U.S. flag An official website of the United States government.
				Here's how you know we're official. **Official websites use .gov** A **.gov** website belongs to an official government organization in the United States.
 **Secure websites use HTTPS** A small lock or **https://** means you’ve safely connected to a .gov website. Share sensitive information only on official, secure websites.


Toggle navigation Toggle search National Ocean Service National Oceanic and Atmospheric Administration
 Search
					Search National Ocean Service


Home
				Ocean Facts

				Topics
					Explore All Topics 
                        Arctic 
                        Careers 
                        Economy 
                        Ecosystems 
                        Geodesy 
                        Hazards 
                        International
                        Navigation 
                        Observations 
                        Ocean 
                        Recreation 
                         Sea Level Rise 
                        Story Maps 
                        Tools
                
                			Education

				
				News
				Podcasts
				Video
				Images
				About Us


1. Home
2. Education
3. Estuaries
4. Classifying Estuaries: By Geology

**Classifying Estuaries: By Geology** Estuaries Tutorial  The features of an estuary are determined by a region's geology, and influenced by physical, chemical, and climatic conditions. For example, movements in the Earth’s crust elevate or lower the coastline, changing the amount of seawater that enters an estuary from the ocean. The coastal elevation also determines the rate of fresh water that flows into an estuary from rivers and streams. The amounts of seawater and fresh water flowing into an estuary are never constant. The quantity of seawater in an estuary changes with the changing tides, and the quantity of fresh water flowing into an estuary increases and decreases with rainfall and snowmelt. Classification by Geology Estuaries are typically classified by their existing geology or their geologic origins (in other words, how they were formed). The four major types of estuaries classified by their geology are drowned river valley, bar-built,  tectonic, and fjords. In geologic time, which is often measured on scales of hundreds of thousands to millions of years, estuaries are often fleeting features of the landscape. In fact, most estuaries are less than 10,000 years old. Drowned river valley estuaries are formed when rising sea levels flood existing river valleys. Bar-built estuaries are characterized by barrier beaches or islands that form parallel to the coastline and separate the estuary from the ocean. Barrier beaches and islands are formed by the accumulation of sand or sediments deposited by ocean waves. Tectonic estuaries occur where the Earth’s tectonic plates run into or fold up underneath each other, creating depressions. Fjords are steep-walled river valleys created by advancing glaciers, which later became flooded with seawater as the glaciers retreated. Drowned River Valley Estuaries The Chesapeake Bay on the East Coast of the United States and Coos Estuary on the West Coast are both coastal plain estuaries. These, and most other coastal plain estuaries in North America, were formed at the end of the last ice age between 10,000-18,000 years ago. As glaciers receded and melted, sea levels rose and inundated low-lying river valleys. Coastal plain estuaries are also called drowned river valleys. Other examples of coastal plain estuaries include the Hudson River in New York, Narragansett Bay in Rhode Island, the Thames River in England, the Ems River in Germany, the Seine River in France, the Si-Kiang River in Hong Kong, and the Murray River in Australia. In the animation below, rising waters flood a low-lying river valley, creating a coastal plain estuary.  The Chesapeake Bay (center) and Delaware Bay (upper right) are both examples of drowned river valley estuaries.*The Chesapeake Bay (center) and Delaware Bay (upper right) are both examples of drowned river valley estuaries. (Credit: NASA)*  Bar-built Estuaries Bar-built or restricted-mouth, estuaries occur when sandbars or barrier islands are built up by ocean waves and currents along coastal areas fed by one or more rivers or streams. The streams or rivers flowing into bar-built estuaries typically have a very low water volume during most of the year. Under these conditions, the bars may grow into barrier beaches or islands and the estuary can become permanently blocked. The area between the coast and the barrier beaches or islands are protected areas of calm water called lagoons. Barrier beaches or islands break the impact of destructive ocean waves before they can reach the estuary and mainland, consequently protecting them. The barrier beaches take the brunt of the waves' force and are sometimes completely washed away, leaving the estuary and coast exposed and vulnerable. During heavy rains, large volumes of water flowing down the river or stream can also completely wash away small bars and reopen the mouth of the estuary. Bar-built estuaries are common along the Gulf Coast of Texas and Florida, in the Netherlands, and in parts of North Carolina. Good examples are Pamlico Sound in North Carolina, Matagorda Bay in Texas, and the Nauset Barrier Beach System on Cape Cod, Massachusetts.  The coast of North Carolina consists of a series of bar-built estuaries. The image above is of Back Sound, the estuary separating Cape Lookout, at the southern portion of the Outer Banks from the mainland of North Carolina.*The coast of North Carolina consists of a series of bar-built estuaries. The image above is of Back Sound, the estuary separating Cape Lookout, at the southern portion of the Outer Banks from the mainland of North Carolina.*  Tectonic Estuaries 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. San Francisco Bay, on the West Coast of the United States, is an excellent example of a tectonic estuary.  The San Francisco Bay in California is an example of a tectonic estuary. (Credit: NASA)*The San Francisco Bay in California is an example of a tectonic estuary. (Credit: NASA)*  Fjords Fjords (pronounced fee-YORDS) are typically long, narrow valleys with steep sides that are created by advancing glaciers. The glaciers leave deep channels carved into the Earth with a shallow, narrow sill near the ocean. When the glaciers retreat, seawater floods the deeply incised valleys, creating estuaries. Fjords tend to have a moderately high input of fresh water from land. In comparison, very little seawater flows into the fjord because the sill prevents the deeper salty waters of the sea from mixing with deep waters of the fjord. This poor water exchange results in stagnant, anoxic (low oxygen) water that builds up on the bottom of the fjord. Not surprisingly, fjords are found in areas that were once covered with glaciers. Glacier Bay in Alaska and the Georgia Basin region of Puget Sound in Washington State are good examples of fjords. Fjords are also found throughout Canada, Chile, New Zealand, Greenland, Norway, Siberia, and Scotland.  DThis satellite image of Glacier Bay Alaska shows many characteristic long narrow fjords that have been carved out of the surrounding terrain by advancing glaciers.*This satellite image of Glacier Bay, Alaska, shows long narrow fjords that have been carved out of the surrounding terrain by advancing glaciers.*
 Estuaries **Lessons** Welcome What is an Estuary? Economy and Environment Ecosystem Services Geology Water Circulation Estuarine Habitats Adaptations to Life in the Estuary Natural Disturbances to Estuaries Human Disturbances to Estuaries Monitoring Estuaries The Future References
  More **Information** Roadmap to Resources Subject Review  The National Science Teaching Association (NSTA) includes this resource in its database. NSTA provides educators and students access to Web-based, educationally appropriate science content that has been formally evaluated by master teachers.
   **Search** Our facts    Get **Social** Social
  **Last updated:** 02/26/21 **Author:** NOAA How to cite this article Contact Us


Feedback Provide Feedback
							Contact Us
							Report an error
 Social Instagram Logo/a>


*Science. Service.Stewardship.*
 About NOS About Us
							NOS Program Offices
							Media Inquiries
							Website Acknowledgements
                            Site Index
							Disclaimer
 More Resources Freedom of Information Act
							Accessibility
							Information Quality
							Privacy Policy
							NOS For Employees
							USA.gov
 Department of Commerce
									National Oceanic and Atmospheric Administration (NOAA)
									National Ocean Service