The source of the River Thames has dried up so much that there are no signs of aquatic life for 10 miles, experts say.
It comes after weeks of dry weather and record-breaking temperatures across the UK.
The source of the Thames has moved east from Kemble, Gloucestershire, to beyond Somerford Keynes.
Scientists say the unprecedented incident is “a stark warning” of what climate change has in store.
Parts of the riverbed in Gloucestershire regularly dry out during the summer, but not to the current extent.
The Rivers Trust director of science and policy Dr Rob Collins said what has happened to the river is “not natural.”
“There is absolutely no aquatic life to be seen for exactly 10 miles down stream. So it’s really troubling to see it in this state,” he said.
“This is going to be the new norm. So we really need to learn to adapt to it, learn to manage water more wisely and become more resilient to droughts and water scarcity.”
The Rivers Trust is calling for accelerated metering, rapid reduction in leakage, support for households to reduce water usage, such as installing low-flow toilets and water butts, and sustainable drainage including rain gardens, wetlands and permeable paving to build up local stores of water underground.
Ben Lord is the owner of the Thames Head Inn near the source of the Thames is situated near the source of the river.
He said he is shocked at its condition
“We are experiencing a prolonged hot dry spell so it’s to be expected that the source would move further downstream,” he said.
“But in 17 years I’ve never known it to be this bad.”
The River Thames is traditionally about 215 miles long, and travels east from the Cotswolds through to London and out into the North Sea.
It is the second-longest river in the UK, after the Severn.