When Michael Jacobs set out to hike Colombia's longest river he knew it might be arduous, but he didn't imagine he'd end up having to do a deal with the devil

The sound of distant gunfire disturbed the early-morning peace of a sunlit valley brilliant with tropical colours. A young woman with long brown hair was blocking the fern-shaded path. She was dressed in camouflage and holding a machine gun. The acronym on her lapels identified her as a member of the Farc, Colombia's most notorious guerrilla army. She was telling us to stop. She'd had orders from the sub-commander.

I was on my way back to the Andean town of San Agustín, together with a friend from Bogotá, Julio, and a local guide who had led us to the remote moorland source of Colombia's longest river, the Magdalena. The source was the culmination of a journey I had been planning for years – to travel the whole length of a river emblematic for me of all that was darkly alluring about South America. But it was only during the journey's final stage that I had had any true sense of danger.

The past three days had been ones of near-constant fear. The ride on horseback along a steep and slippery jungle path had been scary enough, but it was the discovery soon after heading into the mountains that the Farc were around us that had made the tension almost unbearable. Only the night before, a group of heavily armed guerrillas had emerged from complete darkness to surprise us at the isolated farm where we had been staying. An erratically behaved man called Marlon had subjected us to a chaotic sociopolitical discourse, after which the guerrillas had all left, to our relief.

But with the sound of that early-morning gunfire, my worst fears were returning. I was worried we had been caught up in a crossfire with the Colombian army, or else that the guerrillas had heard in the intervening hours that I was not an innocent tourist but rather a writer with contacts in the Colombian government. Jenny, the woman guerrilla detaining us, whom I identified as Marlon's girlfriend, could give us no idea of what was happening. We tried engaging her in conversation. As with most of her companions, she could have been little more than 18, but she revealed she had already been with the Farc for more than 13 years. She was probably one of its many members who had been stolen from their families.

The sub-commander finally arrived, excusing himself for being late on the grounds that he had been practising his shooting. After some pleasantries, he outlined the Farc's plans for building a visitors' centre, for widening and improving the path we were on, and for making the area generally more enticing for tourists. Then he asked a question that enhanced yet more the unreality of the encounter. He wondered if I spoke any English. When I admitted to a reasonable knowledge of the language, he seemed delighted.

"Normally," he said, "every Farc division has its English speaker, but we're without one at present. We need someone to translate a small text for us. Would you be able to help?"

"Of course," I replied, flattered by having been mistaken for a Spaniard, and almost pleased to be of some help. Though I'd had serious doubts about ever returning home, the guerrillas had been unfailingly polite and friendly in their dealings with us.

I glanced at the four-page leaflet that the sub-commander took out of the pocket of his bullet-carrying waistcoat. It was an assembly manual for a gun's laser sight. "My technical English isn't so great," I confessed, "and this is American English, which I find particularly difficult." Perhaps because of my fear, the morally questionable nature of what I had been asked to do had still not dawned on me. "Don't worry," reassured the sub-commander, "we'll show you how a gun works and help you with the more specialist terms."

As we were led off to an improvised hut commanding extensive views over the beautiful valley, the absurdity of the situation we had landed ourselves in mixed with the fear. I was almost repressing a giggle on sitting down at an outdoor table to embark on what Julio called "our homework".

Julio proposed that he would write down my translation, while Jenny gave me the correct technical terms for the different parts of the machine gun she was carrying. I began the task as a true professional, searching for eloquent and fluent Spanish for the clumsy and stilted English. Fortunately I soon realised I didn't have a clue what the pamphlet was about.

Jenny thought it might help if she unpacked the actual laser sight, so she did, as if she were a child who had been handed a new toy. Too impatient to wait until I had worked out the instructions, she attempted hamfistedly to put together its various parts and then attach it to the gun. A wave of paternal tenderness came over me as I watched her, followed by a tense irritation as she started dropping the tiny and doubtless irreplaceable components on the ground. "Please be careful," I admonished. "If we lose any piece we might as well throw the whole thing away."

Michael Jacobs with Farc graffiti near where he was asked to help the guerrillas. Photograph: Christian Escobar Mora

Eventually we started making progress, despite occasional visits from smiling woman friends of Jenny who wanted to talk to us. Was Spain covered in jungle? What food did we eat there? I was being given a recipe for a sweet made from sugar cane and bull's penis when a sweating Marlon appeared, bringing a note of chilling seriousness to the occasion. "Personally," he muttered, inspecting the half-assembled laser sight, "I prefer to kill people at close range, when I can see their faces. But the beauty of this gadget is that you can shoot accurately at 150m, and in the dark."

From that moment on, all I could think about was how to leave the area as quickly as possible. "The homework's finished," announced Julio after I had sped through the manual's final two pages without bothering to check what I was dictating. The sub-commander thanked us greatly, while I apologised for possible inaccuracies. He hoped we would come and see them all again one day, "if you don't mind putting up with our humble tents constructed out of branches and leaves". We thanked him for such a tempting invitation, and then set off again down the mountain.

"We've saved our consciences," whispered Julio once we were out of the guerrillas' sight. He explained that he had made small changes to my translation that had rendered an already garbled text completely useless. Thankfully he had seen what, in my fear, I had put aside. His conscience had prevented him from assisting people who had caused so much suffering and terror in Colombia, who were involved in drug-trafficking, who abducted children. For the next few days I would continue mulling over these implications, until after our return to Bogotá, news came from the area that would make me view the whole incident in yet another light.

We were told by our guide that the army had stepped in and had most likely killed all the guerrillas we had met. I could not get out of my head the image of Jenny fumbling with a laser sight and then dying in the interests of a cause so confused that it had come to embrace, of all things, the promotion of tourism. All feelings about the rights and wrongs of translating for the Farc gave way to something else: an overwhelming sense of pity.

Essentials

The Robber of Memories: a River Journey Through Colombia by Michael Jacobs is published by Granta Books on 1 November at £16.99. To order a copy for £13.59, with free UK p&p, click on the link or call 0330 333 6846