<P> The controversial Three Gorges Dam in China is able to store 22 cubic kilometres of floodwaters on the Yangtze River . The 1954 Yangtze River floods killed 33,000 people and displaced 18 million people from their homes . In 1998 a flood killed 4000 people and 180 million people were affected . The flooding of the reservoir caused over a million people to relocate, then a flood in August 2009 was completely captured by the new reservoir, protecting hundreds of millions of people downstream . </P> <P> Diseases Whilst reservoirs are helpful to humans, they can also be harmful as well . One negative effect is that the reservoirs can become breeding grounds for disease vectors . This holds true especially in tropical areas where mosquitoes (which are vectors for malaria) and snails (which are vectors for Schistosomiasis) can take advantage of this slow flowing water . </P> <P> Resettlement Dams and the creation of reservoirs also require relocation of potentially large human populations if they are constructed close to residential areas . The record for the largest population relocated belongs to the Three Gorges dam built in China . Its reservoir submerged a large area of land, forcing over a million people to relocate . "Dam related relocation affects society in three ways: an economic disaster, human trauma, and social catastrophe", states Dr. Michael Cernea of the World Bank and Dr. Thayer Scudder, a professor at the California Institute of Technology . As well, as resettlement of communities, care must also be taken not to irreparably damage sites of historical or cultural value . The Aswan Dam forced the movement of the Temple at Aswan to prevent its destruction by the flooding of the reservoir . </P> <P> Reservoirs may contribute to changes in the Earth's climate . Warm climate reservoirs generate methane, a greenhouse gas when the reservoirs are stratified, in which the bottom layers are anoxic (i.e. they lack oxygen), leading to degradation of biomass through anaerobic processes . At a dam in Brazil, where the flooded basin is wide and the biomass volume is high the methane produced results in a pollution potential 3.5 times more than an oil - fired power plant would be . A theoretical study has indicated that globally hydroelectric reservoirs may emit 104 million metric tonnes of methane gas annually . Methane gas is a significant contributor to global climate change . </P>

If an ecosystem is changed through a natural disaster