As in North America, the West and East Coasts of South America are very different from a geological viewpoint, which is reflected in the Neotropical coastal wetland distributions (Figure 8.1). Tides on both coasts are semi-diurnal or mixed and mostly of medium range (2–3 m) except for macrotidal areas in the northwest, southeast and at the mouth of the Amazon River (Eisma, 1997). The West Coast, part of the ‘Pacific Ring of Fire’, is tectonically active and bordered by the high Andean mountain ranges, which restrict the amount of lowland available for spread of intertidal wetlands. Here rivers and streams are small, wave erosion is high and earthquakes followed by tsunami waves can liquefy the marsh sediments, resulting in subsidence of up to 1.6 m over large areas (e.g. 200 km in 1979). In contrast, the East Coast passive margin is formed mainly from sedimentary basins and it has extensive low-lying plains crossed by long rivers. There are large deltas at the mouths of the Orinoco, Amazon and São Francisco rivers, which supply sediment to huge mudflats (Figure 8.2). The Amazon River is the source for vast amounts of sediment that coastal currents transport to the giant mudbanks (‘slikke’) off Surinam and French Guiana.