Caño Cristales turns blood-red for a few months each year — not the water, but the riverbed itself. The colour comes from Macarenia clavigera, a mossy aquatic plant that carpets the rock and blazes crimson and wine-red when sunlight and water level fall into a narrow balance: too deep and it stays green, too shallow and it dries out. The plant grows in only a handful of rivers draining the Serranía de la Macarena, an ancient block of quartzite that geologists date to around 1.2 billion years old — a western outlier of the Guiana Shield in central Colombia, where the Andes, Amazon and Orinoco plains meet. Against the red the river shows four more colours at once: yellow sand, green moss, the blue-black of deep pools, and clear water that lets the whole palette through. Colombians call it the River of Five Colours, or the Liquid Rainbow.
There are no roads in. Visitors fly to the town of La Macarena, then travel upriver by boat and finish the last stretch on foot with a licensed guide. The 100-kilometre river lies inside Serranía de la Macarena National Natural Park, and the rules are strict: visitor numbers are capped, sunscreen and insect repellent are banned because they poison the plant, and touching the Macarenia is forbidden. For decades a visit was impossible — the area sat inside FARC guerrilla territory and was closed to outsiders from the late 1980s, reopening to travellers only around 2009 as the conflict eased. Today you hike between low waterfalls and swim in clear pools scoured smooth by the current, the red bed glowing beneath your feet. The display is never guaranteed — it needs the water to settle into a narrow balance between flood and drought, and even then some weeks disappoint. When it holds, few rivers on Earth look anything like it.