People have constructed dams to manage water resources for at least 5000 years. The first dams were used to control floods and to supply water for irrigation and domestic purposes. Later, the energy of rivers was harnessed behind dams to power primary industries directly, and more recently still, hydroelectricity has been generated using water held in reservoirs, allowing the impoundment and regulation of river flow. The modern era of big dams dates from the 1930s and began in the USA with the construction of the 221 m Hoover Dam on the Colorado River. But in the past 50 years there has been a marked escalation in the rate and scale of construction of big dams all over the world, made possible by advances in earthmoving and concrete technology. Initially, these dams were for hydroelectricity generation, and subsequently for multiple purposes – primarily power, irrigation, domestic and industrial water supply, and flood control. Some rivers have been intensively manipulated in this way; North America’s River Columbia, for example, has, since the mid nineteenth century, become the site for no fewer than 19 big dams and more than 60 smaller ones, making it the world’s largest generator of hydroelectricity (Lee, 1989).