The Amazon is the world's largest river in terms of water discharge, with a mean discharge ten times that of the Mississippi and five times that of the next largest river, the Congo (Zaire). The drainage area of the Amazon basin is 2.67 million mi2 (6.9 million km2), nearly double that of the Congo. As an equatorial river, the Amazon flows generally from the northern and central Andes (Colombia, Ecuador, Peru, Bolivia) in an east-northeasterly direction to the Atlantic Ocean, with major tributaries joining it from the north and south. The Amazon River measures about 4,000 mi (6,400 km) from its source to its mouth and is the second longest river in the world. Occupying 38 percent of the total area of South America, the Amazon Basin receives 52 percent of the daily precipitation for the continent. Geologically, the Amazon Basin is constrained by a framework of three distinctive landforms (1) the Andes, (2) the plateaus of the Brazilian craton, and (3) the Amazonian trough. Geochemically, the dissolved load of rare-earth elements (REE) in the main flow is the highest in the world, but some of the tributaries carry much lower quantities of dissolved material because of the bedrock and topography of each catchment.