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The Darling River’s contribution to the Murray

How much water does the Darling River give to the River Murray?

The Darling River connects the entire river network of the northern Murray–Darling Basin with the southern Basin where it joins the River Murray at Wentworth, in south-west New South Wales.

It is the last river to enter the Murray before the Murray reaches the sea in South Australia, about 825 kilometres downstream of the junction. This gives the Darling a special significance in the management of the Murray system.

When viewing the whole of the River Murray system, the Darling contributes on average about 8 per cent of the total volume that enters the Murray from all its tributaries, including the upper Murray.

The Darling River contributes on average about 15 per cent of the total flow in the River Murray at the point where the Darling enters the Murray.

In reality, most years do not deliver an average contribution from the Darling River. Averages mask extreme fluctuations in Darling flows, which are driven by the boom and bust nature of rainfall in the northern Basin.

When flow in the Darling River is low, the Murray system cannot count on a contribution from the Darling River to meet the demands of irrigation farmers and the needs of the river environment.

This page explains

What effect does the Darling have on New South Wales Murray water allocations?

The Darling contributes on average about 8 per cent to the overall volume of the River Murray. It can certainly help to increase the water available to the New South Wales Government for allocation to farmers and other licence holders but its effect on New South Wales Murray water allocations is relatively small over the long term.

The greatest influence on NSW allocations is the flow from the upper Murray catchments.

We know that in the past 20 years more than two-thirds of the reduction in the average inflow to the River Murray system has been due to changes in the upper Murray and Victorian tributaries. All such changes ultimately affect water availability for licence holders.

The way the states have agreed to share the water in the River Murray system between them is set out in the Murray–Darling Basin Agreement, which was first signed more than a hundred years ago and is now part of the Water Act.

How do the Menindee Lakes effect the Darling River’s flow to the Murray?

The volume of flow in the Darling River varies greatly. Flows are typically low, punctuated by short spells of big floods that cannot be stored for later use.

This variability is smoothed out by collecting flows from the Darling into the Menindee Lakes and controlling the release of water for the final 500 kilometre stretch of the Darling before it joins the Murray.

The vast Menindee Lakes have acted as a water storage since they were modified in the 1960s to hold water in reserve and to mitigate the impact of floods—such as the record floods in 1956 that threatened the town of Wentworth. Located in a flat, windy and very hot part of New South Wales, the lakes’ average annual loss of water through evaporation is equivalent to about one Sydney Harbour.

When the volume of water in the Menindee Lakes rises above 640 gigalitres and until it drops to below 480 gigalitres, the water is shared between New South Wales and Victoria. The states, including South Australia, have agreed that the Murray–Darling Basin Authority (MDBA) manages that shared resource as part of the River Murray system.

The MDBA manages the system as efficiently as possible in order to maximise the water available to the states and their licence holders. That means water from the Menindee Lakes helps to meet downstream water demands as it becomes available. The more water is held back in the lakes where evaporation rates are high, the less water is available to the states to allocate to licence holders.

How much water does the Darling River contribute to South Australia’s entitlement?

Through the Murray–Darling Basin Agreement, New South Wales, Victoria and South Australia have agreed to a base entitlement flow of 1,850 gigalitres per year into South Australia. This volume is reduced in dry times but often significantly more than that can flow into the downstream state because water is traded, water for the environment is delivered or more water enters the river than can be held back in storage.

Separating how much water from each dam or tributary, including the Darling, ends up in South Australia’s 1,850 gigalitre entitlement is near impossible—like unscrambling an egg.

The clearest measure of the Darling’s contribution to South Australia is the 15 per cent on average of total flow that the Darling provides to the River Murray at the junction of the rivers, just upstream of the South Australian border.

Where the other 85 per cent of water at the junction comes from depends on where water is available at any given time—runoff directly into the river, releases from the major dams in the high country, or water entering the Murray from the many other rivers and streams.

Updated: 23 Oct 2020