The Tonle Sap River in Cambodia flows North for almost half the year and then South for the rest of the year.

Monsoon rains fill the neighboring Mekong River with so much water that the Tonle Sap river is forced to flow backwards into the Tonle Sap Lake. The lake swells to 5 times its normal size, providing Cambodians with giant Mekong catfish for food.

Overfishing, and damming of the Mekong river may disrupt this important seasonal occurrence. Fishermen have been catching fewer and smaller fish in recent years, and Mekong catfish is now endangered.