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By Dr Francis Chiew 12 May 2020 4 min read

The Murray-Darling River ends its lengthy 2,500 km journey by flowing into Lake Alexandrina and discharging through the narrow Murray Mouth into the sea.

It’s a challenging journey in one of the world’s most variable climate regions, where the inflow from catchments in a wet year can be more than 20 times greater that the inflow in a dry year.

And it’s a system facing profound future challenges to adapt to a drier, hotter climate.

During wet years, there’s generally enough water for everyone. However, during dry periods, which we have encountered more frequently in recent decades, everyone, from communities, farmers and the environment, suffers.

Water use becomes more contested, with often polarising debates. So it’s unsurprising that there’s interest in the water that reaches the Coorong, Lower Lakes and Murray Mouth (CLLMM) at the end of the system in South Australia.

In response, the Murray-Darling Basin Authority (MDBA) initiated an independent review of the relevant science relating to the management of the Lower Lakes. I chaired a panel of five experts to conduct this science review, steered by the Advisory Committee on Social, Economic and Environmental Sciences (ACSEES) for the MDBA.

The region is one of six icon sites in the Murray-Darling Basin and is internationally recognised as a wetlands for migratory birds. Image: Tanya Doody.

The CLLMM, an important environmental, social and economic national asset, has been extensively studied over the past 20 years. It’s one of six icon sites in the Murray-Darling Basin and is internationally recognised as a Ramsar-listed wetland.

The region supports endangered migratory birds, threatened wildlife and rare plants, and has significant importance for the Ngarrindjeri People. Consistent with other parts of the Basin, the CLLMM is a human-modified environment that contains important natural elements.

It’s also been the focus of literally hundreds of research papers and reports covering aspects ranging from geomorphology, hydrodynamics, hydroclimate, ecology, salinity regime, and water management.

The Independent review panel reviewed these studies and spoke to the many technical experts, to synthesise and summarise the knowledge in a report, released Tuesday, May 12.

The water management of the CLLMM has been informed by this extensive body of knowledge, together with ongoing monitoring of flow, salinity and water levels, and hydrological and hydrodynamic modelling of the system.

The review’s first term of reference addressed the salinity history of the Lower Lakes. This is partly driven by the debate in the palaeoecological science community, which analyses preserved diatoms (or algae) and other remains in sediment cores.

The salinity through time is indicated by the abundance of diatoms at the different sediment depths, hence age. These records generally suggest a decline in marine influence over the past 8,000 years, especially the past 1,000 years when the sea level reached its current position.

The weight of evidence points to the main body of the Lower Lakes being largely fresh prior to European settlement, with moderate tidal influence and incursion of sea water during periods of low Murray River inflow. This is supported by the palaeoecological records, water balance estimates, hydrological and hydrodynamic modelling, and traditional knowledge of the Ngarrindjeri People and anecdotal evidence of early explorers and colonists.

We know from observed data and hydrological modelling that the average annual inflow from the Murray River to Lake Alexandrina before development is more than 13,000 gigalitres each year. This large volume of water would fill the lakes on average eight times a year. Upstream development has reduced the river inflow by about half on average, resulting in more frequent incursion of seawater into the Lower Lakes. The barrages were built in 1940 in response to these changes, isolating the Coorong and the sea from the Lower Lakes.

One of the four barrages that were built in the region in 1940, after upstream development reduced the river inflow, resulting in more frequent incursion of seawater into the Lower Lakes. (Image: Kane Aldridge)

The second term of reference addressed the implications of removing the barrages and the potential water savings. Due to the human-induced reduction in inflows, without the barrages the freshwater values in the Lower Lakes can’t be maintained. This would significantly change the ecological character of the Ramsar-listed site, which is a wetland of international importance. This will also impact traditional owner values and other socio-economic values that are reliant on a healthy CLLMM system.

Saltwater ingression upstream into the lower Murray River during prolonged dry periods would also impact water offtake for irrigation and for Adelaide and regional towns, although engineering solutions can overcome this.

Under the Basin Plan, there’s more water for the environment across the Basin, and more water is also reaching the Lower Lakes. Environmental water is managed to achieve a range of outcomes, where water is reused down the river, to sustain a healthy system across the entire Basin and beyond to the coastal ecosystem. Much of the water reaching the Lower Lakes has achieved many benefits along the way.

Monitoring and modelling show the additional water flowing in the Murray River secured under the Basin Plan has enhanced the ecological outcomes in the CLLMM, through building resilience in the system and providing some freshwater flow during dry years. The environmental water to the CLLMM is essential to meet the objectives informed by science and modelling and envisaged under the Basin Plan.

However, the looming challenge for the CLLMM, as well as the long-term future of the Basin, is climate change.

This is addressed in the review’s third term of reference. Along with the need to adapt to a drying climate and reduced runoff, sea level rise will alter the hydrodynamics of the Coorong and Murray Mouth, as well as cause more seawater to flow into the Lower Lakes. Evaporation from the Lakes would be higher.

We need to determine what management options can be implemented that will maintain the balance of the CLLMM. That requires research and a future focus on the adaptation challenges for the Lower Lakes and the entire Basin system.

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