During the summer of 2026, water availability has become a direct variable for electricity markets across Europe. Record-low river levels are reducing hydropower generation, limiting nuclear cooling, and widening the price gap between low-priced solar hours and more expensive evening electricity. The effect is most visible in Central and Southeast Europe, where the Danube supports hydropower generation, nuclear cooling, industrial activity and cross-border transport.
The European Commission’s Joint Research Centre has reported record-low August water levels on the Danube, Rhine, Loire and Po. It said around 50% of the EU and the UK were experiencing some degree of drought, with 9% at the most severe alert level. Forecasts pointed to warmer and drier conditions through August and into September.
Hydrological constraints affecting hydro, nuclear and wind
The situation is not limited to a single-plant or single-fuel outage. It functions as a correlated hydrological shock that reduces hydroelectric output while also constraining cooling-water availability for nuclear plants. Heatwaves then raise air-conditioning demand at the same time that hydro and nuclear availability weaken.
Stagnant high-pressure conditions can also reduce wind generation, shifting the evening system balance toward gas, coal, imports, batteries and demand response. Hydropower output depends on river flows, hydraulic head and turbine efficiency. Run-of-river facilities react quickly to lower discharge because less water passes through turbines, reducing megawatt-hours generated.
Reservoir-based plants have more operational flexibility, but operators must decide whether to use stored water during summer or preserve it for evening peaks, emergency balancing and winter needs. Nuclear plants face a different constraint because the reactor and nuclear fuel can remain fully operational while the conventional portion requires large volumes of cooling water to condense steam after it passes through turbines. Low river levels can also move water intakes and pump systems outside normal operating ranges.
High river temperatures can further restrict how much heated water can be discharged back into the river. In this context, water-flow limits and discharge-temperature limits can affect river-cooled reactors even when fleet-level losses remain relatively small.
Cernavodă shutdowns tied to Danube levels
Romania’s Cernavodă nuclear power plant provides a clear example of how river conditions translate into generation availability changes. Unit 1 had already been shut down as Danube levels deteriorated. Nuclearelectrica began the controlled shutdown of Unit 2 on the morning of 13 August due to the continuing decline in river levels.
Each reactor at Cernavodă has approximately 700 MW of installed capacity, so about 1.4 GW of Romanian baseload generation became unavailable. Both units remained in safe shutdown condition with no reported impact on personnel, the public or the environment. Cernavodă normally provides about one-fifth of Romania’s electricity requirements.
The outage increases import requirements and removes a major source of low-variable-cost generation while raising balancing exposure during evening demand peaks. Nuclearelectrica also obtained force-majeure certificates covering electricity-delivery contracts after severe Danube drought prevented it from meeting all contracted supply obligations from its own generation. The impact therefore extends beyond plant operations into contractual, counterparty and earnings risk.
Paks output reduced as suction elevation falls
Hungary is facing a similar issue at the 2,000 MW Paks nuclear power plant, which normally supplies close to half of domestic electricity. Low Danube levels forced Units 1, 3 and 4 to shut down. Unit 2 operated at half output for 11 days before gradually increasing production on 10 August.
An official government update dated 13 August said Paks was operating at around 500 MW, or 25% of installed capacity, while the Danube was expected to decline again. MVM said the key constraint was not simply total river volume but whether water remained above the effective suction elevation of existing pumps.
The operator stated that operating units require roughly 100 cubic metres per second, while the four shut units require around 2.5 cubic metres per second for residual cooling. Paks was operating with the river approximately 28 centimetres below the previous 2018 record and more than one metre below the century-minimum level assumed when it was designed more than four decades ago.
Drought measures for Paks and downstream Serbian impacts
Hungary approved an emergency river-engineering programme to address intake conditions at Paks. The government approved a bed sill requiring about 145,000 cubic metres of rock, with two 80-metre barges prepared as additional temporary measures. The intervention is expected to cost around HUF6.1 billion.
The government estimated that a complete shutdown of Paks could impose a burden of at least HUF50 billion per month. Downstream, Serbia’s hydropower production is also being cut by Danube shortages reported by Elektroprivreda Srbije.
EPS said Đerdap 1 was producing only around 20% of usual output with inflows falling to approximately 1,400 cubic metres per second. Đerdap 2 was operating at around 30%. Đerdap 1 has 1,140 MW installed capacity and historically received average Danube inflows of around 5,370 cubic metres per second.
EPS expected Serbian electricity demand to approach 100 GWh per day during the heatwave and said around 10% of requirements were being procured from the market to compensate for missing hydroelectric generation. Thermal plants carried a larger share of domestic demand while EPS sought to preserve reservoir stocks and coal inventories ahead of winter.
Nuclear availability spillovers and limits on cross-border imports
Serbia is exposed on two fronts: directly through lower Đerdap production and indirectly through reduced nuclear availability in neighbouring Hungary and Romania. With regional low-cost generation reduced, Serbia faces limits on how much electricity is available for import precisely when its demand is elevated during heatwave periods.
The situation also challenges assumptions that cross-border imports automatically compensate for weather-related outages in individual countries. Interconnectors can transfer electricity but cannot create additional generation when multiple interconnected markets experience the same weather shock at similar times.
The source described Romania, Hungary and Serbia as potentially requiring additional imports during the same evening hours, linking Austria, Slovakia, Croatia and Bulgaria into a shared price-formation chain where scarcity premiums appear based on transmission congestion.
Adequacy assessment in Europe alongside country-level constraints
The wider European power system remains broadly adequate according to ENTSO-E’s pre-summer assessment. It identified no major systemic adequacy threat across most of Europe supported by expanding renewable generation, stronger cross-border coordination and growing battery capacity.
Battery capacity had reached approximately 29 GW, while expected European hydro storage at the beginning of the season was already 18% below June 2025 levels. Summer electricity demand was forecast to rise by around 2.5%
