ephemeral river of southwestern Western Australia. It rises in the hills south of Corrigin as the Avon and flows 224 mi (360 km) northwest and southwest past Northam and Perth to the Indian Ocean at Fremantle. It is known as the Swan only along its lower 60-mi course. The rivers Helena (site of Mundaring Weir) and Canning are left-bank tributaries. Dry during much of the summer and autumn, the river runs through steep gorges and rapids on its upper course, widening at the coast to form two shallow tidal basins (Perth and Melville waters). A causeway was completed across the river at Perth in 1843 and a bridge at Fremantle in 1866. Above Perth, its banks are lined with vineyards and market gardens.

The river in its lower reaches, explored in 1697 by Willem de Vlamingh, was named for the black swans found there. Western Australia's first free settlement was made on its banks in 1829.