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How can coastal subsidence create 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).

Coastal subsidence creates an estuary by producing the same effect as a rise in sea level, which allows marine waters to inundate low-lying coastal areas.1

When a stretch of coastline undergoes broad local subsidence or down-faulting due to tectonic activity, the land surface drops relative to the sea. This process submerges coastal zones, such as river valleys or structural basins, creating a partially enclosed body of water where fresh water from rivers mixes with seawater. A prominent example of an estuary formed by such tectonic subsidence is San Francisco Bay in California.123