<P> A ria (/ riːə / or / riə /) is a coastal inlet formed by the partial submergence of an unglaciated river valley . It is a drowned river valley that remains open to the sea . Typically, rias have a dendritic, treelike outline although they can be straight and without significant branches . This pattern is inherited from the dendritic drainage pattern of the flooded river valley . The drowning of river valleys along a stretch of coast and formation of rias results in an extremely irregular and indented coastline . Often, there are islands, which are summits of partly submerged, pre-existing hill peaks . </P> <P> A ria coast is a coastline having several parallel rias separated by prominent ridges, extending a distance inland . The sea level change that caused the submergence of a river valley may be either eustatic (where global sea levels rise), or isostatic (where the local land sinks). 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 in Devon, England, 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 . </P>

Drowned valley forming and inlet of the sea