Question – When is a lake not a lake?

Dry mudflats at Lake McLarty, March 2022

Answer – when it’s an ephemeral wetland. So, what’s an ephemeral wetland? An ephemeral wetland is one that naturally cycles between wet and dry due to seasonal changes in rainfall and temperature-driven evaporation rates. Lake McLarty is a prime example of this type of wetland, temporarily holding water in the winter through to early summer or after heavy rains.

Previously Lake McLarty has been referred to as “Big Lake” and when it’s full, it is indeed a big lake. The lake bed covers approximately 200 hectares, with a length of 2.3 km and being 1.3 km wide at its widest point. When full it holds the equivalent of about 1000 Olympic swimming pools of water. Lake McLarty is a key component of the Ramsar Site 482 for the habitat it provides for resident and migratory birds. The lake often supports over 20,000 waterbirds, meeting Ramsar Criterion 5 in its own right, however if you were to go there right now expecting to see these numbers of birds you would probably be disappointed.

Due to the ephemeral nature of the lake and the changing water levels the number and species of birds seen varies greatly throughout the year. Over the winter months when the lake is at its fullest and deepest it supports thousands of ducks including Australian shelducks, Pacific black ducks, grey teals, Australasian shovelers, musk ducks, blue-billed ducks as well as swans, grebes, ibis, cormorants, coots, pelicans, silver gulls and terns.

As the water levels decrease during the warmer months the mudflats provide important habitat for migratory shorebirds including sharp tailed sandpipers, curlew sandpipers, red-necked stints, long-toed stints, black-tailed godwits, red knots and common greenshanks. Resident shorebirds including red-capped plovers, red-necked avocets and black-winged stilts also use the rich feeding habitat of the shallow lake and mud flats.

Although Lake McLarty is a naturally ephemeral system there is extreme concern that climate change resulting in increasingly prolonged dry periods will have a detrimental impact on the ecology of the lake. To investigate this, PHCC has recently completed two studies as part of the Saving Lake McLarty project, funded by the State NRM Program: an assessment of the risk posed by potential acid sulfate soils exposed during the dry period; and investigating options for improving surface water capture by the lake and the feasibility of supplementing the lake with other sources of water to extend the time when the lake is wet (i.e. its hydroperiod). The outcomes from both studies are currently being reviewed to determine what action can be taken to enable Lake McLarty to retain its ecological value under the threats posed by climate change

The Saving Lake McLarty project is supported by funding from the Western Australian Government’s State NRM Program.