How do river valleys become rias?
A ria is a coastal inlet formed when the sea floods the lower part of a river valley. A branching former drainage network can produce an indented coastline. Its origin differs from a fjord, which occupies a glacial valley.
The role of relative sea level
Rising sea level or sinking land can allow seawater to advance into a valley. The main valley becomes a larger inlet and tributaries become its branches, preserving aspects of the former drainage pattern. Depth and width depend on the old valley shape, subsequent sediment filling and coastal processes. The sea occupies a three-dimensional valley rather than simply moving a line inland.
Connections with estuaries and fjords
A ria may contain estuarine conditions where fresh and salt water mix, but ria describes landform origin while estuary emphasises the water environment. Fjords are often steeper-sided and glacially overdeepened, yet appearance alone should not determine the distinction. Maps, bathymetry and sediment records help reconstruct development. A quiet present-day appearance does not mean the coast has remained unchanged over time.