A lawyer who is attempting to swim the length of the Thames said there was not enough water to begin at its source.
Mr Pugh blamed drought for the lack of water
Lewis Gordon Pugh blamed the current drought for forcing him to run 20 miles before finding water deep enough to swim in.
The 36-year-old started on Monday at Kemble, Gloucestershire, and will finish at Southend-on-Sea, in Essex.
The 203-mile (327km) journey, which is expected to take 15 days, is to raise awareness about climate change.
He said he was more worried about swans than the whales and dolphins he has encountered in previous long-distant swims.
"They can be very vicious," he told BBC Radio 4's Today programme. "They do get a bit agitated."
He said an assistant would be close by to fend of any birds that might approach him.
He hopes his attempt to become the first person to swim the Thames would help raise awareness about the effects of global warming.
He has completed a string of endurance swims and is the only person to have completed a long-distance swim in all five oceans of the world.
The maritime lawyer, who works in London, constantly seeks new challenges.
The most recent, in May, involved him swimming 0.75 miles (1.2km) in sub-zero temperatures in Nigards Glacier Lake in Norway.