A marsh is a type of wetland.
It is dominated by herbaceous rather than woody plant species.
Marshes are formed in several ways. They can be formed by tides in lowland areas near a coast. Rivers often form marshlands on low lying floodplains and near lakes that flood during the wet season. Some marshes are seasonal and occur when the river is high, flooding grassland areas.
Marshes range in size from a marsh size of a small lake to marshlands that extends for hundreds of square kilometers/miles.
There are 3 types of marshes: freshwater, brackish water, and saltwater.
Ranging greatly in both size and geographic location, freshwater marshes make up the most common form of wetland in North America. They are also the most diverse of the three types of marsh. Freshwater marshes are usually found near the mouths of rivers, along lakes, and are present in areas with low drainage like abandoned oxbow lakes.
Brackish marshes develop by salt marshes where a significant freshwater influx dilutes the seawater to brackish levels of salinity. This commonly happens upstream from salt marshes by estuaries of coastal rivers or near the mouths of coastal rivers with heavy freshwater discharges in the conditions of low tidal ranges.
Saltwater marshes are formed by seawater flooding and draining, which exposes flat areas of intertidal land. They are found around the world in mid to high latitudes, wherever there are sections of protected coastline. It is dominated by dense stands of salt-tolerant plants such as herbs, grasses, or low shrubs.
Marshes serve as ecosystems for plants and animals, as barriers to erosion and as filters between estuaries and oceans.
Familiar marsh plants include cattails, bulrushes, reeds, grasses, and sedges.
Wild rice is of some commercial importance, but true rice is undoubtedly by far the most important marsh plant and supplies a major portion of the world’s grain.
Red-winged blackbirds, rails and great blue herons live in marshes.
Types of bass, pike, walleye and sunfish are common marsh fish.
The Everglades are the largest marsh in the United States and the largest contiguous freshwater marsh in the entire world. The Everglades is about 160 kilometers (100 miles) long and 100 km (60 miles) wide. It spans from Lake Okeechobee to Florida Bay and is often referred to as the “true Everglades” or just “the Glades.” The region covers more than 11,100 square kilometers (4,300 square miles).
The Okavango Delta in Botswana is one of Africa’s largest freshwater marshes. It was named as one of the Seven Natural Wonders of Africa. On 22 June 2014, the Okavango Delta became the 1000th site to be officially inscribed on the UNESCO World Heritage List.
The Mesopotamian Marshlands are located in southern Iraq and Iran. The confluence of the Tigris and Euphrates Rivers create the Mesopotamian Marshlands. The Mesopotamian Marshlands were once the largest wetland ecosystem in the Middle East, covering an area of 15,000 to 20,000 square kilometers. In the 1980s and 1990s, this marshland was drained by upstream dams and water control structures, down to 10% of the original area.
Horicon Marsh is a marsh located in northern Dodge and southern Fond du Lac counties of Wisconsin. It is the site of both a national and a state wildlife refuge.
Some areas, such as the northern Great Plains of the United States, have so many small marshes that they are a characteristic of the landscape.
Some areas of the world 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 the process of returning marshes to the landscape to replace those lost in the past. Restoration can be done on a large scale, such as by allowing rivers to flood naturally in the spring, or on a small scale by returning wetlands to urban landscapes.