Science Discussion: Types of Wetlands

Wetlands is a broad term.
The term “wetlands” is pretty general. If you see that word and think it means “land that is wet,” then you’d be right! But “land that is wet” can mean a lot of things and as you might guess is too vague to be used as the sole descriptor for a given area. To clear up some of the ambiguity in the word “wetland,” scientists classify wetlands into four main types: marshes, bogs, swamps, and fens.

What characteristics did scientists use to define the four types of wetlands?
Wetlands are characterized according to their soils, water regime, and vegetation.
Soil is just what kind of dirt is there. Does it have a lot of clay? A lot of peat (definition: peat is a type of soil composed mainly of decaying plants and mosses)?  Soil also takes into account how much water is held by the dirt. Is the soil usually submerged under standing water, or is it periodically flooded and drained? This aspect of the soil classification is closely tied to the water regime.

Unsurprisingly, when you do a google image search for
Unsurprisingly, when you do a google image search for “soil,” most of the results are either just a pile of brown dirt, or people artistically holding dirt in their hands. I opted for the slightly more visually interesting picture of someone holding dirt. Image courtesy of OSU EESC.

Water regime refers to both what sort of water is keeping the wetland wet, and the periodic variations in total water content (or lack there of).  For types of water keeping the ground wet, scientists are primarily concerned with surface water sources vs groundwater.  There are many types of surface water sources: rainfall, runoff, tides, snowmelt, and streams to name a few. Groundwater is just what is sounds like – water stored underground, usually in water saturated soil or pores in rocks. When the depth at which the ground becomes saturated with water (called the “water table”) is very shallow, the groundwater will feed the surface soil and create wetlands, ponds, lakes, or streams. Describing how groundwater and the water table work in words is awkward and I’m not sure that I did a great job of it. To ensure clarity, here is a helpful picture from the USGS that gives a visual explanation to complement what I tried to explain verbally.

Here is a diagram of how groundwater works, including the level of the water table. Notice how when the level of the land surface dips below the level of the water table, the land begins to fill with pooled water.
This is how groundwater works. Notice how when the level of the land surface dips below the level of the water table, the land begins to fill with pooled water. In this diagram, “surface water” refers to pooled water above the land surface, such as a wetland, river, lake, etc.

One thing that was surprising to me is that whether a wetland is fed by saltwater or freshwater does not necessarily play into its classification. For example, there are many examples of both freshwater and saltwater swamps. However, bogs are exclusively freshwater. The reason freshwater vs saltwater isn’t a unique parameter is because there are both saltwater and freshwater types of surface water (think snowmelt runoff vs ocean tides – both allow water to infiltrate the wetland from above, but one is fresh and the other is salty). For some reason, scientists chose to be more concerned with surface water sources vs groundwater instead of salt vs freshwater when classifying wetlands. I don’t know exactly why that decision was made, but it is what it is.
Classification by variations in total water content is more concerned with questions like whether the wetlands have consistent standing water, or varying water depths dependent on the time of year? Are the wetlands ever totally submerged in water, or is the ground just very soggy all the time? It is the combination of soil type and water regime determines what the vegetation in the wetland is composed of.

Vegetation of the wetland is simply what kinds of plants are best suited to grow there. Different plants prefer different growing conditions (you wouldn’t try to grow a cactus in the rainforest), and so it follows that the different water regimes and soil type combinations lead to different plant populations thriving in the various types wetlands. Some wetlands are well suited to grow large trees, such as red maple, bald cypress, evergreens, or willow. Other wetlands are most hospitable to herbaceous (meaning non-woody) plants. Common herbaceous plants include cattails, pickerelweed, and a variety of mosses.

Examples of different types of vegetation found in Canada. Vegetation ranges from trees to shrubs to grasses to mosses and everything in between.
Examples of different types of vegetation found in Canada. Vegetation ranges from trees to shrubs to grasses to mosses and everything in between.

Okay, so what are the characteristics of the four types of wetland: marshes, bogs, swamps, and fens?

Marshes
Soil:
Marsh soil can be periodically flooded, or have a continual standing, although very shallow, pond on top of it. Soil is consistently saturated with water regardless of whether there is any standing water. 
Water Regime: 
Marshes are supplied by a combination of groundwater and surface water. There are varying ratios of groundwater to surface water contribution which lead to a number of subtypes of marshes. Marshes that have a larger groundwater contribution tend to be wetter for longer portions of the year, or even have permanent shallow standing water. Marshes that rely more heavily on surface water sources tend to be more seasonally wet. For example, a marsh fed mostly by snowmelt runoff is going to be wettest in the winter and spring, and largely dry during the summer and fall. 
Vegetation:
Marshes grow non-woody plants the best. Their wet soils and periodic flooding is usually too much water for woody trees to handle, so plants that don’t mind spending some portion of the year partially submerged in water dominate this wetland ecosystem. 

Photograph of a marsh in New England. Note the abundant grassy, weedy plants and shallow standing water. Also note that there are no trees growing inside the marsh, only on the edges.
Photograph of a marsh in New England. Note the abundant grassy, weedy plants and shallow standing water. Also note that there are no trees growing inside the marsh, only on the edges.

Bogs
Soil:
Bogs are composed of spongy peat soil. They continually produce peat soil as vegetation in the bog dies and decomposes.
Water Regime:
Bogs are exclusively freshwater as they are created by an abundance of rainwater in cool-cold climates (such as the northern United States and Canada). They are not fed by groundwater or any other surface water sources.
Vegetation:
Bogs grow evergreen trees, some shrubs, and a thick carpet of moss. They are densely vegetated.

A beautiful bog in Wisconsin. Not a whole lot of standing water, but abundant vegetation which will eventually be converted into the characteristic soil of bogs: peat. Image courtesy of wetlandsandwater.com
A beautiful bog in Wisconsin. Not a whole lot of standing water, but abundant vegetation which will eventually be converted into the characteristic soil of bogs: peat. Image courtesy of wetlandsandwater.com

Fens
Soil:
Fens, just like bogs, produce a lot of peat soil.
Water Regime:
Fens are fed almost exclusively by groundwater, and also occur most frequently in the northern US and Canada. 
Vegetation:
Fens grow willow and birch, as well as a variety of grasses, reeds, and wildflowers.

Fens are similar to bogs in that they produce peat, but fens are distinct in that they are fed by groundwater. This difference leads to differences in vegetation as well: fens grow willows, grasses, and reeds whereas bogs grow evergreens, shrubs, and mosses.
Fens are similar to bogs in that they produce peat, but fens are distinct in that they are fed by groundwater. This difference leads to differences in vegetation as well: fens grow willows, grasses, and reeds whereas bogs grow evergreens, shrubs, and mosses.

Swamps
Soil:
Swamps have very wet soils, and usually will have a significant amount of standing water over the soil for some or all of the year. They are often found in floodplains (which is just the term for the land along a body of water like a river or bay that floods in high water).
Water Regime:
Swamps receive most of their water input from surface water, such as river flooding and tides. Swamps can be either freshwater or saltwater, depending on what kind of surface water is feeding them.
Vegetation:
Swamps are dominated by trees and shrubs. Common trees in freshwater swamps are bald cypress, swamp white oak, and red maple. Buttonbush and Swamp Rose are the main shrubs that grow in swamps. Saltwater swamps in coastal wetlands grow a unique tree called the Mangrove, which can basically live as a little floating island in fairly deep, salty water. Mangroves are really quite amazing and I highly recommend googling them.

A swamp with large bald cypress trees growing in it.
A swamp with large bald cypress trees growing in it.

That was a lot of information…what are the key points to remember?

  • There are four types of wetland: marshes, bogs, fens, and swamps
  • Wetlands are primarily classified by their soil, water regime, and vegetation
  • Marshes get water from a variety of ways and have wet soil, sometimes with very shallow standing water. They grow grasses and weeds and NO TREES.
  • Bogs make peat and are fed by rainwater. They grow evergreen, shrubs and moss.
  • Fens make peat and are fed by groundwater. They grow willows, grasses, and wildflowers.
  • Swamps have relatively deep standing water and form in floodplains – both freshwater and saltwater. They grow cypress, maple, and shrubs. Coastal swamps grow mangrove trees.

Want to learn more? I found the EPA (Environmental Protection Agency)’s handout on wetland classification to be pretty straightforward and easy to read. As of September 2015, it can be found here.

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