The Canadian Wetland Classification System (National Wetlands Working Group 1997) is based on a hierarchical system, which includes (1) wetland class, (2) wetland form and (3) wetland type. The five wetland "classes" are differentiated by their developmental characteristics and the environment in which they exist. The five classes are: bog, fen, marsh, swamp, and shallow water. Some wetlands accumulate peat (partially-decomposed organic matter) and are called peatlands. Bogs and fens are the dominant peatland classes in Alberta, although some swamps and marshes can also accumulate peat. In contrast, shallow open water wetlands and many marshes and swamps do not accumulate peat.