Georges River, in the southern suburbs of Sydney (Australia) 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 is a landform, often referred to as a drowned river valley. Rias are almost always estuaries. Rias form where sea levels rise relative to the land either as a result of eustatic sea level change (where the global sea levels rise), or isostatic sea level change (where the land sinks). When this happens valleys which were previously at sea level become submerged. The result is often a very large estuary at the mouth of a relatively insignificant river (or else sediments would quickly fill the ria). The Kingsbridge Estuary is an extreme example of a ria forming an estuary disproportionate to the size of its river; no significant river flows into it at all, only a number of small streams.

The southcoast of England is a submergent coastline, and contains many rias, including Portsmouth Harbour, Langstone Harbour, Chichester Harbour, Pagham Harbour, Southampton Water, Poole Harbour, the estuaries of the Exe, Teign and Dart, the Kingsbridge Estuary, and Plymouth Sound in Devon, and the estuaries of the River Fowey and River Fal in Cornwall. Milford Haven in Pembrokeshire, Wales is also a ria.

The east coast of Australia features several rias around Sydney, including Georges River, Port Hacking and Sydney Harbour itself.

The Marlborough Sounds are a large network of rias at the northern tip New Zealand's South Island.

Narragansett Bay, Chesapeake Bay and Delaware Bay, on the east coast of the United States, and San Francisco Bay, on the Pacific coast, are rias. The phenomenon is also common in South America.

The rias in northern Brittany are called Abers; Aber Wrac'h (), Aber Benoît (), Aber Ildut ().

Rias are sometimes confused with fjords. Although both are formed in drowned valleys, fjords are created not by rivers but by glaciers. For instance, a ria north of Rovinj on the western coast of Istria, Croatia, the Lim Bay (Limski kanal in Croatian) is often called "Lim fjord", although it was not actually formed by glacial erosion but by the river Pazin?ica.

See also

• Estuary
• Fjord
• Submergent coastline

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