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Types of Wetlands.

What's the difference between a "swamp" and a "bog"? Or a "marsh"? There really are different, specific meanings to these terms, and it's not just an arbitrary choice to use one word or the other.

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Types of Wetlands

A "wetland" is a place that is not quite land and not quite water. It has characteristics of both, and it has unique characteristics that set it apart from either land or water.

That simply seems self-evident, but it really is important to clarify our terms. In the interest of clarity, I try to use the terms for various types of wetlands correctly. But as with almost anything in today's science, there is some disagreement, and there is always some ambiguity. So, on this page, I'll describe what "most scientists" mean by these terms, and if the way I use the terms is different, I'll tell you why, and why the differences are important. I'll also provide a few examples of each - bog, swamp, and marsh - both from places where I hike and from places that are generally well known. I'll also provide a few examples of well-known ambiguous wetlands that have characteristics of two or more types.

Read on to discover the differences between "swamps," "bogs," and "marshes," and some of the other less-precise terms for types of wetlands.

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Swamp

A "swamp" is a low wetland where water from streams or rivers collects in a shallow flat area before flowing out in another stream or river. A swamp is usually muddy, and frequently supports trees, either water-tolerant trees like mangrove and cypress, or "land trees" on islands within the swamp.

Since the water in a swamp flowed in from a stream, it may be rich in oxygen and nutrients. Therefore, a swamp may support fish and other aquatic animals.

The best example of a swamp in my usual hiking areas is the swamp across U.S. Route 302 from Elephant Head. It receives water from a stream that flows off Mount Willard, and it discharges into the Saco River. Most of the area of the swamp is muddy stands of reeds and rushes, but there are distinct channels of open water within it. Classic swamp.

Swamp near Elephant Head

The swamp across the road from Elephant Head is a classic swamp.

Perhaps the best-known example of a classic swamp in North America is the bayou country of the Mississippi Delta. It receives water from the Mississippi, the Atchafalaya, and other rivers, and discharges into the Gulf of Mexico through several distinct channels. (Maybe it merges into marsh at many places along its fringe, but it's mostly swamp.)

Maybe you thought of Okefenokee or the Everglades? I consider each of them ambiguous. Read on.

Characteristics of a swamp:

  • Receives water from streams or rivers
  • Discharges water in a definite flow which may be streams or rivers, or direct discharge of fresh water into the sea or a lake
  • Muddy bottom, not peaty
  • Often supports trees
  • May be permanent or seasonal

There is some disagreement in the scientific community about the definition of a "swamp," but it often depends on vegetation. At least in North America, a "swamp" is a wetland that is covered with trees and that is not a "bog." My definition is different, as I think it should depend more on geology than on biology. So, "most scientists" would probably call the wetland across from Elephant Head a "marsh," but I call it a swamp. I am a little ambiguous myself, as my definition should call the mangrove areas of the Florida Keys a "marsh," but I usually call it a swamp. Even by my definition, the mangrove "swamps" along the fringes of the Everglades are fairly unambiguously a "swamp."

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Bog

A "bog" is a wetland, often higher than the surrounding land, which holds water by virtue of the absorbency of its thick layer of peat. This peat is the main distinguishing characteristic of a bog, and it may be composed of sphagnum, lichen, or any other accumulated vegetation. A bog receives water only via precipitation, and releases water pretty much in all directions via general seepage.

Because it receives water only as precipitation, a bog is usually poor in oxygen, as well as in nitrogen and other nutrients. Furthermore, the slow decay of peat renders it somewhat acidic. As a result of all this, a bog usually supports few if any fish, and the plants often have special adaptations to obtain nitrogen from other sources. Many bog plants are able to fix nitrogen from the atmosphere, and most insectivorous plants - which obtain nitrogen from the insects they "eat" - are found in bogs.

The best example of a bog in my stomping grounds (I crack myself up) are the two bogs on the northern side of Mount Jackson. No stream flows into them, and only a few small seeps flow out of them in spring and summer. They support no trees, and there is no open water within them. The land trembles when you stomp on it, just as in Okefenokee (try jumping on the trail bridges and see what happens). While the peat here is more lichen than sphagnum, these are nevertheless classic bogs.

Bog on Mount Jackson

The larger and lower of the two bogs on Mount Jackson.

Bog plants

Close-up of bog vegetation, including moss, lichen, sundew, and others.

There are few well-known bogs in North America. I'll have to cite the peat bogs of the British Isles and northwestern Europe.

Okefenokee Swamp is technically a bog. It receives water only from precipitation, and that water is poor in oxygen and nutrients, but it does have distinct discharge into the Suwannee and St. Mary's Rivers. Also, un-boglike, it supports trees and some fish, and it has areas of open water. Still, it is a vast peat sponge higher than the surrounding land, so it is more of a bog than a swamp.

Characteristics of a bog:

  • Receives water from precipitation only
  • Discharges water as general seepage rather than distinct flow
  • Thick layer of peat
  • Low levels of dissolved oxygen and nutrients
  • Water is acidic
  • No trees, or very few
  • No fish, or very few
  • Insectivorous plants
  • Virtually all bogs are permanent, though their outflow may be seasonal

My definition of "bog" agrees pretty well with that used by "most scientists."

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Marsh

A "marsh" is a wetland on the fringes of a body of water. The water may be nearly stationary, exchanging with main body of water only via wave action or convection, or it may flow into and out of the main body if there is a tide. A marsh may also occur in an estuary, where a river spreads out into a shallow, grassy area before emptying into the sea or a lake. A marsh may be separated from the main body of water by sandbars or barrier islands, but water is usually exchanged through large channels which can't really be called rivers.

A marsh may be fresh or saline, depending on the nature of the main body of water, or it may be brackish if it is an estuary where a river meets the sea. It usually has the same properties of dissolved nutrients as the main body of water, but it may be poorer in oxygen if the water exchange is poor. Marshes often serve as breeding grounds for fish that live in the more open water.

Many of the muddy areas along the Nashua River in Mine Falls Park are marshes. Maybe one or two of the reedy places on the fringe of the Mill Pond in Mine Falls Park could also be called marshes.

The fringes of the Everglades, where it meets the Gulf of Mexico, are more like a marsh than a swamp. Perhaps a better example is the vast salt marsh areas behind the barrier islands of the Mid-Atlantic, such as the shallow stretches of Pamlico Sound and the phragmites thickets of Virginia's Tidewater region.

Characteristics of a marsh:

  • Adjacent to a body of open water
  • May or may not have a distinct flow, including a tidal flow
  • Muddy bottom, not peaty
  • No trees
  • May support fish, especially fry
  • May be permanent or seasonal
  • May be fresh, saline, or brackish

My definition of "marsh" is a bit different from the definition used in the scientific community. In North America, a "marsh" is a wetland that is covered with grasses, sedges, and similar vegetation and that is not a "bog." I think the definition should depend more on geology than on biology. So, "most scientists" would probably call the wetland across from Elephant Head a "marsh," but I call it a swamp.

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Other Terms

Other terms for wetlands tend to be regional or archaic and not generally used. Their meanings can be vague or ambiguous, so I stick with the three terms defined above, which are ambiguous enough.

Examples of some of these terms are "fen," "slough," and a whole slough of others.

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Further Reading

Here are a few references elsewhere on the Web that might confuse you further:

  • Child-oriented (and child-authored) ThinkQuest article on Types of Wetlands.
  • Wikipedia article on Wetlands.
  • U.S. Environmental Protection Agency article on Wetlands.
  • Wikipedia article on Swamps.
  • Kildeer Countryside Virtual Wetland Preserve article on Swamps.
  • Wikipedia article on Bogs.
  • Kildeer Countryside Virtual Wetland Preserve article on Bogs.
  • U.S. Environmental Protection Agency article on Bogs.
  • Gulf of Maine Aquarium/Research Institute article on Bogs.
  • Wikipedia article on Marshes.
  • Kildeer Countryside Virtual Wetland Preserve article on Marshes.
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