Mekong River

The Mekong rises on the Tibetan Plateau in China and runs generally southeast across mainland Southeast Asia. In its upper course it passes through Yunnan; farther south it forms sections of the Myanmar–Laos and Laos–Thailand borders, then flows through Laos, Cambodia and Viet Nam. Near Phnom Penh, the Bassac branches from the main river, marking the beginning of the Mekong Delta. A network of distributaries carries water and sediment across the delta plain to the South China Sea. The river channel and its drainage basin are not the same: the wider basin covers about 795,000 square kilometres, including parts of China and Myanmar as well as much of Laos, Thailand, Cambodia and Viet Nam. The Mekong River Commission divides the basin into upper and lower regions, with varied highlands, plateaus, floodplains and delta landscapes.

The Mekong is strongly shaped by seasonal rainfall and the monsoon. Tributaries from wetter uplands contribute to the river's wet-season flow, while the size and timing of seasonal floods vary across the basin. In Cambodia, the Tonle Sap River has an unusual seasonal reversal: during the rainy season, rising Mekong waters push it toward Tonle Sap Lake; in the dry season, lake water drains back toward the Mekong. This exchange expands the lake and its connected wetlands during high water and is important to the ecology and fisheries of the lower basin. The Tonle Sap's cycle also illustrates why river conditions cannot be understood from the main channel alone.

The basin's waters sustain fishing, irrigation, agriculture, transport and household use. Tens of millions of people depend on the river for food, livelihoods and water, and the annual flood cycle connects human activity with wetlands and fish habitats. The Mekong is also notable for its freshwater biodiversity and large-scale fish migrations. These resources are not evenly distributed, and the river crosses national borders, so changes upstream can affect communities and ecosystems downstream. The Mekong River Commission is an intergovernmental body that coordinates cooperation among Cambodia, Laos, Thailand and Viet Nam on shared water resources; it does not include all countries in the river's wider basin.

The Mekong Delta is a low-lying, sediment-built landscape in southern Viet Nam, crossed by distributaries and canals and important for agriculture. Its form depends on the balance between sediment delivered by the river, waves and currents, subsidence and human land use. A peer-reviewed study published in 2015 analyzed satellite observations of shoreline change from 2003 to 2012 and reported extensive erosion along the delta. The researchers discussed declining sediment supply, sand mining and subsidence among the contributing factors, while emphasizing that erosion has multiple interacting causes and cannot be attributed to dams alone. Dams and other infrastructure can affect fish migration and sediment flows, while climate change, land use and water-management choices add pressure to ecosystems and local livelihoods. Their effects vary across places and over time, making basin-wide monitoring and cooperation important.