Tonle Sap (known as the Great Lake) is a combined lake and river system. It is the largest freshwater lake in South East Asia and was designated a UNESCO biosphere in 1997. For most of the year the lake is relatively small, about one meter deep, with an area of 2,700 square kilometers. During the rainy season, however, the river which connects the lake with the Mekong River reverses its flow. Water is pushed up from the Mekong into the lake, increasing its area to 16,000 square kilometers and its depth to up to nine meters. Most of these photos were taken during the rainy season.