Tonlé Sap is the great lake of Cambodia, south of Siem Reap, and the river that drains it changes direction twice a year. From June the monsoon-swollen Mekong pushes water back up the Tonlé Sap river instead of taking it away. The lake spreads from about 2,500 square kilometres to more than 13,000 and deepens from a metre to nine or ten, drowning the forest and fields around its rim. There is no fixed shoreline here, and the villages are built for that. At Kampong Phluk the houses stand on stilts six or seven metres high: at high water your boat goes down the lanes almost level with the doorways, and in the dry months the same houses are far overhead on bare poles. Other settlements do not stand at all. They float — houses, shops, a pagoda, a school, a clinic and a fuel seller, all of it on pontoons.
The flood is also why the lake feeds the country. It drowns the rim forest exactly when the fish are spawning, so the young have kilometres of nursery, and as the water pulls back the catch pours in. The yearly haul is the single biggest source of animal protein in Cambodia. Families work it the way their grandparents did: nets cast from narrow boats, traps set among the reeds, children paddling themselves to school, all within arm's reach of your boat. At high water, September to November, a paddled canoe takes you off the open lake into the drowned forest, where the flood has climbed the trunks and the lowest leaves touch the surface, and your guide works through the green tunnels in near silence. From December the water drains back and strands the fish in shrinking pools. Tens of thousands of storks, pelicans and adjutants then crowd the swamp forest at Prek Toal to nest, their colonies guarded by the people who used to raid them. Upstream dams and riverbed sand mining are flattening the pulse all of this runs on. When the water turns, the floating village is untied and towed to wherever the shore is going to be.