These very large rivers transport massive volumes of freshwater (>10,000m³/s) through flat lowlands, mostly in tropical or subtropical regions. Their very large flow volumes, diverse habitats and slow to moderate flows make them highly productive. High nutrient levels come from upstream catchments and floodplains, with additional productivity contributed by in-channel algae and aquatic plants. Their food webs are complex, with a high diversity of plants and animals, including large-bodied fish, reptiles and mammals.