A fish rescue operation was underway in Surrey after the heatwave caused a dramatic fall in water levels.
Stretches of the River Mole near Dorking have dried completely, exposing the chalk bed.
Fish including chub, pike, perch, dace and eels have become trapped in ever-diminishing pools.
Environment Agency officers fear a critical drop in oxygen levels, causing the fish to suffocate.
In still water like ponds and lakes, oxygen an be topped up from a floating aeration pump, but the process is unsuitable for rivers.
Fishery officers plan to use electricity to stun the fish before collecting them in nets and moving them to deeper water upstream.
The rescue operation by the EA team responsible for south London, Surrey and Kent is the fourth since the heatwaves began.
Joe Kitanosono, fisheries officer, Environment Agency said: "The problem is the river is drying out causing a real issue to fish.
"Like we need air to breathe, fish need water to survive and there is just no water in the river.
"As the level of the river has dropped these fish are stranded.
"What you have to do in extreme circumstances is rescue these fish and take then out and move them to somewhere where the water will stay longer."
Below: Video from Martynas Pranaitis shows the fast flowing River Mole in Surrey in March this year
Want a quick and expert briefing on the biggest news stories? Listen to our latest podcasts to find out What You Need To Know...