Wetlands are an essential part of freshwater systems. Wetlands provide a wide array of goods and services, including flood control, nutrient cycling and retention, carbon storage, water filtering, water storage and aquifer recharge, shoreline protection and erosion control, and a range of food and material products, such as fish, shellfish, timber, and fiber. Wetlands also provide habitat for a large number of species, from waterfowl and fish to invertebrates and plants. Wetlands also have aesthetic and recreational values, although these are harder to quantify. They can include activities, such as bird watching, hiking, fishing, and hunting.

A large part of the world’s population lives in or near floodplain and wetland areas, where the soils are rich in nutrients and are therefore very fertile. As a result of their potential as agricultural land (and also because they are feared as places that harbor disease), wetlands have undergone massive conversion around the world. Modifying wetlands has been a major focus of many development and river regulation plans for decades. Wetlands have been either completely converted to other land uses (often by building drainage ditches and filling in swamps) or their functions have been altered gradually by changing hydrologic regimes and introducing agricultural crops and livestock (Revenga et al. 2000). Data on actual numbers of wetlands converted or modified are not available globally. However, Myers (1997) estimated that half of the wetlands of the world were lost in the 20th century.

This map shows the percentage of the basin area that is covered by wetlands. Eight basins have more than 20 percent of their area covered by wetlands. These basins include, in descending order in terms of percentage wetland area by basin: the Nelson, Mackenzie, and St. Lawrence basins in North America, the Fly and Sepik basins in Papua New Guinea, the Yukon basin in Alaska, the Brahmaputra, and the Mississippi.