Classification of wetlands has been a problematical task, with the commonly accepted definition of what constitutes a wetland being among the major difficulties. A number of national wetland classifications exist. In the 1970s, the Ramsar Convention on Wetlands of International Importance introduced a first attempt to establish an internationally acceptable wetland classification scheme. Ramsar classification The Ramsar classification of wetland types is intended as a means for fast identification of the main types of wetlands for the purposes of the Convention. The wetlands are classified into three major classes: Marine/coastal wetlands Inland wetlands Human-made wetlands These are further subdivided by the type of water: fresh / saline / brackish / alkaline; and may be further classified by the substrate type of other characteristics. National systems of classification Australia Wetlands in Australia that considered to be of national importance are so classified by criteria published in association with the Directory of Important Wetlands in Australia (DIWA). The following list is that used within Australia to classify wetland by type: A—Marine and Coastal Zone wetlands Marine waters—permanent shallow waters less than six metres deep at low tide; includes sea bays, straits Subtidal aquatic beds; includes kelp beds, seagrasses, tropical marine meadows Coral reefs Rocky marine shores; includes rocky offshore islands, sea cliffs Sand, shingle or pebble beaches; includes sand bars, spits, sandy islets Intertidal mud, sand or salt flats Intertidal marshes; includes saltmarshes, salt meadows, saltings, raised salt marshes, tidal brackish and freshwater marshes Intertidal forested wetlands; includes mangrove swamps, nipa swamps, tidal freshwater swamp forests Brackish to saline lagoons and marshes with one or more relatively narrow connections with the sea Freshwater lagoons and marshes in the coastal zone Non-tidal freshwater forested wetlands B—Inland wetlands Permanent rivers and streams; includes waterfalls Seasonal and irregular rivers and streams Inland deltas (permanent) Riverine floodplains; includes river flats, flooded river basins, seasonally flooded grassland, savanna and palm savanna Permanent freshwater lakes (> 8 ha); includes large oxbow lakes Seasonal/intermittent freshwater lakes (> 8 ha), floodplain lakes Permanent saline/brackish lakes Seasonal/intermittent saline lakes Permanent freshwater ponds (< 8 ha), marshes and swamps on inorganic soils; with emergent vegetation waterlogged for at least most of the growing season Seasonal/intermittent freshwater ponds and marshes on inorganic soils; includes sloughs, potholes; seasonally flooded meadows, sedge marshes Lakeshore mudflats in freshwater lakes and ponds Permanent saline/brackish marshes Seasonal saline marshes Shrub swamps; shrub-dominated freshwater marsh, shrub carr, alder thicket