A dam-building spree in China poses the greatest threat to the future of the already beleaguered Mekong, one of the world's major rivers and a key source of water for the region, a U.N. report said Thursday. China is constructing a series of eight dams on the upper half of the Mekong as it passes through high gorges of Yunnan Province, including the recently completed Xiowan Dam, which is more than 290 meters, or about 960 feet high. Its storage capacity is equal to all the Southeast Asia reservoirs combined, the U.N. report said. Laos, meanwhile, has started construction on 23 dams expected to be finished by 2010 on the Mekong and its tributaries, the U.N. said, as a means to spur development and lift the country from poverty. Cambodia and Vietnam also have ambitious dam-building plans.