All the greatest rivers of monsoon Asia – the Indus, Ganga, Brahmaputra, Irrawaddy, Salween, Chao Phraya, Mekong, Yangtze, and Huang Ho – come to life atop the world, in Tibet's high mountains, cold, rocky, and dry, where scruffy nomads herd goats. The rivers descend through deep twisting valleys, gorging themselves on melting snow and monsoon rain from countless tributaries, and they finally settle down to wandering the lush green lowlands of ancient agrarian civilizations, joining the sea in the teeming hot deltaic heartlands of Asia's modern world economy. On the way down from the highlands, rivers change names and identities to mark their passage through the homelands of history, where nature's free-running waters acquire social life in geographies inscribed most powerfully by the great imperial civilizations that came to life in river lowlands, expanding their reach over centuries, upstream and downstream, across the plains, into the highlands. Many smaller, less powerful homelands also evolved in the pathways of imperial expansion, and interactions among all these societies, in their different habitats, produced a vast diversity of connected communities, which collectively entered the historical record ‘during the first millennium CE, [when] nascent centers of … civilization were part of an interacting continuum of communities that occupied, thickly or thinly, the whole of South Asia’. As Sumit Guha goes on to explain,

Over the past two millennia, imperial state territorialism, spreading out from the lowlands, has defined Asia's historical geography. Forest, jungle, mountain, desert, river, ocean, and savannah became peripheral, exotic habitats for people outside core regions of imperial culture filled with intensive farming, commerce, arts, manufacturing, urbanity, and spatial order, where progress meant imposing their imperial design on their surrounding environment.