Abi Jackson spends a week as a Grand Canyon river rat

A: Leap out of bed and retrieve the tarpaulin sheet from my dry bag - it's drizzling and I stupidly ignored advice to sleep with it tucked under my sleeping bag, in case the weather turned.

B: Stay put and hope it's a fleeting shower (and, crucially, avoid that scorpion last seen scooting in my direction).

By the time I've convinced myself the scorpion's probably huddled under a rock by now, the drizzle's escalated to a full-blown torrential storm, my sleeping bag's drenched and rain is pooling between my knees.

Next comes the thunder, and lightening so immense it's as though God's started playing with Earth's light-switch.

I'm soggy and shivering and quite possibly at risk of electrocution, but all I can do...is giggle.

This time last week, I was sat at my office desk, punctuating mundane admin tasks with peeks online and wondering whose turn it was to make tea.

Now, deep inside the Grand Canyon, on the banks of the Colorado River, miles from civilisation, I'm lying under the stars on a metal cot - and at this precise moment, with a three-inch rain puddle for a pillow, it feels downright hilarious!

It's night three of my seven-day adventure with Grand Canyon Whitewater, who offer guided rafting trips covering 187 river miles, ideal for completely inexperienced adventurers like me.

Their rafts are motorised, which means there's no paddling involved, and their articulated design makes them particularly adept at negotiating the rapids.

Incredibly, their Mary Poppins-bag-style bellies somehow also manage to carry enough grub to keep us very well fed for the week - we feast on fajitas, steaks and chilli, among other things - cots for all of us, plentiful drinking water, first aid kits and 'Oscar' (the chemical toilet that's set up behind a bush each night, where we do our 'number-twos').

There are 25 others in my group. A few days earlier, we'd assembled outside Marble Canyon Lodge, a roadside motel in a remote corner of the northern Arizona Desert, giddy with nervous excitement and clutching our rucksacks containing, hopefully, at least the most crucial of the items we'd been told to bring (wide-brimmed hat, suncream, rainsuits, footwear for hiking, fleece for cool evenings).

Ranging in age from 13 (twins Bryan and Evan) to 80-something (their grandpa Fred), we are students, teachers, office workers, a marine biologist and even a brain doctor (Gary). But this week, none of that really matters.

We are equally-ranked team-mates, and we've spent the last two days arm-to-arm on rafts for around seven hours a day, squealing together as we crash through rapids, working together to unload/load supplies and set up camp, and washing and peeing together in the river (peeing on the banks and side-streams is strictly forbidden).

Apart from me and two other Brits, a father and son, everybody's American and it's mainly families.

This isn't the sort of trip you book lightly. It's a long-dreamed-of cross off that bucket list option, the marker of a major milestone, an important birthday, to celebrate surviving an illness, or a fresh start after divorce...

The moment we'd set off from our river starting point, Lees Ferry, however - after donning lifejackets, packing our stuff into dry bags and a brief but thorough safety orientation - all those things had started to drift from our minds. There were other things to concentrate on, like not falling off the raft, and keeping our eyes peeled for bobcats and desert bighorn sheep (there are rattlesnakes but they're seldom seen).

Each day follows a similar pattern: couple of hours on the river, hit the shore somewhere for lunch and a hike, and then back on the raft until we find a spot to set up camp. The guides take turns to cook and we all chip in with clearing and packing up.

Much of our time on the water's spent cruising gently, our guides keeping us entertained with tales of the early explorers who discovered the hard way just how mighty and powerful this river is, and geology lessons about how the canyons formed over billions of years.

There are plenty of rapids too, ranging from fun teasers to full-on, heart-in-your-mouth grade nine to 10 monsters. Hermit and Zoroaster are two of my favourites, but the biggest, Lava, comes on the last full day.

Talk of this bubbling beast, and how it fills even the bravest captains with dread, thrills us all week.

For me though, the height of my fear hits on day six. We pull up for a hike where we're promised we'll be rewarded with the most beautiful natural oasis.

Getting there, however, involves a bit of a trek through a narrow gorge, with a 40ft drop.

I'm not good with heights but there's no way I'm missing out, so off I traipse, merrily clambering up winding rocky paths until we enter the gorge.

The ledge slowly begins to narrow. Suddenly, for about two metres, it's just inches wide, with overhanging rock at about chest-height, which means I'll have to traverse, leaning outwards over that 40ft drop.

I'm petrified. Every cell in my body screams that this is a terrible idea. I'm clumsy enough on 3m-wide pavements. How the hell do I expect to pull this off?!

It's quite handy, then, that Gary, who cures brain disorders for a living and is therefore a pretty cool cucumber, is behind me. Somehow, his calm encouragement unlocks my frozen limbs and I begin to edge along the ledge.

I make it - and yes, the oasis around the next corner is spectacular - but all I can think about is the fact I have to tackle that ledge again on the way back! So while everybody else frolics in the natural spring, chirping with delight, in the middle of the Grand Canyon, I sit down and do some deep-breathing exercises.

Obviously, I survive the way back too, then immediately afterwards, burst into tears. The relief!

I am not somebody who cries often, but that day, my eyeballs give the Colorado River a run for their money.

People say you can't come to the Grand Canyon without being emotionally effected. I'd sobbed the day before too, during a hike at mile 75 (I'd just been overwhelmed by how beautiful the rock was).

Lava is so ferocious, its roar echoes through the canyon for miles. When we get near, we pull up on the banks and the guides hop out to do a recce, leaving us alone, wondering whether they're just pulling our legs.

But they're not.

And despite the fact we're now, of course, old hands at this outdoor survival malarkey (real beds and hot water - who needs those?!), when we finally begin our approach, we are filled with excited terror again. We're a bunch of regular Joes whose lives are suddenly entirely in the hands of Mother Nature and some pretty awesome river guides - and it feels amazing.

TRAVEL FACTS

Abi Jackson was a guest of the Arizona Tourist Board. Visit visitarizona.com

A seven-day full canyon motor rafting adventure with Grand Canyon Whitewater (grandcanyonwhitewater.com) costs from 2,535pp US dollars (approx £1,637), based on 2015 departure dates. Includes helicopter ride out of the Canyon and flight back to Marble Canyon at end of the tour.

Rooms at Marble Canyon Lodge (marblecanyoncompany.com), where it's advised you stay the night before, from approx £45 per night.

British Airways (britishairways.com) offers return direct flights from London Heathrow to Phoenix from £743pp.