Advantages from building dams include delivering more water and energy to local populations, expanding surrounding riparian habitats and controlling floods, while disadvantages include harm to the plants and animals in local environments, decrease in natural diversity and structural instability of the nearby riverbed and riverbanks. Dam construction directly influences the surrounding environment, and impacts economies and societies too. Proponents support dam building on the basis that dams, large and small, bring jobs and revenue into local economies, and help societies modernize and develop greater complexity.
Since the times of ancient Egypt, societies around the world have used dams for transporting water and irrigating crops. They bring critical supplies of water to people in surrounding areas, and allow the growth of crops and plants. Dams can even provide political stability internally, helping governments allocate and control distribution of water among residents. They improve political and economic relations among neighboring countries too, when dams cross national borders.
Dams also provide novel sources of renewable energy in the form of hydroelectric power. Their construction might improve quality of life for some species in local environments, including non-native organisms, but can negatively influence environments as well. Dams change the nutrient content in waters downstream and alter water temperatures, which in turn affects the growth of populations. They sometimes necessitate costly repairs and maintenance, and dam breaks create safety hazards.