Dams are planned, constructed, and operated to meet human needs - generation of energy, irrigated agricultural production, flood control, public and industrial supply, supply of drinking water, and various other purposes. Dams impound water in reservoirs during times of high flood that can be used for human requirements during times with inadequate natural flows. Positive impacts of dams are improved flood control, improved welfare resulting from new access to irrigation and drinking water. Without dams there would be insufficient food to feed the world’s people and energy would be generated by burning fossil fuels that produce greenhouse gases.

Despite this progress there remain significant concerns about the environmental impacts of dams. These environmental impacts are complex and far reaching, remote of the dam, and may occur in time with the dam construction or later and may lead to a loss of biodiversity and of productivity of natural resources.

This bulletin compiles improvements in knowledge and state of the art technology to avoid or mitigate environmental impacts of dams on the natural ecosystem as well as to the people that depend upon them for their livelihood and also addresses the mitigation of environmental impacts on dams and reservoirs.