Reversing river

Every six months the Tonlé Sap River reverses direction.

Tonlé Sap River
Taguelmoust, CC BY-SA 3.0, via Wikimedia Commons

Back in August I wrote about the historical reversals of the Amazon and Chicago rivers, one because of the rise of the Andes and the other because of a whole lot of earthworks and determination. Well, there’s a river in Cambodia that reverses direction every six months.

The Mekong River flows south from Tibet down to the South China Sea, passing through almost every country of continental Southeast Asia (Laos, Thailand, Myanmar, Cambodia, and Vietnam). The Tonlé Sap River connects to the Mekong within Cambodia’s capital city, Phnom Penh. The Royal Palace in that city is perched right over the meeting point of the two rivers. At the other end of Tonlé Sap River, 115 kilometres away, is the freshwater Tonlé Sap Lake. Here’s a map:

Tonlé Sap map
Mkummu, CC BY-SA 3.0, via Wikimedia Commons

For half of each year, Tonlé Sap River drains into the Mekong. The lake that is its source gradually empties, shrinking from sixteen thousand square kilometres in early November down to just twenty-five hundred in late April. But then, in May, the monsoon waters hit the Mekong.

The point where the two rivers meet is very flat. When the Mekong swells in size, Tonlé Sap River reverses direction completely. Instead of flowing from the lake, it flows into the lake. Over the next six months Tonlé Sap Lake is refilled, restored to its former glory… and then, once the Mekong’s flow drops, Tonlé Sap River reverses direction back again. The cycle begins once more.

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