What Are Wetlands? - Definition & Types

Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Also called Earth's kidneys, wetlands are dynamic and unique environments. Found all over the world, wetlands come in many different types and serve a variety of important functions.


Wetlands are dynamic aquatic ecosystems found all over the world. A wetland is an area of land that is saturated with water either permanently or seasonally. Wetlands can be freshwater, brackish (partly salty), or saline (very salty).

Wetlands are known as the earth's 'kidneys' because, like your kidneys, they serve the very important function of filtering water. As water moves through a wetland, the sediments and pollutants 'stick' in the wetland, making the water cleaner. Wetlands also help reduce flooding and prevent shoreline erosion.

Wetlands may be natural or human-made. The only continent on Earth that does not contain natural wetlands is Antarctica. Human-made wetlands may be constructed for water management purposes in urban areas. What is unique about all wetlands is that they contain vegetation that is specially adapted to such wet conditions. Plants that live in wetlands must be quite tolerant of wet, productive soils.


There are four main types of wetlands based on their location, water salinity, and dominant vegetation.

Marshes are wetlands that are usually permanently saturated, and can be saline or freshwater. Marshes can be tidal or non-tidal. Tidal marshes occur along coastlines and are affected by changing tides, while non-tidal marshes occur along streams. Tidal marshes are often brackish or saline, and the vegetation found in this type of wetland is adapted to both wet and salty conditions. Non-tidal marshes are usually freshwater, but can also be brackish. Non-tidal marshes are the most common wetland in North America.

Swamps are freshwater wetlands that are dominated by woody plants. There are many different types of swamps, and they become saturated with freshwater from stream and river runoff. The easily recognizable cypress tree is found in forested swamps.

Cypress trees in a forested swamp
cypress trees

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