The villagers are used to seasonal fluctuations of the water table and even to flooding during the wet season, but since the dam construction, the river levels have been changing frequently and erratically. Diurnal fluctuations of the water table have made fishing and farming difficult, if not impossible.
The basin's entire ecosystem is in danger, with many varieties of riverine plants having dwindled because of unnatural flooding, and bird and turtle nests having been swept away."
The Lower Mekong mainstream dams in Laos, Thailand, and Cambodia, by one estimate published in a December 2008 issue of the MRC’s journal, Catch and Culture, will block the spawning migration of 70 percent or more of the most commercially important species, imperilling the food security and livelihoods of millions, most of them in Cambodia and the Mekong Delta. A more recent preliminary “technical note” also published by the MRC provisionally estimates that if all eleven Lower Mekong dams are built, the population of the main commercial species, will decline by less than 22 percent in all. However, the study notes that the impact will be disproportionately borne by Cambodia (43 percent decline) and Vietnam, affecting populations that are the poorest and most dependent on fish for their protein needs. The study estimates that Thailand will be little affected while Laos may experience an increase of up to 25 percent, presumably because of the expansion of the currently narrow mainstream wetlands.
The greatest downstream ecological impact will be felt in Cambodia’s Tonle Sap Great Lake that connects to the Mekong mainstream at Phnom Penh, and Vietnam’s Mekong Delta. Upstream dams, including two planned by Cambodia itself, will seriously degrade the Tonle Sap, by far Cambodia’s most important fishery and the seasonal “nursery” of many of the Mekong’s most important fish species. The Mekong Delta of Vietnam will also be severely impacted. The Delta, which produces about 52 percent of Vietnam’s rice and most of its aquaculture fish and shrimp exports, will suffer major consequences from the upstream dams, including the capture of vital silt in dam reservoirs, pollution from mining operations, and a large increase in the use of chemical fertilizers and increased effluents from accelerated urbanization and a decrease in natural soil replenishment."