Case study 2 of TWO contrasting ecosystems at risk - the Murray-Darling Basin

What this dot point is asking

NESA requires TWO contrasting case studies of ecosystems at risk. The Murray-Darling Basin provides a strong contrast to the Great Barrier Reef: freshwater rather than marine, multi-state rather than single-state, with dominant stresses (water extraction, salinity, invasive carp) that are domestic rather than global. Strong responses are precise about the basin's hydrology, its specific threatened communities, and the Basin Plan management framework.

The ecosystem

The Murray-Darling Basin covers 1 million km2 across NSW, Victoria, Queensland, South Australia, and the Australian Capital Territory. It is 14 percent of the Australian landmass. The basin's three main river systems (Murray, Darling-Baaka, Murrumbidgee) and their tributaries support:

• 2.6 million people, including Australia's largest inland city Albury-Wodonga.
• Around 40 percent of Australia's gross agricultural value (cotton, rice, wheat, dairy, horticulture, wine).
• 16 wetlands of international significance under the Ramsar Convention, including the Macquarie Marshes, the Gwydir Wetlands, the Hattah Lakes, the Coorong, and the Lower Lakes (Alexandrina and Albert).

Biophysical setting

• Hydrosphere. Long average rainfall around 530 mm, but with 94 percent returning to atmosphere through evapotranspiration. Runoff naturally around 24,000 GL per year; under regulation around 12,000-15,000 GL per year.
• Atmosphere. Hot summers, cool winters, with strong ENSO-driven inter-annual variability. The 2001-2009 Millennium Drought reduced rainfall by 12 percent below average.
• Lithosphere. Mostly sedimentary basin with some volcanic and basement rock outcrops. Soils generally low in phosphorus and prone to salinity when water tables rise.
• Biosphere. Around 35 native fish species (Murray cod, golden perch, silver perch, Macquarie perch), 53 native frog species, 130 waterbird species, river red gum and black box woodland (300,000-plus ha), lignum and chenopod shrublands, native pastures.

The risk

Water extraction and river regulation

The Basin has been extensively developed for irrigation since the 1880s. By the late 1990s, around 12,500 GL was being extracted annually, against natural runoff of around 12,400 GL to the sea. The Murray Mouth closed during the Millennium Drought; sustained by emergency dredging.

Major dams: Hume (3,000 GL), Dartmouth (3,856 GL), Burrinjuck (1,026 GL), Menindee Lakes (1,800 GL), Eildon (3,335 GL). The dams replaced natural flood-drought cycles with constant low flows, eliminating the flood pulses that wetland species depend on.

Floodplain forests have lost recruitment. Around 75 percent of river red gum forests along the Murray showed canopy decline during the Millennium Drought.

Dryland salinity

Land clearing for agriculture replaced deep-rooted native vegetation with shallow-rooted annuals. Water tables rose. Salt accumulated in surface soils. Around 2 million ha of Basin land is affected by dryland salinity, with around 70,000 ha of severely affected land lost from production. Salinity loading to the Murray reached around 600 t/day before salt interception schemes brought it down.

Algal blooms

Nutrient enrichment from fertilisers and animal waste, combined with low summer flows, produced major blue-green algal blooms (cyanobacteria). The 1991 Darling River bloom was 1,000 km long. Blooms are toxic to humans and livestock.

Fish kills

Major fish kills at Menindee Lakes on the Darling-Baaka in December 2018, January 2019, and February 2023. Estimated over 1 million fish killed in the 2018-19 event. Caused by combined low flows, high temperatures, and algal blooms creating low-oxygen conditions.

Invasive species

European carp dominate 70-90 percent of fish biomass in the Murray-Darling. Carp disturb sediment, increase turbidity, and outcompete natives. The National Carp Control Plan has investigated biocontrol (a carp-specific herpesvirus) but the release has been paused.

Other invasives: redfin perch, gambusia (mosquitofish), goats, rabbits, foxes, weeds (Mimosa pigra, blackberry, willow species along rivers).

Climate change

The Bureau of Meteorology projects 5-15 percent reduction in southern Murray-Darling rainfall by 2050 and greater inter-annual variability. The 2001-2009 Millennium Drought and the 2017-2020 drought were both consistent with this trajectory.

Management

Governance

• Murray-Darling Basin Authority (MDBA). Federal authority established 2008, replacing the older Murray-Darling Basin Commission. Independent statutory body.
• Basin states. NSW, VIC, QLD, SA, ACT each retain water management functions within nationally agreed frameworks.
• Commonwealth Environmental Water Holder (CEWH). Manages around 2,800 GL of environmental water entitlements purchased from irrigators.

The Basin Plan (2012)

The largest restructure of Australian water policy in a century. Key elements:

• Sustainable Diversion Limit (SDL). Total extraction capped at 10,873 GL/year, a 2,750 GL reduction on previous allocations.
• Water recovery. Achieved through buybacks ($3.1 billion spent), on-farm efficiency upgrades, and infrastructure investment.
• Environmental Water Holdings. Around 2,800 GL held by the Commonwealth Environmental Water Holder, deployed strategically to support environmental outcomes.
• Constraints management. Removing physical barriers to environmental flows (river bank infrastructure, low-lying private property that floods at higher flows).
• Compliance and metering. Tighter water-meter requirements and enforcement after the 2017 Four Corners "Pumped" investigation.

The 2012 plan was extended by the 2023 Restoring Our Rivers Act, which committed to additional water recovery and extended timelines for some components.

Salinity management

The Murray-Darling Basin Salinity and Drainage Strategy (1988, revised) and the Basin Salinity Management Strategy (2001-2015, 2030) have driven salt interception schemes that intercept around 500 t/day of salt before it reaches the Murray. Total salinity load to the Murray Mouth has been substantially reduced.

Aboriginal water entitlements

The Aboriginal Water Entitlements Program (2019) provides $40 million to acquire cultural water entitlements, recognising First Nations interests in the water cycle that European management ignored for over a century. Limited to date but growing.

Wetland and biodiversity programs

• Living Murray. Federal-state initiative targeting six iconic sites (the Coorong, Hattah Lakes, Barmah-Millewa Forest, Gunbower-Koondrook-Perricoota, Lindsay-Mulcra-Wallpolla, Chowilla).
• Ramsar wetland management. Site-specific plans for the 16 listed wetlands.
• Fish ladders and cold-water pollution mitigation. Allow fish movement past dams; release water from warmer reservoir levels to reduce thermal stress on natives downstream.

Drought response

Demand management (water restrictions), capital city desalination plants ($10 billion combined investment), regional pipelines, drought-relief programs for farmers.

Effectiveness

Significant achievements:

• Total extraction reduced by approximately 2,100 GL since 2012.
• Salinity at the Murray Mouth reduced by around 50 percent.
• Some wetland recovery in years with adequate environmental flows.
• Strengthened compliance and monitoring.

Persistent failures:

• Northern Basin (NSW Darling-Baaka) water recovery has lagged. Menindee Lakes fish kills illustrate continued ecological collapse.
• Climate change reducing total available water faster than extraction reductions can compensate.
• Carp dominance unsolved.
• Indigenous water rights remain limited.

The 2018 Productivity Commission and 2019 Royal Commission into the Basin Plan identified continued problems with implementation, monitoring, and compliance. The 2024 Basin Plan progress assessment found mixed progress.

Why this case study works

The data are very public (MDBA annual reports, ABS, CSIRO, ABARES). The stresses are predominantly human-induced and domestic, contrasting with the GBR's global climate driver. The management framework is the textbook example of multi-jurisdictional environmental management. Pair with the Great Barrier Reef for HSC essays that ask you to compare ecosystems at risk.