The Mekong is one of the world's major rivers. It is the 13th-longest in the world, and the 10th-largest by volume (discharging 475 km³ of water annually). Its estimated length is 4,023 km. It drains an area of 795,000 km²: From Tibet it runs through China's Yunnan province, Myanmar, Thailand, Laos, Cambodia and Vietnam. All except China and Myanmar belong to the Mekong River Commission. The extreme seasonal variations in flow and the presence of rapids and waterfalls have made navigation extremely difficult.