Rias and fjords are both types of drowned valleys that form as a result of changes in sea level, particularly through processes of glaciation and subsequent sea level rise.

While both rias and fjords result from the flooding of pre-existing valleys due to rising sea levels, the processes that create these valleys are distinct. Rias develop in river valleys, whereas fjords originate in valleys shaped by glacial activity.

Rias typically form in regions that have undergone tectonic activity. When tectonic forces uplift the land, river valleys are created. Over time, these valleys are eroded by the river, leading to wide, flat valley floors and gently sloping sides. When sea levels rise—often as a consequence of melting ice caps—these river valleys can become inundated, resulting in the formation of a ria. A notable example of a ria is the Dart Estuary in Devon, England.

In contrast, fjords are formed in areas that have experienced significant glaciation. Glaciers carve out deep, U-shaped valleys characterized by steep sides and flat or slightly rounded valley floors. Upon the retreat of the glacier, these valleys typically take on a deep, narrow shape. If sea levels rise subsequently, these glacial valleys can become flooded, resulting in the creation of a fjord. The steep sides of a fjord are a direct consequence of the glacier’s erosive power, and the depth of the fjord often exceeds that of the surrounding sea. The Sognefjord in Norway, recognized as the world’s second-longest fjord, serves as an excellent example of this phenomenon.

In summary, both rias and fjords are formed due to changes in sea level; however, the processes that create the valleys they flood differ significantly. Rias emerge from river valleys, while fjords originate from valleys sculpted by glaciers.