Río Celeste, Costa Rica: Why the River Is Blue and How to Hike It

Río Celeste in northern Costa Rica runs a milky sky-blue through green rainforest, and nobody adds anything to it. The colour is pure physics: tiny mineral particles scatter sunlight the way the atmosphere colours the sky. The river is the star attraction of Tenorio Volcano National Park, and a forest trail leads past its waterfall, lagoon and the spot where the blue suddenly begins.

Why Río Celeste is blue

For years guidebooks explained the colour as a chemical reaction between sulphur and calcium carbonate. In 2013 a team led by Max Chavarría of the University of Costa Rica tested the water and published the real answer in the journal PLOS ONE.

Río Celeste is formed where two colourless streams meet. The Río Buenavista is almost neutral, with a pH of about 6.8, and carries very fine particles of aluminosilicate minerals, around 184 nanometres across. The Quebrada Agria is acidic, with a pH of about 3.1. Their meeting point is known as El Teñidero, roughly "the dyeing place".

When the two streams mix, the water becomes more acidic, with a pH of about 5. The change in acidity removes the electrical charge that kept the mineral particles apart, so they clump together into larger aggregates of about 566 nanometres. Particles of that size scatter blue light especially strongly. This effect is called Mie scattering. The researchers found no copper, cobalt or other coloured compounds in the water. Their light-scattering model matched the measured sky-blue almost exactly.

Because the colour depends on particles and light, the shade varies from day to day. Rain, sediment and cloud all change what you see, and bright light brings out the strongest turquoise.

Tenorio Volcano National Park

The park covers about 129 km² (50 sq mi) of Guanacaste Province and forms part of the Arenal Tempisque Conservation Area. Tenorio Volcano rises above dense cloud and rainforest, and the area has thermal springs and small geysers.

The main visitor trail, described on the SINAC Tenorio Volcano National Park page, leads to the Río Celeste waterfall, where the blue water drops into a round pool framed by forest. Farther along are Laguna Azul, a blue lagoon, and finally the Teñidero, where you can see the clear and the blue streams meet.

Hiking the Río Celeste trail

Allow half a day. The trail is muddy for much of the year, and the waterfall viewpoint is reached by a steep descent that you have to climb back up. Wear shoes with good grip, carry rain gear and water, and start early so you have time for the full route.

Swimming is not allowed anywhere in the park's rivers and pools, so enjoy the water from the viewpoints and bridges. Older photos show people standing at the base of the waterfall, but the pool is now protected.

Many visitors stay in the small town of Bijagua or drive in from La Fortuna; the park lies about 42 km northeast of Fortuna. Check the current opening days and fees on the SINAC page before you go.

More rivers and waterfalls with a twist

For another unusual river colour created by a completely different process, read about Caño Cristales in Colombia. Our Mauritius underwater waterfall article explains a famous optical illusion. Heading north from Costa Rica? Our Nicaragua guide covers cities, volcanoes, lakes and islands.

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