Floating villages, which are mainly found at the mouths of the major rivers where they empty into Tonle Sap, represent another major type of village community in Cambodia. These communities are characterized by self-subsistence and are specialized in fishing activities. Surplus catch is used to purchase other essential foodstuffs and tools. Inhabitants live on moveable boathouses grouped into tight clusters. They maintain their position by moving seasonally with the rise and fall of the level of the lake. The village composition of these floating communities is more loosely structured than that of subsistence farmers on the plains. Their ethnic composition is quite varied, but the majority are Khmer and Vietnamese, with some Cham.