hydrological dam are widely used to generate electricity. at present, hydrological power is deemed be clean energy, because it does not have greenhouse gas emission. however, many reports and researches show that hydrological dam has negative impacts on water and creatures. this paper discusses the negative impacts of three gorges dam and la grande complex for changes of mercury level. the three gorges hydrological dam is the largest hydrological dam in the world that located at yangzi river in china, but after building the dam, the mercury level is increasing rapidly; the number of fishes that live in the yangtze river is decreasing and the water quality is also decreasing. similarly, in canada, the la grande hydroelectric complex also has the same negative impacts as the three gorges hydrological dam.