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.

The secret to the Tonle Sap Lake's amazing productivity is what's known as a monotonal flood pulsed system. Normally, the Tonle Sap river flows into the Mekong river. But during the annual monsoon, the Mekong swells so much it pushes water back into the Tonle Sap river, and from there into Tonle Sap Lake, Eyler says.

"That causes an expansion of the lake to about five times its dry season size," he says. "It not only brings in a lot of water, but massive amounts of sediment that form the basis of a food web to feed the fish and fish eggs and fish larvae that go into the lake and find habitats to thrive."

It's not just fish that depend on the bounty of the lake, but also endangered water birds from around the world that migrate there at this time of year. And when the floodwaters recede, the Tonle Sap river reverses course again, taking the sediment and the fish and their eggs back into the Mekong and from there to spawning grounds upriver in Laos and Thailand, and downriver in Vietnam.

Normally, this process takes several months to play out. But this year, in part because of the drought, it lasted just six weeks, Eyler says. In a country where 70% of the people rely on fish for their protein intake, that's a huge problem: Less water means fewer fish. Now, that's the scene playing out on both the Tonle Sap lake and river.

"Last year, when the fishermen came, they'd have five or ten kilos [11-22 pounds] of fish to sell, but this year, it's just one or two," says 39-year-old fish broker Khout Phany. It's just after daybreak, and she sits under an umbrella at the water's edge in the village of Chhnok Tru at the southern end of the lake. She clutches a thick wad of Cambodian bank notes as fishermen motor up to her with their catch.

"The lower level of the water is causing a problem for fishing," she says.

Fishermen aren't showing up, because they know the fish this year will be smaller and harder to find, she adds. 31-year-old Sam Sokeng has just brought her his catch. He's spent two nights on the water, he says, with little to show for it.

"Last year, I could earn almost 75 dollars a day," he says. "This year, I barely make enough to pay for gas and bait."

Another fisherman—39-year-old Tim Chhoeun—says he and his wife were out all night.

"Last year, I could catch about ten kilos of fish a day," he says. "But today, you see I caught only three."

His wife, 40-year-old Chhum South, says if things don't get better, she'll have to go work on a Chinese company's cassava plantation to earn enough money to survive. The couple have four children, aged 10 to 18.

"I don't want them to be fishermen," Chhoeun says. "I tell them to learn hard and find a good job and not be like me."

A few hundred yards away, Phap Phalla sits on the floor of her home, which rests on stilts because of the seasonal flooding. Peering through the floorboards, you can see the muddy brown water about eight feet below.

This time last year, she says, the water was so high it flooded her entire home. She came to live here just after the end of the murderous reign of the Khmer Rouge more than 40 years ago. Her husband is the local school administrator. She works with fishermen and a local NGO that tries to improve their livelihoods through conservation efforts. This year, she says, is the worst she's ever seen.

"Next month should be the height of the fishing season," she says. "But the water and the fish are already gone. So, what will the fishermen catch?"

Many, she says, will have to move to the cities—or find work abroad as manual laborers—to survive. A coalition of NGOs she works with has also had some success, she says, helping fishermen find new jobs as small businessmen with funding from the European Union. But she worries about another drought next year. She blames the Tonle Sap's current state on climate change, overfishing and upstream dams in neighboring Laos and China that impede fish migration, sediment and water from flowing downriver. And more hydropower dams are planned.

"If the eleven Lower Mekong mainstream dams are built, two in Cambodia and nine in Laos, it's really a game-over situation for the mightiness of the Mekong," says the Stimson Center's Eyler. "For the natural provisions of the river, for the ability of the river to regenerate the land around it to produce this...[the] world's largest inland fish catch."

Environmentalists are pleading for the dams' construction — some of which could begin by next year — to be suspended. They want Laos and Cambodia to look at alternative forms of energy such as solar and wind that are less harmful to the environment. If that doesn't happen, says one Cambodian natural resources consultant, Ham Oudum, "The Tonle Sap is screwed."

And so, too, may be the food security and livelihoods of millions who've depended on the lake and on the Mekong for decades.

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