A festival timed by a river
For part of every year the Tonlé Sap runs backwards. The monsoon lifts the Mekong faster than the lake can drain, the flow stops, and then reverses, pushing water inland until the lake is several times its dry-season size.
Bon Om Touk falls when that reverses again at the end of the rains. It is not an anniversary of anything historical; it marks something the river is doing that week.
The races
Long boats, crewed by dozens of rowers, race over several days. Villages and pagodas field their own, and the boats are built and maintained locally rather than centrally, which is why the fleet looks different every year.
The largest racing is on the water at Phnom Penh. Around Siem Reap the festival is smaller and closer to its agricultural roots, which many find the better version of it.
Why it matters beyond the week
The flood pulse the festival marks is the engine of the Tonlé Sap fishery, one of the most productive inland fisheries anywhere, and the reason a city the size of Angkor could be supported here at all.
Upstream dams and changing rainfall have made the pulse less reliable in recent years, which has turned a celebration into something closer to a reckoning.
