Ria

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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 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.

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 comes from Galician language, as rias are present all along the Galician coast. It relates to the word rio (river). The 19 Galician rias are divided in three main groups: Rias Altas (Upper Rias), Rias Medias (Middle Rias) and Rias Baixas (Lower Rias).

Further examples

• England: The south coast of England is a submergent coastline which 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.

• Wales: Milford Haven in Pembrokeshire is a ria.

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

• New Zealand: Rias of various scales abound on the eastern shores of the upper North Island. On the west coast, in contrast, they are fewer but larger; Kaipara Harbour is the country's largest, and the Hokianga Harbour, further north, is of historical significance to the native Māori people. The Marlborough Sounds at the northern tip of the South Island form a large network of rias.

• United States: Narragansett Bay, the Chesapeake Bay, and Delaware Bay on the east coast of the United States and San Francisco Bay, on the Pacific coast, are rias. So is Willapa Bay and Grays Harbor in WA. The phenomenon is also common in South America.

The Maza bridge crosses the Ria of San Vicente de la Barquera located in Cantabria, Spain

• Brittany: The rias in northern Brittany are called Abers; Aber Wrac'h (48°35′59″N 4°32′58″W / 48.599807°N 4.549376°W / 48.599807; -4.549376 (Aber Wrac'h)), Aber Benoît (48°33′46″N 4°34′48″W / 48.562747°N 4.579905°W / 48.562747; -4.579905 (Aber Benoît)), Aber Ildut (48°28′22″N 4°44′59″W / 48.472649°N 4.749602°W / 48.472649; -4.749602 (Aber Ildut)).

• Andalusia: Ria of Carreras, Ria of Huelva.

• Asturias: Ria of Aviles, Ria of Ribadeo, Ria of Navia, Ria of Villaviciosa, Ria of Ribadesella, Ria of Llanes, Ria of Tina Mayor.

• Basque Country: Ria of Bilbao, mouth of the rivers Nervión, Ibaizabal and Cadagua.

• Cantabria: Ria of Tina Mayor, Ria of Tina Menor, Ría de San Vicente de la Barquera, Ría of la Rabia, Ría of San Martín de la Arena, Ría of Mogro, Ría of Solía, Ría of Carmen, Ría of Boo, Ría of Tijero, Ría of Cubas, Ría de Ajo, Ría of Cabo Quejo, Ría of Treto, Ría of Oriñón.

• Galicia: the Rias Baixas, like the Ria of Vigo, Ria of Pontevedra, Ria de Arosa, Ria of Muros and Noia, Ria of Corcubion and Cee. And the Rias Altas like the Ria of Coronne, Ria of Ares and Betanzos, Ria of Cedeira, Ria of O Barqueiro, Ria of Ferrol, Ria of Ortigueira, Ria of Viveiro, Ria of Foz and Ria of Eo.

See also

• Estuary
• Fjord
• Submergent coastline