Forests and other vegetation are crucial components of the watershed, as they maintain water quality and moderate water flows, reducing runoff during high-water periods and maintaining flow during dry periods. In addition, forests provide habitat for many terrestrial species. In many floodplain areas, forests also provide much of the food and breeding grounds on which fish and other species depend. The extent of historical deforestation is thus a useful indicator of watershed degradation.

This map shows the percentage of original forest cover remaining in each basin. This percentage was calculated by dividing the extent of current forest cover by the extent of original forest cover for each basin. Current forest refers to closed canopy forest in existence today. Original forest cover refers to an estimate of the extent of closed canopy forest in existence 8,000 years ago, assuming current climate conditions. Forty-two watersheds have lost more than 75% of their original forest cover. Fifteen of these have lost more than 95% of their original forests. Most of these basins, with the exception of the Tigris and Euphrates, are found in Africa, Central America, and Europe. In terms of area of forest loss, nine basins have lost more than 500,000 km2 of forest, including among others the Mekong, Ganges, Amazon, Paraná, Ob, Volga, and the Mississippi River basins. The Yangtze and the Congo have lost more than 1 million km2 of forest each.

It is important to note that some areas denoted as having remaining original forest cover, particularly in Western North America, Mexico, and Scandinavia, are actually covered by intensely managed fiber plantations. The management practices in these plantations can have detrimental effects on water quality and terrestrial and aquatic species. Therefore, these data should be interpreted with caution.