Río Agrio

The Río Agrio is a volcanic acid river that originates on the flanks of the Tenorio volcano and carries dissolved sulphur compounds and volcanic minerals downstream to its confluence with the Río Buenavista. At the exact point where the two rivers merge, the combined water turns the distinctive turquoise-blue colour visible throughout Río Celeste. The scientific explanation — a light-scattering phenomenon caused by the mineral concentration at that specific dilution ratio — does not diminish the visual effect of watching clear water become blue at a single point in the river. The Agrio itself is pale and colourless upstream of the confluence, the water carrying a faint sulphur smell and a slightly acidic character. The confluence point is accessible via the Tenorio Volcano National Park trail system. The geology here is legible — you can stand at the point of colour change and watch it happen in real time as the two currents mix.