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Why do estuaries form when sea level rises over river valleys?

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continental shelf The broad, gentle pitch of the continental shelf gives way to the relatively steep continental slope. The more gradual transition to the abyssal plain is a sediment-filled region called the continental rise. The continental shelf, slope, and rise are collectively called the continental margin.

Estuaries form when sea level rises over river valleys primarily because of the geological history of river entrenchment during periods of lower sea level.12

During glacial stages, such as the Wisconsin Glacial Stage, massive amounts of water were locked in ice, causing global sea levels to drop significantly. As the baselevel declined, rivers were rejuvenated and entrenched their downstream reaches, carving deeper valleys into the continental shelf.123

When the climate warmed and glaciers melted, the rising sea level inundated these entrenched river valleys, creating "drowned river estuaries". These environments are characterized by the mixing of freshwater and seawater. While this process is a primary cause of estuary formation, it is important to note that estuaries can also be created by tectonic activity, such as coastal subsidence, or by the development of barrier islands that enclose coastal waters.124