The Rio Azul, the blue river, 10km short of the Brazilian border. Over the past two weeks, we have all but crossed the lands of the ancient Bolivian Moxos civilisation by horse, jeep and canoe. This is the point where we must decide whether to go on or turn back. We have reached the green mass of jungle on my map, which is empty except for the words savajes gentios, which my dictionary translates roughly as “crowds of savages”, but I rather think means that this area is an indigenous Indian reservation.

We have left the canoe behind for this last stage. The idea had been to cross overland to the Azul, float downstream to Brazil then pack the boat up into rucksacks and trek back on foot. But we can’t face this last bit; it would mean carrying 40kg each. So, we’re at a river with no boat, and leaving it behind has turned out to be the right thing to do. It’s not that the Azul is dry. Far from it. In places it is deep and clear with shoals of tucanare that our driver, Pepelucho, is catching with consummate ease.

Irgen, our guide, says this would be an excellent place to find an anaconda: a rainforest river where the canopy closes over the water. There are tracks of peccary, tapir and jaguar. A flock of toucans is squealing nearby. Perfection.

Except: just 100 metres on from the furthest point Pepelucho was able to push the pick-up truck through the vegetation-choked track, the Rio Azul stops. Fallen trees block the flow and, machete-hacking a good distance both up and downstream, we find more blockages. Nowhere can we find more than 50 metres of clear water, and the tributary stream which was to have been our alternative route is dried up.

More intriguingly, perhaps a little disturbing, are the remains of a shelter and the hundreds of cut-open Brazil nut kernels that we find not far from the track. Irgen judges the pods to have been opened some time ago, but the ridge pole tied with vines between two tree trunks is newly, built, probably by More Indians. We have come into their land and we don’t have permission.

Pepelucho wants to know if Irgen, Julian and I still want to be left here.

Yes, we say. The decision is unanimous, but Irgen is clearly unsettled. He sites our camp some distance from the road in a thicket of banana-like platanillo palms. He does not mark our trail there. And when we set out next morning to gather Brazil nuts, he makes us take down our hammocks and hides the packs among the platanillos out of view of any passer-by. We return in the afternoon half expecting to find our equipment rummaged through or people waiting for us. Though it has not been said, we know we will not continue onwards from here. It’s not that we fear the More. But Irgen is a forest Indian himself, a Tacana. He says that if we met them, things would be “awkward”. So tomorrow we will trek out. We will walk for three or four days and hope to encounter some of the abundant wildlife whose tracks we are finding everywhere. We will hope not to meet the More.

Simon Chapman is head of physics at Morecambe high school and author of the Explorers Wanted! series for children (Egmont) and The Monster of the Madidi (Aurum Press). His final letter from Bolivia will appear in next week’s TES. Keep up with the Knowsley project at www.spiritofthejaguar.org.uk