Amazon River

Long, long ago, the Amazon flowed the other way!

Then the Andes Mountains grew tall. Rain washed mud and sand down from the mountains. Little by little, the land filled up, and the river turned to flow east to the ocean.

Today the Amazon flows east, but for a long time it flowed the other way. Geologists at the University of North Carolina found tiny mineral grains in the middle of South America that could only have come from old mountains in the east. They showed that over 65 million years ago the river ran west, away from the Atlantic.

As the Andes Mountains rose in the west, rivers carried huge amounts of mud and sand off them. A 2014 computer study by Victor Sacek of the University of São Paulo found that this sediment filled low lakes along the mountains and built a slope down toward the Atlantic. Scientists in Brazil reported in 2019 that the Amazon was already flowing east by about 8.5 million years ago.

A 2006 study by Russell Mapes and Drew Coleman of the University of North Carolina, working with Brazilian colleagues, dated zircon grains across about 80 percent of the basin. Sediment in central South America came from now-eroded eastern highlands raised in the Cretaceous (145 to 65 million years ago) as South America and Africa separated, so the river then ran west. A low north-south ridge, the Purus Arch, rose in the middle of the continent and divided the drainage; as the Andes grew, their sediment filled the basin until the river breached the arch and took its present course. The team described three stages of drainage since the mid-Cretaceous.

The timing and cause of the final reversal are debated. A 2014 model by Victor Sacek of the University of São Paulo attributed it mainly to erosion and sedimentation from the Andes, against an earlier proposal that mantle movements reshaped the land. A 2019 University of São Paulo study, reported by FAPESP, dated rocks of the Solimões Formation and found evidence that the Amazon was already flowing to the Atlantic by about 8.5 million years ago, after flowing west during most of the Miocene. The young, easily eroded Andes still supply the sediment that makes the Amazon a muddy whitewater river.

Every year, the Amazon gets much higher, then lower again.

At the city of Manaus, the water goes up about as high as a three-floor building. Many houses there stand on tall posts, called stilts.

The Amazon's most striking rhythm is its flood pulse: every year the water rises and falls. Amazon Waters reports that the yearly change in river level is usually between 4 and 15 meters. At Manaus, in the heart of the Amazon, it averages about 10 meters, the height of a three-story building.

The floods spread over the floodplain, the low land beside the river, and flood the forest each year. Houses built on stilts are common on the floodplain. Near Manaus the water peaks in June and July. In the Andean foothills, rivers can rise 4 to 9 meters in a single day after heavy rain, then drop back within a day or two.

Amazon Waters, drawing on the Smithsonian Atlas of the Amazon (Goulding and colleagues, 2003), puts average annual river-level fluctuations at 4 to 15 m, greatest (10 to 15 m) between the middle Madeira and the middle Juruá, 8 to 10 m upstream of the Madeira mouth and about 10 m at Manaus, falling to under 2 m in the tidal estuary. Extreme recorded ranges are about 1.5 to 2 times the average. Because the pulse is fairly predictable, it is called a flood pulse.

The flood does not move as one wave. The first peak comes in April and May near the Xingu mouth, about 200 km from the Atlantic; the next in May near Iquitos; and the last near Manaus in June and July, about two months after the rainiest period, because of a backwater effect between northern and southern tributaries. In the Andean foothills levels can rise 4 to 9 m in one day. Floodplain bank erosion is so active that it returns more sediment to the river than the net flux past Óbidos, about 900 km from the sea.

The Amazon is a very big river. Big rivers can be stronger than they look.

Always wear a life jacket in a boat, and stay close to a grown-up. Watch river dolphins and caimans from far away. Leave plants and animals where they live.

Big rivers can hide dangerous currents, and rivers in the Andean foothills can rise fast after heavy rain. The CDC says life jackets lower the risk of drowning when boating, for people of all ages, and that children should wear one around natural water with a grown-up watching closely. Floating toys are not safety devices. For riverboat trips in Peru, the UK government's travel advice suggests checking the operator's credentials with the iPeru information service.

People are working to protect the river, but Amazon Waters warns that the river itself is poorly protected for its size. One bright spot is the Mamirauá Sustainable Development Reserve, where the Solimões meets the Japurá: more than 10,000 people live in or near it, and residents and scientists together set zones, with about a third kept as a protected zone. UNESCO says Jaú and Mamirauá need well-defined plans for visitors.

The CDC's drowning-prevention advice fits Amazon river travel: lakes and rivers have hidden hazards such as dangerous currents; life jackets reduce the risk of drowning while boating for all ages and swimming abilities; children should wear them for all activities in and around natural water, under close and constant adult supervision; air-filled or foam toys are not safety devices. Amazon Waters also notes that rivers in the Andean foothills can rise 4 to 9 m in a single day. For Amazon riverboat cruises in Peru, the UK Foreign, Commonwealth and Development Office suggests checking operators' credentials with the iPeru information service.

Conservation is uneven. Amazon Waters reports that the river is very poorly protected for its size, with no functional protected areas along its lower 2,000 km and the most modified floodplains on the lower Solimões near Manaus and below the Rio Negro. It calls Mamirauá, at the Solimões–Japurá confluence, the only functional conservation unit alongside the river: more than 11,000 km², nearly all floodplain, zoned by residents and academics with about one-third designated a protected zone, though illegal logging is hard to prevent. UNESCO adds that Brazil's national parks have advisory boards and its sustainable development reserves have legislative councils, and that Jaú and Mamirauá need public use plans for tourism and research.