Marshes often form in low-lying areas adjacent to water bodies. Water levels typically vary from a few inches to 3 feet, with areas sometimes drying out completely. Periodic flooding brings nutrients and minerals, which settle in the slow moving waters to form rich marsh sediments. Soils are further enriched by the decomposition of marsh plants, which forms peat. Marshes serve to recharge groundwater, filter water of excess nutrients, moderate stream flow and reduce downstream flooding and erosion.
Marshes are differentiated from swamps by the amount of water cover and the types of vegetation present. Whereas swamps generally contain more water and are dominated by woody vegetation, marshes are shallower, usually treeless, and characterized by soft-stemmed vegetation. Common marsh plants include lilies, reeds, sedges, grasses and rushes. Most marsh plants reproduce by spreading roots, as opposed to through seeding. Since soils tend to contain little oxygen, many marsh plants have large leaves and internal structures to store oxygen drawn from the air.
Marshes are particularly productive habitats for birds, fish, small mammals, amphibians and aquatic insects. Insects thrive due to the large amount of standing water. For birds, the slow-moving, shallow water provides an excellent hunting ground, especially considering the numerous species of insects, amphibians and fish present. Due to the abundance of food, many bird species also use marshes as breeding grounds. Many small mammals congregate in marsh ecosystems, including muskrat, mink, beaver, raccoons and otters.
Coastal salt marshes are characterized by daily tidal flow. Salinity within the marsh varies depending on location relative to the incoming tides, with upper marsh areas receiving relatively less tidal inflow. Like freshwater marshes, salt marshes are shallow with slow moving water and rich sediments. Due to variations in salinity and water level, tidal marshes often contain numerous microhabitats that can harbor a diverse group of plant and animal species in a relatively small area. Unfortunately, many coastal marshes are threatened by human development, nutrient runoff from agriculture and diversion of river water.