Blog | 19th Dec, 2022

5 ways climate change is impacting our rivers

We know that burning polluting coal and gas is making our climate hotter and drier. But what does this mean for our rivers?

As big fossil fuel companies continue to burn coal and gas, our rivers are having to survive in a climate that is getting hotter and drier.

Over the long-term, a hotter climate means less rainfall, less water flowing through our rivers and less flooding into wetlands and billabongs.

But the impacts are more complex than this. Warmer air also holds more moisture, so when it finally condenses and turns into rain, it’s more likely to fall as an intense burst. This is why, even as annual rainfall declines, we’re seeing more intense flash flooding events [1].

Because of the way we’ve manipulated the natural rhythms of rivers, for example by taking too much water away and capturing it in weirs and dams, there are other consequences too.

Climate change is occurring on top of decades of over-extraction. It means rivers are now being squeezed even tighter – and the methods we’ve been using in recent years to keep them alive are becoming harder to rely on.

Here we explain 5 ways climate change is impacting our rivers, and all the people, wildlife and wetlands they support:

  1. Less rainfall leads to less water flowing through our rivers
  2. Harder to use water set aside to keep wetlands alive
  3. Longer dry periods
  4. Fewer small and medium floods
  5. More pressure on some rivers

1. Less rainfall leads to less water flowing through 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.

2. Harder to use water set aside to keep wetlands alive

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 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.

3. Longer dry periods

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].

River Red Gums can tolerate around nine years without a good flood. When water finally flows over riverbanks and across the floodplain, these incredible trees can regenerate rapidly. This is how arid and semi-arid wetlands survive. Some plants are remarkably drought-tolerant, waiting quietly in wetlands, holding life in bulbs, rhizomes and tubers. Others have learned to recover quickly when wet conditions return.

This resilience is magnificent, but it’s also conditional. 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. But with longer, more frequent periods without water, the trees struggle to bounce back.

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].

Floods allow native fish to travel to floodplain wetlands. It’s where perch and Murray Cod often go to breed. Waterbirds look for floods, many nesting and breeding in flooded River Red Gums.

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 — building resilience for wetlands and communities.

4. Fewer small and medium sized floods

Longer dry periods also mean less frequent small and medium floods.  

This could spell disaster for many of the birds and fish that rely on wetlands to survive. There are fish species with short life-spans, like the Spangled Perch, that 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. 

But the impacts of prolonged dry periods cascade through the whole river system. 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, turning water the colour of black tea. These smaller blackwater events occur naturally and provide nutrients to aquatic life.  

But hypoxic black water – severe events with very low oxygen in the water – are 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. As the bacteria get to work breaking it down, it sucks up a lot of oxygen.  

The impact is heart-breaking. Murray crayfish crawl out of the river to survive and yellowbelly fish suffocate. In the 2022 floods, 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.

5. More pressure on some rivers

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.

The solution?

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

 

References

  1. Victoria’s Climate Science Report 2019, page 32.
  2. Interim Inspector-General of Murray-Darling Basin Water Resources 2020, Impact of lower inflows on state shares under the Murray–Darling Basin Agreement, Canberra, page 7.
  3. Zhang, L, Zheng, HX, Teng, J, Chiew, FHS, and Post DA (2020). Plausible Hydroclimate Futures for the Murray-Darling Basin. A report for the Murray–Darling Basin Authority, CSIRO, Australia. 34pp. Study completed March 2020.
  4. Ibid.
  5. Dr Avril Horne, A Prof Angus Webb, Dr Libby Rumpff Meghan Mussehl, Dr Keirnan Fowler, Andrew John (2020). Kaiela (Lower Goulburn River) Environmental Flows Study Final Report, page 72.