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The Tonle Sap, located in the Cambodia, is perhaps the most incredible and unique geographical system in the world.

Sap in Khmer means great, as in large. Tonle translates into both river and lake, depending on context. In the case of the Tonle Sap, it means both because the Tonle Sap is a river/lake system, but I will refer to them as lake and river to avoid confusion.

The Tonle Sap Lake is located roughly in the middle of Cambodia and runs, via the Tonle Sap River, into the Mekong. It is at the confluence of these two rivers, that the city of Phnom Penh, the capital, is located. The Mekong then continues its way into Vietnam, and eventually the South China Sea.

Now the mighty Mekong isn’t called such for no reason. During the rainy season from mid-May to mid-October, the Mekong river changes level an at an incredible rate and to extremes, in some locations, by as much as 30 meters! The Mekong is the fifth longest river in the world and starts in East Tibet. It has many tributaries and before reaching Cambodia, it passes between Burma, Laos and Thailand.

Now by the time the turbulent waters reach Phnom Penh, the force of the river is so incredible that it actually causes the Tonle Sap River to flow backwards! In doing so, it also causes the Tonle Sap Lake to flood.

Now, don’t go thinking that either the Tonle Sap River or Lake are small by any means. A trip along the river to the lake takes about 2 hours by speedboat. During the dry season the Tonle Sap Lake has a size of 3,000km2 with a depth of 1 m, during the wet season, it has a size of 10,000km2 with a depth of 12 . That’s about 2 and half times its original size and many more times the volume. Imagine any lake near you expanding that much. Compare and contrast.

The Tonle Sap is largely responsible for the location and success of the ancient Khmer Empire, with Angkor Wat, the most important temple, located about 10 kilometers from it’s shores. And today it plays a vital role in the region’s economy; the yearly flooding provides water for rice irrigation as well as a spawning area for a variety of fish life.

The system further prevents excessive flooding further down along the Mekong, at the Mekong River Delta. If it wasn’t for this natural water reservoir, the lands south of Phnom Penh would be much more devastated, and more permanently, by flooding.

A Natural wonder of hydrology.

When the Tonle Sap changes directions in November, once again flowing in the direction of the sea, the Khmers celebrate with long-boat races and mark the event with a 5 day festival appropriately called the Water Festival. Thousands of people swarm into the city to watch the boats, usually numbering about 200, compete for top prize, awarded by King Sihanouk himself.

The changing of the direction of the water, unfortunately, can not be seen. I spent countless hours along the riverside in Phnom Penh, hoping to be there when it happened. Along with others, I expected the river to grind to a halt and immediately start to flow, dramatically, in the opposite direction. However, the event takes place over a period of several days, during which the water simply seems to stagnate and slowly reverse its course.

Despite the lack of drama, this system remains astounding.

Cambodian Ministry of Tourism provided the numbers, the rest was me

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