Submergent coastlines are stretches along the coast that have been inundated by the sea due to a relative rise in sea levels. This occurs due to either isostacy or eustacy.

Submergent coastline are the opposite of emergent coastline

Emergent coastline

Emergent coastlines are stretches along the coast that have been exposed by the sea due to a relative fall in sea levels. This occurs due to either isostacy or eustacy....

s which have experienced a relative fall in sea-levels.

Features of a submergent coastline are:

• drowned river valleys or ria
  Ria
  A ria 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...
  
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• drowned glaciated valleys or fjord
  Fjord
  Geologically, a fjord is a long, narrow inlet with steep sides or cliffs, created in a valley carved by glacial activity.-Formation:A fjord is formed when a glacier cuts a U-shaped valley by abrasion of the surrounding bedrock. Glacial melting is accompanied by rebound of Earth's crust as the ice...
  
  s.

Estuaries

Estuary

An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....

are often the drowned mouths of rivers.