Earth's largest tropical rainforest surrounds the river with the greatest mean flow. Trees return moisture to the air, rivers enter flooded forest, and both systems change sharply from west to east and season to season.
There is more than one valid Amazon boundary. The river's strict drainage basin covers about 6.3 million km²; ecological Amazonia extends differently, and Brazil's Legal Amazon is a government reporting region — with Brazil holding roughly 60% of the forest.
Plot-based models estimate about 390 billion large tree stems and roughly 13,600–16,000 tree species, but those are extrapolations from a tiny sampled area, not a forest census. Just 227 modelled "hyperdominant" species account for about half of those stems.
The river pours about a fifth of all the fresh water that reaches the oceans. More than 30 million people call the wider region home, including hundreds of Indigenous groups.
Trees return moisture to the air — an atmospheric pathway often called flying rivers — and seasonal floods turn forest into waterways.
A plot-based model found that just 227 tree species accounted for about half of Amazonia's estimated large stems — even though thousands of tree species may live across the region.
The tallest tree ever found here tops 80 metres — about a 25-storey building.
Models estimate that local evaporation and transpiration supply roughly 25–40% of rain over the Amazon basin.
Sediment-rich whitewater floods create várzea forest; nutrient-poor blackwater and clearwater floods create forms of igapó.
Female Amazon river dolphins with calves use flooded várzea at high water.
Giant armadillos dig burrows that dozens of other forest animals later use as shelter.
Field Notes
A Forest in Four Layers
Rainforests are often introduced as four overlapping layers rather than rigid floors. Emergent crowns rise into full sun, where macaws and harpy eagles fly.
A dense canopy captures much of the light; how many animals and plants gather there changes with the stand. Even that green roof varies — some stands open after disturbance, others stay closed for decades.
The understory grows in shade as vines and young trees reach upward. On the floor, where little sun lands, ants, fungi, roots and other recyclers break down what falls.
Height and structure shift with soil, water, disturbance and forest type — várzea, igapó and terra firme do not share one diagram.
Research Report
The Flying Rivers
Most Amazon moisture first arrives from the Atlantic. After rain reaches the forest, roots draw up water and leaves return it to the air.
Models estimate that local evaporation and transpiration supply roughly 25–40% of rain over the basin, with the exact share changing by season and method. Winds rain some of that moisture again over the forest and carry some southward — an atmospheric pathway often called a flying river.
Expert Dossier
The Flooded Forest
Seasonal river rises can move the boundary between water and forest by metres. Sediment-rich whitewater floods várzea; nutrient-poor blackwater and clearwater flood forms of igapó, creating different soils and tree communities.
Fruit-eating fish enter the trees to feed and spread seeds, while female Amazon river dolphins with calves use flooded várzea. Flood height and duration vary greatly by river — there is no single Amazon flood.
Jaguars still need vast, connected territories across this mosaic. Explorers learn the local river first: flood peaks differ from one tributary to the next.