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Why doesn’t the MDBA allow for the mouth of the Murray to close?

Some people are concerned about the amount of water that flows out of the River Murray and into the sea. They worry that this water is being wasted and could be put to better use.

This page explains why letting water flow out to sea is necessary.

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The Murray mouth needs to remain open so that excess salt and sediment can be carried out to sea

Naturally the Murray mouth has always stayed open even during severe previous droughts like the Federation Drought.

Without development, the water running downstream would have kept the Murray mouth open all the time, but river regulation and water extraction have combined to reduce these flows to a point where they can often no longer do this. The Murray mouth closed for the first time in 1981 and dredging has been undertaken periodically since then to keep it open.

The salt of the Murray–Darling Basin derives from millions of years of rainfall and the weathering of rocks and ancient ocean sediments. The flat terrain, low rainfall and high evaporation rates that are seen across most of the Basin favour the accumulation of salt in the landscape, while the groundwater and river systems only slowly return the salt to the ocean. By the time water has travelled through the whole Basin to the mouth of the Murray, it contains much more salt than it did at the source. When the water flows through the Murray mouth, the salt is carried out to sea, removing it from the river system.

The mouth of the River Murray is the only point in the Murray–Darling Basin where the river system connects to the sea. This means it is the only place where salt can naturally leave the Basin’s river system.

Changes that humans have made to the Basin through land clearing, irrigation and agriculture, as well as urban and industrial uses have increased the amount of salt in the water. And because people use more water from the Basin than they used to, less water reaches the mouth of the Murray. This leaves more salt in the Basin and increases salinity levels.

To dilute salty water and flush it from the system, water needs to be released and allowed to flow into the sea. The mouth of the Murray River is the only place where this can happen.

If this extra salt is not removed from the rivers, the level of salt in the water will keep rising. Water with high salinity levels may impact or even kill some of the plants and animals that inhabit the river and its surroundings. Farmers would be unable to grow crops, and the water would eventually become undrinkable for humans without expensive treatment.

Water flowing out to sea also transports sediment, nutrients and organic matter from the river and feeds the marine ecosystem in the Southern Ocean surrounding the Murray mouth.

Keeping the Murray mouth open protects the Lower Lakes

The Coorong, Lower Lakes and Murray Mouth are Ramsar-listed, meaning they are internationally recognised as representative, rare or unique, and important for conservation. They need fresh water to be healthy.

This region is home to a huge variety of fish, birds and other wildlife. Due to long periods of drought and reduced flows from the River Murray leading to excess salinity (saltiness), the wetlands have been severely degraded. To improve the health of the wetlands, the mouth of the Murray must remain open so salt can be carried out to sea. The exchange and mixing of fresh water from the river upstream with saline water from the ocean is critical to maintaining a healthy and productive estuarine environment that makes the area internationally important.

Some people believe that the Lower Lakes have always naturally filled with sea water. In fact, there is clear evidence that before European settlement the Lakes were almost always filled with fresh water, and only contained significant amounts of sea water during rare periods of low flows.

Unique species also rely on the mouth remaining open—seven fish species that require access to fresh and saline water to complete their life cycle. Values of the Ngarrindjeri Nation and other socio-economic values (e.g. tourism and fisheries) are also reliant on the health of the Coorong and Lower Lakes.

What you need to know

  • Salinity in the river increases in times of drought.
  • It is important for water to flow through the whole river system to prevent salinity from building up in the lower Murray.
  • Sometimes water levels are managed to prevent salt spikes, and other water quality issues, that can kill plants, wildlife and crops
  • Salt interception schemes divert salt away from the river and nearby landscapes, however they can’t stop all the salt from entering the rivers.

Questions you might have

“Why does the river need to flow into the Lower Lakes? Shouldn’t they naturally be full of seawater?”

Evidence from fossils shows that for the past 7,000 years the Lower Lakes have mostly been freshwater. It was only in times of extreme dryness that water levels fell below sea level, causing sea water to enter the lakes. Allowing the Lower Lakes to become permanently estuarine (a mixture of fresh and salty water) would damage delicate and unique ecosystems, including endangered wildlife.

“Isn’t it wasteful to let water flow out to sea when it could be used by struggling farmers?”

When water is released so it can flow to the sea, it’s doing important work. For the river system to remain healthy and able to sustain life in the Basin, water needs to flow out to sea to prevent salinity and sediment from building up to dangerous levels.

As water flows through the Murray–Darling system, carrying away excess salinity and sediment, providing clean drinking water, a healthy river for farming along the whole river system and recreational benefits, such as better conditions for swimming and water sports, as it moves downstream. It also provides environmental benefits, providing water to ecosystems that need it. By the time the water reaches the Murray mouth, it has carried out several tasks that are necessary to keep the Basin healthy. Water can be used at multiple sections along the river, as it travels downstream.

“If we build ‘Lock Zero’ will we have better control of the water? Would there be more water available for people in New South Wales?”

Building ‘Lock Zero’ and decreasing flows out of the Murray mouth would have significant negative effects on the Basin system as a whole and would not necessarily benefit anyone in the longer term. The whole river system may gradually become more and more salty, until no one would be able to use the water.

Lock Zero is the name for a potential lock near Wellington, downstream from Lock One. Building Lock Zero and allowing the Lower Lakes to fill with seawater would cause irreversible damage to the ecosystems of these regions. Plants and animals would die, and this internationally recognised region could be lost forever.

The water that currently flows out of the mouth of the Murray does more than supply the Lower Lakes with fresh water. It also removes salinity and other sediment from the river system and provides water for environmental and recreational uses.

Learn more about why the Murray mouth needs to remain open

Updated: 24 Sep 2020