The source of the River Thames has moved five miles downstream for the first time in its history.
While parts of the riverbed in Gloucestershire regularly dry out during the summer, experts said it was a worrying sign of the impact of the climate crisis to see the Thames begin flowing so far downstream.
Christine Colvin, advocacy and engagement director of the Rivers Trust, said: “What we’re seeing at the source of the iconic River Thames is sadly emblematic of the situation we’re facing across the country, now and in future.
“The climate crisis is leading, and will lead, to more extreme weather including droughts and heatwaves.
“This poses a grave threat to rivers and, as a result, the wider landscape.”
The source of the Thames has moved east from Kemble, just south of Cirencester, to beyond Somerford Keynes.
Parish council chairman John Whitwell said the shallow riverbed at Somerford Keynes tended to dry out most summers.
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, and is used for the annual boat race between Oxford and Cambridge.