China has notified countries on the lower Mekong River that it will reduce water flow to allow maintenance of its electricity grid.
Environmental NGOs have warned that a sharp drop in water levels will seriously impact the river.
The Mekong River Commission, an intergovernmental organization comprising Cambodia, Laos, Thailand and Vietnam, said it received the notification on Tuesday.
The notification came as part of a new data-sharing pact that China and the Mekong countries agreed to in October.
It says the water discharge from a Chinese dam to downstream countries would drop to 1,000 cubic meters per second, almost half of the normal flow.
The Mekong River Commission secretariat says data-sharing could assist future forecasts for the river, which people in the basin depend on for their daily lives.
But Beijing's notification was received five days after the drop in outflow was first detected by a US-funded monitoring organization, Mekong Dam Monitor, on December 31. The organization criticized China for failing to notify downstream countries.
In recent years, falling water levels has become a serious issue in the lower basins. It has been pointed out that this is related to the upstream dams.
A local NGO in Thailand says the reduced water supply will be devastating to the river's ecosystem and to livelihoods, especially in the dry season.