Cambodia covers an area of 181,035 square kilometers with the maximum extents of 600km east-west and 450km north-south. The country is bordered to the north by Thailand and Laos, to the west by Thailand, to the east and south by Vietnam, and to the south and south-west by the Gulf of Thailand. Much of Cambodia is relatively flat with vast plains. The largest of all are the deltas created by the Tonle Sap and the Mekong Rivers. On the edge of these deltas are mountain ranges. The highest peaks are found in the mountain ranges in the north and south of Cambodia which can range from 700m to 1813m above the sea level.

Most of the major rivers and river systems in Cambodia drain into the Tonle Sap or into the Mekong River. Some rivers to the west flow into the Gulf of Thailand. The Mekong River in Cambodia flows southward from the Cambodia - Laos border passing through Cambodia and Vietnam emptying into the sea. At Phnom Penh, the Mekong splits its current into the Mekong (upper) and the Tonle Bassac (lower).

In September or in October, the Mekong River fed by heaven monsoon rains increases its level to a point higher than that of the Tonle Sap. Then we'll see the water flowing into the Tonle Sap Lake. At other times, when the Mekong water is lower the flow reverses, and water flows out of the Great Lake.