In the twentieth century, humans transformed the planet's rivers. On every continent, save Antarctica, they dammed, diverted, and depleted rivers. On local, regional, and continental levels, the pathways and uses of water changed, if not always in kind, then in intensity, location, and scale. Two ecologists, Mats Dynesius and Christer Nilsson, reported in the closing decade of the twentieth century that over three-quarters of the total water discharge of the 139 largest rivers in the Northern Hemisphere are “strongly or moderately affected by fragmentation of the river channels by dams and by water regulation resulting from reservoir operation, interbasin diversion, and irrigation. … These conditions indicate that many types of river ecosystems have been lost and that the populations of many riverine species have become highly fragmented.”

The aim was not to fragment but to create a new order. In emerging nation–states, in empires and colonies, in capitalist and communist societies, political elites applied new technologies of power to rivers and lakes. Dreams of a hydraulic order sought to correct past ills, to raise wealth, to impose control over nature and others. Floods would be stopped, rushing, wasting water would be harnessed with hydroelectric dams, and arid lands and reservoirs would be linked with irrigation systems. A new order of previously unimaginable scope was placed on rivers almost everywhere. Over the course of the twentieth century, humans increased their annual withdrawals from rivers and lakes ninefold.