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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 submergent coastal landform, often known as a drowned valley or 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 south coast 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.

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 Pazinčica.

The word "ria" is of Portuguese or Galician origin. Rias can be found also in Asturias in Spain and in Portugal (Aveiro) but the most important are on the northern and western coast of Galicia.

In northern Galicia, fronting on the Cantabrian Sea, the Rias Altas comprise the Ria de Ribadeo, the Ria de Foz, the Ria de Viveiro, the Ria do Barqueiro, and the Ria de Ortigueira. On the west coast — the so-called Rias Medias and Rías Baixas — we find the Ria de Ferrol, Ria de Ares y de Betanzos, Ria da Coruña, Ria de Corme e Laxe, Ria de Camariñas, Ria de Muros e Noia, Ria de Arousa, Ria de Pontevedra, and Ria de Vigo.

The rias have shaped the development of Galicia as the population tended to cluster on their banks. A Coruña, Pontevedra, and Vigo are major ports. Fishing, and the production of shellfish on platforms (bateas) in the rias, are well-developed. The Rias region is one of high rainfall and moderate temperatures — between 0°C and 30°C. The sceneries and the beaches along these calm, relatively warm waters, attract tourists to such resorts as Sanxenxo and A Toxa.

• Aber
• Estuaries
• Submergent coastline
• Coastal
• Landform
• Plate tectonics