WRI's study of 145 primary and secondary watersheds around the world estimated that 42 of these watersheds have lost more than 75 percent of their original forest cover - the closed forests that are believed to have existed in the pre-agricultural era.

Fifteen of these have lost more than 95 percent of their original forest cover. Most of these basins, with the exception of the Tigris and the Euphrates, are found in Africa, Central America, and Europe.

Large basins, with very extensive forest cover, have lost a relatively small fraction of their original forest, but the absolute losses are large. Nine basins, including the Amazon, Ganges, Mekong, Mississipi, Paraná, and Volga river basins, have lost more than 500,000 km2. The Yangtze and the Congo basins have each lost more than 1 million km2 of forest (Revenga et al., 1998:1-13).