In the Cambodian capital, Phnom Penh, where the Tonle Sap and Mekong rivers meet, workers are putting the final touches on the grandstand for Bon Om Touk, the annual water festival which begins this weekend. It’s a huge party—the country pretty much shuts down for the three-day holiday, with dragon boat races and plenty of drink and dance.

It’s a celebration of the water’s bounty. This year, though, there will be less to celebrate.

Upriver dams and a devastating drought this year brought the Mekong to its lowest level ever recorded. And the combination has left the Tonle Sap Lake—Southeast Asia’s largest—in crisis.

“It’s the beating heart of the Mekong,” says Brian Eyler, director of the Stimson Center’s Southeast Asia program and the author of the new book Last Days of the Mighty Mekong.

“The life of the Mekong comes out of the Tonle Sap lake”, he says. “It produces 500,000 tons of fish each year for the people of Cambodia and 2.6 million tons of fish caught throughout the rest of the Mekong basin,” which also includes Thailand, Vietnam and Laos.

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