Tonlé Sap is the largest freshwater lake in Southeast Asia (fig.). It dimensions swell and shrink up to five times its original size, depending on the seasonal monsoons and the flow of the Tonlé Sap River which in the  Southeast converges with the Mekhong River.

In the rainy season, when the level of the Mekhong rises rapidly, it reverses the natural flow of the Tonlé Sap River, causing it to flow upstream, into the lake.

Due to the sedimentation that this brings along its waters are high in nutrients, making it one of the most fertile inland fisheries in the entire world.