You twist the tap, and a stream of pure water glints briefly in the light before twirling down into the dark hole of the drain. Most of us give little thought to where that water comes from or where it goes to, but there's no question it's one of the essentials of civilization. No human settlement can last long without a water supply—not a city or a village or a farm or even a campsite. Thus, the infrastructure for delivering fresh water and disposing of waste water is found wherever people live.

Almost all of the earth's water—some 97 percent—is in the oceans. The ice sheets of Antarctica and Greenland account for another 2 percent. That leaves only 1 percent for liquid fresh water, and most of that is deep underground, in the invisible reservoirs called aquifers. It seems hard to believe—especially when you're crossing the Mississippi or standing on the shores of Lake Superior—but all the rivers and lakes on earth hold only 0.1 percent of the world's water supply.

That small fraction of the water, however, is extraordinarily important because it circulates. Solar energy powers a planetary-scale distillery, in which water evaporates from the ocean surface and falls to earth again as rain or snow. The water is purified by this process, since evaporation leaves behind salts and other contaminants. Furthermore, when the water is raised above sea level, it acquires potential energy—a strong urge to flow back downhill again. That urge is exploited in canals, aqueducts, pipelines, and other structures that divert the water to where it's needed. Also, some of the gravitational energy can be converted into electricity when falling water is made to turn turbines and generators.

This chapter describes three broad categories of waterworks: first, dams, levees, and other structures for controlling natural watercourses; second, systems for collecting, storing, purifying, and distributing drinking water; and third, the corresponding systems for collecting, treating, and disposing of sewage. Water has such a central role in human life that waterworks also turn up in several other parts of this book. Irrigation is discussed in Chapter 3, on agriculture; hydroelectric plants have their place in Chapter 5, on power and energy; waterway navigation and all things nautical are taken up in Chapter 12, on shipping.