A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees are used to manage or prevent water flow into specific land regions. Hydropower and pumped-storage hydroelectricity are often used in conjunction with dams to generate electricity. A dam can also be used to collect water or for storage of water which can be evenly distributed between locations. When a dam is constructed, be it for hydropower or water supply, the destruction is highly visible. But the environmental impacts of a dam stretch much further downstream than the location of the actual dam site.

The environmental consequences of large dams are numerous and varied, and includes direct impacts to the biological, chemical and physical properties of rivers and riparian (or “stream-side”) environments. The damming of rivers produces dramatic and far- reaching environmental changes: of the first order are hydrological changes, including water quality; the second order affects channel morphology and aquatic, riparian, and floodplain vegetation; and the third order includes vertebrates and fish. Thus there are important linkages between the physical, chemical, and biological components of river systems that are disordered when a river is impounded.

The most significant changes take place in the hydrological regime which, in turn, affects many other natural processes. Stream flow is most influenced by reservoirs with long-term and marked seasonal regulation; the upstream levels can change over a year by amounts ranging from a few tens of centimetres to tens of metres and in some reservoirs by over a hundred metres. Marked variations in level and water exchange rate affect the flow, temperature, and hydrochemical and hydrobiological regimes of reservoir and downstream rivers as well as silting and changes in beds and banks. The flow of suspended and dissolved materials, including mineral biogenic substances and trace elements, also drops sharply.