The Amazon river has over 1,100 tributaries of which several are themselves over 1000 km long. The forest may have as many as 120 species of tree in an area the size of the standard international soccer pitch. Not to be outdone, the Amazon river system has more species of freshwater fish than any other, and the biggest freshwater fishery in the world.
Accounting for one-fifth of the world's total river flow, the Amazon is still over 3km wide 1000 km from the sea. At the mouth, the estuary is 240km, with a main steam alone is wider (at 80km) than the English channel. It discharges 300,000 m³ (that would be 3 billion liter bottles) of water every second. There is so much force to the system that fresh water can be had 100 km from the coast. Along with it is some 3 million m³ of mud and sediment.
All this the result of a river system so complex that it contains one-third of all the freshwater on the planet and has so many tributaries and stream that there are five linear km of water for every one square kilometer of land. And it's a system that pulses too – rising and falling in response to its own rainy seasons and to those of the far distant Andes. The result are water levels that can rise and fall by 9m of the year – which is why when traveling in Amazonia you often see houses on stilts, floating petrol stations and cows being raised on anchored wooden rafts.
The region has an interesting, and quite complicated, geological history. The region's first big river flowed east to west, not west-to-east like today. Scientists have called it the Nozama (who says academics don't have a sense of humor). Over the next 60 million years, this was followed by huge shallow seas and a massive fresh-water lake before the current system established itself. As a result the rivers of Amazonia have members of such ancient evolutionary lineages as bony-tongued fish (the arapraima) and side-necked terrapins. The shallow seas also left their mark – today's Amazonian rivers have freshwater dolphins, herring, stingrays, puffer and needle fish. There are even sweetwater jellyfish, sponges and clams. Superimpose this on an explosive radiation of catfish, tetras and cichlids and you have an evolutionary powerhouse of stunning beauty and diversity. Sadly, its one few visitors ever get to really appreciate.
Rather more obvious, though, is the forest. Even the casualest of glances through the airplane window reveals a multitude of species (all those shades of green). But its more complex still, for there are many types of Amazonian forest. Not only is there the never-flooded rainforest (locally called terra firme), there is also palm swamp (usually dominated by a palm called burití and therefore called a buritizal), there are also white-sand scrub forests which are very open and dominated by things that look like rhododendron bushes, and have a lot of moss and lichens on the ground (carnivorous plants too if you look closely). These are called campinas. Then there are two types of flooded forest: that on the margins of black-water rivers like the Negro and its tributaries is called igapó, while the corresponding form on the banks of white-water rivers like the Solimões and Amazon is called várzea. Both have to deal with being flooded for up to nine months each year, but they look very different. White-waters rivers are sediment-rich and so have broad floodplains, which means that a várzea forest can extend a km or more from the riverbank. Sediment starved blackwaters rivers offer no such largesse, and the igaoó will be a thin ribbon, a couple of hundred meters wide at the most. But both are gloriously lit, tranquil, fascinating and quite unlike any other tropical forest you will ever see. Such flooded forests occur only in the Amazon.
There is little to climb in the Amazon. The few scattered mini-mounts are usually exceedingly ancient rock and in highly remote places. They are worth the trip, though, as the views can be superb and the moss-crusted forests on their peaks have a weird beauty all of their own.