How do flooded river valleys form estuaries?

Flooded river valleys, often referred to as drowned river estuaries or rias, form through a combination of river entrenchment and subsequent sea-level rise.123

During periods of lower sea level—such as the maximum of the Wisconsin Glacial Stage approximately 18,000 years ago—rivers adjusted to a lower baselevel by deepening and entrenching their downstream reaches into the continental shelf. As global temperatures increased and glaciers melted, the resulting rise in sea level caused marine waters to invade and inundate these previously carved valleys.12

In some coastal regions, such as Norway and British Columbia, these valleys were further deepened by glaciers before being flooded, creating deep, narrow fjords. This process of submergence transforms the lower portion of a river system into an estuarine environment where fresh water and seawater mix.123

Are you interested in learning more about how different types of estuaries are classified based on their water-mixing processes?