Estuaries and their surrounding wetlands are bodies of water that usually form where rivers meet the sea. The brackish water found in an estuary is a mixture of fresh water that drains from the land (through watersheds) and salty sea water. There are five major types of estuaries that are classified based on their geologic origins
Image Sources: Google Maps
These estuaries were once river valleys, but when the glaciers melted some of the coastal streams and rivers flooded and nearby land became covered with water. A great example would be the Chesapeake Bay in Virginia.
Sandbars and barrier islands were built up by ocean currents and waves in coastal areas have created protected areas fed by small streams and rivers. The barrier islands of the US Atlantic Coastline would be an example.
Delta’s are created at the mouths of large rivers from sediment and silt deposits that are not washed away by currents or waves. When the river flow is restricted by the delta, estuaries may form. The estuary in the Mississippi River delta estuary is an example of this.
Estuaries that form in basins along major cracks or large land sinks are known as tectonic estuaries. Most of these develop due to earthquakes and are seen along fault lines, like San Francisco Bay.
Advancing glaciers carved out long, narrow, with steep sides and when the glaciers melted, sea water came rushing in. This is how Kachemak Bay in Alaska formed.