PART I: What is an estuary?

Introduction to Estuaries

In most coastal communities, the local "estuary" is called by another name: San Francisco Bay, Chesapeake Bay, Puget Sound, Boston Harbor, Cook Inlet -- all "estuaries." But what makes them so, and why are they important to the country and to the 110 million Americans who live near their shores?

Estuaries are bodies of water along our coasts that are formed when fresh water from rivers flows into and mixes with salt water from the ocean. In estuaries, the fresh river water is blocked from streaming into the open ocean by either surrounding mainland, peninsulas, barrier islands, or fringing salt marshes. This mixing of fresh and salt water creates a unique environment that brims with life of all kinds -- a transition zone between the land and sea known as an estuary. The estuary gathers and holds an abundance of life-giving nutrients from the land and from the ocean, forming an ecosystem that contains more life per square inch than the richest Midwest farmland.

Why are healthy estuaries important to us? Estuaries are a critical source for much of our ocean life. Their bounty forms a natural wonder that offers the more than 50% of Americans who live near estuaries, and the millions who visit, a wealth of recreational opportunities. Estuaries provide essential habitat for over 75 percent of our nation's commercial fish catch. Commercial and recreational fishing, boating and tourism also provide more than 28 million jobs. Fishing alone generates $111 billion yearly in economic activity.

The tourism value of estuaries to local and regional economies is significant. In 1993 more than 180 million Americans visited coastal waters nationwide -- nearly 70 percent of the U.S. population. And, in one year alone (1991), 73 million people in the U.S. spent more than $10 billion on recreational boating products and services.

Estuaries are important to our quality of life and our health for reasons other than jobs, healthy economies, and recreational opportunities. The local bay or sound often serves as the focal point for community life and traditions, hosting everything from harvest festivals to busy ports. They also protect water quality, are a center for research and education, and help stem the erosion of our shoreline communities.

Estuaries, in short, are national treasures -- vital ecological and community resources whose health affects our health and the vibrancy of our communities and economy.

1. Summarize the characteristics that distinguish an estuary from other bodies of water.

2. List a few reasons why estuaries are important and why we should know how they work.

Other web sites with helpful information about estuaries include:

Types of Estuaries

Every estuary is different. Some are deep and some are shallow. Some have big areas and some are quite small. Some have large openings to the sea and others have small openings. Some have rivers that input lots of fresh water while others have rivers with only trickles of fresh water. Some are surrounded by human settlements and others are very sparsely developed.

To understand how estuaries work, we must understand how fresh and salt water mixes, or circulates. This mixture has a huge impact on what organisms can live in the estuary, and on how toxic substances are retained or flushed out. How an estuary circulates depends on its size and shape, the amount of fresh water entering into it, the range of tidal action, and on human modifications.

Based on their natural characteristics, estuaries can be divided into four main types. Keep in mind that an estuary can be different types at different locations (for example, the ocean end versus the river end), and at different seasons (for example, the rainy season versus the dry season). Because salt water is more dense than fresh water, estuaries tend to contain fresher water at their surfaces and saltier water at their bottoms.

a. Salt-wedge estuary:

This type forms where a large river inputs lots of fresh water in an area where the tidal range is low or moderate (Figure 1a). The exiting fresh water holds back a wedge of intruding seawater. Because fresh water is less dense than salt water, the river water remains near the top and flows seaward. The seawater wedge moves seaward during falling tides or times of strong river flow; it moves landward during rising tides. Mixing occurs at the boundary between the two water masses and can be especially concentrated at the Bay's bottom, where fresh and salt water meet. This type has a strong salinity gradient from surface to bottom and is found in the mouths of major rivers, such as the Mississippi, or where rivers enter other estuaries, such as the mouth of the Sacramento into the San Francisco Bay.

b. Well-mixed estuary:

This type forms where rivers input smaller amounts of fresh water and the tidal range is moderate to high (Figure 1b). Whereas in the salt-wedge type there is a strong salinity gradient from surface to bottom, in the well-mixed type salinity is more constant from surface to bottom. As in the other types of estuaries, there is a seaward salinity gradient from fresher water landward to saltier water seaward. Well-mixed conditions tend to occur in shallow estuaries, such as the mouth of the Columbia River, where tidal currents can thoroughly mix river and sea water.

c. Partially-mixed estuary:

This type is transitional between salt-wedge and well-mixed types, and occurs in deeper estuaries with moderately strong tidal currents and greater amounts of river inflow (Figure 1c). Mixing is driven both by tidal turbulence and river flow. The salinity gradient is transitional between types a and b. Examples are the main body of San Francisco Bay and Puget Sound.

d. Reverse estuary:

This type forms along arid coasts when rivers cease to flow and the evaporation of seawater in the uppermost reaches cause water to flow from the ocean into the estuary, producing a salinity gradient of increasing salinity from the ocean to the estuary's upper reaches (Figure 1d). Reverse estuaries, sometimes called lagoons, are common on the Pacific coast of Mexico's Baja Peninsula and along the U.S. Gulf Coast. Because of high evaporation, salinity values in the estuary often exceed that of the open ocean.





Figure 1. Types of estuaries. Salinity values in each type show the amount of mixing between fresh water (0 parts / thousand) and sea water (33 parts / thousand). Solid lines represent lines of equal salinity. Note that 33 parts per thousand = 3.3 parts per hundred (3.3%). Diagram from Garrison, 1995, Essentials of Oceanography: Wadsworth Publishing Company.






Some Estuaries in the U.S.

Click here to go to the web site for the National Estuarine Research Reserve System (NERRS). Choose an estuary from the list of reserves. Read information about the estuary.

3. Briefly describe some interesting features about the estuary you chose.

4. Based on the description (particularly tidal range and river flow), what type of estuary (Figure 1 above) do you think exists at your site?

Like Rivers, Estuaries have Watersheds

Because rivers feed fresh water into estuaries, the size and condition of the river's watershed can have a large impact on the estuary itself. For example, if there is agriculture or other industries within the watershed, rivers will carry chemicals associated with these activities, as well as fresh water, into the estuary. Rivers with large watersheds are likely to contribute larger amounts of fresh water to estuaries than rivers with small watersheds.

Click here to go to a map of watersheds for the estuaries within the National Estuary Program, operated by the Environmental Protection Agency. Observe the variation in watershed size for some of the estuaries around the country. Choose one of the estuaries and click on its watershed to obtain information about this site.

5. Based on the information available for the site you have chosen, what activities in the watershed may be posing problems for the estuary's health?


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