The thirsty summer: what Europe’s heatwaves are doing to its freshwaters

The thirsty summer: what Europe’s heatwaves are doing to its freshwaters In late July, a mammoth’s jaw and tusks appeared on an exposed Danube riverbank near Ryahovo, in Bulgaria. The river had dropped far enough to give back an animal from a colder world. Falling water in Budapest uncovered an unexploded wartime bomb. At Prahovo, in Serbia, people swam beside the hull of a sunken German warship. The media ran these stories as curiosities. They perhaps work better as a stratigraphy. A low river gives up its own history and ours, layer by layer, drought by drought. Central Europe has long had a way of reading drought. Hunger stones lie in the beds of the Elbe and the Rhine, and dozens survive between Bohemia and Saxony. The stone at Děčín carries marks from 1536 to 2003. Its best known inscription tells the reader to weep on seeing it, and is usually dated to 1616. Hydrologists at the Czech Hydrometeorological Institute have shown otherwise. It was cut around 1904 and recut in 1911 by a local man, Franz Mayer. Their larger finding matters more than the correction. The people who cut these marks were recording the exact annual minimum, and the marks sit within four centimetres of the gauge readings. Five centuries of careful amateur hydrology, carved at the waterline, by people who understood that the low water mark was worth documenting. Read together, the stones show a slow decline in minima since the late eighteenth century. Another Elbe stone offers another prophecy. When this stone goes under again, it says, life will blossom. In the midst of another heatwave European summer, how are old rules about its freshwaters being rewritten? The thirsty atmosphere Drought, in the popular imagination, means the rain stopped. The World Weather Attribution study published on 23rd July found something more complex. In western Europe, the low April to June rainfall carried no long-term climate signal at all. The rain was ordinary. The heat was not, and the heat changed what the rain did. Potential evapotranspiration tracks how much moisture the atmosphere can pull from soils, plants and open water. Evaporative demand climbs exponentially with temperature. A warmer sky is a thirstier one. The WWA found such conditions in western Europe had become roughly eighty times more likely under human-caused warming, and around forty times in the east. Agricultural soil drought is now about five times more likely in the west and eleven times more likely in the east. The threshold has shifted. Rainfall deficits that once passed without incident now tip catchments into drought. An event of this rarity would have registered as moderate or severe in a climate 1.4°C cooler. It now registers as extreme. The events also arrive earlier each year. Germany called 2018 a ‘Jahrhundertsommer’ (or ‘summer of the century’) after the Rhine spent 132 days at low water. This July the Danube sat at a level it did not reach until October that year. Europe has endured four heatwaves this summer already. A river heatwave Until recently we had no precise language for what heat does to a river. That changed fast. Three pieces of research in the past seven months have made riverine heatwaves a measurable thing. A paper in Nature Communications in January mapped river heatwaves worldwide. Intensity and duration are both climbing, and river water temperatures rose 0.2°C per decade across the study baseline. Among the sharpest historical trends, the Rhine and Danube stand out for intensity, and the Rhine and Elbe for frequency. A second paper, in Nature Geoscience, was published on 30th July. It covers 796 river basins across the United States and central Europe. Between the 1980s and the 2010s, riverine heatwaves rose 114% in frequency, 148% in duration and 95% in intensity. Atmospheric heatwaves rose by roughly a quarter to a third. Rivers are heating faster than the air above them. The two trends increasingly arrive together. Compound events have tripled in four decades. When both hit at once, water temperature climbs a further 16% and dissolved oxygen drops another 2.9% against a river heatwave alone. Abhinav Dengri and Peter Greve reported a matching European pattern at the European Geosciences Assembly assembly in May, tracking compound low-flow and heatwave events back to 1960. Central and eastern Europe show the sharpest rises. The longest events, the ones that hurt ecosystems most, are multiplying fastest. The physiology is blunt. Oxygen dissolves less readily in warm water, and fish metabolism roughly doubles for every 10°C rise, so demand climbs as supply falls. Salmonids start to struggle around 20°C. Mortality follows near 25°C. Low flows cut the turbulence that re-aerates water, and stretches go still. Warm, slow, sunlit water suits algal blooms, which shade out submerged plants, then collapse, and bacteria strip the last oxygen within hours. The Nature Geoscience authors put the trajectory in stark terms. River systems may be shifting from episodic extremes to chronic thermal stress. Their colleagues in January said it more directly: extremes as we have understood them will stop being extreme. Rivers already in trouble None of this heat stress arrives in healthy freshwater systems. In April the IUCN published its updated European Red List of Freshwater Fishes, assessing all 558 native species. Some 42% face extinction and another 18% sit close behind. Migratory fish fare worst, their routes cut by an estimated million barriers. A global assessment launched in March put their decline at roughly 81% since 1970, and named the Danube a priority basin. We have seen this pattern before. In 2018 Swiss authorities pulled about a tonne of dead grayling from the Rhine as temperatures passed 25°C, roughly 30% of the annual catch. By 2022 the Atlantic salmon that restoration managers had spent decades nursing back to the river could not reach their spawning grounds. That August a bloom of the golden alga Prymnesium parvum killed around a thousand tonnes of fish on the Oder, along with most of its mussels and snails, across 500 kilometres. The analysis found no single culprit. The disaster needed chronic salt and nutrient pollution, acute heat and low discharge, all at once. A river carrying that much stress has little capacity left to absorb constant rolling heatwaves. What it costs: economies, wildfires and groundwater The Rhine has fallen to its lowest level since records began at Kaub in 1880. Navigable depth there dropped to 23 centimetres on Wednesday, under the previous record of 25 set in 2018, and forecasters expect it to fall further. The smaller tributaries feeding the river have run dry too, so recovery needs sustained rain rather than a cool week. Barges run at a fifth of capacity. Hungary shut down the Paks Nuclear Power Plant on Sunday as a result of the drought, the first full closure in the plant’s 44-year history, stripping 40% of national energy generating capacity. More than 100 Hungarian towns and villages sit under water restrictions. Romania declared a state of emergency and detonated 180kg of explosives on the Bala canal to clear a rock and buy its remaining reactor a few days. The cruise ship Viking Ullur grounded on a Bulgarian sandbank on 28th July, and border police took off 186 passengers after those aboard ran out of food and water. Fire and water make one crisis. The European Forest Fires Information System recorded 434,976 hectares burnt across the EU by 29th July, compared to 346,836 at the same point in 2025, the worst year on record. Dry soils feed fire. Fire strips the vegetation that holds water and shades streams. Groundwater is the quiet emergency. Slovak hydrologists report spring yields and groundwater below 20% of the long-term norm. Aquifers are so often out of sight, out of mind, but they keep headwaters flowing between rains, and take years to refill. An ECB working paper this summer modelled 1,117 EU regions and found a compound heat and drought event on the 2022 scale would cut agricultural value added by an average of 4.54 percentage points. Related ECB work finds over 40% of euro area bank lending sits with firms exposed to drought and dependent on surface water. A companion study led by Sehrish Usman at Mannheim put the summer of 2025 at €43 billion in short-term losses, rising towards €126 billion by 2029, and called that conservative. The study excludes wildfires, and it excludes compound events where heat and drought arrive together. In other words, it leaves out the exact mechanisms driving this summer. Catchments built for a climate that has gone Specialists from Wetlands International Europe, WWF and the European Environmental Bureau identified the structural causes of drought at the end of July. Europe’s worsening exposure comes as much from two centuries of drainage as from the rate of warming. Communities across the continent have drained wetlands, straightened rivers, cut off floodplains, dried peat and sealed soils. Each decision made sense within its own frame of flood defence, navigation or farm improvement. Together they stripped out the systems that hold water in wet months and release it slowly in dry ones. Europe engineered its catchments to drain. It now needs them to retain. Desertification already reaches thirteen member states. Evidence is not the obstacle. Germany’s KliMaWerk programme found large-scale water retention more durable than isolated technical fixes, and named governance as the limiting factor. Jan Verkade of Deltares offered a useful reframing when he told CNN that droughts are the new floods. His colleague Marjolein Mens, speaking of Dutch conditions in July, said the country was already living through what modellers had mapped for 2050, arriving faster than expected and fitting the worst-case climate scenario. Water in the Rhine and Meuse stood at around 25°C that month, with blue-green algae spreading and fish dying. In other words, our predictions of a climate crisis future are arriving now, and there is an urgent need for action. A rewetted peatland is an investment in a river’s condition decades out, and restoration timelines assume a reasonably steady target. The target has moved twenty-five years closer. Strategies for managing drought So what can be done in this time of drought and wildfire? In the Danube Delta, 700,000 hectares across Romania, Ukraine and Moldova, decades of damming made floods worse and the loss of native grazers let combustible vegetation build up. Reintroducing bison and other herbivores and reconnecting floodplains cut both wildfire and flood risk, and improved water availability for local farmers, which is what turned residents into stakeholders in the wetland’s survival. The timing favours action. The Commission’s call for evidence on its climate resilience framework closes on 4 September. Member States are finalising National Restoration Plans, and the European Water Resilience Strategy folds water resilience into them. UNCCD COP17 meets in Ulaanbaatar this month. The obstacle is political, and the diagnosis is not new. A 2016 survey of European drought policy found the continent still managing drought as a crisis rather than a risk, nine years after the Commission had called for exactly that shift. The Water Framework Directive governs water quality rather than quantity, and was drafted with northern Europe in mind. Member states were found to favour hard flood engineering over natural retention. The only EU instrument aimed squarely at drought is a Communication, which carries less weight than a Directive. Drought management plans remain voluntary, which is the same gap the World Weather Attribution team identified again last month. Twenty years of accurate diagnosis has not produced an obligation. Teresa Ribera, the Commission’s executive vice-president for the clean, just and competitive transition, called the June heatwave a dramatic warning and blamed the retreat from environmental ambition on what she termed nonsense and ideologically driven lies from vested interests. The risk those NGO authors flagged is a framework that assesses risk without the mandate or money to reduce it. Reading the riverbed I keep returning to the mammoth in the Danube mud. It took a record drought to make it visible. The bones lay there the whole time, and our sightline moved. The same will hold for what this summer costs ecologically. Fish kills, collapsed invertebrate communities, reaches that go anoxic in August and come back thinner in spring: most of it goes unnoticed and much goes unrecorded. It registers instead as a small downward shift in what people expect a river to hold. That is how shifting baselines work. Substitution, generation by generation, until nobody can say what has gone. A summer like this briefly makes the invisible visible. Not only the bones and the shipwrecks. It shows the underlying condition. These rivers lost their capacity to hold water, buffer heat and absorb shock. People did that deliberately, over time, through the decisions they made. Other people can undo it through different ones. The Danube will rise this autumn and cover the mammoth, the bomb and the warship. The conditions that exposed them remain the same. The memory is fresh now. The political consultation is open now. What gets decided in the next four weeks will shape these rivers for decades. Comments are closed.