Geography
One Cambodian river flows backward for half of every year
The Tonle Sap River normally drains its lake south into the Mekong. But when monsoon rains swell the Mekong each summer, the larger river forces the Tonle Sap to reverse course and flow north instead, pumping the lake up to six times its dry-season size. When the Mekong subsides in autumn, the Tonle Sap flips back and drains the swollen lake out again — a twice-yearly reversal found nowhere else at this scale.