Hydroelectricity is a major source of electricity in Turkey, due to its mountainous landscape and many rivers. The country's main river basins are those of the Euphrates and the Tigris. Over 700 hydropower plants have been built, and hydroelectricity makes up about 30% of the country's electricity generating capacity. Annual generation varies greatly, and in rainy years lots of hydroelectric power can be generated. Government policies have generally supported dam construction, but some dams are controversial in neighbouring countries, while others raise concerns about damage to the environment and wildlife of Turkey.