As a trans-boundary river, the Mekong river runs through China's Yunnan Province, Myanmar, Laos, Thailand, Cambodia, and Vietnam. At Phnom Penh the river is joined on the right bank by the river and lake system the Tonlé Sap. When the Mekong is low, the Tonle Sap is a tributary: water flows from the lake and river into the Mekong. When the Mekong floods, the flow reverses: the floodwaters of the Mekong flow up the Tonle Sap.