unspoiled valleys in neighboring sichuan show us how it might once have looked. it's the sort of natural abundance that is necessary to support an emerging civilization. how could a landscape with such potential have been reduced to this? when chinese scientists and civil engineers began to survey the area, they realized that several thousand years of agricultural exploitation had denuded the hills and valleys of vegetation. the relentless grazing of domestic animals on the slopes meant that there was no chance for young trees and shrubs to grow. the rainfall no longer seeped into the earth, but simply washed down the hillsides, taking the soil with it. over millennia, this progressively destroyed the region's fertility. when this happens over an area as extensive as the plateau, millions of tons of silt are swept down into the yellow river, which gets its name from the color of the fine loess soil. the mounting quantities of silt clog up the river, impeding its flow, contributing to the floods that give the river another name: china's sorrow. in some areas, crea