The Amazon Basin is the largest basin on the planet and also one of the least understood. Its drainage area covers more than one third of the South American continent, and its discharge contributes almost one fifth of the total discharge of all rivers of the world. The headwaters of the Amazon River are located about 100 km from the Pacific Ocean and it runs more than 6 000 km before draining into the Atlantic Ocean. In addition, the Amazon has 15 tributaries, including the Tocantins River, that measure more than 1 000 km in length. The Madeira and Negro rivers are the most important tributaries, contributing with more than one third of the total water discharge. The Amazon Basin contains a complex system of vegetation, including the most extensive and preserved rainforest in the world. The rainforest, known as the Amazon Rainforest, is not confined to the Amazon Basin but also extends into the Orinoco Basin and other small basins located between the mouths of the Orinoco and Amazon rivers. In addition, savannah and tundra like vegetation can also be found.