It was not a river. It was scarcely a stream. The Ruisseau des Quartes, Marcourt, Belgium. An unlovely and unremarkable tributary of the Ourthe, itself a tributary of the mighty Meuse, which thunders from France through Belgium and the Netherlands and on to the chilly oblivion of the North Sea. It was barely 2 metres wide, boggy in places, just 5cm deep in others. The parents dropping off their children at the United World Colleges summer camp on 10 July 2021 hopped over it as they lugged bags to the dormitories.

Fourteen-year-old Benjamin Van Bunderen Robberechts was nervous on the drive down. He would have to take a Covid test on arrival and he worried it would be positive. Belgium was beginning to relax restrictions and Benjamin was desperate to socialise with other teenagers. But the test was negative; soon, Benjamin was dropping off his things in his dorm and meeting his other campmates. And there was Rosa.

Rosa Reichel was 15, from Denmark and Germany by way of New York, but her family lived in Brussels. Dyed red hair and black eyeliner and chunky silver necklaces. She tapped Benjamin on the shoulder and told him a dirty joke. Just like that, they were friends.

The girl Benjamin met that day laughed a lot: loudly, happily. If you gathered Rosa’s friends in a room and asked them to describe one thing about Rosa, without question it would be her laugh. If you gave them more words, her friends would say how much fun she was, but also how caring: how Rosa was always the person who noticed when people were feeling low and tried to cheer them up. They felt that she was someone you could rely on, someone who brightened any room she was in. She stood up for her friends and for causes she believed in. At first, she might have been a little shy around you, but when she opened up, she would share her humour, her values, herself.

Benjamin was a little over­whelmed by her. “She was the greatest person I ever met,” he says.

The rain was hammering down on Wednesday 14 July. During their morning workshops, Benjamin and Rosa exchanged looks, as if to say: this is stupid. The teenagers played ping pong and grouched about the food. By the afternoon, they were getting restless. A few, including Rosa, decided to go for a run in the rain.

“How was the run?” Benjamin asked when she returned. “Wet,” she said.

There were no adults around, but other children were outside. It was raining, but it didn’t feel dangerous. Benjamin and Rosa went outside, stood on a bridge over the stream and watched the water. The ground was muddy. Rosa slipped. Benjamin caught her before she fell. “The last thing she said to me was: ‘Benjamin, what would I do without you?’”

Seconds later, the fields flooded with a terrifying rush of water. Rosa was dragged away. Benjamin jumped in after her. He caught her and grabbed at branches with his free arm. He remembers seeing his sandals float away, one after another. There was a fence pole sticking out of the bank. He lunged at it, still holding Rosa with his other arm. “But then a bigger wave came and she slipped out of my hands.”

Across Wallonia, the primarily French-speaking region in the south of Belgium, the rain hammered all through 13 July. In the worst-affected areas, between 200mm and 300mm fell in only 72 hours. Members of the European Flood Awareness System in the UK had sent warnings to the Belgian authorities on 12 July, but it appears these were not sufficiently heeded.

The catastrophe unfolded on the morning of 14 July. In the city of Liège – once at the centre of the Industrial Revolution, a place where coal was mined and iron was forged and copper was refined, for export via the Meuse all around the world – members of the city council attended a Bastille Day ceremony in honour of the French resistance at the Parc d’Avroy. Senior figures kept taking phone calls and stepping away. The military commander of the province excused himself.

Seven miles away, in Trooz, a humdrum deindustrialised town with 8,500 inhabitants, the River Vesdre burst its banks at noon. It rose 6 metres in total. The mayor, Fabien Beltran, had been up all night trying to deal with a mudslide on the outskirts of the city. His wife was in hospital with a brain aneurysm; this was the last thing he needed.

The city’s communications servers were housed in a basement in the town hall, which flooded within an hour. By 1.30pm, all the roads were impassable. When Beltran’s phone got a signal, which was seldom, it buzzed incessantly with panicked calls from residents. They were standing on their kitchen tables, with the water up to their waist. Beltran called the army, who told him help was on its way. Eight hours later, a few soldiers arrived with small dinghies. Beltran had 2,200 people who needed to be moved to safety. It was impossible. Parents were on rooftops with their children; they weren’t sure whether to jump into the water. Beltran had no one to send them. It was the worst moment of his life.

Upstream, in Eupen, the Vesdre dam was straining. In normal times, it holds 25m cubic metres of water, but after the heavy rainfall it was cradling an extra 13.4m cubic metres. If it burst, 38.4m cubic metres would barrel through the Vesdre valley towns of Limbourg, Verviers, Pepinster, Trooz and Chaudfontaine. On the evening of 14 July, a decision was made to release water. It flowed into the Vesdre initially at a rate of 5 cubic metres a second, which gradually increased until it rushed through at 150 cubic metres a second. On 15 July, just after midnight, seismometers shuddered with the roar of a flash flood.

It thundered along the Vesdre, then into the Ourthe and the Meuse. It pulverised bridges, roads, warehouses, lorries, factories, cars and shops. Buildings were torn in half like loaves of bread. Bridges crumpled like cans in a recycling bin. The water carried metre-long blocks of butter, now foul-smelling and contaminated with oil, and chocolate moulds from the Galler factory in Chaudfontaine.

The next day, 15 July, dawned cold and wet. In Liège, Dolhain and Eupen, the mayors evacuated the cities, amid fears a dam near Liège would break. But in Trooz and Pepinster, it was too late for that: people were already trapped. Members of a jetski club offered Beltran their services. If they drowned, he could be liable for their deaths. But there were children on roofs, so he said yes. At great personal risk, the jetskiers rescued people. But not everyone could be saved.

In Trooz, a 20-year-old man died trying to cross the road. Two elderly people had heart attacks. In Liège, two people drowned in their houses. In all, 39 people died in Belgium before the waters receded.

On the morning of 15 July, an alert flashed on the mobile phones of people in the affected areas. “Be alert,” it read. “Flooding on the banks of the Meuse. Evacuate if possible, or take shelter upstairs.”

It took three days for rescuers to find Rosa’s body four miles downstream. Benjamin tried to be hopeful, to think that maybe she was sitting in a tree, but he knew in his heart that she was dead. “The water was a monster,” he says.

For months, he barely left his bedroom. He tried going to school, but on his first day back someone asked him how his summer was and it broke him. He avoided looking at bodies of water. He felt that Rosa’s death was his fault. “I think maybe I was the one who said: ‘Let’s go outside.’ I didn’t rescue her,” says Benjamin, now 17, in a quiet voice.

He went over every memory he had of Rosa, their every interaction in the five days he had known her. How she said his aviator sunglasses made him look like Tom Cruise in Top Gun. How they joked that they were like husband and wife. How he taught her how to play Rock Around the Clock on his bass guitar. “Every time I thought of Rosa, I would be pulled into the river in my mind,” he says.

It didn’t take long for Benjamin to make the connection between Rosa’s death and the climate emergency. “It pretty quickly fell into place,” he says. On 10 October 2021, Benjamin joined protesters marching through Brussels to call for climate justice before Cop26. He was with a group of Rosa’s friends, all dressed in red.

“Politicians die of old age,” read their banner. “Rosa died of climate change.”

Philippe Duquesnoy was meant to be on holiday with his partner, 45-year-old Inge Van Tendeloo, in July 2021, but they couldn’t go because of the pandemic. The 51-year-old factory manager saw the floods on the news. He lives in Kessel, in the Flemish part of Belgium, an hour and a half’s drive away. He decided to offer his services as a volunteer.

On the first day, he cooked for people on an old army kitchen. He started organising the volunteers and sending them to flooded houses. Residents would point him out – that guy with the round glasses, they would say, he’ll help you – and the volunteers turned to him, awaiting orders.

“That’s the way it started,” says Duquesnoy. It’s January 2024 and we are in Trooz. Walking through the town with Duquesnoy and Van Tendeloo is like walking around with a celebrity: they cannot go five paces without being stopped. Everywhere, slices of cake, thimbles of homemade kirsch and cups of coffee are pressed upon them. In a bakery, a man insists on buying them – and me, because I am with them – lunch. They have a special sign that lets them park anywhere they want. Duquesnoy jokes that if Van Tendeloo ever kicked him out, he would be able to live in Trooz for nothing for years.

This largesse repays what, in retrospect, seems a bottomless, unfathomable kindness. For two years, Duquesnoy and Van Tendeloo spent every weekend – often both days – rebuilding Trooz, a town they had never visited before the floods and in which they had no family. With the rest of their volunteers, a 120-strong collective named #TeamEclairs, they cleaned, painted, tiled and fixed roofs. (The name is a nod to their favourite pastries, but also the speed with which they worked – éclair means flash of lightning in French.)

In this town, they are vastly more popular than Beltran, a softly spoken man with a hangdog expression and a black sense of humour. “Many people are very tough with me,” sighs Beltran. We are in his office, in a portable building in the car park of an old car museum. Nearly three years on, city hall still hasn’t been rebuilt. “They think it’s my own responsibility, what happened. It’s very hard for me every day on the Facebook: the city don’t do anything! The mayor is not there. Every day. Every day!” This will be Beltran’s last term. “I resign,” he says, laughing. “Oh yes.”

I arrive in Trooz after a few days of heavy rain. People are anxious about the water level; they check a flood warning app incessantly. Some will not walk by the river; the sound of rainfall triggers painful memories. After the floods, about 1,000 people moved away, although 600 have returned. Those that remain mutter darkly about the dam and how its mismanagement, not the rain, was responsible for the catastrophic flows. Trooz is a post-industrial town. People here are not wealthy. About 40% of the population were not insured. They received some assistance from the government, but mostly relied upon the volunteers.

Duquesnoy is a jovial wisecracker in a threadbare #TeamEclairs hoodie. Van Tendeloo has short fair hair and radiates warmth. They take me to the house of Jennifer Klar, a 39-year-old single mother who works in the public sector. It’s a gutted-out wreck. Klar’s home was flooded, then looted. She had to split the insurance payout with her ex-husband, so she received only half of what she needed to rebuild. She bought a cheaper house nearby that was also flood-damaged, but she says a builder took her money and disappeared before completing the work, leaving the roof exposed and the house uninhabitable.

Duquesnoy heard through his contacts in late 2023 that she was in trouble. Even though #TeamEclairs had officially disbanded, he agreed to help her.

Until the renovations are completed, Klar and her daughters, 15, 10 and six, are sleeping at her mother’s flat. Klar is on the sofa and the girls share a double bed; her mother stays at a friend’s. “The girls are wondering if they will ever have a normal life like before,” says Klar, a petite blond woman with manicured nails and tiny, delicate tattoos. “Because it’s taken two and a half years and they don’t see an end to the situation. For the oldest, it’s the most difficult. Because she understands everything.”

Blinking heavily, Klar tells me that she promised her daughters they would be in the new house by Christmas 2023, but she couldn’t make it happen. They didn’t even put up a Christmas tree – no one felt like celebrating. She starts to cry.

“Sometimes, people ask me: ‘How’s the situation right now?’” says Duquesnoy quietly, as Van Tendeloo comforts Klar on the other side of the room. “And the situation is that a lot of people are psychologically very down.” Some were suicidal before #TeamEclairs showed up. “That’s something for my life,” he says. “To know you’ve helped someone in that way. And that’s the reason we’re still here.”

In neighbouring Chaudfontaine, a 34-year-old secretary, Jennifer Koremans, shows us around her newly refurbished home, which is high-spec and modern, but has the antiseptic feel of an Airbnb. Everything is new. There are no photos or pictures – all gone in the flood – and the only colour comes from a bowl of tangerines on the kitchen table. Koremans was also the victim of a cowboy builder, she says, losing €29,000 (about £25,000). #TeamEclairs spent nine months refurbishing her home, for nothing.

“They came from heaven to help me, without any reason,” says Koremans, eyes brimming. “That’s how I started to want to live again.”

On the drive back, I ask Duquesnoy whether he thinks he is going to heaven. “You don’t know what I did before!” he roars, slapping the steering wheel. “Maybe God will consider it.”

There is so much kindness, friendship and love in this town. Volunteers have formed romantic relationships with local people – there is even a #TeamEclairs baby, a boy called Fonske. But there is also a sense that this must never happen again. The city authorities are in the process of buying land to be repurposed as flood plains. A now abandoned social housing project, situated in a meander of the Vesdre, will be torn down. But there is only €40m in the municipal budget for this, not nearly enough. And some of the houses that will be demolished have already been rebuilt by their owners, at great cost.

On an individual level, many are not prepared for the floods to return. They have rebuilt their homes exactly as they were, without flood-resilience measures. Electrics and boilers are downstairs. Bedrooms are on the ground floor. There are no flood-proof walls, doors or windows.

All across Trooz and the surrounding towns, people talk of the dam. Had it not been full, had it not been opened so suddenly, this calamity would not have happened. Yes, there would have been flooding, but not on this scale. The authorities, they think, will not make the same mistake twice. There will be future flooding, yes – these towns are used to 30cm, even 60cm, of water – but the bombe à eau that exploded in the Vesdre valley on the night of 15 July will never be seen again.

Beltran believes this is magical thinking, but he is OK with it. “If this idea can reassure them, it’s good for me,” he says. “It’s like a religion. In religion, if you are thinking there is someone over there taking care of you, no problem.” The real reason, says the beleaguered mayor – whom I am interviewing on a Saturday morning, because he works most Saturdays anyway, who met 2,500 people in their homes after the floods while his wife was in hospital, yet still gets hammered on Facebook – “was not human error. It was climate change.”

A criminal inquiry is under way into the alleged mismanagement of the Vesdre dam, but Liège university’s professor of urban planning, Jacques Teller, knows the real cause of the flooding: it’s the rain, stupid.

“Of course the dam did not help,” says Teller, a spry, compact, fast-talking man. “But the main driver is the rain.” Fabian Docquier, the director of dams for the region, couldn’t agree more. “We mustn’t reverse the roles,” he told the Belgian newspaper Sudinfo. “It’s not the Vesdre dam that flooded the valley, but these exceptional rains.”

The climatologist Prof Xavier Fettweis has calculated that the floods that took place in Belgium in July 2021 would have been impossible before 2014 and were made possible only because of the climate crisis. Hot air retains water, which increases precipitation; a 1C temperature increase means that 7% more water is retained in the air. Global heating also means that areas of low pressure stay in the same place for longer. The 2021 floods were caused by a low-pressure system over central Europe, leading to sustained rainfall over large areas. Belgium was badly affected, as was Germany.

The topography of the Vesdre valley makes it particularly susceptible to flooding. Three major rivers converge – the Vesdre, the Ourthe and the Meuse – as well as their tributaries, while the Hautes-Fagnes, the highest hills in Belgium, trap clouds, concentrating rainfall into a densely populated region.

According to Fettweis’s models, a flooding event on the same scale as July 2021 – or even larger – will take place once or twice in the Vesdre valley before 2050.

How do you tell a community of people who have lost everything – whose neighbours died, who fought insurance companies and predatory builders and only now, falteringly, are beginning to get their lives back on track – that these floods may recur not only once, but twice, in the next 25 years?

Teller has tried, interviewing survivors in the immediate aftermath of the floods. “A lot of people, at the end of the interview, told me: ‘I will rebuild this time. But I could not do it another time,’” says Teller. How did local people take the news? “Very badly. They were totally convinced it could not occur any more.”

We are in Pepinster, standing on the riverbank, shifting to stay warm. It’s so cold that Teller tries to give me his gloves more than once. Facing us is what was a two‑storey brick house, now derelict, with an enormous hole in the front wall.

People like to believe that it was the dam, says Teller, “because you keep control”. It’s a forgivable folly, but a folly nonetheless. Here, in the Vesdre valley, layers of folly accumulate like high tide marks on the riverbank. Houses are built right on the water, sometimes with subterranean garages. The bridges crossing the river have columns that are easily clogged with debris. The land has been planted with pine trees, which drain, rather than absorb, water.

Teller points out some older houses, built in or before the 19th century, which are raised half a metre above the ground. “People were not stupid,” says Teller. But this knowledge was lost with the rapid industrialisation of the valley in the 19th century. By the 1950s, people had started building upstream, driving more water to low-lying ground. “People felt protected by the dam,” says Teller. “It created a false confidence.”

Central Liège did not flood in 2021, because the city authorities installed pumping stations in 1926, after the Meuse burst its banks and flooded the city centre. You can still see the water marks on the cathedral. But Teller says hard flood defences across such a large area are not practicable. “When you have these walls, you have to be sure they will never be breached,” says Teller. “Because when they’re breached, it’s terrible. It’s a catastrophe. It’s going very fast. It’s better to let the water get higher progressively than build a wall, giving people a false feeling of safety.”

Teller is advising the authorities on how to rebuild the region. They are mostly receptive to his advice, but there is a limit to his influence. In Limbourg, we walk around an abandoned social housing development. “They had the brilliant idea to build social housing here,” says Teller, sarcastically. “After the flood, the social housing company that owns the land initially rebuilt the homes, installing new window frames, before the city authorities halted the reconstruction. Now, the city will tear down the development, but in nearby Verviers, a private developer hopes to redevelop the city centre to include underground car parking by the Vesdre. “What is in their mind?” Teller laughs. “I cannot understand.”

The inconvenient reality is that the Vesdre valley will certainly flood again, badly, within the next 25 years, maybe twice. When the waters come – and they will come – many people will not be prepared.

“People want to forget,” says Teller. “It’s normal. The memory is already progressively erased.”

When I meet Benjamin, he has just returned from Cop28, where he spoke at 10 events and met a senior adviser to António Guterres, the secretary general of the UN. He is now a full-time activist, or, as he prefers to put it, a “climate diplomat”; his campaign is called Climate Justice for Rosa. (Benjamin home-schools himself and hopes to attend university next year.) Already, he has succeeded in persuading the EU to honour the global victims of the climate crisis, with a day of remembrance held annually on 15 July, but he wants it to be policy at a UN level.

Benjamin has the weary cynicism of someone twice his age. “Cop was horrible,” he says, picking at a pain au chocolat in a bistro in Brussels. “The end result was so bad.” He has received a death threat. “It wasn’t a bad one,” he says with a shrug.

He knows all the Belgian politicians and he knows their tricks. “Always take the picture at the end of the conversation, otherwise they’ll use you for the photo and disappear,” he says. He dreads interviews about Rosa, but he never turns them down, because he feels it’s important to tell her story. “I haven’t lost all hope yet,” he says. Benjamin hates it when journalists ask him if their relationship was romantic. “I don’t know,” he says flatly. “I don’t want to think about it too much, because I will badly emotionally hurt myself.”

It seems to me, meeting Benjamin, that two children fell into that stream on 14 July, but the one who survived was no longer a child.

We leave our pastries and walk to an alder tree planted in Brussels’ Ixelles district – Rosa’s tree. Today, 7 January, would have been Rosa’s 18th birthday. Her childhood best friend, Freya Devlin, 18, and Rosa’s teenage cousins are busy preparing the tree for a memorial event that evening.

“It’s like time stopped for her, but kept going for everyone else,” says Devlin. “How do you process that?” She wears one of Rosa’s silver necklaces around her neck – she had to unknot it from a mass of jewellery taken from her body after she died. “I miss her laugh a lot,” she says. “I miss talking to her. She was really, really cool. I think I was always in awe of how cool she was. Still am, to be honest.”

Benjamin pulls out a box of chalk and begins drawing roses on the pavement. Rosa’s uncle Søren cleans litter away. “Ben is so good,” Søren says, watching him. “He’s so persistent, so stubborn in this important struggle.” The teenagers light fairy lights that flicker in the sub-zero breeze. “There’s so much love around her,” says Søren of his niece.

By 6pm, there are about 50 of us – Rosa’s extended family, her friends from school and camp and their family and friends. One mother attends on behalf of her daughter, who is away at university. “It’s really shaped these kids,” she says to another parent.

Rosa’s immediate family – her mother, father and two brothers – arrive. Her mother embraces everyone individually and gives them a rose. The family link arms and look up at the tree’s branches, now festooned in brightly coloured pom-poms and paper lanterns. They stand there in silence, faces tight with pain. Then, one by one, they step forward and deposit Rosa’s roses at her tree. We follow suit. As they are leaving, Rosa’s father sticks a Climate Justice for Rosa sticker on an adjacent lamp-post.

The sticker is still there the next month, when Benjamin sends me a photograph of the tree: it has snowed and someone has built a snowman in front of it.

The snowman melts. Rosa’s friends start talking about university; #TeamEclairs clean out Klar’s house; Beltran earmarks houses for demolition while the angry Facebook messages pile up in his inbox; and climatologists announce that this January was the warmest on record. But for Rosa, now and always, time stands still.