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How does a drowned river valley become an estuary?

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Diagram showing four main types of estuaries: salt wedge, partially mixed, vertically homogeneous, and fjord. Each panel illustrates river and seawater mixing patterns, with arrows indicating the direction of saltwater and freshwater flow. — estuary Four main types of estuaries: (A) salt wedge estuary, (B) partially mixed estuary, (C) vertically homogeneous estuary, and (D) fjord (black arrows indicate salt water and white arrows fresh).

A drowned river valley becomes an estuary primarily through the rise in sea level that followed the melting of continental glaciers after the last glacial maximum, approximately 18,000 years ago.12

During periods of glaciation, sea levels were significantly lower, causing rivers to entrench their channels and deepen their valleys into the continental shelf. As the climate warmed and glaciers dissipated, the rising marine waters flooded these entrenched river valleys, transforming them into estuarine environments.12

These features are often referred to as rias when formed by the submergence of a river valley. In instances where the valleys were previously carved or deepened by glacial erosion, the resulting drowned valleys are known as fjords. Additionally, tectonic activity, such as the downfaulting or subsidence of a coastal zone, can produce the same effect as a rise in sea level, similarly creating an estuarine basin.312