The largest freshwater lake in Southeast Asia, Tonle Sap, due south of Siem Reap, supports a population of over a million people, the majority of whom live from rice cultivation and fishing, in stilted or floating villages around its shoreline. During the annual monsoons the lake swells up to six times as the waters of the Mekong, unable to pass through the relatively narrow channel at Phnom Penh, back up and flood the plains further north. The restoration of flow into the Mekong, at the end of the rainy season, is celebrated by the Water and Moon Festival, when thousands participate in canoe races and religious rituals on the water.
Tonle Sap also attracts huge numbers of wetland birds, among them rare or endangered species such as the spot-billed pelican. The best way to observe the extraordinary conflagration is a visit to the Prek Toal Sanctuary, at the northwestern corner of the lake. Although reachable in a day trip from Siem Reap, it’s well worth spending the night in one of the basic stilted cottages at the reserve in order to be there at dawn and dusk, when the vast flocks of painted storks and black-headed ibis are at their liveliest.