Drakes Estero, Point Reyes National Seashore, California
Tidal channels and mudflats exposed at low tide in the upper reaches of Drakes Estero, a drowned river valley. During the Pleistocene glacial epochs, worldwide sea levels were lowered as much as 425 feet. Water was removed from the oceans to form vast continental glaciers. During periods of lowered sea level, valleys were eroded and deepened in shallow offshore and coastal regions. As the glaciers melted and sea level rose again, the deeper valleys were flooded. A drowned river valley is an estuary - a coastal water body that is open to the ocean and is diluted by fresh water from the land.
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