Every year the Amazon and its tributaries rise by 10 to 15 m (33 to 49 ft) and spill into the Amazon floodplain, the belt of seasonally flooded forest, grassland and lakes that lines the rivers in Brazil, Peru, Colombia, Bolivia and their neighbours. As they do, the area under water in the basin grows from about 110,000 km² in an average dry season to about 350,000 km² in the wet season. Floodplains of the sediment-rich whitewater rivers alone cover more than 300,000 km², about 180,000 km² of it várzea forest, while the nutrient-poor blackwater rivers such as the Rio Negro flood a second type of forest called igapó. It is the most extensive flooded forest in the world, and its best-known protected area is the 11,240 km² Mamirauá Sustainable Development Reserve near Tefé, a Ramsar wetland that forms part of the Central Amazon Conservation Complex World Heritage Site, together with Anavilhanas and Jaú national parks on the Rio Negro. Everything that lives here, from the tambaqui fish to the riverside farmers, runs on the same calendar of rising, high, falling and low water.
The floodplain is not a single habitat. Várzea forests are flooded by whitewater rivers such as the Solimões-Amazon, the Purus and the Madeira, which carry Andean silt and leave fertile soil behind each year; that makes them some of the most productive land in Amazonia and, historically, its most densely settled. Igapó is flooded by blackwater rivers, stained the colour of strong tea by humic acids from sandy soils. Its soils are sandy, acidic and poorer than the várzea's, and like other flooded forests it holds fewer plant species than dry-land forest, but trees there can stand submerged for up to six months of the year.
Within the várzea, height above the river decides everything. Low várzea sits under more than 3 m of water on average for more than 50 days a year; high várzea floods to less than 3 m for fewer than 50 days. The plants sort themselves along that gradient, which is why a short boat trip from a river channel can pass through several forest types.
November to February: the rising water
The water year begins with the rains. At Manaus, where the Rio Negro meets the Solimões, rainfall climbs from about 114 mm in October to 188 mm in November and 254 mm in December, according to the 1991–2020 normals of Brazil's national meteorological institute. Many branches of the Amazon start to rise in November and keep rising until June, though the tributaries are not synchronised: the Madeira rises and falls about two months earlier than most of the system, while the Rio Negro does not start rising until February or March.
The first rise triggers the most important fish migration in the basin. Tambaqui, a seed-eating fish shaped like a large piranha that can reach 1.1 m and 44 kg, gather in large schools and move into the whitewater rivers to spawn between November and February. Their larvae drift in the main channels, including the Amazon itself. The arapaima, or pirarucu, one of the largest freshwater fish in the world at up to 3 m long, times its breeding to the same moment: it digs a nest about 50 cm wide and 15 cm deep in muddy bottoms while the water is still low or just beginning to rise, so that the eggs hatch as the flood spreads.
At the mouth, the highest spring tides bring another kind of rising water, strongest around the March and September equinoxes. When the Atlantic tide is funnelled into the estuary it creates the pororoca, a tidal bore whose waves usually reach up to 4 m, with some accounts giving 7.6 m, and can travel as far as 800 km inland.
March to June: the high water, when the forest stands in the river
By April, the wettest month at Manaus with 331 mm, the floodplain is close to full. The Amazon's discharge to the Atlantic, which averaged 209,000 m³/s between 1973 and 1990, can reach 300,000 m³/s in the rainy season. Land that was walked on a few months earlier is now under several metres of water, and only the upper parts of the trees show above it in places along the stretch between the great bend at Canaria and the Rio Negro.
For the animals, high water is a time of plenty and of space. With predators dispersed or confined to smaller patches of dry ground, fish and other aquatic animals breed. Adult tambaqui spend four to seven months in the flooded forests of white, clear and black water rivers, feeding on fallen fruit and seeds; in one study of high-water stomach contents, fruit made up 78 to 98% of the diet. The fish that eat this fruit are among the animals that spread the seeds of floodplain trees. Amazonian manatees and river dolphins move into the várzea too. The pink river dolphin, the largest river dolphin, has unfused neck vertebrae that let it turn its head 90 degrees, and long fins that let it manoeuvre between flooded trunks. The sexes use the flood differently: males return to the main river channels while the water is still high, but females and calves stay in the flooded areas as long as possible, probably to avoid aggressive males and predators. Young arapaima, hatched as the water rose, use the high-water months to grow.
July to September: the falling water
When the rain eases, the water starts to drain. Manaus receives only 67 mm in July, 56 mm in August and 79 mm in September. As levels drop, tambaqui leave the forest for the main river channels and, to a lesser extent, the floodplain lakes. The dolphins follow them, and through the dry season they keep to the main river channels.
The lakes that the falling river leaves behind become traps. Cut off from the main channel, they lose oxygen. This is the arapaima's season: it breathes air and can survive in oxbow lakes with dissolved oxygen as low as 0.5 ppm, and in isolated lakes it becomes the top predator, picking off sluggish prey. It also has to surface to breathe, which is what makes it easy to harpoon. Overfishing led Brazil to ban arapaima fishing in 1996; since 1999, managed fishing has been allowed in designated areas, and in a sample of 10 such areas the count rose from 2,500 fish in 1999 to over 170,000 in 2017.
October and November: low water on the várzea
At the bottom of the cycle, around 110,000 km² of the basin is still under water, but much of the várzea is dry ground again. Forest floors that sat under several metres of river in May are now covered in leaf litter, and the fertile silt left on the banks becomes farmland. Riverside farmers plant rice, maize, beans, peppers and bananas on the fresh deposits. The soils are rich at first but lose fertility after two or three years of cultivation, and the next flood both renews them and sets the limit on what can be grown before the water returns. Cattle ranching on natural floodplain pasture has long been a principal activity, and in some areas, such as the floodplains of Amapá, herds have switched to water buffalo, which cope better with wet ground.
Low water is lean for the fish that fattened in the forest. After the flood, around 10% of a tambaqui's body weight is stored fat, and it needs it: in one sample of low-water fish, about 14% had empty stomachs, roughly ten times the high-water figure, and zooplankton made up about 70% of what the rest had eaten. Mamirauá shows how people fit into this calendar: the reserve's estimated 6,306 residents include caboclo, Ticuna, Kambeba and Kokama communities, and the reserve also protects caiman from illegal hunting for their skins.
When the cycle broke: the droughts of 2023 and 2024
The floodplain is built for a low-water season, not for one that never ends. In 2023 the combination of El Niño, a warm tropical North Atlantic and global warming produced the lowest rainfall in over 40 years in eight Brazilian states between July and September. The Rio Negro basin came close to its lowest recorded levels. In Amazonas state, 42 of 62 municipalities declared drought emergencies and about 250,000 people were affected, as crops and fish died and long queues for humanitarian aid formed on the riverbanks. In Lake Tefé, near Mamirauá, the water reached 39.1 °C, and about 150 tucuxi and pink river dolphins died.
The effects reached far beyond the floodplain. The Santo Antônio hydroelectric plant on the Madeira, Brazil's fourth largest, had to stop generating in 2023 for the first time since it opened in 2012. In August 2024, several rivers in the south-west Amazon hit record lows for the time of year in the first weeks of the dry season, and low water on the Madeira again cut output from two of Brazil's largest hydro plants.
Dams add a slower pressure on the same calendar. The Amazon's main stem is still undammed, but around 412 dams operate on its tributaries, 151 of them on six of the main ones, and hundreds more are planned; scientists warn they block fish spawning runs and cut the flow of nutrients that make the várzea so productive. A floodplain that depends on a predictable rise of 10 m or more each year is exposed to anything that flattens the curve. How the river itself behaves is covered in our complete guide to the Amazon River, the dolphins that follow the flood into the trees have their own guide to Amazon river dolphins, and further south the Paraguay River drives a similar flood pulse through the Pantanal wetlands.