upright=1.2|thumb|right|A submergent landform: the drowned river valley ([[ria) of Georges River in the greater Sydney area, Australia]]
Submergent coastlines or drowned coastlines are stretches along the coast that have been inundated by the sea by a relative rise in sea levels from either isostacy or eustacy.
Submergent coastline are the opposite of emergent coastlines, which have experienced a relative fall in sea levels.
Many submergent coastlines were formed by the end of the Last Glacial Period (LGP), when glacial retreat caused both global sea level rise and also localised changes to land height.
Formation
Submergent coastlines form either when sea level rises or the land level fall. This can be caused by isostatic or eustatic change. Both isostatic and eustatic change can be caused by a variety of reasons.
Isostatic change could be due to post-glacial adjustment. Rias commonly have a widening funnel shape and gradually increasing depth as they move towards the coast. The widening and deepening of the ria towards the sea usually causes an exaggerated tidal effect within the estuary.