A dam is a structure constructed across waterways to retain water in a reservoir, or man-made lake. Dams are a tool used by humans to harness the power of water for millennia. The oldest dam is thought to be the Jawa Dam in the Black Desert of Jordan constructed sometime in the 4th millennium B.C.E.

Dams allow people: to store water for consumption or irrigation; to improve the navigation of a waterway; to generate electricity; to control flooding up or downstream; to fish; or for recreation. Often a single dam will be used for multiple purposes.

Although dams provide people with many benefits, they are not perfect. They block rivers which prevent natural fish migration patterns which in return impacts species populations. Dams slow the flow speeds of water in rivers which allows sediment to build up and confuses natural signals to species such as salmon and river herring who depend on natural seasonal flow patterns to guide them to breeding grounds. Further, these slower flowing streams can decrease water quality due to warming water temperatures that could lead to algal blooms and oxygen-depleted waters. Finally, dams change habitats. First, by drowning land immediately upstream and changing upstream beds from rocky to sandy, silty, or muddy. Often the land taken for the reservoirs belonged to and had cultural significance to indigenous peoples. Finally, by limiting the amount of water flowing downstream, less water is available for the people or animals and plants to use.

While they often create them, waterways do not stop at political boundaries. So anything that impacts a river upstream will also impact it downstream, whether this is point source pollution or the construction of a dam. When a country upstream builds dams that limit the amount of water available to countries downstream the relationship between the two places can become strained and shift power dynamics critical to peace.

One example of this is the Rio Grande in the arid western United States. The Rio Grande runs approximately 3,060 kilometers (1,900 miles) and crosses three U.S. states before it forms the border between the United States and Mexico. This river also has 15 dams and kilometers of canals shifting water to other locations. Due to the high water needs of the region which has been experiencing increasing drought conditions in the last several decades, water in the Rio Grande no longer reaches the mouth of the river–which historically empties into the Gulf of Mexico–consistently. While the water in the river is essential to maintaining life and agriculture in New Mexico, Colorado, and Texas, the damming and diversion of water often prevents it from reaching the Gulf.

This map layer illustrates global dams and classifies them by their primary purpose. The size of the icon notes the relative amount of water able to be stored within the dam reservoir (when full). This dataset is from Global Dam Watch.

What is the dam nearest to you?