Classifying Estuaries: By Geology

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.

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.

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.

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.