Many of the small river basins across Europe are also overheating, the results show, particularly during the summer season. In the worst-affected river basins the heat is having a serious impact on ecosystems.

It is well known that power plants – particularly those that use "once-through" cooling – are responsible for warming rivers, and local studies have identified specific hot spots. Until now, however, no one had quantified thermal pollution from power plants globally.

After quantifying the mean annual heat rejection rates from power plants worldwide, Catherine Raptis from ETH Zurich in Switzerland and colleagues combined these with a global hydrological-water temperature model. This allowed the researchers to estimate the increase in riverine temperature caused by power plants directly, and compare rivers across the world.

Their results showed that the Mississippi river receives the highest total amount of heat emissions, with nearly two-thirds coming from coal-fired power stations, and nearly one-third from nuclear power stations. It also has the highest number of instances where the commonly set 3°C excess temperature increase limit is equalled or exceeded.

The Rhine receives one-fifth of the Mississippi's thermal emissions, mostly from nuclear power plants. But it is the most thermally polluted river basin in the world in terms of amount of flow impacted, with one third of its total flow experiencing a temperature increase of 5°C or more on average over the year.

"The thermal pollution in the Rhine is alarming," said Raptis. "There are many thermoelectric power plants with once-through cooling systems in the watershed."

Salmonid species are particularly affected by thermal pollution. In European rivers any temperature rise over 1.5°C is likely to disturb these fish. During the late 19th century the Rhine was the largest salmon-bearing river in Europe, with around 250,000 of these prized fish caught from the river every year. By the 1950s the combination of industrial pollution and river dams on the Rhine had pretty much wiped out the salmon population. Since the 1990s the river has been cleaned up and fish-ladders inserted at the dams, and the salmon has started to return. The numbers are still far from their 19th century peak, however, and the new findings support the idea that thermal pollution may be stressing the salmon population in the Rhine today.

The results, which are published in Environmental Research Letters (ERL), reveal that smaller river basins in Europe, such as the Weser and the Po, also showed signs of severe thermal pollution, with temperature increases of 3°C or more for 49% and 81% of river flow, respectively, from July to September.

In previous work, Raptis and her colleagues showed that thermal pollution is likely to be an increasing problem in many rivers, particularly those in China and India, where power plants with "once-through" cooling technology are still being constructed.

Meanwhile, the US, Russia, Canada, France and Germany experience some of the highest rates of thermal emissions today; they have a large number of "once through" cooling power plants along their rivers, often constructed during the 1970s and 1980s. Many of these countries are looking to phase out power plants with "once through" cooling, but in light of the increasing demand for power, replacing them is not a trivial task.

There are always trade-offs associated with using alternative cooling technologies. "If we opt for (wet) cooling towers then the thermal pollution is negligible, but the freshwater consumption is much higher than in once-through cooling," said Raptis. "If we choose dry cooling technologies, we lose out on efficiency and hence end up increasing the fuel consumption for a certain power output. If, where possible, we build the new power plant at the coast and use ‘once through’ cooling we are simply shifting the burden of the thermal emissions to the sea, especially in areas where dilution might be limited."

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