For a few months a year the river Caño Cristales runs red, and the colour is not in the water but under it. It lies in the Serranía de la Macarena, an isolated massif in central Colombia, and what turns it red is a plant. Macarenia clavigera is a riverweed: it grips bare rock in fast water with fine red fronds that comb the current, and it flowers while submerged. The plant only reddens once the water has dropped far enough to let sunlight through, so the river burns brightest between flood and drought, from late July into November. Red is only one of the colours. Yellow sand, green moss and the blue-black of the deep pools lie alongside it under clear water, and the stretch where all of them meet at once is what gave the river its name. The rock under all this is quartzite laid down about 1.2 billion years ago, far older than the Andes rising to the west.
You follow the rock along the water with a guide. The river spills over ledges in low cascades, some pouring straight onto the red beds below, and you can stand in the cool spray between rounded quartzite boulders. The current has scoured smooth basins and potholes out of the stone, and you can swim in those and nowhere else. Crouched by the shallows you get a close look at the thing behind all of it: the plant is a few centimetres tall and looks more like a red moss than anything that flowers, and touching it is not allowed. None of this could be visited for about two decades. The river lay inside territory held by FARC guerrillas, and it opened again only as the region was secured, from around 2009. It still closes for half the year so the plant can recover. The window is narrow even then: a little more water or a little less, and the river that gets called the most beautiful in the world is a clear stream over old stone.