When these high costs, delays and risks of low river flows are factored into calculations of the costs of electricity it can be seen that hydropower is now an expensive form of power generation. Hydropower should not be considered as clean power because of the destruction of river ecosystems and its many social impacts. Internationally, private investors in power projects are largely avoiding large dams and prefer to invest in cheaper and less risky gas-fired power plants. The priority before building new power plants should always be to improve the efficiency of existing energy supply and use. When new power plants are clearly needed, most environmentalists favor the use of solar and wind power, which are now on the verge of becoming commercially viable.
Small dams can be a sustainable and economic source of electricity, especially in rural areas.
Dams can stop regular annual floods but often fail to hold back exceptionally large floods. Because dams lead people to believe that floods are controlled, they lead to increased development of floodplains. When a large flood does come, damages caused are often greater than they would have been without the dam. Irrigation systems around the world are in general very wasteful of water. The cheapest and most effective way of providing more water to cities is therefore to increase the efficiency of irrigated agriculture.