What have the recent floods meant for the Coorong, Lower Lakes and Murray Mouth?

Recent floods flowing down the Murray River have well and truly reached the Coorong, Lower Lakes and Mouth of the Murray. All 593 openings across 5 barrages, which for the last 80 years have controlled flows at the Mouth of the Murray, were fully open over the summer, with freshwater, saltwater, fish and other creatures flowing between the ocean and Lower Lakes. Now, as we reach the end of this flooding season, flows are dropping, and the barrages are starting to close — it’s time to explore what the effect has been on the Lower Murray ecosystem.

The ecological status of the Coorong, Lower Lakes and Mouth of the Murray River is a complex Anthropocene issue. The term ‘Anthropocene’ refers to today’s current geological period (or epoch). Its reference to ‘anthropo’, the Greek word for ‘human being’, relates to the immense impacts that human beings have had, and are continuing to have, on our environment. The Lower Lakes of the Murray River are no exception. Over the last century, these complex ecosystems have been modified significantly by human impacts and water management infrastructure, such as upstream dams and downstream barrages.

History of the Coorong

The Coorong, Lower Lakes and Mouth of the Murray River (see Figure 1 below) are a unique part of the South Australian landscape, and the dramatic end point of the Murray-Darling Basin. As a vast combination of lakes, rivers and wetlands, the management and care of these water bodies affect the millions of freshwater animals and plants that inhabit them. The Coorong is well known for its importance both nationally and internationally to waterbirds, but less so for its highly diverse fish population, which is unique within the Murray-Darling Basin. Many of the fish endemic to this area are species of national and state conservation significance and are important to the Traditional Owners of the site, the Ngarrindjeri.

In recognition of the ecological significance of the area, the Coorong and the Lower Lakes are listed as a Ramsar site. Ramsar sites are wetlands of international ecological importance, and are internationally protected under the Ramsar Convention, which aims to prevent the loss of wetlands globally and endorse the implementation of best-practice management.

The Murray barrages were constructed between 1935 and 1940 to keep salt water from entering the lower reaches of the system as demand across the Basin increased. Prior to the barrages, sea water could move up to 250km upstream.

Figure 1. Map of barrages in the Lower Lakes and pipelines along the Murray River, South Australia. Source: MDBA.

Flows over the barrages from the Lower Lakes into the Coorong. Photo: South Australian Research and Development Institute (SARDI).

The barrages transformed the lower Murray River from an estuarine, brackish system that fluctuated with inputs of freshwater and saltwater tidal movements, to a largely stable, freshwater system.

There are five Lower Lakes barrages, all of which were designed to preserve freshwater during droughts when upstream regulation removed too much water. It was imagined that preventing saltwater intrusion into the Lakes’ system would ensure a reliable water source and reduce salinity levels. In addition to the barrages, large scale river regulation and water storage infrastructure was built throughout the Murray River system, further preventing fish passage and water flow.

We now know that the ecological impacts of the barrages have been significant. The Murray barrages were not originally designed with fish passage in mind and have hindered diadromous species from completing their lifecycles. Water flowing swiftly through open barrage gates presents a physical barrier to the upstream migration of fish. Similarly, when closed, they create a barrier for fish to move between the Murray-Darling Basin and the Coorong or Southern Ocean.

Diadromous species require migration between freshwater and marine environments to complete their life cycle. Within the Coorong and Lakes, five species of diadromous fish can be found: the Short-headed lamprey, (Mordacia mordax), Pouched lamprey (Geotria australis), Common galaxias (Galaxias maculatus), Southern shortfin eel (Anguilla australis) and Congolli (Pseudaphritis urvillii).

Adult Pouched Lamprey. Image credit: SARDI.

Adult Pouched Lamprey. Image credit: South Australian Research and Development Institute (SARDI).

Congoli. Image credit: Ivor Stuart.

Prior to the barrages, European carp were not present in the Coorong and Lower Lakes as they cannot reproduce in brackish water. With fresher water, however, carp have been able to inhabit the system and thrive in large infestations, much as they have done right across the Basin. In addition, the plants, birds, fish, and bugs that relied on the fluctuations between salt and freshwater have declined, with some species, like the water plant Ruppia Tuberosa, becoming endangered.

The endangered Ruppia Tuberosa. Image credit: MDBA.

Pelicans in the Coorong. Source: Jochen Kaempf

Pelicans in the Coorong. Image credit: Jochen Kaempf.

Over the past decade, environmental water flows have been essential in keeping plants and animals alive that may have otherwise disappeared during the Millenium Drought. Environmental water has also been critical in enabling and maintaining salt exports from the Murray River out through the Murray Mouth — recent studies by the Lower Murray Flow-Monitoring, Evaluation and Research (Flow-MER) team found in a preliminary analysis that 1.2 million tons of salt was flushed out of the system through the Murray Mouth between July-December 2022, during the high flows. Prior to the barrages, this would have occurred naturally as the saltwater would gradually dilute as it worked its way up the river. For the migration of diadromous species, fishways have been installed in some of the barrages, enabling them to migrate once again. Early signs show that this is occurring, but what about the future?

What does this summer’s flooding change?

This summer, the Coorong, Lower Lakes and Murray Mouth experienced high flows as a result of major flooding occurring throughout the Murray Darling Basin. For the first time in many years, dredges were not needed to keep the Murray Mouth open. In this way, nature is essentially making water management decisions for us, as the barrages cannot hold back the water that is currently in the system. In recent weeks, the flooding has reduced; the flow rate into South Australia on 14th February was 48.5 gigalitres per day, compared to the peak of 185.9 gigalitres per day at Christmas time.

The studies conducted by the Flow-MER team found that the high flows were beneficial for plants, animals and the ecosystem generally, including upstream, with the flows assisting the generation and maintenance of floodplain vegetation like River red gums and Blackbox gums, and the germination of seeds across the lower Murray floodplains. High flows are also important to support the breeding cycle of waterbirds and fish. Native fish, such as Golden perch, rely on high water flows to trigger spawning, whilst Murray cod showed improved recruitment in fast flowing habitats (see the Lower Murray River Quarterly Newsletter for more information).

The mixing of flood and sea water temporarily reinstated estuarine habitat; however, the debate continues whether this is something we need to enable in the future by reintroducing tidal movements into the system. Some believe freshwater security should be given priority. Moreover, if the Coorong and Lower Lakes were to become estuarine, there are concerns that the area will lose its Ramsar status, as this was granted on the basis of it being a freshwater system. What is certain is that this debate will continue for some time…

Construction of fish passage structure at the Coorong. Photo credit: Environment South Australia

Construction of fish passage structure at the Coorong. Image credit: Environment South Australia.

Members of the National Fish Strategy meet with commercial fishers in the Coorong. Photo credit: Fern Hames

Members of the National Fish Strategy meet with commercial fishers in the Coorong. Image credit: Fern Hames.

So, what does this mean for native fish?

Some native fish will thrive after these floods, and others will be negatively impacted. One major concern is mass fish deaths resulting from floodwaters inundated with decomposing material, like leaf litter, creating low oxygen levels. Fish need oxygenated water to survive, and with low oxygen levels looking likely, native fish species may be prone to major fish kills. This has unfortunately already occurred upstream of the Lakes in the Darling River at Menindee, where stagnant deoxygenated flood waters were met with high temperatures in March 2023.

Nature may continue to make some decisions for us, as rain continues to fall in parts of the Murray-Darling Basin, while other areas are seeing drier conditions. With the La Niña weather pattern coming to an end this year, it’s unlikely that flows will top the barrages again. However, if it does, there could be beneficial outcomes for the plants and animals that need estuarine conditions, such as our native Diadromous species, as the freshwater and seawater meet. We will be keen to see what ecological outcomes have resulted from the recent flooding as water flows recede at the Mouth of the Murray.

We know that outcomes for the Coorong, Lower Lakes and Murray Mouth will continue to be negotiated with environmental, economic and cultural requirements in consideration. Sometimes, however, nature may make the decision for us, and we will have to wait to see what happens next.