Drowned valley coasts are also known as estuaries. Estuaries are confined bodies of water that occupy the drowned valleys of rivers that are not currently building open-coast deltas. The estuarine environment can also be defined as the complex of lagoon-bay-inlet-tidal flat and marsh that make up 80 to 90 percent of the U.S. Atlantic and Gulf Coasts.
During periods of lowered sea level, rivers incised the lower reaches of their valleys and discharged increasing amounts of sediment out onto the shelf. Deltas accumulated and fluvial channels were cut, dissecting parts of the delta plain. At the lowest stands of sea level, estuaries almost disappeared and were confined to river valleys. When sea level rose again, the valleys were flooded and the estuaries reappeared.
The largest drowned valley coasts in the U.S. are the Chesapeake and Delaware Bays on the Atlantic coast. Barriers and drowned valleys continue south to Miami, Florida, along the Atlantic coast.