The Tonlé Sap lake is fed mostly from the Mekong river, when the water flows “backwords” into the lake. At the end of the dry season, the water level is only about 1 m. This increases to 9-14 m at the end of the rain season. The annual rainfall is between 1,000 and 3,000 mm, in some places 4000 mm. The huge change during the seasons cause the size of the lake to vary between 2 500 and 16 000 km2.
By the 2030s, hydropower development upstream in China and Laos may lead to large-scale changes in habitat and culture.
The Tonlé Sap river runs 120 km down to Phnom Penh. Thus, the 300 km boat trip from Phnom Penh to Siem Riep takes 8 hours with on speedboat running at 20 – 30 knots.