Environmental responses to a significant River Murray flood

The Millenium drought was a prolonged period of dry conditions experienced in the Murray-Darling Basin between 1996 and 2010.

The drought conditions had a severe impact on the Murray-Darling Basin, its agriculturally rich areas, communities and natural habitats.

In the CLLMM region, flows from the River Murray were not sufficient to keep the Murray Mouth open, and full-time dredging was needed to maintain the connection of the River Murray, Lower Lakes and Coorong to the sea. This connectivity is critical to move sediment, salt and nutrients out of the Basin, while also enabling animals that need to move between marine and freshwater environments to complete their life cycles.

For First Nations, the drought had a significant impact on cultural values, including the exposure of burial grounds, loss of culturally significant habitat and impacts on important totemic species (known as Ngartji to the Ngarrindjeri).

The Lower Lakes began to dry up as River Murray flows continued to decrease and the drought continued. Low water levels reduced available aquatic habitat for endangered and threatened fish species. Other fish species that migrate between freshwater and marine habitats to breed were unable to complete their life cycles. Suitable habitat and food for native and migratory birds was reduced.

The dry lake beds exposed acid sulfate soils which resulted in large areas of water becoming acidic, and acidic dust being blown onto land and into communities.

The Murray Mouth had to be dredged around-the-clock to ensure salt, nutrients and pollutants could be flushed out of the river system.

Much of the Coorong became too salty for many native plants and animals to survive, with the Coorong South Lagoon becoming five times saltier than the sea.

The drought broke in the Murray-Darling Basin in 2010, and water levels in the Lower Lakes increased again, reconnecting habitats and reducing salinity in some areas. However, the impact of the Millennium Drought across the CLLMM region has been felt for much of the past decade, with some habitats and species yet to fully recover to pre-Millennium Drought conditions.

There have been 4 major Murray-Darling Basin flood events in recorded history occurring in South Australia [1Gigalitre (GL) = 1 billion litres]:

Over the period of the Millennium Drought (1996-2010), flows into South Australia averaged less than 9 GL per day, with lowest flow recorded as 0.7 GL per day on 3 June 2007. In 2022-2023, the flood peaked at 190 GL per day.

The 2022-2023 flood event had a major impact on the aquatic environment in the Lower River Murray. High flow events such as these provide important changes to the environmental conditions that support native fish spawning, bird breeding and plants dispersing.

To assess the environmental response to the 2022-2023 flood, the South Australian Government through the Department for Environment and Water funded a series of research projects with the Goyder Institute for Water Research. The collaborative and multi-disciplinary work brought together researchers from the University of Adelaide, Flinders University and the South Australian Research and Development Institute (SARDI).

The work focused on two key areas:

1. The immediate environmental impact of the flood waters in the Riverland region of South Australia, in particular the Katarapko and Pike floodplain areas.
2. The environmental response to the flood event within the Coorong, Lower Lakes and Murray Mouth region, including the impacts of the flood waters on marine habitats and species.

The research has provided valuable insights into the health and functioning of the Lower River Murray ecosystems in South Australia following the 2022-2023 flood event, informing water management, conservation strategies and decision making.

The 2022-2023 flood event provided much needed water to areas of South Australian floodplain that have not received water for over 60 years. The flood carried rich nutrients, salt and sediment down the Murray-Darling Basin system, through the Lower River Murray and into the CLLMM region.

Ecosystems are very sensitive to changes in both water quality and sediment quality. To assess the immediate environmental impact of the 2022-2023 flood, this research focussed on two aspects that are very important to aquatic ecosystems: salinity and dissolved oxygen.

Salinity is a measure of the salt concentration in water. It can be assessed using various methods, each providing different insights into water chemistry. One common unit for expressing salinity is parts per thousand (ppt), which denotes the amount of salt per thousand grams of water.

• Drinking water is typically <1 ppt
• Sea water is typically around 35 ppt
• The Dead Sea is typically 290 ppt

(35 ppt means about 3.5% of the weight of seawater comes from the dissolved salts)

Another important measurement of salinity is electrical conductivity (EC), which reflects the water’s ability to conduct an electric current. Higher salinity increases water's conductivity because salt ions enhance its conductive properties.

While measuring salinity in parts per thousand (ppt) directly quantifies salt concentration, EC provides an indirect measure of salinity, useful for real-time monitoring and in various scientific and environmental applications. Both methods are crucial for understanding aquatic ecosystems, managing water quality, and studying oceanographic phenomena.

Locations were sampled across the entire Coorong to better understand any changes in the Ruppia Community as a result of the 2022-2023 flood event.

The continued monitoring of the Lower River Murray and CLLMM region is essential to understand long term environmental impacts following flood, drought and climate change.

The high flows removed sand and sediment around the Murray Mouth, widening the mouth of the river. This led to removing the immediate need to dredge for a period of time. Flow through the Murray Mouth is important to flush salt and pollutants from the entire river system and keep water levels and salinity in the Coorong at healthy levels.

During the flood event a plume of freshwater and sediment extended into the Southern Ocean from the Murray Mouth, at least 40 km offshore. The nutrients that were carried into the Southern Ocean resulted in an algal bloom. Algal blooms are caused when higher nutrient loads occur as the algae can grow rapidly when the nutrients are available.

Algal blooms can have negative impacts on the environment, for example when some algae produce toxins that can make people and animals sick; or by using up oxygen in the water resulting in dead zones with no animals and plants because of the low oxygen levels.

However, increased amounts of algae can also have positive environmental effects, for example by providing fuel for animals further up the food chain.

The nutrients that entered the marine environment as the flood waters moved through the Murray Mouth into the Southern Ocean were firstly used by algae which then fuelled higher levels of the marine ecosystem, from microscopic animals (plankton), then small fish such as anchovies, through to predators such as tuna, sharks and birds.

Whilst there were initially some negative visual impacts of the flood waters, such as sediment-living (benthic) marine animals dying due to low salinities and sediment-rich water along the Goolwa and Middleton beaches, these freshwater flows carried with them abundant nutrients and carbon from of the Murray-Darling Basin. These nutrients and carbon provide energy sources for the plants and animals living in the marine environment.

Productivity contained within the flood waters could be observed through the response of carp throughout the Lower River Murray area as described previously, above. A large number of carp passed through the Murray Mouth into the Southern Ocean where, despite the lower-than-normal salinities resulting from the flood waters, the carp and other freshwater species were unable to survive. Thousands of carp washed up on the southern beaches of the Fleurieu Peninsula, an event which was covered in the local media.

The 2022-2023 River Murray flood event in South Australia was the third largest flood event in recorded history. It resulted in wide-ranging, short-term changes in the environment, which impacted on the animals and plants living in the river and its associated floodplains and wetlands.

The immediate environmental response in the Katarapko and Pike floodplain areas included reduced shallow groundwater salinities, with some saline groundwater discharging from the floodplains into the river at specific locations. Dissolved oxygen levels did not reduce to critical levels and any low oxygen events that did occur came into South Australia from further upstream.

In the Coorong, major short-term changes associated with the large volume of freshwater passing through the barrages increased water levels and lagoon connectivity, and reduced salinity. Away from the immediate flow over the barrages, nutrients reduced and sediment quality improved. The flood event produced both positive and negative environmental responses:

Positive environmental outcomes in the Coorong following the 2022-2023 flood event:

• Reduced salinity enabled increased productivity of fish and invertebrates and aquatic plants.
• A greater biodiversity of plants, fish and invertebrates was found throughout the Coorong and many species became more widespread.
• Large numbers of formerly widespread species also returned to the Coorong South Lagoon as connectivity and moderate conditions influenced the South Lagoon.

Negative environmental outcomes in the Coorong following the 2022-2023 flood event

• The direct impact of physical scouring, high water levels and changing water chemistry lead to the loss of some ecosystem functions.
• During periods where salinity change was dramatic, some species were not able to survive the reduction to much lower salinities, changing the composition of species in these areas with saline tolerant species being replaced by species able to tolerate much lower salinities.

Climate change is expected to result in increased frequency and duration of extreme weather events. Understanding how the South Australian Lower River Murray responded to the third highest river flows on record has been critical to help plan for an uncertain future under climate change.

Based on the observed responses to the 2022-2023 flood event, further positive responses to the Coorong ecosystem can be expected if the region experiences further moderate to good freshwater flows from the River Murray in the next couple of years.

The positive environmental responses to the flood event show that the Coorong ecosystem does have signs of resilience. However, the short-term nature of these positive environmental responses highlight the need for ongoing freshwater flows entering the Coorong.

Please see the fact sheets and report available on the Goyder Institute's website:  Outputs – Goyder Institute