Dams, aqueducts, and canals are widely used to store and transport water, and to support both instream uses such as recreation, navigation, hydropower, etc., and offstream uses such as urban and agricultural water supply. The rapidly growing size and number of these structures is a key feature of “hard path” water management in the twentieth and twenty-first centuries.

While these types of “gray infrastructure” can provide many benefits, they can also have severe impacts on aquatic ecosystems and human communities, and have provoked significant opposition in many cases. In the face of increased understanding of the negative consequences of dams and similar technologies, some places are even starting to remove dams. But globally the boom in large dam construction is far from over.