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Wetlands are areas where the presence of water determines or influences most, if not all, of an area's biogeochemistry—that is, the biological, physical, and chemical characteristics of a particular site, according to the . The amount of water present in a wetland can vary greatly.

Different plant communities may be found in different types of wetlands, with each species adapted to the local hydrology. Wetland plants are often referred to as hydrophytes (hydro = water, phyte = plant) because they are specially adapted to grow in saturated soils. Many bird, insect, and other wildlife species are completely dependent on wetlands for critical stages in their life cycles, while many other species make use of wetlands for feeding, resting, or other life activities.

A marsh is a type of wetland, an area of land where water covers ground for long periods of time. Unlike swamps, which are dominated by trees, marshes are usually treeless and dominated by grasses and other herbaceous plants, according to . 

The Forest Preserves of Cook County “Usually a permanent body of water, surrounded by and interspersed with vegetation. A hemi-marsh has an equal amount of ��һ�Թ�open water and plants, creating sheltered nooks for waterfowl to nest.”

According to the , a hemi-marsh is the primary wetland type needed by nearly all the Calumet’s declining wetland birds, including the Black-crowned night-heron, common gallinule, least bittern, and other rare species. Marshes may be dominated by submersed, floating-leaved, or emergent vegetation, including cattails, pondweeds, water lilies, and various sedges, rushes, spike rushes, grasses, and forbs (broad-leaved flowering plants). Marshes can be subcategorized into

The Wetlands Initiative says a hemi-marsh is found in deeper water and is characterized by an ��һ�Թ�open mix of emergent and/or floating-leaved vegetation interspersed with a submersed plant community. The submersed community may consist of species like sago pondweed, coontail, and wild celery, while the emergent or floating-leaved group can include deeper water species like broad-leaved cattail, American lotus, white water lily, and common bur reed. Big Marsh Park is a hemi-marsh.