The first stage in the formation of a tectonic estuary is when the rapid movement of the Earth’s crust causes a large piece of land to sink, or subside, producing a depression or basin. These drastic changes typically occur along fault lines during earthquakes. If the depression sinks below sea level, ocean water may rush in and fill it. The same geological forces that create these depressions often form a series of natural channels that drain fresh water from nearby rivers and streams into these newly formed basins. The mixture of seawater and fresh water creates a tectonic estuary. Estuaries formed in this manner are typically very deep and surrounded by mountainous areas. San Francisco Bay, on the West Coast of the United States, is an excellent example of a tectonic estuary.
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