The singer is part of an all-star group braving crocodiles, hippos and huge rapids for a five-day, 68 mile (109.4 kilometres) journey along the Zambezi River, in aid of Comic Relief.
The participants, who will travel the route in boats and rafts, arrived in Africa for the trip over the weekend (26-27Jan13), and in an interview conducted before she set off, Chisholm admitted she is worried about being out of touch with her three-year-old daughter Scarlet.
She tells Britain's Hello! magazine, "When we're on the river we're pretty much out there on our own. We're not taking our phones on the river - we're going to have our hands full with oars, anyway.
"It'll be the longest I've ever not had contact with her, so I'm quite nervous. That will be a challenge in itself. But she's well looked after here (in Britain); she's got grandparents and friends and daddy around, so she'll be absolutely fine."
Chisholm also reveals she will be asking her former bandmates to donate sponsorship money, adding: "I'm sure all the Spice Girls will be sponsoring me. I'll be tapping them all up for some money."
The BT Red Nose Challenge: Hell And High Water expedition is being staged ahead of this year's (13) Comic Relief Red Nose Day on 15 March (13), and is expected to raise $1.6 million (£1 million) to help educate children in Zambia.