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 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).
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.