What is an estuary?

Mt. Susitna and the upper reaches of Cook Inlet’s Knik Arm. Image from Wikipedia Commons

An estuary is a partially enclosed body of water, and its surrounding coastal habitats, where saltwater from the ocean mixes with fresh water from rivers or streams. The picture to the right shows Knik Arm at the northern most reaches of Cook Inlet. Here, fresh water from the Knik River mixes with salt water coming in from lower Cook Inlet and the Gulf of Alaska.

Types of Estuaries

Estuaries can be classified by the geological forces that create them.

  • Coastal Plains: When glaciers receded after the last ice age and sea level rose, seawater penetrated river valleys.Chesapeake Bay, MD is a a coastal plain estuary.
  • Tectonic: Significant tectonic movements can lead to the formation of a large depression or basin. As sea water floods in it will meet up with inflowing streams forming an estuary. San Francisco Bay, CA, is an example of a tectonic estuary.
  • Bar Built– Bar-built estuaries are formed by the accumulation of sediment around the mouth of a river. They can occur when sandbars or barrier islands are built up by ocean waves or by the deposition of river sediment at the mouth of the river. Pamlico Sound, NC is an example of a bar built estuary.
  • Fjord– Head south from Anchorage and in about two hours you will find yourself in the town of Seward, the gateway to Resurrection Bay, AK. Resurrection Bay is a classic fjord. The Bay is long and narrow with deep walls that have been cut out by the advancement of Glaciers. The water is significantly deeper (almost 1,000 feet) than the surrounding continental shelf.

Estuaries can also be classified by how fresh and salt water mix in the estuary. To learn more about each type of estuary you can check out NOAA’s -What is an estuary and Classifying estuaries by water circulation (animations)

  • Salt wedge- found at mouths of large rivers (e.g., Mississippi), the large fresh water influx “holds back” salt water at depth creating a salt wedge, little fresh and salt water mixing
  • Partially Mixed- deeper than well mixed, more shallow than Fjord, good fresh and salt water mixing, salinity changes both horizontally and vertically, big tidal influence
  • Vertically Mixed- typically shallow with strong tidal influx of salt water, extensive mixing of salt and fresh water, uniform salinity from top to bottom, water gets saltier the closer you get to the mouth of the estuary
  • Fjord- Deep, highly stratified water chemistry, fresh water flows out at surface, salt water flows in at depth, little mixing of fresh and salt water, anoxic conditions at depth

Water Chemistry in Estuaries

Dissolved Oxygen, nutrients, and salinity are all highly variable in estuaries. The variation of each of these is based on several factors such as: freshwater input, tidal ranges, depth reach of the estuary, pollution, run off, and sewage from human settlements. Estuaries are often major sources of seafood for the surrounding communities, but they are also often the place where sewage and industrial waste is flushed. Some of the worlds largest cities, sit unsurprisingly on some of the worlds most polluted and degraded estuaries.

Life in an Estuary

The combination of active mixing and nutrient input means that Estuaries can be more biologically productive than any other marine ecosystem. As such they can also be home to a large variety of organisms. Yet life in an estuary is no picnic. Large changes in salinity, in particular, mean that estuarial species have to be very adaptable. Organisms like crabs, clams, mussels, and worms are particularly common. (image fromWikipedia)

A rich array of habitats surround estuaries. The type of habitat is usually determined by the local geology and climate. Habitats associated with estuaries include salt marshes, mangrove forests, mud flats, tidal streams, rocky intertidal shores, reefs, and barrier beaches.

Salt marshes are daily flooded with saltwater by incoming tides. They are dominated by salt tolerant grasses and halophytes. The ground is primarily made of decomposing plant material and peat. Salt marshes have extraordinary biological importance, particularly to migrating birds.

In the tropical and subtropical regions where the sea surface temperatures never fall below 16 degrees C, mangrove forests may dominate estuaries. Mangrove forests serve as nurseries to developing fish. They stabilize soils, limit erosion, and protect coastlines during strong storms.

Where the shorelines of estuaries are protected from waves, clay and other fine particles can accumulate in expansive mudflats. These areas may look lifeless, but they provide important habitat for many species of burrowing worms and bivalves. They are important feeding areas for fish and seabirds.

The Economics of Estuaries

Estuaries, have historically provided the focal point around which coastal communities developed. Most of the worlds largest cities have been build on estuaries. These early developing towns and cities took advantage of the food availability, effective waste disposal, and high quality of life that estuaries provided.

Estuaries have significant economic importance. They provide habitat for commercial and sport fisheries. The are important routes for shipping and commerce. Coastal recreation and tourism generates hundreds of billions of dollars for the US economy each year.

The Roles of Estuaries in Ecological Systems

Estuaries do far more for our economy than supporting industries and providing jobs. Estuaries provide significant “ecological services” which directly benefit the surrounding communities. For example, estuaries protect landowners from flood waters and stabilize shorelines during storms, they play a major role in improving water quality by removing excess nutrients and pollution, and they provide habitat for countless plant an animal species. To learn more check out NOAA: Why estuaries are so important.

Estuary Destruction in the United States

Many of the worlds largest cities sit on estuaries. They have historically represented close access to food and an easy means to dispose of waste. Today these estuarial cities are centers of commerce and shipping. Habitat in estuaries has been destroyed with little regard for its many economic values and quality-of-life benefits. In just the United States, more than half (roughly 55 million acres) of wetlands have been destroyed. Estuaries have suffered damage in three major ways. First dredging, draining, and habitat destruction. In the United States, nearly 40% of estuarial habitat has been destroyed in this way, with some localities seeing much higher damage. The second source of destruction is the introduction of non-native invasive species. These species often outcompete local species and can lead to local extinctions. The third source of destruction is chemical pollution, particularly from the unintentional runoff of petroleum and agricultural wastes and To get more details go to

Oil drilling platform in Lower Cook Inlet, the Alaska Range in the backdrop. Image from: AK Business Magazine

Cook Inlet

You and I live within a few miles of an estuary. Cook Inlet is a tidal estuary that includes approximately 20,000 km2 of mudflats, rocky shoals and deep canyons. Several larger river systems, including the Susitna, Matanuska, Knik, Kasilof, and Kenai drain into Cook Inlet. Its watershed includes almost 100,000 square kilometers and most of the human population of Alaska.

Cook Inlet is big enough to contain several types of geological and salt water mixing features. Parts of Cook Inlet are Coastal Plain, parts are formed by the dynamic plate tectonics of the area. There are examples of tight glacier fjords and bar built estuaries.

Cook Inlet supports tremendous commercial and sport fisheries. Tourism, oil and gas production, and shipping exist in a delicate balance with the native wildlife. Many species thrive in this balance, while others have struggled. The Cook Inlet Beluga Whale was declared an endangered species in 2008, and may be facing extinction.

Tidal Currents in Cook Inlet, click to animate, image from Marine Ecosystem Dynamics Modeling


Questions to Research:

  1. What is an estuary? Write a description of an estuary in your own words.
  2. Use the text above to describe four types of geological estuaries. Based on those descriptions, what type of estuary is Cook Inlet? Resurrection Bay (Seward) is deep, narrow, and surrounded by glaciers. What type of estuary does that make it?
  3. Examine some data from the Pomhom Rocks Light House in the Providence River Estuary in Rhode Island. Make for yourself a copy of this spreadsheet and make a graph that compares depth vs dissolved oxygen (DO), temperature, and salinity for Pohom Rocks.
  4. Write a two sentence summary of your graph giving your best explanation of why these factors change with depth.
  5. Using the same spreadsheet, examine the second set of data from a site at South Prudence Island. Create a graph that compares depth vs dissolved oxygen (DO), temperature, and salinity for South Prudence Island.
  6. Write a two sentence summary of the how and why this site differs from Poham Rocks. (Hint: South Prudence Island is closer to the open ocean than the Providence River Site.)
  7. Explain two reasons why estuaries are so ecologically important and economically important.
  8. In our coastal states, more than half (roughly 55 million acres) of wetlands have been destroyed. Because of the on going threats discussed above, restoration has been a priority. Explain what the National Estuarine Research Reserve System (NERRS) is and the goal of the Estuary Restoration Act (ERA).
  9. GAK1 is a sampling station at the mouth of Resurrection Bay. It has been sampled for temperature and water chemistry each month since 1970. Describe two observations that can be made about Resurrection Bay from this long term study. (see image below)
  10. Do your best to explain why warmer conditions at GAK1 may be leading to lower salinity at the surface.
GAK 1 Anomalies

GAK 1 Anomalies, image from UAF