Río Celeste

The exact metre where a clear river turns blue

The river is not blue. Nothing in it is blue, no dye, no mineral, no dissolved copper. The colour is made by the size of particles suspended in the water and by what that size does to light.

A team led by Erick Castellon and Max Chavarria published the explanation in 2013. Two streams meet here: the volcanic Rio Buenavista and the acidic Quebrada Agria. Where they mix, the change in chemistry makes colloidal aluminosilicate particles clump together, and once those clumps reach a certain size they scatter sunlight in the way described by Mie theory, throwing back the sky-blue that gives the river its name. Upstream of the confluence the water is ordinary. Downstream it is not. There is a specific point on the ground where it changes, and you can stand at it.

That is a better story than a legend would be, and the place has one of those too, but the physics is the thing worth carrying up the trail.

The river runs through Tenorio Volcano National Park in Guanacaste, about 42 kilometres northeast of Fortuna, part of the Arenal Tempisque Conservation Area, with the volcano itself added to the park in 1995. Thermal springs and small geysers are scattered through the area along with waterfalls and lagoons. The upper park is primary cloud forest and the lower ground is rainforest. Tapir live here, and puma, though almost nobody sees the latter. A ranger station at the base of the volcano has the basic information, and the Lago Las Dantas trail climbs to the summit through forest.