What is a delta?

A delta is a triangular or fan-shaped landform created where a river drops its sediment as it enters the sea, a lake, or another water body. The river fans out into multiple smaller channels (distributaries), depositing sand, silt, and clay to build new land that grows seaward over time.

True deltas are uncommon in Australia compared with other continents because most Australian rivers carry relatively little sediment, the country has largely tectonically stable coastlines, and tidal currents on the north and south coasts redistribute sediment instead of letting it accumulate. The Murray mouth, Australia's largest river outlet, has a small delta system much reduced by upstream water extraction. The Burdekin River in north Queensland has a substantial delta on the Coral Sea coast, supporting one of Australia's largest sugarcane irrigation areas.

Other notable Australian deltas include those of the Fitzroy and Daly Rivers in northern Australia, the Ord River in Western Australia (much altered by the Ord River Irrigation Scheme), and the Shoalhaven River in New South Wales. Several inland rivers form internal deltas where they flood out into the Lake Eyre Basin, including Cooper Creek's Innamincka delta and the Diamantina-Georgina system.

Deltas in Australia are important for agriculture (especially sugarcane, rice, and cattle), for fisheries (deltas nurture estuarine fish stocks), and for biodiversity (delta wetlands support migratory waterbirds along the East Asian-Australasian Flyway). They are also vulnerable to sea-level rise, with the low-lying deltas of the Burdekin, the Fitzroy, and the Daly already affected by saltwater intrusion. Coastal management plans across northern Australia increasingly account for the long-term retreat of delta land.