A drought stretching from the Rhine delta to the lower Danube cut hydropower output across seven European countries by about one fifth between April and July 2026, according to an analysis of river flow, generation and price data published by pv magazine on 10 August. Nuclear plants on the Danube also reduced output as river flows fell to their lowest levels in seven years, while wholesale power prices rose steadily through the summer.
The analysis, by consultancy WSP, compared 2026 with the 2019 to 2025 average using Copernicus river discharge modelling and generation data from the ENTSO-E Transparency Platform.
Rivers at record lows
Modelled discharge of the Rhine at Lobith, where it enters the Netherlands, averaged 81% of its 2019 to 2025 norm in April, 66% in May, 58% in June and 37% in July. The July average of 850 cubic metres per second was below the lowest single July day of the entire baseline period, 880 cubic metres per second, recorded in the drought of 2022.
In France, the Garonne at Tonneins averaged 5% of its normal July flow, the Loire at Saumur 13% and the Rhône at Beaucaire 41%. On the Danube, flow through the Iron Gates, the river's largest hydropower complex, was 58% of normal in May, 54% in June and 48% in July.
Hydro losses
The seven corridor systems, Austria, Italy, Romania, Serbia, Hungary, Slovakia and France, produced roughly 13 TWh less hydropower between April and July than the 2019 to 2025 average, a shortfall of about one fifth. In July, Hungary's hydro output was down 53%, Italy's 38%, Austria's and Serbia's 34% and Romania's 32%. France moved from a wet spring, with output 6% to 7% above normal in April and May, to 16% and 17% below normal in June and July.
Other systems had much better conditions. Portuguese hydro ran 117% above normal in July after a wet first half, and Spain was 16% to 44% above normal by month. Swiss output started 64% above normal in April on strong snowmelt before converging to 5% above by July. Together, these wetter systems added about 9 TWh against the baseline, offsetting roughly 70% of the corridor's loss at continental level, though not where it was needed.
By mid-July, Italian and Romanian reservoir storage was closer to the 2015 to 2025 minimum than the median, while Spanish reservoirs were near the top of their historical range.
Nuclear on the Danube
At Cernavoda in Romania, output fell to about 15 GWh a day over the last three days of July, against a July norm near 29 GWh. At Paks in Hungary, output fell from 38 GWh a day on 28 July to 21 GWh on 31 July, a 45% cut in three days. Bulgarian nuclear output was 22% below its June norm. The analysis noted that riverside air temperatures at Paks exceeded 32°C on only six July days, suggesting the reductions tracked river conditions more closely than air temperature.
French nuclear output, by contrast, ran 10% to 15% above its 2019 to 2025 norm in every month from April to July, despite heat-related curtailments at individual plants such as Golfech on the Garonne. The fleet's lowest days, 792 GWh in late June and 831 GWh on 12 July, were modest dips against an average of about 945 GWh a day.
How systems compensated
Italy replaced 3.6 TWh of lost hydro, compared with 2025, with 4.1 TWh of additional gas-fired generation and 1.1 TWh of new solar. Austria covered a 2.4 TWh hydro shortfall mainly with imports, which were 1.6 TWh higher than a year earlier, plus 0.9 TWh of additional solar. Switzerland combined 1.4 TWh of extra net imports with higher nuclear output. Serbia cut lignite generation by roughly 0.7 TWh and raised imports. Romania's consumption ran about 1.2 TWh below 2025.
Solar output grew year on year in every corridor market reporting to ENTSO-E, from 0.3 TWh in Hungary to 1.1 TWh in Italy, and by 5.4 TWh in Spain. Solar was the main generation source in the analysis whose output was not directly constrained by river conditions.
Rhine shipping and coal
Low water on the Rhine limits how much barges can carry, raising transport costs for coal, oil products and other goods. German coal generation showed no clear link to Rhine levels in the data, which the analysis attributed to plants drawing on fuel stocks and rail alternatives, so the effect appeared as higher costs rather than lost output.
Prices
Day-ahead power prices rose through the summer. July averages reached €122 per MWh in Hungary, against €103 a year earlier, €120 in Romania, €117 in Austria against €88, and €117 in Switzerland against €91. France averaged €95 per MWh against €58 in July 2025. Hungarian, Romanian and Serbian daily averages exceeded €225 per MWh on 1 July during a heat spike, and Swiss daily prices averaged €134 per MWh from 22 to 31 July as rivers emptied.
The analysis noted that drought-related price effects tend to persist longer than those from short weather events, because rivers recover slowly.
Implications for winter
Low reservoirs at the end of summer matter for winter supply. Hydro operators in the Alps and the Balkans normally hold water in storage for the colder months, when demand is higher and solar output is low. If reservoirs start the autumn depleted, systems may rely more on gas and imports during the winter, which feeds into forward prices and gas demand.
The climate dimension
The drought followed a record June heatwave in western Europe. Climate scientists have documented an increase in the frequency of hot, dry summers in central and southern Europe. Hydropower and river-cooled thermal plants are both exposed, and the 2026 data show how both can be constrained at the same time in the same region. Systems with diverse generation, strong interconnections and growing solar capacity absorbed the shock more easily than those dependent on a single river.
What to watch
Key items include reservoir levels heading into autumn, particularly in Italy and Romania, the duration of low flows on the Rhine and Danube, gas demand for power generation and the effect on winter price expectations. Copernicus is expected to publish its assessment of the European summer in September.