Except for the smaller rivers in the southeast, most of the major rivers and river systems in Cambodia drain into the Tonle Sap or into the Mekong River. The Cardamom Mountains and Elephant Range form a separate drainage divide. To the east the rivers flow into the Tonle Sap, while on the west they flow into the Gulf of Thailand. Toward the southern end of the Elephant Mountains, however, because of the topography, some small rivers flow southward on the eastern side of the divide.

The Mekong River in Cambodia flows southward from the CambodiaLaos border to a point below Kracheh city, where it turns west for about 50 kilometers and then turns southwest to Phnom Penh. Extensive rapids run above Kracheh city. From Kampong Cham the gradient slopes very gently, and inundation of areas along the river occurs at flood stage--June through November--through breaks in the natural levees that have built up along its course. At Phnom Penh four major water courses meet at a point called the Chattomukh (Four Faces). The Mekong River flows in from the northeast and the Tonle Sab--a river emanating from the Tonle Sap--flows in from the northwest. They divide into two parallel channels, the Mekong River proper and the Basak River, and flow independently through the delta areas of Cambodia and Vietnam to the South China Sea.

The flow of water into the Tonle Sab is seasonal. In September or in October, the flow of the Mekong River, fed by monsoon rains, increases to a point where its outlets through the delta cannot handle the enormous volume of water. At this point, the water pushes northward up the Tonle Sab and empties into the Tonle Sap, thereby increasing the size of the lake from about 2,590 square kilometers to about 24,605 square kilometers at the height of the flooding (see fig. 6). After the Mekong's waters crest--when its downstream channels can handle the volume of water--the flow reverses, and water flows out of the engorged lake.

As the level of the Tonle Sap retreats, it deposits a new layer of sediment. The annual flooding, combined with poor drainage immediately around the lake, transforms the surrounding area into marshlands unusable for agricultural purposes during the dry season. The sediment deposited into the lake during the Mekong's flood stage appears to be greater than the quantity carried away later by the Tonle Sab River. Gradual silting of the lake would seem to be occurring; during low-water level, it is only about 1.5 meters deep, while at flood stage it is between 10 and 15 meters deep.