Geography of Cambodia
Hydrology
Hydrology
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[thumb\|**Floating homes on the Mekong**](/wiki/File:Mekong_floating_homes.jpg "Mekong floating homes.jpg")
[thumb\|An overview of [drainage divides](/wiki/Drainage_divide "Drainage divide")](/wiki/File:Drainage_Cambodia.jpg "Drainage Cambodia.jpg")
[thumb\|Mekong tributary, [Stung Treng Province](/wiki/Stung_Treng_Province "Stung Treng Province")](/wiki/File:Mekong_tributary_Cambodia.jpg "Mekong tributary Cambodia.jpg")
The Mekong River and its tributaries comprise one of the largest river systems
in the world. The central Tonle Sap, the *Great Lake* has several input rivers, the most important being the Tonle Sap River, which contributes 62% of the total water supply during the rainy season. Direct rainfall on the lake and the other rivers in the sub\-basin contribute the remaining 38%. Major rivers are the [Sen river](/wiki/Stung_Sen_Wildlife_Sanctuary "Stung Sen Wildlife Sanctuary"), [Sreng River](/wiki/Sreng_River "Sreng River"), [Stung Pouthisat River](/wiki/Stung_Pouthisat_River "Stung Pouthisat River"), [Sisophon River](/wiki/Sisophon_River "Sisophon River"), [Mongkol Borei River](/wiki/Mongkol_Borei_River "Mongkol Borei River"), and [Sangkae River](/wiki/Sangkae_River "Sangkae River").

Smaller rivers in the southeast, the Cardamom Mountains and Elephant Range form separate [drainage divides](/wiki/Drainage_divide "Drainage divide"). To the east the rivers flow into the Tonle Sap, as in the south\-west rivers flow into the [Gulf of Thailand](/wiki/Gulf_of_Thailand "Gulf of Thailand"). Toward the southern slopes of the Elephant Mountains, small rivers flow south\-eastward on the eastern side of the divide.

The Mekong River flows southward from the Cambodia\-Laos border to a point south of [Kratié (town)](/wiki/Krati%C3%A9_%28town%29 "Kratié (town)"), where it turns west for about  and then turns southwest towards Phnom Penh. Extensive rapids run north of [Kratie city](/wiki/Krati%C3%A9_%28town%29 "Kratié (town)"). From [Kampong Cham Province](/wiki/Kampong_Cham_Province "Kampong Cham Province") the gradient slopes very gently, and inundation of areas along the river occurs at flood stage. From June through November—through breaks in the natural [levees](/wiki/Levee "Levee") 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 Sap river emanates from the Tonle Sap—flows in from the northwest. They divide into two parallel channels, the Mekong River proper and the [Bassac River](/wiki/Bassac_River "Bassac River"), and flow independently through the delta areas of Cambodia and Vietnam to the [South China Sea](/wiki/South_China_Sea "South China Sea").

The flow of water into the Tonle Sap is seasonal. In spring, the flow of the Mekong River, fed by monsoon rains, increases to a point where its outlets through the delta can't handle the enormous volume of water. At this point, the water pushes northward up the Tonle Sap river and empties into the Tonle Sap lake, thereby increasing the size of the lake from about  to about  at the height of the flooding. 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 Sap River. Gradual silting of the lake would seem to be occurring; during low\-water level, it is only about  deep, while at flood stage it is between  deep.


What happens to the Tonle Sap lake during the rainy season?