thumb|300px|Marshlands are often noted within [[wetlands, as seen here in the New Jersey Meadowlands at Lyndhurst, New Jersey, U.S.]]
thumb|upright=1.35|right|Marsh in shallow water on a lakeshore
In ecology, a marsh is a wetland that is dominated by herbaceous plants rather than by woody plants. More in general, the word can be used for any low-lying and seasonally waterlogged terrain. In Europe and in agricultural literature low-lying meadows that require draining and embanked polderlands are also referred to as marshes or marshland.
Marshes can often be found at the edges of lakes and streams, where they form a transition between the aquatic and terrestrial ecosystems. They are often dominated by grasses, rushes or reeds. If woody plants are present they tend to be low-growing shrubs, and the marsh is sometimes called a carr. This form of vegetation is what differentiates marshes from other types of wetland such as swamps, which are dominated by trees, and mires, which are wetlands that have accumulated deposits of acidic peat.
Marshes provide habitats for many kinds of invertebrates, fish, amphibians, waterfowl and aquatic mammals. Moreover, they have an outsized influence on climate resilience of coastal areas and waterways, absorbing high tides and other water changes due to extreme weather. Marshes have extremely high levels of biological production, some of the highest in the world, and therefore are important in supporting fisheries. Marshes (and other wetlands) are able to absorb water during periods of heavy rainfall and slowly release it into waterways and therefore reduce the magnitude of flooding. Marshes also provide the services of tourism, recreation, education, and research.
Salt marshes are most commonly found in lagoons, estuaries, and on the sheltered side of a shingle or sandspit. The currents there carry the fine particles around to the quiet side of the spit, and sediment begins to build up. These locations allow the marshes to absorb the excess nutrients from the water running through them before they reach the oceans and estuaries.
Freshwater tidal marshes
Although considered a freshwater marsh, the ocean tides affect this form of marsh. However, without the stresses of salinity at work in its saltwater counterpart, the diversity of the plants and animals that live in and use freshwater tidal marshes is much higher than in salt marshes. The most severe threats to this form of marsh are the increasing size and pollution of the cities surrounding them. They are regularly flooded but are often dry in the summer.
Vernal pools
Vernal pools are a type of marsh found only seasonally in shallow depressions in the land. They can be covered in shallow water, but in the summer and fall, they can be completely dry. In western North America, vernal pools tend to form in open grasslands, whereas in the east, they often occur in forested landscapes. Further south, vernal pools form in pine savannas and flatwoods. Many amphibian species depend upon vernal pools for spring breeding; these ponds provide a habitat free from fish, which eat the eggs and young of amphibians. Similar temporary ponds occur in other world ecosystems, where they may have local names. However, vernal pool can be applied to all such temporary pool ecosystems. Like vernal pools, they are only present at certain times of the year and generally have a circular shape. As the playa dries during the summer, conspicuous plant zonation develops along the shoreline.
Prairie potholes
thumb|201x201px|[[Vernal pools are ponded only during the wetter part of the year.]]
thumb|upright=0.9|right|Aerial view of prairie potholes
Prairie potholes are found in northern North America, such as the Prairie Pothole Region. Glaciers once covered these landscapes, and as a result, shallow depressions were formed in great numbers. These depressions fill with water in the spring. They provide important breeding habitats for many species of waterfowl. Some pools only occur seasonally, while others retain enough water to be present all year.
Riverine wetlands
Many kinds of marsh occur along the fringes of large rivers. The different types are produced by factors such as water level, nutrients, ice scour, and waves.
Embanked marshlands
Large tracts of tidal marsh have been embanked and artificially drained. They are usually known by the Dutch name of polders. In Northern Germany and Scandinavia they are called Marschland, Marsch or marsk; in France marais maritime. In the Netherlands and Belgium, they are designated as marine clay districts. In East Anglia, a region in the East of England, the embanked marshes are also known as Fens.
Restoration
Some areas have already lost 90% of their wetlands, including marshes. They have been drained to create agricultural land or filled to accommodate urban sprawl. Restoration is returning marshes to the landscape to replace those lost in the past.