THE MEKONG The Mekong River is nearly five thousand kilometres long and it flows through six different countries: China, Laos, Myanmar, Thailand, Cambodia and Vietnam. It is the twelfth-longest river in the world. At Phnom Penh, the capital of Cambodia, the river branches out to form a huge delta. Large fluctuations in rainfall between the wet and dry seasons mean that the level of the river also varies significantly. At Phnom Penh, that variation is so large – up to ten metres – that the flow of the tributary, the Tonlé Sap, changes twice a year and the lake of the same name is transformed into the largest in South-East Asia in the wet season. Many people depend on the Mekong. The river is the source of fish and rice is grown in the delta. There are already seven dams in the main river in China; two are being built in Laos. Nine dams are planned in Laos and Cambodia. The dams are a source of concern for fishermen. And they block the flow of sediment needed to prevent the erosion of the Vietnamese coast. Another challenge is land subsidence. The residents of the vast cities in and on the edges of the delta with water take groundwater from the subsurface. Researchers from Deltares are investigating flood risks, the salinisation of groundwater and coastal erosion, and they are getting involved in devising solutions and working on the design of a warning system.