The singer is taking part in an expedition along the continent's fourth biggest river to raise funds for British charity Comic Relief.
Chisholm and several other celebrities set off on the 68-mile (109.4 kilometres) journey over the weekend (26-27Jan13), and were briefed by experts ahead of their first time battling the river's rapids on Tuesday - but just hours into the challenge Chisholm and her co-rider, comic Jack Dee, got into trouble.
The pair lost control of the raft and the dangerous currents dragged them through trees before the boat capsized and they had to be plucked to safety.
A series of updates on the charity's official Twitter.com reads, "Jack and Mel lost control of their dinghy, were dragged through trees by the white water & capsized before being rescued.
"Everyone is safe and well, but the last two hours have been incredibly dramatic. Everyone is shaken but physically fine now - but that was as frightening a morning as you'd ever want to have."