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Why were Amazon River bottomlands able to support dense Indigenous populations before European conquest?

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Map of South America and the Caribbean showing the distribution of Aboriginal cultural groups, with regions color-coded by cultural type including Andean, Central American, tropical forest, and various nomadic cultures such as aquatic nomads, forest hunters and gatherers, and shellfish gatherers. Major group names are labeled across the continent and surrounding islands. — South American forest Indian Distribution of aboriginal South American and circum-Caribbean cultural groups.

Before the European conquest, the Amazon River bottomlands were able to support dense, sedentary Indigenous populations primarily due to the high agricultural potential of the várzea floodplains.12

These areas possess nutrient-rich soils that are naturally rejuvenated each year by the deposit of fertile silt as floodwaters recede. This environment allowed Indigenous groups to practice intensive root-crop farming, which was effectively supplemented by fishing and the hunting of aquatic mammals and reptiles.12

Furthermore, Indigenous peoples significantly enhanced the local landscape through their own activities. They created terra preta dos indios ("black earth of the Indians"), a superior and localized soil type that developed on the sites of prehistoric settlements, further increasing the land's long-term productivity. These combined factors provided the stable food resources necessary to sustain larger, more concentrated populations compared to the small, seminomadic tribes that inhabited the more elevated, less fertile upland areas.132