During the monsoon season the flooded Mekong river reverses the flow of the Tonle Sap river, sending it pouring upstream into the Great Lake. The water level of the lake rises as much as 29 feet (9 meters) during this time.

(Text adapted from and photograph shot on assignment for, but not published in, "The Mekong: River of Terror and Hope," December 1968, National Geographic magazine)