The next 24 hours will show whether the release of extra water into the Wye has been enough to save the river’s salmon stocks.

The Wye & Usk Foundation urged Welsh Water to release large amounts of water from reservoirs in the Elan valley in the headwaters of the Wye at the end of last week to increase the flow and so cool down the water in the river’s lower stretches.

“We also asked farmers to reduce abstraction from the river, and not to use spray guns,” the foundation’s chief executive Simon Evans said.

“There is a high chance of thousands of salmon on the river dying, which they do when the temperature is above 27°C,” he explained.

“It’s a perfect storm. We had a dry spring, with just enough rain so far this year for gardens and farms, but not enough for the river.

“As a result, some salmon haven’t yet made it upstream to Wales, they are stuck south of Ross-on-Wye, and that’s the section that’s most exposed to the current heat.”

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The river levels are lower than in 1976, when its waters passed the critical 27°C temperature and thousands of fish died.

“We will know by the end of tomorrow if it’s been enough to save them this time,” Mr Evans said.

The River Wye at Ross-on-Wye, and inset, Simon EvansThe Hereford Times Save the Wye campaign calls for more concerted action to protect the river.