Port Jackson, also referred to as Sydney Harbour, is a ria, or drowned river valley. The deeply indented shape of the ria reflects the dendritic pattern of drainage that existed before the rise in sea level that flooded the valley.
A ria (/ˈriːə/;[1]Galician: ría, feminine noun derived from río, river) is a coastal inlet formed by the partial submergence of an unglaciated river valley. It is a drowned river valley that remains open to the sea.