The Liquid Forest


July 31, 2012


Tropical rainforest rivers are often overwhelming to the first-time visitor because of their size and abundance. Even more perplexing is the ability of tropical rivers to fork into large branches, forming giant islands that can be easily confused with the mainland. It is sometimes nearly impossible to distinguish which is the main fork of the river.

Overhead pictures of tropical rivers reveal another curious aspect: the meandering course. A river will twist and turn, sometimes turning almost 180 degrees back on itself. The lack of slope and the clay-like soils of many tropical regions allow rivers to have virtually free rein over their direction.

Bend in the Javari river in the Amazon rainforest. Photo by Rhett A. Butler

The volume of water flowing through tropical forests, coupled with the soils and varying water levels, can create great river cliffs over 100 feet high, even at regular water levels. These clay banks form an important part of the local ecology in parts of the Amazon. Macaws gather by the hundreds on some of these banks to ingest minerals that bind to and detoxify chemicals in the fruits they consume.

With their huge volume, large rivers like the Amazon transport tremendous quantities of wood and debris. It is common to see giant logs and trees passing, though sometimes natural meadow rafts, complete with trees and animals and sometimes a shack, are seen floating downstream. Often river navigation is complicated by massive logjams that form in river channels. The Rio Madeira, a sizeable tributary of the Amazon, gets its name from the large amounts of wood that pass down the river. These logjams, along with sunken wood, provide a critical habitat for fish and other aquatic animals.

Sungai Utik creek in Borneo. Photo by Rhett A. Butler


Tropical streams and creeks are even more variable than tropical rivers and can change from a virtually dry river bed to a raging torrent 30 feet deep in a matter of hours during a heavy rain. Smaller streams and creeks are often invisible by air because they flow beneath the rainforest canopy. Despite their inconspicuousness, these waterways house an astounding array of animal life. Creeks are common in the rainforest and provide an important niche for certain fish, amphibian, and insect species in addition to providing an important source of water for other forest floor dwellers.

Some of these creeks, especially in lowland Amazonia, can be surprisingly deep with a U-shaped riverbed. The clay substrate helps these creeks keep their form and seemingly defy the laws of physics.

Amazon River where it forms the border between Peru and Colombia. Photo by Rhett A. Butler



The Amazon River is the most voluminous river on Earth, eleven times the volume of the Mississippi, and drains an area equivalent in size to the United States. During the high-water season, the river's mouth may be 300 miles wide, and up to 500 billion cubic feet per day (5,787,037 cubic feet/sec) may flow into the Atlantic. For reference, the Amazon's daily freshwater discharge into the Atlantic is enough to supply New York City's freshwater needs for nine years. The force of the current, from sheer water volume alone and virtually no gradient, causes the current to continue flowing 125 miles out to sea before mixing with Atlantic salt water. Early sailors could drink fresh water out of the ocean before sighting the South American continent.

The river current carries tons of suspended sediment, causing the characteristic muddy whitewater appearance. It is calculated that 106 million cubic feet of suspended sediment are swept into the ocean each day. The result from the silt deposited at the mouth of the Amazon, is Majaro island, the world's largest river island, about the size of Switzerland.

Despite its immense size, the Amazon today is being affected by human activities, including deforestation, which can affect sediment levels and water flows; dams, which disrupt nutrient cycles, water flow, and fish migration; and climate change, which is causing more extreme conditions including flood and drought.


The Congo River (formerly Zaire River), is Africa's most powerful river and the second most voluminous river (not counting the Madeira and Negro which are considered part of the Amazon) in the world, with 1,500,000 cubic feet of water passing out of its mouth every second. It is the fifth longest river in the world, draining a basin of nearly 1.5 million square miles.

The river is best known for its role in history. Known as the heart of darkness by Joseph Conrad, the river and surrounding rainforest have long been known for brutal colonialism and ensuing conflict.

The river itself is as turbulent as its history, though it begins peacefully enough in the savannas just south of Lake Tanganyika. Gradually the river widens and picks up speed until it enters the "Gates of Hell," a 75-mile long canyon of impassable rapids. The river emerges again, surrounded by lush tropical rainforest, as the Lualaba or Upper Congo. During the course of its journey through the foreboding rainforest, the river crosses the equator twice. Because the watershed of the Congo drains from both the Northern and Southern Hemispheres, it does not have as great seasonal fluctuations in water level as other great rivers. Its flow is relatively stable because part of its watershed is always in the zone of rain. The Upper Congo abruptly ends with Stanley Falls, a 60-mile stretch of rapids.

Stanley Falls gives way to the Middle Congo, a 1,000-mile stretch of navigable river, nine miles wide in some parts. Along this quiet stretch of river is the city of Kinsangani, a city known for violence since Belgian colonial days. Near the end of the Middle Congo, the river slows to a virtual standstill for 20 miles, a section known as Stanley or Malebo Pool. Here the river is 15 miles wide and flanked by the capital cities of Kinshasa and Brazzaville. The peace of the pool is suddenly shattered by Livingstone Falls, a series of rapids and cataracts 220 miles long. There are some 32 cataracts, having as much power as all the river and falls in the United States combined. The final 100 miles to the Atlantic Ocean from the end of the falls is fully navigable.


The 2,590-mile (4,170-km) course of Africa's third largest river was one of the great mysteries of Africa until the mid-nineteenth century. From its origins in Guinea less than 150 miles (240 km) from the Atlantic, the river heads north into the Sahara desert. At Timbuktu, the legendary city of gold, the river turns east, then abruptly south back towards the Gulf of Guinea. The river splits into 23 real mouths in the coastal mangrove forests of Nigeria, and some of these are only navigable by canoe. The Niger Delta is one of the world's largest wetlands, covering more than 7,700 square miles (20,000 square km), and houses Africa's largest mangrove forest.

It is the river's great arc and seeming lack of a mouth that made its course so elusive. It was long speculated that the river was a tributary of the Nile or Zaire (Congo) River.

The river was first thoroughly explored by the Scotsman Mungo Park, who drowned in a rapid during an expedition. The mouth of the river was discovered in 1830 and by the turn of the century had become the focus of European attention for its rich oil deposits. Today, Nigeria depends on these oil reserves to fuel its economy.

The forests of the Niger River delta and Nigeria overall are fast declining. According to the Food and Agriculture Organization of the United Nations (FAO), Nigeria has the world's highest deforestation rate of primary forests. Between 2000 and 2005 the country lost 55.7 percent of its primary forests—defined as forests with no visible signs of past or present human activities.

Rainfall over the Amazon river. Photo by Rhett A. Butler

Review questions:

  • What is the world's largest river by volume?

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Continued / Next: Flooding, Low Water, High Water

  • Forbath, P. conveys the greatness and rich history of the Congo (Zaire) River in The River Congo, Boston: Houghtin Mifflin Company, 1977.