The NWI is mapping all wetlands and deepwater habitats throughout the United States. Therefore, this inventory includes lakes, rivers, and streams as well as marshes, bogs, swamps, etc. The structure of the classification system is hierarchical, progressing from systems and subsystems at the most general levels to classes, subclasses, and modifying terms. Each system may be subdivided into a subsystem which further defines the characteristics. Classes describe the general appearance of the habitat in terms of either the dominant life form of the vegetation or physiography and composition of the substrate.
Subclasses are finer distinctions under each class.
For example, the code PEM1Fh describes a marshy area that has been artificially created through the use of a dike, or impoundment. Palustrine - See definition under Ponds and Marshes. Erect rooted, herbaceous, wetland plants, excluding mosses and lichens. Semipermanently flooded - Water stands throughout the growing season in lost years. When surface water is absent, the water table is usually at or very near the surface of the land.