Rainfall that occurs in the Amazon originates from weather patterns that travel from the East, in the Atlantic Ocean, to west, where they move across the Amazon lowlands and approach the Andes mountains. At the same time, daily transpiration of Amazonian trees releases a large quantity of moisture into the air. As this moisture is pushed towards the Andes, it is squeezed into narrow valleys, such as the Kosnipata Valley shown below. The clouds converge and condense into rain as they hit the mountains, which trap this rain and direct it to th etributaries of the Amazon River. Nearly 20% of all the freshwater runoff to the oceans on the planet travel through the Amazon River.
This rain benefits Amazonian rainforests, which require significant moisture for most of the year. Forests use this moisture for photosynthesis, and afterwards, the water vapor is released into the air. Deforestation in the Amazon inhibits this pattern, and dries out forests in the western part of the continent. Scientists estimate that 90% of the current Amazon must stay intact in order for this moisture cycle to continue and for forests to stay healthy.