The Comic Relief charity challenge on the Zambezi River took a wrong turn when comic Dara O'Briain and Olympic athlete Phillips Idowu were swept away from the rest of their team.
The other celebrities, including Melanie C and Jack Dee, lost sight of the pair on Tuesday as the river carried them off before O'Briain fell out of the boat and was left hanging on to a tree for around an hour before their guides came to their rescue.
Radio 1 DJ Greg James, who is also taking part in the challenge with actress Chelsee Healey, said he was left "shaken" by the pair's near miss.
He said: "We went on some rapids with two people per canoe and it all went horribly wrong.
"We actually lost Dara and Phillips - the rapid was so fierce and fast-moving that they got taken down a different channel of the river to the rest of us, into a different area with loads of different rapids, and had to try to stop themselves and hold off in a calm area.
"They were away for an hour, the guides could see them but we had no idea where they were."
There was also trouble for Dee and former Spice Girl Melanie, who ended up trapped beneath their canoe when it capsized and had to be rescued.
Dee said: "We've lost our boat, it's wrapped around the tree we hit. My helmet is covered in scratches.
"Luckily I'm not very bright so it's not a problem, but we could have been properly bashed up.
"I'm going to have to stop doing these things; I don't know why Comic Relief hate me so much. They're doing their best to do me in out here."
The group will be paddling for up to eight hours a day during the BT Red Nose Challenge: Hell and High Water.
During the 111km (68-mile) journey, they will face rapids known as Gnashing Jaws of Death, The Washing Machine and Oblivion on their way to the world's largest waterfall, Victoria Falls.
The challenge will see them shunning luxury accommodation and instead camping on cliff-tops and watery riverbanks as they nurse blisters and rest their aching limbs.
Armed guards will accompany them to protect them from wild animals.
The journey, which it is hoped will raise £1 million, is being staged ahead of this year's Red Nose Day on March 15 with the proceeds being used to educate children in Zambia.
The stars can be sponsored at rednoseday.com/zambezi.
Red Nose Day celebrates its 25th anniversary this year, having raised more than £600 million for people in the UK and Africa.