The Tonl? Sap is a vast lake in Cambodiaˇs central northwest, surrounded by a fertile plain. The Sap River runs from the lakeˇs southeastern end to join the Mekong in Phnom Penh, some 100km (63 miles) distant. During the dry months, roughly between November and May, the lake is at its smallest, though it still covers 2,500 to 3,000sq km (965 to 1,160sq miles). When the rains fall, though, from mid-May through October, a unique hydrographic phenomenon occurs. The rising waters of the Mekong cause the flow of the Sap River to reverse. During this period, the Tonl? Sap increases in surface area, sometimes to well in excess of 10,000sq km (3,860sq miles). At its lowest most of the lake is less than 2m (6. 5ft) deep, and can resemble a marsh criss-crossed by navigable channels, but when at its fullest, its depth increases to as much as 14m (45ft), and it gains up to 70km (44 miles) in width.