A ria is a submergent coastal Landform, often known as a drowned valley or drowned river valley. Rias are almost always estuaries. Rias form when sea levels rise or Plate tectonics cause coastal levels to fall. 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. The Kingsbridge Estuary is an extreme example of a ria forming an estuary disproportionate to the size of its river, because no 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, Chichester 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.
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. A ria north of Rovinj on the western coast of Istria, Croatia, the Lim Bay, Limski kanal in Croatian is often called "Lim fjord" though the real Limfjord is in Denmark.
In northern Galicia, fronting on the Cantabrian Sea, the Rias Altas comprise the Ria de Ribadeo, the Ria de Viveiro, and the Ria de Ortigueira. On the west coast — the so-called Rias Médias and Rias 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 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.