Wetlands can occur wherever water affects the soil. Not only are there therefore many kinds of wetlands, but their size and shape is very variable. Wetlands can include small pools in deserts and seepage areas on mountainsides, or they can be long but narrow strips on shorelines of large lakes (Figure 3), or they may cover vast river floodplains (Figure 4) and expanses of northern plains. The two largest wetlands in the world (both >750 000 km2) are the West Siberian lowland and the Amazon River basin. The West Siberian Lowland consists largely of fens and bogs, but marshes occur along rivers, particularly in the more southern regions (Figure 5). The Amazon is a tropical lowland with freshwater swamps and marshes containing more kinds of trees and fish than any other region of the world.
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