Freshwater wetlands are diverse habitats supporting a range of plants and animals with distinct requirements for and varying ability to thrive in saturated soil and standing water. Freshwater, non-tidal wetlands, also called palustrine wetlands, are commonly grouped into three general categories according to the dominant vegetation they support: marshes, swamps, and bogs. Marshes, also called emergent wetlands because the vegetation emerges out of standing water, are characterized by soft-stemmed herbs and grasses plants like arrow arum, cattails, and rushes. Swamps are home to water-loving woody plants and are often called scrub-shrub wetlands if dominated by low woody plants, like dogwood, willow, buttonbush and elderberry, or called forested wetlands if dominated by trees such as, sweet gum, white ash, red maple and pin oak, and shade tolerant shrubs like spicebush and chokecherry. Bogs, as well as fens and vernal pools, form around watertight, rain-fed basins, such as depressions left by glaciers. They are usually particularly sensitive to stresses from development and urbanization since they are not buffered by connections to other larger wetland systems and can also be very small.
Another approach to categorizing freshwater wetlands that is useful in the Bronx is based on hydrologic regime and position in the landscape, or hydrogeomorphology. Depressional wetlands are found in topographic low points that may or may not have inlets or outlets, and can be located at various places in the watershed. These wetlands may have standing water, open water, or only saturated soil for some period during the growing period. In the Bronx Zoo, some depressional wetlands have been constructed to provide exhibit space (as shown in the photo), but others existed historically, though they now are home to zoo animals or have been modified to better withstand additional animal use and runoff. Riverine wetlands are found in the floodplains of larger streams and rivers, as they are in the Bronx Forest Floodplain. These wetlands in the Bronx are forested, supporting pin oak, and red maple and dogwood, as well as spring ephemerals such as skunk cabbage. Fringe wetlands, located along the edges of ponds, lakes and estuaries, are found at the edges of the impoundments on the Bronx River. At the forest edge in the New York Botanic Garden and the Bronx Zoo, for example, buttonbush, elderberry, arrow arum, lizards tail, and iris grow in standing water much of the year.Despite their limited size and extent, all freshwater wetlands in the Bronx play an important local role in offering habitat for fish, waterfowl, amphibian and reptiles, providing flood storage, retardation, detention, and erosion control, and helping to protect water quality. They also play a particularly important role in providing recreational and aesthetic benefits by offering beauty and nature in a dense urban environment.