Why does the direction of the Sab River change with the seasons?

The direction of the Sab River changes seasonally due to the hydrological relationship between the Mekong River and the Tonle Sap lake.1

During the rainy season (mid-May to early October), the Mekong’s water volume increases significantly due to monsoonal rainfall. This enormous volume of water causes the river to back up into the Sab River, forcing it to flow northwestward into the Tonle Sap, which dramatically expands the lake's surface area and depth.123

As the rainy season ends and the water level of the Mekong falls during the dry season, the process reverses. The water stored in the Tonle Sap then drains back down through the Sab River into the Mekong, switching the flow to a southeastward direction. This annual cycle is a critical natural phenomenon that supports one of the world's richest freshwater fish populations.13