It"s a strange paradox: the earth"s water supply is constantly growing, yet humankind--or so scientists now warn us--is facing a potentially catastrophic shortage of potable water in the next century. "Fresh water," writes Canadian scientist Pielou, "will turn out to be the factor that limits population growth," largely because so much of that growing supply is locked up in arctic ice or lies deep beneath the surface of the earth. In her thoughtful survey of the physics and chemistry of water, Pielou introduces readers to such concepts as the water cycle, in which rainwater becomes groundwater and eventually returns to the sky from whence it came; examines the economics of water surpluses and deficits in the natural world; and studies the formation and behavior of rivers and lakes, among many other topics. Building more dams, she warns, is no solution to the impending water crisis: "Channelization and diversion both have the effect of increasing a river"s rate of discharge (equivalently, speeding its flow), which may be as undesirable as decreasing the discharge (slowing the flow)." Her thoroughly interesting natural history of fresh water in all its forms makes for a fine introduction to hydrology and for a solid contribution to environmental literacy. --Gregory McNamee