Similar to the situation in India, the Mekong depends on a mix of snow/glacial melt from the Himalayas, and on seasonal monsoon rains. Both are becoming less reliable, and countries in the headwaters, including China and Laos, are going on massive dam-building binges. (Disclaimer: This website looks like a reputable source of journalism, but I am not familiar with it.)

The crisis began when critical monsoon rains, which usually start in late May in the Mekong region, failed to arrive. Dry conditions, driven by the El Niño weather phenomenon and exacerbated by climate change, persisted well into July. At that time, observers say, the situation was made worse by hydropower dam operators upstream, in China and Laos,withholding water for their own purposes...

Originating in the Tibetan highlands, the Mekong River flows through six Asian countries, including China, Myanmar, Thailand, Laos, Cambodia, and Vietnam, before emptying into the South China Sea. The river basin is home to the largest inland fishery in the world and more than 60 million people depend on it for their livelihoods.