The recent heatwave has seen River Wye water temperatures reach dangerously high levels - with more blistering heat set to hit the area next weekend.
In a bid to try to saver as many salmon in the river as possible, the Wye and Usk Foundation requested extra water to be released from the Elan reservoirs.
Within four hours of the request the flow was increased followed by a further increase in flow over the weekend.
The foundation urged farmers to refrain from abstracting from the main stem in a bid to get as much water as possible to the section below Monmouth, where much of this year’s salmon run is still holed up.
The amount of oxygen in water is directly proportional to its temperature.
Thousands of salmon died in 1976 when water temperatures reached 27.2 degrees. In 2022 around 100 salmon died when water temperatures reached 25.5 degrees.
Ross on Wye was 24.6 degrees on July 11 with three more very hot days forecast.
The foundation said: "Sadly, we expect that some salmon will die as water temperatures are still elevated after the last heatwave.
"A third heat wave so early in the year is abnormal. It is unprecedented. This is a real threat and shows how we need cool our rivers.
"This can be done by increasing the amount of cool water coming from seeps and springs, and by shading the tributary network. This is what WUF and our partners are doing."
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