How River Floods Work

Water that fell on mountains weeks ago is now standing in rice fields and city streets. Since mid-September 2026, floods have reached fifty-one Thai provinces, and by the twelfth of October seventy-one people had died.

Floods like this are not bad luck. They follow rules: where rain falls, what the ground does with it, and how a whole country's water finds its way to the sea.

This year's floods spread across several river basins. This film follows the largest of them, the Chao Phraya, link by link, from the clouds to the tide.

It starts over the Indian Ocean. Each year from about mid-May, the south-west monsoon carries warm, wet sea air over mainland Southeast Asia.

When that air rises over hills and heats over land, its water vapour condenses into cloud and falls as rain. Thailand's rainy season runs until about mid-October.

The rain peaks in August and September. By then, the land has been drinking for three months.

A raindrop that lands on soil has two roads. It can soak in, filling the gaps between grains, or it can run across the surface toward the nearest stream.

Soil can only take in water at a certain rate. In a drizzle, almost everything soaks in. In a downpour, rain arrives faster than the ground can swallow it, and the extra runs off.

Try it. Switch between drizzle and downpour and watch where the drops go.

There is a second way to make runoff. Below every field lies the water table, the level under which the ground is already full of water. After weeks of rain it rises until the soil is saturated from below, and then even gentle rain has nowhere to go.

Press a week of rain and watch the blue layer climb. As it does, the share of water running straight to the river grows.

That is why the same storm is harmless in June and dangerous in September. And where the ground is paved over, almost nothing soaks in, wet season or dry.

Hydrologists draw a flood as a curve: how much water passes a point in the river, hour by hour, after a storm. Rain falls at the start; the river rises later.

In a forested valley, much of the rain soaks in and seeps out slowly, so the peak comes late and stays low. Now switch to a paved city.

Roofs, roads and drains send water to the river at once. In one US Geological Survey study around Washington, D.C., urban development raised flood peaks by about two to nearly eight times.

Now zoom out. In the northern mountains, four rivers collect the runoff: the Ping and the Wang, the Yom and the Nan.

The Wang joins the Ping, the Yom joins the Nan, and at Nakhon Sawan the two meet to become the Chao Phraya, which runs about three hundred and seventy kilometres to the sea.

Altogether the basin covers almost a hundred and sixty thousand square kilometres, about a third of Thailand. Press Send a storm: rain from four valleys pours into one river, and it swells far more than any of them alone.

Two giant dams sit high on the funnel's branches. Bhumibol, finished in 1964 on the Ping, can hold about thirteen and a half billion cubic metres. Sirikit, finished in 1972 on the Nan, holds about nine and a half billion.

Each has two jobs that pull against each other. Farmers need water stored for the dry season. Flood control needs empty space for the storms to come.

Try the rainy season both ways. Keep the dam full, and once it is full it must pass on everything that flows in, all at once. Release early, and the flood downstream stays small, but the dry season starts with less water.

In 2010 rain was scarce, and by June the reservoirs were at record lows. Keeping water became the priority.

Then 2011 turned. In March, rain over northern Thailand was more than four times the normal amount. In three months, Bhumibol collected over eight billion cubic metres and filled.

Storms kept coming. By early October most dams were near or over capacity and had to release more, adding water to a river that was already overflowing.

At the bottom of the funnel lies Bangkok, built on the flat mud of the Chao Phraya delta. Most of the city stands only half a metre to two metres above the sea.

Here the river is tidal. Once or twice a day the gulf rises and pushes back up the channel, so floodwater coming down can only drain when the tide goes out.

Studies of Bangkok's historic floods found that the highest monthly tides, and possibly storm surges, can make a river flood worse, just as the peak flow from the north arrives.

The delta has another problem: it sinks. Its soft clay is squeezed when water is pumped out of the layers below, and the ground above settles.

In the 1980s, with factories and homes pumping groundwater freely, parts of Bangkok were sinking about ten centimetres a year. Press the pump and watch the street drop toward the tide.

Laws, groundwater charges and close monitoring slowed it. By 2020 most of the city was sinking about one centimetre a year or less, but the lost height does not come back.

In 2011 every link gave way together. Soaked soil, a full funnel, brimming dams and a low delta. The water spread south for months, from late July until the middle of January.

Eight hundred and fifteen people died, and thirteen point six million were affected. North of Bangkok, barriers around industrial estates in Ayutthaya and nearby provinces failed, and seven major estates went under, in places three metres deep.

Thailand made about a quarter of the world's hard drives, so the shortage was felt worldwide. The World Bank put the damage at forty-six and a half billion dollars, then the fourth costliest disaster on record.

Will a flood like 2011 come back? Engineers answer with odds. A “hundred-year flood” does not come once a century. It is a flood with a one per cent chance of happening in any given year.

Every year starts with the same odds, so two can come back to back. Over thirty years, the chance of at least one is about one in four.

Roll thirty years for twenty towns and count how many get hit. Then try a ten-year flood: almost every town meets one.

After Bangkok flooded in 1995, King Bhumibol proposed an idea named after a monkey storing bananas in its cheeks: kaem ling.

Ponds, lakes and canals take in floodwater when rain is heavy. Floodgates to the sea stay shut while the tide is high, and open when the sea drops below the water in the canals.

Watch the gate follow the tide, then add the monkey cheeks and see how much lower the river runs. The same idea now shapes parks built to hold rain.

The floods of 2026 came at the peak of the rainy season, after Bangkok had a month of rain in a few days. Every link in this film decides how bad a year like that becomes.

None of these links can be removed. But each can be made stronger: by keeping room in reservoirs, leaving space for water, and building with the flood in mind.

The water that reaches the Gulf of Thailand tomorrow fell on the mountains weeks ago. Understanding the chain is how a country gets ready for it.