CHAN KNOWS IT’S TIME TO MOVE WHEN WATER TOUCHES THE FLOOR OF HER HOUSE.

Cambodia

Twice a year, the Tonle Sap River switches back and flows in reverse. The sudden rush of water swells Tonle Sap Lake and submerges forests and fields nine meters deep, turning local fishermen’s yards into swimming holes for rare freshwater dolphins and 300-kilogram catfish. “When water reaches the bamboo that makes the floor of the house, it is time to move,” says Poe Chan, 55. “Otherwise the bed would be underwater.”

Poe has lived on Tonle Sap Lake in Siem Reap, Cambodia, all her life; today she sells sweet steamed coconut-rice cakes on the side of the road. She bathes in the lake, washes her clothes and dishes in it, fishes in it, and relieves herself in it. She moves when it moves: once the lake begins to rise and spread, Poe’s little stilt house in the shallows can be packed up, loaded onto a motorbike trailer and driven a few kilometers north to the mountainside within three hours.

Poe is one of only a few remaining villagers. Those who could afford it have already abandoned Siem Reap’s shifting lakeside for the city. Worldwide, so many rural residents are on the move that by 2050, seven out of every 10 people will live in cities, leaving behind only their unluckiest neighbors. “If you have a little money, you can move to the city,” says Poe. “But if you’re poor, you stay in the same place, generation after generation.” Three-quarters of the world’s poorest remain in rural areas, sometimes without the very infrastructure necessary to move house, like roads and schools. “There was no education here when I was growing up,” says Poe. “I don’t know how to read and I’ve never seen a map.”