A river that turns bright blue at a specific point you can stand and look at. The colour is not a dye and not a mineral in solution: it is Mie scattering, light bouncing off colloidal aluminosilicate particles suspended in the water. Upstream, two clear streams. The volcanic Río Buenavista meets the acid Quebrada Agria, the acid makes the particles clump together to the right size, and from that confluence down the river is blue. The junction is called El Teñidor, the dyer. It is in Tenorio Volcano National Park, 129 square kilometres established in 1976 with the volcano itself added in 1995, about 26 miles northeast of La Fortuna in Guanacaste.
Tenorio, Costa Rica
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