Cambodia lies in the lower Mekong River basin and occupies some of the flattest and lowest land in Southeast Asia. Three-quarters of the country consists of a broad, very gently rolling lowland called the Cambodian Plain. In several places, the plain is fringed by uplands. In the southwest, the Cardamom Mountains, reaching more than 6,000 feet (1,800 m), and the Elephant Range separate the interior from the coast. Along the northern border with Thailand the Dangrek Range rises sharply above the plain. Eastern Cambodia's uplands are mostly extensions of the mountains and plateaus of Vietnam.
The Mekong, one of Asia's greatest rivers, drains nearly all of the country. The Mekong runs generally southward from the Laos border, passing through many rapids north of the town of Kratie. Southward, the river widens as it winds through the Cambodian Plain, eventually splitting into two main branches before entering its delta in Vietnam.
When summer monsoon rains raise the Mekong's level as much as 40 feet (12 m), part of the river's flow backs up into the Tonle Sap, Southeast Asia's largest lake. The lake then increases considerably in size and depth, flooding the surrounding land. By November the waters of the lake have receded, leaving behind rich alluvial soil and shallow ponds teeming with fish.
Cambodia has a humid, tropical climate. Average temperatures are high all year, varying between about 75º and 90° F. (24º and 32° C.). Most of the rain comes during the monsoon season, from May through October. Coastal areas annually receive as much as 150 inches (3,800 mm) of rain; much less falls in the interior. At Phnom Penh, the national capital, annual rainfall is about 55 inches (1,400 mm); northern areas get somewhat more.