STORY: Britain's Met Office said this July was the driest for England since 1935 with average rainfall, at 23.1 millimetres (0.9 inches), just 35% of the average for the month. Some parts of the country saw the driest July ever.
The River Thames stretches 215 miles (356 kilometres) across southern England, from Gloucestershire in the west through the heart of London, before entering the sea at Essex to the east.
The natural spring that supplies the river, known as the source, dries up most summers. But this year the dry riverbed reaches significantly further downstream than in previous years, according to observations by conservation experts.
"The Thames would normally be at its source - and there's a nice pub next to it - would be about 15 kilometres back upstream," Alisdair Naull, an engagement officer at the Rivers Trust, told Reuters while standing in a small section of the Thames in Cricklade, about 80 km west of London.
"It's very, very shallow here ... but you don't have to go much further up this little bit of the Thames to find yourself stood on dry ground. And really, that is ground that should still be wet and should always be wet."
Naull said the shallow, warmer water contained less oxygen, which fish and other wildlife needed to thrive.
Unprecedented heat and a lack of rainfall has pushed two water companies in the country's south to announce temporary bans on the use of hosepipes and sprinkler systems. Thames Water, which supplies 15 million customers across London and south east England, said it was planning to introduce the similar curbs.
A four-day "extreme heat" warning came into force in parts of England and Wales on Thursday. The Met Office issued its first such warning ever last month, when temperatures breached 40C (104F) for the first time.