Climate change，pollution，overuse of water and development are killing some of the world’s most famous rivers including China’s Yangtze．India’s Ganges and Africa’s Nile．WWF said on Tuesday．At the global launch of its report “World’s Top 10 Rivers at Risk”，the group said many rivers could dry out, affecting hundreds of millions of people and killing unique aquatic（水生的）life． “If these rivers die，millions will lose their livelihoods，biodiversity（生物多样性）will be destroyed on a massive scale，there will be less fresh water and agriculture，resulting in less food security，”said．Rayi Singh，secretary—general of WWF—India．The report launched ahead of “World Water Day” today，also cited the Rio Grande in the United States，the Mekong and Indus in Asia，Europe’s Danube, La Plata in South America and Australia’s Murray—Darling as in need of greater protection． Rivers are the world’s main source of fresh water and WWF says about half of the available supply is already being used up．Dams have destroyed habitats and cut rivers off from their flood plains．while climate change could affect the seasonal water flows that feed them，the report said．Fish populations，the top source of protein and overall life support for hundreds of thousands of communities worldwide，are also being threatened, it found．．The Yangtze basin is one of the most polluted rivers in the world because of decades of heavy industrialization，damming and huge influxes（流入）of sediment（积淀）from land conversion． Climate change，including higher temperatures，also means serious consequences for fishery productivity，water supply and political security in Africa’s arid Nile basin．Tributaries（支流）flowing into the Ganges are drying up because of irrigation，WWF said． 小题1:hat is the text mainly about?
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