There can be various reasons why onstream dams are constructed and operated on rivers. Some dams are built to create new water supplies for irrigation or domestic use that can be stored in the reservoirs behind the impoundments. Other dams are built to generate hydro-electric energy that can be produced by running the water stored in reservoirs through turbines. There are also dams built to regulate flows to avoid downstream flooding during storms. More often than not, there are “multipurpose” onstream dams that are designed to serve a mix of water supply, energy, and flood control purposes.

Globally, the presence of onstream dams is pervasive and extensive. A report by the World Commission on Dams in 2000 found that there were more than 45,000 dams in more than 150 countries.1

Some dams are geographically located in watersheds within a single nation, while other dams are located in watersheds that span multiple nations. For example, in North America, the Columbia River/Snake River watershed spans Canada and the United States, and the Colorado River watershed spans Mexico and the United States.