Late in the rainy season, the sky often clears before the danger has passed. Rain that fell upstream days ago is still travelling down the river, and for people living near the lower reaches, the highest water can arrive after the rain has stopped.
Rain and river do not keep the same clock
Rain is measured where it falls, on the day it falls. A river collects that rain from a whole basin and carries it downhill, through tributaries, reservoirs and floodplains, at the speed water can actually move.
So the river at any one town is answering to rain that fell somewhere else, some time ago. A dry morning in the town says little about what the river is about to bring.
Why the lower river peaks later
Water from many tributaries joins on the way down. Each arrives at its own time, and near the lower end of a large river they add up, so the flow can keep growing for days after the last heavy rain upstream.
The wider the basin, the longer that delay. A short, steep stream can rise and fall within hours, while a large lowland river can take far longer to carry the same storm to the sea.
Floodplains and fields along the way slow the water down too. They hold some of it for a while and release it back to the river later, which spreads the flood out in time instead of letting it pass quickly.
The tide meets the river at the mouth
The last part of the journey depends on the sea. At high tide, sea water pushes into the river mouth and the river cannot drain as fast as before, so water backs up through the lowest stretch of the river.
When a high tide arrives while the river is already carrying a lot of water from upstream, the two effects stack. Places near the coast can then see water rise even though no rain is falling there at all.
What to watch when the sky is clear
Measured water levels upstream are a better guide than the weather overhead. A gauge further up the same river shows what is already on its way, before it reaches you.
Tide tables for the river mouth matter too, if you live near the coast. They are published well in advance, so the timing of the highest tides is not a surprise.
What this means for planning
The end of the rain is a good time to check, not to relax. Crops close to harvest, stock kept on low ground and vehicles parked near the river are worth a second look while the lower river is still rising.
Moving things to higher ground costs little when water is still upstream, and much more once it has arrived.
Where to follow official information
For warnings and evacuation orders in Thailand, follow the Department of Disaster Prevention and Mitigation (DDPM), the Thai Meteorological Department and your local authorities. The DDPM hotline is 1784; for medical emergencies, call 1669.