Once a year, due to the enormous amount of rain, the level of the Mekong River rises so much that the Tonle Sap River can no longer flow into it. The river reverses its direction and flows north into the lake, causing it to swell to triple its size. Fish abound, and silt created by the river's reversal, make soil quite fertile. The central half of Cambodia is a flat rolling plain. The Dangrek Mountains are located on the northern border with Thailand.
Sandy beaches dot the south coast.
Highlands lie to the east along the Vietnamese border. Beach at Sihanoukville. Natural vegetation grows lush and thick in Cambodia. Its eastern highlands are covered with thick tropical rainforest. Out of a plane it looks like a bumpy, woven green blanket completely covering the ground.