Cambodia is a country of forested mountains and well-watered plains. The central part of the country forms a gigantic basin for the Tonle Sap, or Great Lake, and the Mekong River, which flows down from Laos to the southern border with Vietnam. Between the Tonle Sap and the Gulf of Thailand lie the Cardamom Mountains and the Elephant Range, which rise abruptly from the sea and from the eastern plains. In the north, the Dangrek Mountains, 320 km (200 mi) long and 300 to 750 m (1,000–2,500 ft) high, mark the Thailand frontier. The short coastline has an important natural harbor, Kompong Som Bay (Chhâk Kâmpóng Saôm), where the port of Kompong Som (Kâmpóng Saôm, formerly Sihanoukville) is located.
The Mekong and the Tonle Sap dominate the life and economy of Cambodia. The Mekong overflows during the rainy season, deposits vast quantities of alluvial soil, and, backing toward the Tonle Sap, causes that lake to increase in size from about 2,590 sq km (1000 sq mi) to almost 24,605 sq km (9,500 sq mi).