Keeping the Murray Mouth open
The Murray Mouth, near Goolwa in South Australia, is a dynamic system influenced by the flow of River Murray water through the barrages and tidal movement from the Southern Ocean. When River Murray flows to South Australia are low, barrage releases are low and sand deposits occur inside the mouth causing restrictions and increasing the risk of closure.
It is important that the mouth remains open to maintain connectivity between the river, the Coorong and the Southern Ocean, to discharge salt and other nutrients out to sea, and to maintain healthy ecosystems in the Coorong.
Periods of low flow have presented management challenges in the past. At the beginning of the Millennium Drought in 2002 the mouth threatened to close, and dredging was required for eight years to keep it open. Dredging is considered the most effective method of keeping the mouth open in terms of cost and environmental outcomes, when compared with other intervention methods.
Maintaining an open mouth is a key objective under the Murray-Daring Basin Plan, which was adopted in 2012. The Basin Plan seeks to ensure that the mouth remains open without the need for dredging 95 percent of the time under the 3,200 GL water recovery scenario, which is expected to be achieved by 2019.
In the meantime, the mouth is again facing the risk of closure during the 2014-15 summer. On 17 October 2014, the Murray-Darling Basin Ministerial Council agreed to provide $4 million for a dredging program, with the cost being shared equally between the Commonwealth, South Australian, Victorian and New South Wales governments.
The Government of South Australia is working with other jurisdictions and has obtained all the necessary approvals to undertake dredging.
Dredging commenced on 9 January 2015.
How will this affect you?
While dredging activity is taking place, there are some changes to public access to the mouth.
Boat and other watercraft traffic is permitted during daylight hours when it is safe. Temporary closure of the Goolwa and Tauwitchere channels may be required from time to time. Buoys, either connected by a surface cable and rod, or individually anchored, will mark the extent of the exclusion zone around dredges and associated equipment.
The dredges are operating continuously, day and night, and will display the appropriate lights and shapes as prescribed by the River Murray Traffic Regulations. Dredge operators can be contacted on VHF14.
The northern tip of the Younghusband Peninsula in the Coorong National Park is closed to the public. For details, including a map of the closed area, see the Coorong National Park closure web page.
Penalties for entering the dredging exclusion zone or the closed part of the Coorong National Park will be enforced.
Why is the mouth at risk of closing?
Sand accumulates naturally in the mouth under low flow conditions. High flows coming down the River Murray scour sand from the mouth and flush it seaward.
The mouth has become constricted since late 2013 as a result of drier conditions and reduced inflows to the River Murray. These conditions have limited the water available to prevent sand deposition and to scour sand from the mouth.
Flows of around 75 gigalitres (GL) a day over the barrages are required to scour substantial volumes of sand from the mouth and wash it out to sea. Even though the high flows of 2010 scoured a significant volume of sand from the mouth, sand has progressively built up again. Flows of 2 GL a day or more have been maintained, reducing the rate of sand accumulation, but the volume is not enough to remove the risk of mouth closure. Now, a steady build-up of sand has occurred and much greater flows are required to halt or reverse this trend.
What is the dredging plan?
The dredging plan – which includes factors such as location, volume and timing – considers the current and likely future discharge from the barrages and the rate of sand accumulation in the mouth. This plan will be refined as dredging continues.
The dredging program is designed to remove accumulated sand from the mouth and the two channels (Goolwa and Tauwitchere). The cleared channels and the mouth should allow tidal variations into the channels and the Coorong. Sufficient tidal variation allows for the exchange of well-oxygenated and cooling seawater which is needed for the environmental health of the Coorong.
The dredging program is aimed at maintaining the two channels at an optimum size. This will be achieved by having a dredge operating in each channel. Dredging will start in the Tauwitchere Channel first.
Why hasn't the Basin Plan ensured the mouth would remain open?
The increasing risk of mouth closure highlights the importance of implementing the Murray-Darling Basin Plan and achieving full recovery of environmental water. However, even when we recover all of the water stipulated in the Basin Plan, expected to start from 2019, dredging is still likely to be required for five years in every 100.
As at 31 August 2014, around 1,500 GL of environmental water was available for the Commonwealth Environmental Water Holder to use. The Basin Plan sets a target of 2,750 GL by 2019 with potential to increase this by another 450 GL.
Has the Murray Mouth been dredged in the past?
The mouth was dredged for eight years from 2002 to 2010, during the Millennium Drought, to keep it open. The dredging program reintroduced tidal variations in the Tauwitchere and Goolwa Channels, and allowed cool, well-oxygenated sea water to enter the Coorong.
Dredging stopped when higher flows returned in 2010 and conditions at the mouth have continued to be closely monitored while maintaining barrage releases since the return of flows.
DEWNR and its partners SA Water and the Murray-Darling Basin Authority are continuing to monitor conditions at the mouth.
Increased monitoring activities are now being carried out, including:
- hydrographic surveys to monitor mouth bed level changes and sand movement approximately every three weeks (previously every six)
- aerial inspections and photography approximately every three weeks (previously every six)
- monitoring Diurnal Tidal Ratios every month
- water levels and water quality monitoring nearly continuously at telemetered stations.
Department of Environment, Water and Natural Resources
SA Water; Murray-Darling Basin Authority
The Coorong Lower Lakes and Murray Mouth Recovery Project is funded by the South Australian Government’s Murray Futures program and the Australian Government.