5 ways climate change is impacting our rivers

We’re already seeing evidence of less rainfall in Victoria.

In the Murray-Darling Basin, inflows have dropped 50 percent over the past 20 years compared to the century before [2]. And it is predicted to get even drier. By 2046, flows might drop to half the levels we saw between 1975-2005 – and even lower in periods of extended drought [3].

'We need a serious conversation': River flows could decline 40% in Australia's foodbowl

Sydney Morning Herald

River flows in Australia's food bowl, the Murray Darling Basin, will decline by as much as 40 per cent over the next 50 years under the current trajectory of global warming, one of Australia's top hydrologists has warned.

This means the river has less water flowing through it, making it more difficult for fish and turtles to survive. It also means that salt in estuaries at the river mouth doesn’t get flushed out to sea. Estuaries are some of our most diverse and ecologically important ecosystems – places like the Gippsland Lakes and Lower Lakes of the Murray – and a build-up of saltwater can have disastrous consequences. During the Millennium Drought, the Coorong became five times saltier than the ocean. Waterbird numbers were halved while invading marine tubeworms attached themselves to the backs of turtles, until they were weighed down and drowned.

When salt builds up, it also starts encroaching upstream, which is why scientists say a river dies ‘from the mouth up’. You can learn more in this great video from our friends at Conservation SA.

Victoria’s rivers have been heavily modified in the last 100 years. The amount of water rivers get is now carefully monitored, and the timing is controlled by big structures like dams that release the water. However, climate change is making it more difficult to deliver the water our rivers need through this method.

We have limits on how much water can be taken away from rivers, often called caps or diversion limits, and targets for how much needs to be set aside for the river – what’s called ‘environmental water’.

This water is released from dams to support healthy river life. A particular challenge is getting water onto the floodplain, which is critical for birds and fish that breed in wetlands. Small and medium flooding used to occur regularly enough to spill onto the floodplain. But to do this now, water managers need to ‘piggy-back’ environmental water on top of other large flows from rainfall.

The problem is, with less rainfall the river will rarely reach a high enough level to get water onto the floodplain. This could mean that even though we have environmental water, it can’t reach the places that need it most. Without water, wetlands can’t support big breeding seasons for waterbirds and River Red Gum forests could die.

Global heating means that, on average, conditions will be hotter and drier. Projections show a decline in average flows and more frequent periods of drought [4].

This could have particularly devastating impacts for wetlands. Wetland ecosystems, and the plants and animals that live there, have adapted over tens of thousands of years to the boom-and bust cycles of flood and drought. It means they can survive many years without a good drink – but their resilience has limits. Wetlands that have adapted to periodic extremes of drought depend on an otherwise-normal cycle of flooding. For River red gums, this means flooding around every three years. If there are longer, more frequent periods without water, the trees struggle to bounce back. During the Millennium Drought, 75% of River red gums in the middle stretch of the Murray River were found stressed, dead or dying.

In a hotter, drier climate, these extended droughts may be brutally long. On the Goulburn, for example, modelling 110 years in a dry climate scenario shows that only 12% of years might have flows big enough to inundate the floodplain – and this includes a devastating 53-year dry period [5].

As periods of relief become increasingly rare, we can’t hold out hope that these ecosystems will bounce back in wetter years. We need to attend to the disaster that’s already here and build resilience for wetlands and communities.

Longer dry periods mean less frequent small and medium floods.  

These floods are vital for the health of rivers and wetlands, and the survival of many fish and bird species. Fish with short life-spans, like the Spangled Perch, can become locally extinct if a wetland isn’t flooded for 3 years. Others like the Murray Cod rely on a wet floodplain for spawning habitat – and without this won’t be able to breed. Many migratory birds, travelling from as far away as Siberia, stop to feed in crucial wetlands in the Murray-Darling Basin on their journey.

Without regular small and medium floods, there is also a much greater risk of severe blackwater when a larger, more erratic flood sweeps across the landscape. Leaf litter and bark naturally builds up on the floodplain over consecutive dry years. With regular rainfall or small floods, these organic materials would be washed into waterways where bacteria and microorganisms break them down. These smaller blackwater events occur naturally and provide nutrients to aquatic life.  

It’s hypoxic black water – severe events with very low oxygen in the water – that is the issue. The more years the floodplain stays dry, the more that material builds up. With a large flood, that organic matter washes into the river all at once and the bacteria suck up too much oxygen as they break it down. The impact is heart-breaking. Murray crayfish crawl out of the river to survive and yellowbelly fish suffocate. When the Murray River flooded in 2022, communities found hundreds of old Murray-cod washed up dead from the blackwater.

Native fish suffocating as parts of Murray-Darling Basin turn toxic from unprecedented floodwaters

ABC

Native fish are desperately seeking refuge because parts of the Murray-Darling Basin have turned toxic due to hypoxic blackwater. Everything that lives in the affected water is literally suffocating.

Already, small floods aren’t occurring regularly enough because lots of water is held back in big dams for large-scale irrigation demand in summer. Climate change will intensify these risks.

As average flows across the Basin decline, some rivers will be forced to work harder to meet excessive irrigator demand for water – particularly in Northern Victorian.

We saw this from 2017 to 2019. When too much water taken by irrigators in the northern Basin ultimately stopped the Darling-Baaka’s flow, the Murray had to make up for lost water. River operators looked to tributaries like the Goulburn to work harder. This meant pushing massive volumes of water down the Goulburn in summer – a time when the river should naturally have low flows. These high summer flows have been devastating: eroding riverbanks, flooding young platypus from their burrows and killing young native fish.

The conditions have only begun to be addressed with limits on how much water can be pushed down the river. But in a drier climate, water from the Goulburn will become a bigger slice in a smaller pie. The pressures of high irrigation demand will become more acute.

A hotter, drier climate presents a frightening preview of what our future could look like. But it’s one we can respond to with strong climate action and water management now.

To care for rivers, our solutions need to run deep. We need to make water use more fair and less exploitative. And we need to do it in our lifetime. That means:

1. Setting aside enough water for rivers to survive
2. Letting it flow like a river instead of an irrigation channel
3. Making water management fair, starting by returning water to Traditional Owners and reigning in the market

Join the campaign to save our rivers