The lake that runs backwards for half the year

For most of the year, the Tonle Sap River flows in the direction rivers usually do, carrying water out of the Tonle Sap lake south-east into the Mekong near Phnom Penh. Then, roughly every June, as the monsoon swells the Mekong itself, the pressure of that larger river forces the smaller Tonle Sap into reverse, pushing water back up into the lake instead of out of it. It is one of very few river systems on Earth that behaves this way on a predictable annual cycle, and it is the reason Cambodia's largest lake is, depending on the month, a dramatically different size.

A lake that breathes

In the dry season, the Tonle Sap covers somewhere in the region of two to three thousand square kilometres. Once the reversed flow has been filling it for several months, typically peaking around September or October, it can expand to several times that area, flooding surrounding forest and farmland and turning what were, months earlier, dry fields into open water. The cycle then reverses again as the rains ease and the Mekong's level drops, the river flow returns to its normal direction, and the lake slowly empties back down to its dry-season size, leaving newly fertile, silt-enriched land behind it for farming.

Villages built to rise and fall with it

Communities living on and around the lake have adapted to this seasonal swing in a fairly direct way: houses built on tall stilts in areas that flood only partially, and entire floating villages, homes, shops, schools and temples built on rafts or boats, in areas that flood completely. As the water rises, these floating settlements simply move, following the retreating or advancing shoreline, an arrangement that looks improvised to a first-time visitor but represents a settled, generations-old way of living with a lake that will not stay the same size for long.

A nursery for the whole country

The flooded forest that Tonle Sap's expansion creates each year functions as an enormous breeding ground for freshwater fish, and the lake's catch makes up a very large share of Cambodia's protein supply, a dependence that has made the health of the Tonle Sap system a genuine national concern rather than simply an ecological curiosity. Upstream dam construction on the Mekong, across several countries, has begun to affect the timing and strength of the annual flood pulse that the lake, and the fishery built on it, relies on.

A phenomenon confirmed relatively recently

Local fishing communities had understood the lake's seasonal reversal in practical terms for generations, timing their movements and their nets around it long before it was documented in Western scientific literature, and formal hydrological study of the mechanism, including satellite measurement of the flow reversal's timing and volume, is a comparatively modern addition to knowledge that was already, in a working sense, centuries old. It is a useful reminder that "discovery" in cases like this usually means a new audience learning something, rather than a genuinely new fact entering the world.

Seeing it from Siem Reap

Most visitors reach the lake from Siem Reap, a short drive from the temples of Angkor, and boat trips out onto the water or into one of the floating villages are commonly combined with a temple-focused stay for exactly that reason: Angkor's builders depended on broadly the same hydrology, drawing on rainfall and reservoirs to support a population the lake's fish stocks would also, in large part, have fed. Kompong Khleang, one of the larger settlements on the lake's northern shore, is often favoured over villages closer to Siem Reap precisely because it sees fewer boat tours, and its houses, raised on stilts of six metres or more, give a clearer sense of just how far the water rises here than a shorter, busier village visit generally can.