What is an estuary?

An estuary is the part of a river where it meets the sea, where freshwater and salt water mix and tides flow up the river channel. Australia has around 1,000 named estuaries, ranging from large drowned river valleys such as Sydney Harbour to small intermittently closed-and-opened lakes and lagoons.

Estuaries are biologically among the most productive ecosystems on Earth. They mix nutrients washed down from the land with sunlight in shallow warm water, supporting algae that feed an extraordinary food web of fish, crabs, prawns, oysters, and birds. About 75 per cent of Australia's commercially fished species spend at least part of their life cycle in estuaries. Mangroves, saltmarsh, and seagrass beds line many Australian estuaries and stabilise their banks against erosion.

Australian estuaries vary widely in form. Drowned river valleys such as Sydney Harbour, Port Hacking, Botany Bay, the Hawkesbury, the Tamar, and Port Davey were created when rising sea levels flooded river valleys at the end of the last ice age. Wave-dominated estuaries, common along the exposed southern coast, often have a mobile sand bar at the mouth. Tide-dominated estuaries, such as Port Phillip Bay, Spencer Gulf, and the Gulf of Carpentaria, are wide and tidally flushed. Tropical estuaries in northern Australia typically have extensive mangrove forests, and many are home to saltwater crocodiles.

Many of Australia's largest cities sit on estuaries: Sydney on Port Jackson, Melbourne on Port Phillip Bay, Brisbane on the Brisbane River estuary, Perth on the Swan River, Adelaide on the Port River, Hobart on the Derwent, and Darwin on Darwin Harbour. Estuary management combines federal Ramsar listings, state coastal-zone planning, and local catchment-management authorities, with stormwater quality, sedimentation, and seagrass loss the biggest concerns.