Puddles in the bed of the Darling River are a sign of an ecosystem in crisis. Jeremy Buckingham/Flickr
The deaths of millions of fish in the lower Darling River system over the past few weeks should come as no surprise. Quite apart from specific warnings given to the NSW government by their own specialists in 2013, scientists have been warning of devastation since the 1990s.
Put simply, ecological evidence shows the Barwon-Darling River is not meant to dry out to disconnected pools – even during drought conditions. Water diversions have disrupted the natural balance of wetlands that support massive ecosystems.
Unless we allow flows to resume, we’re in danger of seeing one of the worst environmental catastrophes in Australia.
Dryland river
The Barwon-Darling River is a “dryland river”, which means it is naturally prone to periods of extensive low flow punctuated by periods of flooding.
However, the presence of certain iconic river animals within its channels tell us that a dry river bed is not normal for this system. The murray cod, dead versions of which have recently bought graziers to tears and politicians to retch, are the sentinels of permanent deep waterholes and river channels – you just don’t find them in rivers that dry out regularly.
Less conspicuous is the large river mussel, Alathyria jacksoni, an inhabitant of this system for thousands of years. Its shells are abundant in aboriginal middens along the banks. These invertebrates are unable to tolerate low flows and low oxygen, and while dead fish will float (for a while), shoals of river mussels are probably dead on the river bed.
This extensive drying event will cause regional extinction of a whole raft of riverine species and impact others, such as the rakali. We are witnessing an ecosystem in collapse.
Read more: We need more than just extra water to save the Murray-Darling Basin
Catastrophic drying
We can see the effects of permanent drying around the world. The most famous example is the drying of the Aral Sea in Central Asia. Once the world’s fourth largest inland lake, it was reduced to less than 10% of its original volume after years of water extraction for irrigation.
The visual results of this exploitation still shock: images of large fishing boats stranded in a sea of sand, abandoned fishing villages, and a vastly changed microclimate for the regions surrounding the now-dry seabed. Its draining has been described as “the world’s worst environmental disaster”.
So, what does the Aral Sea and its major tributaries and the Darling River system with its tributary rivers have in common? Quite a lot, actually. They both have limited access to the outside world: the Aral Sea basin has no outflow to the sea, and while the Darling River system connects to the River Murray at times of high flow, most of its water is held within a vast network of wetlands and floodplain channels. Both are semi-arid. More worryingly, both have more the 50% of their average inflows extracted for irrigation.
There is one striking difference between them. The Aral Sea was a permanent inland lake and its disappearance was visually obvious. The wetlands and floodplains of the Barwon-Darling are mostly ephemeral, and the extent of their drying is therefore hard to visualise.
Read more: The Murray Darling Basin Plan is not delivering – there’s no more time to waste
All the main tributaries of the Darling River have floodplain wetland complexes in their lower reaches (such as the Gwydir Wetlands, Macquarie Marshes and Narran Lakes). When the rivers flow they absorb the water from upstream, filling before releasing water downstream to the next wetland complex; the wetlands acting like a series of tipping buckets. Regular river flows are essential for these sponge-like wetlands.
So, how has this hydrological harmony of regular flows and fill-and-spill wetlands changed? And how does this relate to the massive fish kills we are seeing in the lower Darling system?
Read more: How is oxygen ‘sucked out’ of our waterways?
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