Murray-Darling Basin 2022-23 flood environmental response – Immediate Investigations

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Project Partners:

The University of Adelaide, SARDI, and CSIRO

Research Theme:

N/A

Status:

Project Overview

The 2022-2023 River Murray high flow event was the second highest flow in recorded history. This high flow event represents a once-in-a-generation opportunity to understand how the South Australian river and coastal environments respond to the high flows.

Critical knowledge gaps currently remain regarding environmental responses to high flow events, and filling these knowledge gaps will greatly assist in the management of riverine, floodplain, wetland and other aquatic habitats in South Australia, particularly with regards to potential future responses to environmental water management and climate change.

On 13 December 2022, all operational barrages near the Murray Mouth were opened, creating 2,000 km of free-flowing river following the removal of the weirs up to Yarrawonga (Victoria). While the high flows will create many positive environmental responses, blackwater events (water with low oxygen levels) and potential pest species responses will accompany the high flows (and may persist for a time after high flows). These present a significant risk to endangered and threatened species throughout South Australian River Murray ecosystems.

This project, funded by SA Department for Environment and Water (DEW), aims to understand exactly how the habitats and species respond to the high flows and their associated risks.

Progress Update and Key Findings

How are things looking down at the Murray Mouth?

  • In the summer of 2022-23, very large volumes of freshwater flowed out the mouth and into the adjacent marine environment. At this time, the plume of freshwater extended at least 40 km offshore (that’s the distance from Adelaide to Gawler).
  • Throughout 2023, freshwater has continued to flow to the sea, all be it in lower volumes.
  • Whilst there were initially some visual negative impacts of freshwater, e.g. benthic marine animals dying due to low salinities and brown water along the Goolwa and Middleton beaches, these freshwater flows carried with them abundant nutrients and carbon from the floodplains of the MDB. These terrestrial energy sources likely lead to a more productive marine ecosystem.
  • The nutrients are firstly used by algae which then fuel higher levels of the marine ecosystem, from small fish like anchovies, through to tuna, sharks and fish-eating birds.
  • At present, we are testing a range of nearshore (e.g. cockles and mullet) and offshore (e.g. snapper and tuna) marine animals to see if carbon and nutrients from throughout the MDB have fuelled their growth.

  

What did the research around blackwater risks find

  • Dissolved oxygen was maintained at satisfactory levels (>90% saturation) at the time of sampling (Feb 2023) in nearly all 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 comparison of the February 2023 survey data to data from continuous dissolved oxygen monitoring stations over a longer time period shows that lower dissolved oxygen concentrations were present on the rising phase of the flood in late 2022.
  • It is considered that these hypoxic conditions were largely driven by low dissolved oxygen River Murray water entering South Australia from upstream states.

 

What impact did the floods have on salinity levels

  • Instream geophysics surveys conducted along Katarapko Creek and the River Murray at Bookpurnong showed significant change in subsurface conductivity within the riverbed between last surveys (2015 for Kat and 2019 for River Murray) and February 2023 in response to the 2022-2023 flood event- i.e., subsurface freshening.
  • Subsurface conductivity features beneath the riverbed still align between the previous and latest surveys, but there is an overall decrease in the conductivity with depth. This suggests that there are discrete zones along the river reaches that are receiving more saline groundwater discharge from the adjacent floodplain aquifers.
  • Despite the reduction in subsurface conductivity in response to the flood event there is still considerable saline groundwater beneath the floodplains discharging to the river. Several natural and or managed flood events would need to occur to ‘flush’ and reduce the salt loads towards the river.

 

What did your research find out about carp movements and breeding

  • Inundated floodplains are known to be favoured spawning and nursery habitats for carp.
  • In association with flooding in 2022/23, there was a notable breeding response from carp throughout the South Australian Murray.
  • Based on long-term datasets from River and wetland habitats, as of autumn 2023, carp were present in the greatest abundance in the past 20 years.
  • While response was widespread, greatest abundances were detected in the Riverland and Lower Lakes.
  • The majority of individuals were young-of-year spawned during the flood, but a substantial portion were 1 year old fish spawned during high flow in 2021/22.
  • A study of adult carp movement, using acoustic telemetry, was ongoing during the flood. There were extensive movements 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) and downstream (e.g. from Lock 4 to downstream of Lock 1) directions

Project Impacts

This project collected time critical data and information to fill knowledge gaps and document environmental responses to the 2022-2023 River Murray high flow event. The four projects aim to fill priority knowledge gaps to help inform the future management of South Australia’s key habitats and species.

  1. The water quality of the River Murray flow, and the influence of the freshwater flows on marine habitats and species outside of the Murray Mouth.
  2. How salt moves and is flushed from the floodplain into groundwater during and after extensive flooding.
  3. The risks associated with blackwater.
  4. Responses in carp breeding and abundance.

Outcomes of the research will improve understanding of environmental responses to a wider range of flow conditions, which in turn will inform future environmental water management, including improved forecasting of river conditions, monitoring and environmental watering priorities.

Research Outputs

No Available Content

Project News

Researchers have been out looking at how the 2022–23 floods have impacted the River Murray, its floodplains and the area around the Murray Mouth where the floodwaters pour out into the Southern Ocean.  Goyder Institute researchers have
The Goyder Institute and Department for Environment and Water have launched a series of research projects to determine how South Australian River Murray environments are responding to the 2022-2023 high flow event. The projects will fill