The stated aims of the Murray-Darling Basin Plan include improving water quality in the Lower Lakes, and "restoring" the health of the basin's rivers. To achieve this, about 20 per cent (2,500 GL) of the water that was available to consumptive users a decade ago, has been diverted to the environment.
Despite this taking away of irrigation water, the plan is not achieving its aims, and many of the basin's major rivers have now stopped flowing. Increased "environmental flows" (especially in the lower basin) have mainly been a wasteful investment in evaporation and in watering the sea.
Every year over 800 GL (Sydney Harbour only holds 500GL) of fresh water is lost through evaporationand seepage from South Australia's Lower Lakes (reflecting that their average depth is only 2.9 metres). Thus with trades of Murray River water recently reaching a high of $970 a mega-litre and exceeding $500 since mid 2018, you are talking about freshwater losses valued at $500 million to a billion dollars annually at recent prices. The loss of water for irrigation resulted in reduced agricultural production and in the ruin of some local communities in upstream irrigation areas. (The Gross Value of Irrigated Agriculture in the Murray-Darling Basin was $7.2 billion in 2016-17.)
The vast (140,000 hectare) Lower Lakes would be far healthier if their natural estuarine system was allowed to function (by removing or strategically opening the barrages built to block tidal flows). In times of drought, opening the barrages to allow sea water in (instead of using valuable Murray freshwater) can stop the lake levels from dropping to environmentally disastrous lows, and from accumulating excessive levels of salt. During the droughty period from 2006 to 2009, the level of the Lower Lakes fell to 1.1 metres below sea level.
Evidence shows that, before European settlement, the Lower Lakes were predominantly fresh the majority of the time, because inflows of freshwater were generally sufficient to prevent seawater from creeping too far in. Following dry spells, when flows are low, the Lower Lakes get saltier. (For example, in 1830 Captain Sturt recorded that water in Lake Alexandrina was undrinkable.) The Murray river itself was recorded as having ceased flowing on at least three occasions since white settlement.
There are five barrages that separate Lake Alexandrina from the Goolwa Channel and the Coorong. Back in the 1930's, the construction of these barrages was opposed by many South Australian graziers and fishers due to the expected impact on fishing as a livelihood, the loss of low lying grazing lands through inundation, and expected increased siltation of the Murray mouth.
Back then the Lower Lakes had healthier water and a thriving commercial mulloway fishery. (Mulloway travel between the ocean and fresh water as part of their lifecycle.) Prior to the barrages in 1938/1939 almost 600 tons of butterfish (mulloway) were caught in the Lower Lakes and Coorong. Since the closing of the barrages in 1940, the mulloway catch plummeted to less than 70 tonnes. Desirable freshwater species such as silver perch and Murray cod, once common in the lower lakes, have now also become rare. The commercial catch from the (now permanently "freshwater") lakes is dominated by other species including low-value European carp. Carppopulations (being less adapted to drought) boomed in the Murray Darling Basin following artificial environmentalriverflows.
Evaporation losses also occur in upstream (short-term) swamp areas, which should be low priority areas for watering during severe drought. It has been reported that the Basin Authority was running the Murray River at "hideous levels", losing 6200 megalitres a day for 141 days straight (last spring and summer) to over-bank flows. It is claimed that, by running the river over capacity (to replace missing flows from the Darling), the MDBA was responsible for losses exceeding 870,000ML of water into surrounding forest wetland.
The Barmah Choke, which was once a restriction that could carry over 9000ML a day, can now only carry 7800ML, because the banks are collapsing and the river has partially silted up. In addition to environmental flows, another major cause of high flows in the lower Murray is the transfer of water from upstream use to almond growing farther downstream, flows which the Murray struggles to deliver.
All this begs the question of what state our rivers and lakes would be in, if Australia had been left in its natural state without storage dams or irrigation. This can largely be determined by looking at recent and historic water flows.
Currently much of the Murray system (in Victoria and southern NSW) has plentiful water. In Victoria the main dams currently are nearly all high, with many 80 to 100 per cent full following a wet winter and spring.
NSW is a different story. In the Murrumbidgee catchment Blowering Dam at the end of October was at 55 per centand Burrinjuck only 33 per cent. In the Lachlan system, Wyangala was only 19 per cent full. In the Macquarie, Burrendong was down to only 4 per cent, while in northern NSW most of the major dams are also at levels below 5 per cent.
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