Southeast Asia's largest lake, Tonle Sap , is in Cambodia and is connected to the Mekong by a short river, also called Tonle Sap. For most of the time this river flows from lake Tonle Sap into the Mekong. However, during the Southeast Asian rainy season from June to October when the Mekong drains large areas of Southeast Asia, the Tonle Sap river flows from the Mekong back into lake Tonle Sap thus causing enormous floods in the area surrounding the lake. During this time, lake Tonle Sap can swell to more than twice its regular size. Central Cambodia is a fertile plain .
Mountain ranges in the shape of a semicircle form a natural boundary with Thailand.
The highest mountain in Cambodia is Phnom Aural in the Cardamon range, at a height of 1,813 metres. To date these mountain ranges are comparatively densely covered with forest and are only sparsely populated. All three are still operating areas of the Khmer Rouge guerrillas. The southern coastal strip has never been of importance for the Cambodian economy. It is separated from the central plain by difficult terrain.