Rainforests help maintain the water cycle. Its role is to add water to the atmosphere through the process of transpiration (where they release water from their leaves during photosynthesis). This moisture contributes to the formation of rain clouds which release the water back on the rainforest.
 
In the Amazon, 50-80% of moisture remains in the ecosystem's water cycle. One square meter of the ocean surface evaporates one litre of water. A tree releases 8-10 times more moisture into the atmosphere than the equivalent area of the ocean (GCP, 2011). When the forests are cut down, less moisture goes into the atmosphere and rainfall declines and sometimes leads to drought.

Tropical rainforests are also home to some of the largest rivers in the world, like the Amazon, Madeira, Negro, Orinoco, (South America), Mekong (Asia) and Zaire or Congo (Africa), because of the tremendous amount of precipitation their watersheds receive. These mega-rivers are fed by countless smaller tributaries, streams and creeks. For example, the Amazon alone has some 1,100 tributaries, 17 of which are over 1,000 miles long. Although large tropical rivers are fairly uniform in appearance and water composition, their tributaries vary greatly. Many tropical rivers and streams have extreme high and low water levels that occur at different parts of the year (Mongabay, 2011).
 
Tropical waters, whether they are giant rivers, streams or lakes, are almost as rich in animal species as the rainforests that surround them. But they, too, are increasingly threatened by human activities, including pollution, siltation resulting from deforestation, hydroelectric projects, and over-harvesting of resident species.