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Murray Mouth (Coordinates: ) is the point at which the River Murray meets the southern Southern Ocean. The Murray Mouth's location is changeable. Historical records show that the channel out to sea moves along the sand dunes over time. At times of greater river flow and rough seas, the two bodies of water would erode the sand dunes to create a new channel leaving the old one to silt and disappear.
The mouth is between two sandhill peninsulas. Sir Richard Peninsula on the northwest separates the Goolwa channel (the main river channel) from the ocean. The much longer Younghusband Peninsula separates the Coorong from the ocean on the southeast of the mouth.
The Murray Mouth is separated from Lake Alexandrina by a row of low islands. The largest one, directly facing the mouth, is Hindmarsh Island. A series of barrages join the islands, separating the salt water from the fresh water of the lakes and river. The barrages can be opened during high river flow.
The Murray River became dammed by uplift of over 250 metres (820 ft) in the Grampians in Victoria during the Pleistocene about 2.5 Mya. The dam formed Lake Bungunnia, which reached 40,000 square kilometres (15,000 sq mi) and deposited the Blanchetown Clay therein. Higher rainfall of at least 500 millimetres (20 inches) per year kept the lake filled at first, but during later times, rainfall was insufficient, and saline lakes formed, depositing dolomite. Even today, some saline lakes remain as a remnant of the vast lake. The Murray River carved out a new path to the sea via Murray Bridge, replacing its previous exit at Portland, Victoria. The lake was gone by about 0.7 Mya.
River Murray water is used by farmers for irrigation in four of Australia's states, as well as supplying water to most towns along the river, and many further away through various pipelines. It has been widely accepted that too much water is being extracted; however business and political concerns make it difficult to remedy the problem. A visible symptom of the over-extraction of river water is the closing of the Murray Mouth.
Since October 2002, two dredging machines have operated at the Murray Mouth, moving sand from the channel to maintain a minimal flow from the sea and into the Coorong's lagoon system. Without the 24-hour dredging, the mouth would silt up and close, cutting the supply of fresh seawater into the Coorong, which would then warm up, stagnate and die.
In mid-2006 the dredging was scaled back as a result of the improved conditions at the mouth. Now only one dredging machine operates, and permission was granted to one commercial operator to navigate the channel between Goolwa and the Coorong, passing the mouth.
- South Coast Story, John C Tolley, Rowett Print, Mt Compass SA, 1968 ISBN 0-9587964-3-2