Environmental responses to a significant River Murray flood
New research and knowledge highlights how the environment of the Lower River Murray responded to the 2022-2023 flood event.
The Murray-Darling Basin
The Murray-Darling Basin is Australia's largest river system flowing through over 1,000,000 km 2 across five states and territories: Queensland, New South Wales, the Australian Capital Territory, Victoria and South Australia.
A total of 23 rivers flow within the Basin, with the two main rivers being the River Murray and the Darling River. The Darling River forms in Queensland while the River Murray begins in the Snowy Mountains in Victoria.
Large rainfall events and floods as well as prolonged dry periods and drought are a part of the Basin's natural cycle. Climate change is expected to change the frequency, intensity and duration of extreme events within the Basin.
About 2.6 million people live within the Basin, including 50 different First Nations, and the river systems are of significant social, cultural, environment and economic importance.
Approximately 40% of Australia's agricultural produce comes from within the Basin, including 100% of rice, 74% grapes and 30% of dairy produce.
The Basin is home to 16 internationally significant wetlands, 35 endangered species and 120 species of native and migratory birds.
The Murray and Darling Rivers join at Wentworth in New South Wales, flowing for a further 100km before it reaches the South Australian border.
The River Murray in South Australia
Within South Australia, the River Murray flows for approximately 640 km from the border to the sea, and its associated floodplains and wetlands supports an estimated 50 fish species and 2,000 native plant species. There are three Ramsar-listed wetlands of International Importance within South Australia: The Riverland Ramsar Site (incorporating Chowilla floodplain and Lindsay-Walpolla Islands); Banrock Station wetland complex; and the Coorong, Lake Alexandrina and Albert wetland.
The River Murray is an important water source for South Australia, supporting agriculture, industry, recreation and tourism, and urban areas. It provides a critical economic resource while also being of significant cultural and spiritual importance to First Nations peoples that have lived around the River Murray and its wetlands for many thousands of years.
The Coorong Lower Lakes and Murray Mouth
The Coorong, Lower Lakes and Murray Mouth (CLLMM) in South Australia is the only location in the Murray-Darling Basin where the river meets the sea. The River Murray flows into two large lakes (Alexandrina and Albert) before flowing into the Coorong and through the Murray Mouth into the Southern Ocean. Stretching 110 km southeast of the Murray Mouth, the Coorong is a relatively shallow (∼1.3 m deep) and narrow (∼2 km wide), lagoonal system. The name Coorong is derived from the Ngarrindjeri word Kurangk, which means 'long narrow neck'.
In the 1930s, a series of five barrages were constructed between the Coorong and Lake Alexandrina to stop saltwater incursions into the Lower Lakes. These barrages are operated to maintain water levels in Lake Alexandrina and control freshwater releases into the Coorong and out of the Murray Mouth.
The CLLMM region supports a rich diversity of plants and animals within ecosystems that include freshwater streams and lakes, estuarine, marine and hypersaline habitats.
It is a culturally significant area for the Ngarrindjeri Nation and the First Nations of the South East, and is recognised as a Ramsar Wetland of International Importance.
The Millenium Drought
Rainfall across Australia from 1 November 2001 to 31 October 2009 during the Millennium Drought. Rainfall within the Murray-Darling Basin ranged between below average to lowest on record during this period (source: Australian Bureau of Meteorology).
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).
1996
The Millenium Drought began.
2002
Murray Mouth almost closed, full-time dredging commenced
2002 - 2007
Lower than average rainfall within the Murray-Darling Basin.
2010-2011
Heavy rainfall in the Basin marks the end of the Millennium Drought as flows return throughout the system.
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.
Impact of the Millenium Drought seen at Goolwa.
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.
Graphs showing 1. Lake Alexandrina's water level during the Millenium drought, plunging to 1.1 metres below sea level in April 2009 and 2. River Murray flows into South Australia.
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.
River Murray Floods in South Australia
There have been 4 major Murray-Darling Basin flood events in recorded history occurring in South Australia [1Gigalitre (GL) = 1 billion litres]:
1931
Flows across the South Australian border peaked at 210 GL per day.
1956
Largest flood in recorded history. Flows across the South Australian border peaked at 341 GL per day.
1974
Flows across the South Australian border peaked at 182 GL per day.
2022 - 2023
Third highest flood ever recorded in South Australia, peaking at 190 GL per day.
The 2022-2023 River Murray Flood Event
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).
Timelapse of the Riverland flood waters in 2022-2023
The work focused on two key areas:
- The immediate environmental impact of the flood waters in the Riverland region of South Australia, in particular the Katarapko and Pike floodplain areas.
- 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.
Measuring the Environmental Impact of the 2022-2023 Flood Event
Assessing the immediate environmental impact of the flood event
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.
Sampling sites along the Riverland including the Chowilla, Pike and Katarapko floodplains, to measure nutrient levels and oxygen levels in the water.
Salinity
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.
Dissolved oxygen and blackwater
Most aquatic organisms require oxygen to survive. Oxygen dissolves in water through wind and agitation of the water surface, as well as from aquatic plants which produce oxygen as a by-product of photosynthesis.
The amount of dissolved oxygen in water can be very important for aquatic organisms as when levels become too low, many organisms cannot survive.
Oxygen that is dissolved in the water is measured as both saturation (%) and concentration (mg/L). The higher the percentage saturation and the higher the concentration, the more oxygen there is in the water.
'Blackwater' is a term used when high levels of organic material and tannins in a river discolour the water making it appear black. The water can then become very low in oxygen (hypoxic) as bacteria in the water eat the organic matter and consume oxygen.
Blackwater events can often occur during or after a flood event as flood waters enter the floodplain where organic material and tannins may have accumulated for some time since the last time the floodplain was wet. Increased bacterial activity can then use up the oxygen in the water, creating blackwater which can flow downstream, killing aquatic organisms and can result in significant fish kills.
Water quality following the 2022-2023 flood event
Salinity
A special type of surveying method called geophysics was used to examine the salinity of shallow groundwater within Katarapko Creek and the River Murray channel. This method examines the conductivity of the water under the ground to give an indication of salinity - the lower the electrical conductivity (or EC), the lower the salinity.
Surveys conducted along Katarapko Creek and the River Murray at Bookpurnong showed significant changes in subsurface conductivity within the riverbed between previous surveys (2015 for Katarapko and 2019 for River Murray) and February 2023. This means that the flood event resulted in lower salinities in the riverbed and shallow groundwater areas of the floodplain at these monitoring locations.
However, results also showed that there are discrete zones along the river reaches that are receiving more saline groundwater from the adjacent floodplain aquifers - i.e. more saline groundwater was moving from the floodplain and discharging into the river system during the 2022-2023 flood event.
Despite the flood event reducing salinity in the shallow groundwater close to the river, there was still considerable saline groundwater beneath the floodplains which was discharging to the river. Several natural and or managed flood events would need to occur to 'flush' and reduce the salt loads that moved towards the river.
Dissolved Oxygen
In February 2023, just after the peak of the flood had passed the South Australian border, dissolved oxygen was maintained at satisfactory levels (>90% saturation) in nearly all sampling sites across the large Chowilla, Pike and Katarapko floodplains and associated river reaches. These conditions were suitable for maintaining the health of fish and invertebrate (e.g. yabbies) populations.
However, comparing the February 2023 flood response data to data from continuous dissolved oxygen monitoring stations over a longer time period showed that lower dissolved oxygen concentrations were present as the flood was rising in late 2022. These hypoxic conditions were likely driven by low dissolved oxygen River Murray water entering South Australia from upstream.
The response of carp to the 2022-2023 River Murray flood event
Carp are an invasive and widespread pest species throughout the Murray-Darling Basin. Inundated floodplains are the favoured spawning and nursery habitats for carp.
Following the 2022-2023 flood event, there was a notable breeding response from carp throughout the South Australian River Murray. Based on long-term datasets from river and wetland habitats, in autumn 2023, carp were present in the greatest abundance in the past 20 years.
While the carp response was widespread, the greatest abundances were detected in the Riverland and Lower Lakes regions.
The majority of individuals were young-of-year spawned during the flood, but a substantial portion were 1 year old fish that had been spawned during a previous high flow event in 2021-2022.
A study of adult carp movement, using special acoustic tags, was ongoing during the 2022-2023 flood event. There were extensive movements of carp between river and floodplain habitats, and several instances of long-distance movements in both upstream (e.g. from Lock 6 to Menindee on the Darling River) and downstream (e.g. from Lock 4 to downstream of Lock 1) directions.
The environmental response in the Coorong to the 2022-2023 flood event
The Coorong
The Coorong is a shallow and wide estuarine lagoonal system that stretches for 110 km along the coast of South Australia. It has a natural salinity gradient ranging from freshwater close to Lake Alexandrina where flows pass through the barrages, to hypersaline in the Coorong South Lagoon. During the Millennium Drought, salinity in the South Lagoon was five times that of seawater.
Freshwater flows from the River Murray help maintain an open Murray Mouth connecting the River Murray to the sea and allowing the export of salt, nutrients and sediment from the Murray-Darling Basin, and keeping salinity lower in the Coorong.
Measuring the environmental response of the Coorong to the 2022-2023 flood event
Assessing the environmental response of the Coorong had a different focus to assessing the immediate environmental response to the 2022-2023 flood event.
Investigations in the Coorong focussed on:
- Salinity changes throughout the Coorong
- Sediment conditions, including nutrients, organic content and the presence of monosulfidic black oozes (see below)
- Responses of fish, macroinvertebrates (bugs in the mud) and seagrasses.
The impact of the flood on the marine environment in terms of water quality and impacts on key species of marine animals was also investigated.
Salinity
During the period of the 2022-2023 flood event, salinities in the Coorong were reduced from a long-term average of around 55 ppt (1998-2020) in the North Lagoon to around 10 ppt, and from around 100 ppt (pre-2022) in the South Lagoon to around 60 ppt. These changes in salinity had an impact on the communities of organisms living in the Coorong, as different animals and plants can tolerate different salinity levels as well as how quickly salinity may change.
Sediment quality
Monosulfidic black oozes (MBOs) are a form of soil/sediment type typically found in waterways when specific environmental conditions exist.
They are black gel-like materials that are enriched in monosulfides, predominantly iron monosulfide (FeS), but also other compounds including hydrogen sulfide (H 2 S).
Monosulfidic black ooze (MBO) or monosulfidic material forms under low oxygen (anoxic) conditions, where oxygen is unavailable for bacteria to breakdown organic matter (such as in sediments and/or under of algal mats).
High levels of MBO occurred following the 2022-23 flood waters when increased algal blooms in the Coorong died, and bacteria used oxygen to decompose the organic carbon to form carbon dioxide (CO 2 ), gaining energy through this process.
From winter to late spring 2023 there was visible evidence of improved sediment conditions, with an ‘oxic’ zone (oxygen present, indicated by light brown sediment colour), at most sampling locations along the Coorong North and South Lagoons. Small amounts of Monosulfidic Black Ooze was observed at these locations. In general, lower organic carbon and total nutrient levels were observed in the sediment in response to the 2022-2023 flood event.
By summer 2024, a lowering of total nutrients in the water column occurred (due to flushing from the flood) along with increased uptake of nutrients in the sediment by different organisms. Overall, sediment quality improved as a result of the 2022-2023 flood event.
Nutrients such as phosphorus and nitrogen (in the form of nitrate, nitrite and ammonium) are very important for aquatic environments as they fuel plant and algae growth, and they can be stored in sediments. If nutrient levels are very high, then under certain conditions, algal blooms can occur.
Measuring the concentrations of nutrients within the water present within the sediment itself (porewater) showed that porewater phosphate was highest at a sampling location downstream of the Mundoo Barrage. This was potentially due to higher inputs of nutrients in the floodwater from the Murray-Darling Basin.
Following the 2022-2023 flood event, nitrate was found in sediment from many parts of the Coorong system, including the South Lagoon. This finding for the South Lagoon is very positive, as nitrate, an important nutrient associated with healthy sediment nutrient cycling processes, was generally not found in the South Lagoon prior to the flood event.
This presence of nitrate is consistent with increased activity (bioturbation) of macroinvertebrates (bugs) in the sediment, along with sediment oxygenation such as that provided by the roots of aquatic plants.
Another important nutrient, ammonium, increased in the South Lagoon which is consistent with previous findings. Both ammonium and sulfide were higher in November 2023, potentially due to the flood effects having receded and higher temperatures resulting in more organic matter decomposition and sulfide production.
Reponses of Fish in the Coorong
Following the 2022-23 flood event, fish species richness (the number of species) increased substantially in Autumn 2023 in the Coorong South Lagoon to nine species, the highest observed since 2007. The number of species present in the Coorong North Lagoon after the 2022-2023 flood event reduced to 15 in comparison to the peak of 18 species observed in response to high flows that occurred in 2021- 2022. Despite this slight reduction, it was still the second highest species richness recorded in this region since monitoring began in 2007.
In autumn 2023 in the Murray Estuary, fish species numbers declined considerably following the 2022-2023 flood event, mainly due to the reduction of marine, marine/estuarine and marine-estuarine opportunistic species, noting that salinity came close to that of freshwater in this estuarine region during the flood event.
By spring 2023, fish species richness remained stable, and had increased slightly in the South Lagoon. Sandy sprat was the most abundant species throughout the Coorong, followed by smallmouth hardyhead. These two small-bodied fish species provide import prey for fish-feeding birds and also larger fish.
The 2022-2023 flood event reduced salinity levels in the Coorong and increased estuarine habitat and food resources, particularly in the North and South Lagoons. This had an overall positive effect on fish populations in the Coorong.
Responses of macroinvertebrates in the Coorong
Macroinvertebrates (worms, cockles, and other critters that live in or on the sediment) are very important “ecosystem engineers” in that they can change the physical properties of habitats that they are present in. They are also important food sources for many birds and fish.
Many macroinvertebrate species have been absent from the South Lagoon due to the high salinities (hypersalinities) typically present in that region.
Following the 2022-2023 flood event some species of macroinvertebrates recolonised into the South Lagoon as salinities reduced to levels that were able to be tolerated by a wider range of species. The diversity and abundance of macroinvertebrates were highest in the North Lagoon, providing a source for colonisation of the South Lagoon.
In autumn and winter 2023, almost no macroinvertebrates were seen in samples at the Murray Mouth region. Usually, the Murray Mouth region has diverse and abundant macroinvertebrate communities.
During winter 2023, some macroinvertebrates (a bivalve, Spisula trigonella, and a snail, Salinator sp.) had established part way into the South Lagoon where they are not usually present, suggesting that the conditions in these areas had become more favourable for these species.
A type of worm, Capitella spp. were observed in the South Lagoon, where they had also occurred before the flood event. Chironomid larvae (a type of fly larvae) are often the only macroinvertebrates present in the South Lagoon in recent history, and these were observed at several sites in the South Lagoon following the 2022-2023 flood event.
By spring 2023, sites in the South Lagoon had some small worms (Capitella), snails and bivalves present.
By Summer 2023-2024, salinity in the Murray Mouth region ranged between freshwater to brackish (between freshwater and seawater salinity), and some macroinvertebrates re-appeared, but very few molluscs were seen. Salinities in the North Lagoon during this time ranged from brackish to hypersaline and this region appeared to contain the highest diversity and abundance of macroinvertebrates. This included several size classes of the bivalves Hiatula alba and Spisula trigonella, indicating that reproduction and recruitment was occurring.
The South Lagoon had returned to hypersaline conditions by summer 2023-2024, but several macroinvertebrates, including the worm Capitella and the bivalve Spisula trigonella were recorded, as well as chironomid (fly) larvae typical for this lagoon.
Seagrasses
Seagrasses are aquatic flowering plants that live underwater. They have a root and rhizome system, produce flowers and set seeds. They provide many benefits to the ecosystem, including: being an important food sources for many organisms; removing nutrients from the water and sediment; storing carbon by removing carbon dioxide from the water; stabilising sediments and reducing erosion; providing oxygen to the water and sediments; and providing habitat for a range of other species.
The Coorong is an ecosystem that ranges from estuarine to marine to hypersaline to extreme hyper saline throughout the year and in this habitat several seagrasses and other aquatic plants form an important foundational ecosystem. The more common aquatic plant and seagrass species in the Coorong are:
- Ruppia tuberosa has become dominant in the Coorong ecosystem as salinity levels have increased to be persistently hypersaline and extremely hypersaline
- Ruppia megacarpa is known to have been present in the Coorong for thousands of years but recently has been absent since being lost during the Millennium Drought
- Althenia cylindrocarpa (below), another aquatic flowering plant that tolerates the extreme hypersalinity.
Flowering Althenia cylindrocarpa also shrouded by filamentous algae
- Lamprothamnium papulosum, Stonewort, a charophytic algal species that is known to occur across the Coorong and adjacent water bodies from historical records and herbarium collections.
The change in seagrasses community composition indicates that since European settlement, it is likely that the overall change in flow of freshwater to the system is from extraction and the building of the barrages.
The Ruppia Community is a community of submerged aquatic plants which occurs in the Coorong. It plays an important role in the southern Coorong where it is recognised as a key resource to the Coorong ecosystem.
The current Ruppia Community consists of three species of submerged aquatic plants: Ruppia tuberosa, Ruppia megacarpa and Althenia cylindrocarpa. These aquatic plants live their whole lifecycle under the water and tolerate a range of salinities.
In the past, other species of aquatic plants have been recorded in parts of the Coorong including the seagrass, Zostera muelleri and the stonewort, Lamprothamnium papulosum.
The species of aquatic plants known to grow in the Coorong over the past 30 years are those that tolerate the very high salinities that occur each year.
What did the 2022-2023 Flood Event mean for the seagrasses of the Coorong?
Flood waters increased the overall water depth in the southern Coorong and this extended the duration that mudflats were inundated. The reduced salinities and water flow also improved sediment quality.
The changed conditions caused by the 2022-2023 flood event led to enhanced growth of the Ruppia Community. When water levels were higher and salinity lower, the annual summer die-off of aquatic plants on the mudflats and shallow water was reduced.
- Plants of the Ruppia Community grew over a wider depth range that typically occurs.
- There was an extended flowering period in 2022-2023, due to the persistent higher water levels.
- Higher biomass of the Ruppia Community was measured in deeper water.
The flood event led to increased seagrass biomass and distribution, however as the salinity reduced to become estuarine or fresh for extended periods, the Ruppia Community experienced changes in community structure and there were changes to the seed bank.
- Ruppia tuberosa and Althenia cylindrocarpa, which dominate the hypersalinity tolerant Ruppia Community, died back
- Ruppia megacarpa recruited into North Lagoon sites and began to appear in the Ruppia Community of the South Lagoon
- The seeds of Ruppia tuberosa appeared 'soft' as the seed coat did not harden fully. This led to concerns that the seed bank would be short lived or even non-viable under some conditions
- There was a short and patchy flowering event in Spring 2023 (the flowering season following the flood event)
- There was increased distribution of algae in the South Lagoon, although surface algal mats that are often present were not as prevalent as in recent years.
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.
Sites within the Coorong sampled for assessing the Ruppia Community and its response to 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 effects of the 2022-2023 Flood Event on the Marine Environment
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.
Conclusions
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.
What can we expect in the future?
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.
For more detailed information on the environmental response to the 2022-2023 River Murray flood event
Please see the fact sheets and report available on the Goyder Institute's website: Outputs – Goyder Institute
