IN South Australia the Coorong fishermen say that before irrigation, before the weirs, locks, levees and barrages (sea dykes), the Murray River would flog down from September until maybe Christmas, filling the lagoon, then out the mouth. By Christmas, flow had usually slowed and water levels dropped right down. Then when the South Westerly wind picked up the sea would pour in through the mouth and work its way across the lake.
So Lake Alexandrina was fresh in spring and summer, but salty by autumn.
What the old fishermen describe is an estuary: a transition zone. The Murray River had a barrier estuary with a central lagoon, Lake Alexandrina, and a sand barrier, the Younghusband Peninsula. A single, narrow and shallow inlet that often closes over is also a characteristic of barrier estuaries.
There are many barrier estuaries along the southern Australian coastline including Lake Illawarra just south of Wollongong. According to the Lake Illawarra Authority’s management guide, freshwater flows into the lake from the escarpment and salty water from the ocean tides and therefore it is an estuary. There are 70, of these Intermittently Closed and Open Lakes and Lagoons, known by the acronym ICOLL, from Sydney to the Victorian border. A significant issue is management of the risk of flooding when their ‘mouths’ close over. NSW State government policy doesn’t support the artificial opening of ICOLLs.
The South Australian government insists the Murray mouth, which is the inlet to Lake Alexandrina, be kept open. In fact since European settlement there have been many schemes devised to change the Murray’s mouth to make it deeper and wider, including through blasting and dredging and more recently through water reform.
There is this invented narrative that the Murray’s mouth closes over because greedy upstream irrigators have stolen all the water, but the reality is Barrier Estuary’s close over naturally. The long-term solution is to remove the Murray Mouth barrages – the sea dykes – that have interrupted the evolution of this system and so let it develop to a fully mature state. Mature barrier estuaries tend to be fully tidal.
The sea dykes dammed the estuary making it totally dependent on river flows. Stopped the tide. Limiting natural scouring of the sea mouth in spring by the river flow and in autumn by the Southern Ocean. Not surprisingly the hydrology and geomorphology of the Murray’s mouth has changed with sand that used to shoal behind the mouth consolidating into Bird Island that continues to grow and may one day permanently plug the Murray’s mouth.
Indeed Federal Water Minister Tony Burke can buy back all the water from all the irrigators across the entire Murray Darling, but this will have very little real impact on the Murray’s mouth.
To learn more about barrier estuaries and why Lake Alexandrina was once part of a wave-dominated barrier estuary and which sea dyke should be removed first read my latest technical paper ‘Plugging the Murray River’s Mouth: The Interrupted Evolution of a Barrier Estuary’