01 How you turn a river around
The mechanism is almost disappointingly simple, and that is why it works. The Tonlé Sap river connects the lake of the same name, to the northwest, with the Mekong, at the city. The only thing deciding which way the water runs is which of the two ends is higher at each point in the year.
From May or June onwards, the monsoon swells the Mekong and its level rises above the lake's. The water cannot get out, so it goes in: the river runs northwest and gradually fills the basin. The reversed flow peaks in November. Then the Mekong falls, its level drops below the lake's and everything empties the other way between December and April.
02 A lake that multiplies by six
The lake's figures give the real measure of the phenomenon. In the dry season it covers between 2,500 and 3,000 square kilometres at around one metre deep: it is, literally, an enormous and extremely shallow sheet of water. At its maximum it reaches 16,000 square kilometres and nine to fourteen metres.
From one metre deep to fourteen, and from 2,500 square kilometres to 16,000. It is the same lake with six times the surface.
The maximum volume is around 80 cubic kilometres, and the river's mean annual reverse flow is about 30, that is, roughly half the lake's maximum volume. And the level rises about ten metres from one season to the other. That means whole kilometres of forest go under water every year, and that its trees are adapted to spending months submerged.
03 Why this feeds a country
Here is the part that turns a curiosity into something important. When the water invades the forest, it floods an enormous surface of vegetation, leaf litter and soil, and that generates an extraordinary quantity of food and shelter. Fish come in with the flood, spawn and grow in that submerged forest, and leave when the water withdraws.
The result is that Cambodia produces around 400,000 tonnes of freshwater fish a year, and most of it comes from this system. UNESCO declared it a biosphere reserve in 1997. This is not a pretty landscape with a secondary function: it is one of the most productive inland fisheries there is.
04 What you see and from where
In the city, the place to see it is the river promenade itself, and what you can appreciate is the level and the direction of the current, not a spectacle. Two things are worth watching: how far the water reaches up the bank's slope, and which way the branches and floating debris drift, which is the simplest way to read the direction of flow.
And an honest caveat is in order. The figures in this article are averages of a historical regime, and in recent years that pulse has weakened: flow regulation upstream and droughts have produced smaller and shorter floods than normal. What you see today may fall short of the numbers, and that gap is itself part of the story.
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