Every May, when the rains come, water levels in the Mekong start to rise.
When the river flows into Phnom Penh it meets another river that drains from a lake in central Cambodia.
So full is the Mekong that it reverses that river’s flow, forcing water back upstream and expanding the lake more than five-fold.
This is the Tonle Sap, the largest freshwater lake in South East Asia. Cambodians call it the Great Lake.
It is an area of extraordinarily rich biodiversity and a key breeding ground for fish, which migrate upstream from the Mekong to spawn in seasonally-flooded forest areas.
The lake is vital to Cambodia. It provides two-thirds of the country’s protein and more than one million people depend on it directly for their livelihoods.
But the lake faces serious threats.
Cambodia’s population has risen rapidly and pressure on resources has increased. Fish stocks are threatened by over-exploitation and illegal fishing methods.
Farmers and developers have taken advantage of weak governance to seize and drain land in the flooded forest, destroying key wildlife habitats and polluting the lake.
More trees have been felled for domestic use by local people, some of whom have been hunting rare wildlife to compensate for smaller fish catches.
Last year, prime minister Hun Sen warned of a “serious environmental disaster” if the problems were not addressed.
The Asian Development Bank-financed Tonle Sap Environmental Management Project (TSEMP) is leading efforts to do that.
Eight-years ago, more than half the lots on the lake allocated to commercial fishing were released to local communities.
Part of TSEMP’s work is helping villages create legally-recognised community fisheries to protect and preserve their own resources. More than 170 of these groups have now been set up.
Soer Tao is deputy head of the community fishery in Kampong Klaeng, on the lake’s northeast shore.
The village is home to about 10,000 people living in stilted houses to cope with the seasonal flooding. Some 85% of residents depend on fishing for their livelihoods.
Ten-years ago, Soer Tao says, illegal fishing and destruction of the forest were causing serious problems to villagers. But local management of resources is bringing benefits.
The village boundaries have been formally set. Residents patrol the area and if people are fishing illegally or if developers are trying to encroach into the flooded forest, they should now be better positioned to tackle the problem.
The village has also established a fish sanctuary, 300 metres by 30 metres, where fish can spawn during the dry season. It is marked by red flags and guarded at each end.
When the flooding comes, the fish will swim out -hopefully in greater numbers every year.
“The fish sanctuary will protect the fish as livelihoods for everyone,” Soer Tao said.
But it is not just about protecting fisheries.
Preak Toal is a floating village. Everything floats, even the school and the petrol station, and everyone depends on the lake to live.
Now projects are being set up to help families diversify their livelihoods away from the lake in a bid to reduce pressure on resources.
Former poachers patrol a biosphere reserve, guarding the rare water birds that they used to hunt. Tourists pay to enter and local families use pedalos to show the day-trippers around.
Some residents have built floating gardens for fruit and vegetables, while others are growing mushrooms in their floating houses. One group is trying to turn water hyacinth into charcoal-like fuel.
But the initiatives are, of course, not perfect. It is still much simpler for villagers to get firewood from the forests and to sell fish for quick profit.
Dr Neou Bonheur, director of TSEMP, admits that trying to promote environmental awareness to those struggling to make a living can be difficult.
“It is hard,” he says, “but when we teach them not to cut the forest because it is a breeding ground for the fish, they see the benefits of that.”
The villagers, he says, are not the greatest challenge.
“Now we are at a turning point -rice and fuel prices are up and there is a tendency to look for resources such as land, not from the communities but from outside groups who want to claim areas for development.
“That’s the most difficult thing for us, the people who damage the communities and fisheries in that way.”
Community resource management was put in place at the right time, he says, but it must be strengthened to ensure local people have a permanent voice.
He describes efforts to date as “so far, so good”, but says they must be sustained.
“We cannot say it is now enough -we have to continue to work hard on many areas.”
But there is one key issue Cambodia cannot control.
China, Thailand and Laos all want to dam the Mekong for hydropower, something experts say could have a serious effect on the seasonal influx of water and wildlife into the lake.
“We are a downstream country and less powerful compared to upstream countries,” says Dr Bonheur. “We can only hope that through dialogue, Cambodia can voice its concern.”
“The Tonle Sap is a great asset for Cambodia. We must protect it at all cost.”