Why does Cambodia's Tonle Sap River change direction?

The reversal comes down to water level. During the monsoon the Mekong swells and, at Phnom Penh, rises above the level of the Tonle Sap Lake. Water follows the slope and flows up the Tonle Sap River into the lake. When the rains subside and the Mekong drops below the lake and its floodplain, the river turns around and drains the lake back into the Mekong. The two changes of direction each year mark the start and end of the flood. Sources: [savethemekong.org](https://www.savethemekong.org/en/stories/why-the-tonle-sap-reverses), [tandfonline.com](https://www.tandfonline.com/doi/pdf/10.2489/jswc.73.3.60A). [1] [2]

The Tonle Sap River is roughly 120 kilometers long and links the lake to the Mekong at Phnom Penh. In the dry season it flows in the ordinary direction, from the lake to the Mekong. According to the Mekong River Commission material summarized by Save the Mekong, the reverse flow usually starts in late May and peaks in late August at around 10,000 cubic meters per second. A journal article by Olson and Morton gives a similar picture, with inflow beginning in May or June as the Mekong floods and backs up into the river and lake ([tandfonline.com](https://www.tandfonline.com/doi/pdf/10.2489/jswc.73.3.60A)). Around late September the current turns back toward Phnom Penh ([savethemekong.org](https://www.savethemekong.org/en/stories/why-the-tonle-sap-reverses)). [1] [2]

The exact timing depends on each year's flood. The sources also differ slightly on details: Olson and Morton put the wet season at mid-May to early October and the river length at 147 km in one passage and 120 km in another, while the other sources give about 120 km. [2] [1]

The lake works as a natural reservoir. The Mekong River Commission says its area grows six-fold, from 2,500 to 15,000 square kilometers, its mean depth rises from 1 meter to 6 to 9 meters, and its volume increases from under 1.5 cubic kilometers to between 60 and 70 ([mrcmekong.org](https://www.mrcmekong.org/wp-content/uploads/2024/08/MRC-Management-Information-Booklet-Series-No.2-The-Flow-of-the-Mekong.pdf)). The Stimson Center describes expansion to five times the dry-season area and sixty times the volume ([stimson.org](https://www.stimson.org/2024/mekong-floodpulse/)). Olson and Morton give different, larger figures for the dry and wet extents. These numbers vary by source, year and how the lake is measured, so they are best read as approximate. [3] [4] [2]

The incoming water carries sediment, nutrients, fish eggs and larvae into the lake, where young fish feed on the flooded forests and floodplain and then migrate out as the water falls ([stimson.org](https://www.stimson.org/2024/mekong-floodpulse/), [nationalgeographic.com](https://www.nationalgeographic.com/animals/article/mekong-river-fish-migrations)). The Mekong River Commission lists further benefits downstream: the lake moderates peak flood flows in the wet season, adds water for dry-season irrigation, and increases fresh water reaching the sea, which reduces saltwater intrusion into the Mekong Delta in Vietnam ([mrcmekong.org](https://www.mrcmekong.org/wp-content/uploads/2024/08/MRC-Management-Information-Booklet-Series-No.2-The-Flow-of-the-Mekong.pdf)). National Geographic calls it one of only a handful of rivers worldwide to reverse this way, while another source describes it as unmatched in scale. [4] [5] [3] [1]