or on the lakes themselves. This rain can generate flows along the relatively short Bremmer River, Finniss River and Currency Creek, along with other streams that enter Lake Alexandrina from the west. However, the contribution of these tributaries is very small compared with the inflow from the Murray.
Water level fluctuations
The water level upstream of the barrages has a nominal Full Supply Level of 0.75 m above sea level. However, the level is varied through operations that must take account of considerable seasonal inflow and evaporation variability, as well as the impacts of low flows during droughts that can limit the amount of water that flows into the Coorong. The barrages cause an increase in water level of approximately 50 cm as far upstream as Lock 1 at Blanchetown (274 km upstream).
Generally, the level of the Lower Lakes fluctuates on an annual cycle. This is driven by higher extractions and evaporation through summer and autumn, and lower extractions and decreased evaporation through winter into spring. As a result, the level begins to increase from late winter to late spring or early summer. In years of full South Australian entitlement, the aim is to bring the lakes to at least the nominal Full Supply Level prior to hot weather beginning. From summer through to autumn, the level diminishes and operations aim to prevent levels falling to 0.35 m (above sea level). However, this is sometimes unavoidable in sustained periods of drought and low inflow, as was the case during the millenium drought.
During the most severe periods of this drought (particularly during 2008 and 2009), the level fell below sea level causing considerable community hardship and environmental problems.
There are 5 barrages:
- Goolwa Barrage
- Mundoo Barrage
- Boundary Creek Barrage
- Ewe Island
- Tauwitchere Barrage.
Stoplogs — beams that are stacked on top of each other in slots in the barrages — are typically used in conjunction with a small number of radial gates and automatic gates in some places to control the water level or flow rate. During periods of low river flow, stoplogs and gates stop the flow of water to help maintain lake levels. When high inflows arrive, they are removed to allow water to pass into the Coorong and out to sea. To prevent salt water entering and maintain optimal upstream water levels, ongoing regulation is required for intermediate flows.
Goolwa Barrage is the deepest of the structures. It is constructed on fine sand and silt, and is founded on timber piles and sheet piling up to 14 m deep. The Barrage contains a 30.5 m by 6.1 m lock chamber.
Ewe Island and Tauwitchere Barrages are built on a calcareous (calcium carbonate) reef, across wide and shallow channels with earth embankments at each end. At Tauwitchere, a 13.7 m by 3.8 m lock is provided for use by fishing boats.
The Mundoo Barrage and Boundary Creek Barrage are the shortest of the Barrages. Founded on a limestone reef, these structures have no provision to allow for navigation.
Around 486 ha of the Sir Richard Peninsula — a sand spit to the west of the Murray Mouth — has been reclaimed to minimise sand drift entering the river between the barrages and the Murray Mouth, and to maintain the protecting peninsula between the Goolwa Barrage and the sea.
The purposes of the barrages are to:
- reduce salinity levels in the lower reaches of the River Murray and the associated lakes
- limit 'reverse flows' of seawater into the lakes during storms, high swells and high tides
- stabilise and raise the river level to provide irrigation (by gravitation) along the reclaimed river flats between Wellington and Mannum
- manage releases to control water levels, water quality and environmental outcomes in the lower lakes, the Coorong and the Murray Mouth
- concentrate releases to the ocean to a small area by scouring a channel for navigation during low flows
- provide a supply of water that can be pumped to Adelaide and the south-eastern corner of South Australia
- enable social, recreational and cultural uses of the Lower Lakes.
While the Lower Lakes barrages are MDBA assets, they are operated in collaboration with South Australian water agencies.