Río Celeste

In northern Costa Rica, two clear rivers meet at a place called El Teñidero (“the dyer”) to suddenly create an intensely colorful third river: the deep, rich turquoise Río Celeste. Why? 

The poetic answer is that Río Celeste is where the gods washed their brushes after painting the sky. It is a beautiful image, however, what is really happening in the water?

For years, the going scientific thought was that a chemical reaction caused the color change. Río Celeste (meaning “sky-blue river”) is in the dense rain forest of Costa Rica’s Tenorio Volcano National Park, so it was logical to conclude that the combination of thermal steam, sulfur, and calcium carbonate created the sudden, dynamic color. 

In 2013, a group of researchers from the Universidad de Costa Rica decided to get their hands and feet wet to find out for sure. 

The first thing they observed was that the water didn’t remain turquoise when removed from its source. It was clear. 

In the river itself, they noticed that a white substance covered the rocks. They also found large amounts of the substance in Río Buenavista, one of Río Celeste’s feeder rivers. They looked at the white substance under an electron microscope and determined that it was a mineral consisting of aluminum, silicon and oxygen. Aluminum and silicon caught their attention because these elements, with particles of the correct size, suspended in the water, might cause the reflecting light from the sun to create the optical effect that the water was blue. Were the particles the right size?

It turns out how we see colors is largely determined by particle size. The relationship between the size of the particles and the wavelengths of visible light are what cause us to see certain colors. That is why the sky looks different to us at different times of the day: molecules and small particles in the atmosphere change the direction of light rays, causing them to scatter.

In Río Buenavista, the source for the white mineral, the particles were roughly 184 nanometers (nm). Not big enough.

However, in Río Celeste, they averaged 566 nm, which was big enough for the Mie Scattering effect that could cause the blue color.

Why was the size so different in the two rivers?

That’s where the second feeder river, Quebrada Agria (“sour creek”), comes in. As its name suggests, Quebrada Agria has high acidity. 

The researchers determined that where Río Buenavista and Quebrada Agria meet, the drop in pH causes the colloidal aluminosilicate particles to aggregate. One provides the mineral material while the other provides the acidic environment to enlarge the particles so that sunlight scatters to reflect turquoise.

Río Buenavista and Quebrada Agria work together to create the beautiful colors of the enchanting river visited by thousands of people each year. Without what each river brings, there would be no Río Celeste. Sometimes collaboration is necessary to make things of great beauty.

SOURCES

Castellón, E. Martinez, M. Madrigal-Carballo, S. Arias, ML. Vargas, WE. Chavarría, M. “Scattering of Light by Colloidal Aluminosilicate Particles Produces the Unusual Sky-Ble Color of Rio Celeste (Tenorio Volcano Complex, Costa Rica).” University of Zurich: September 18, 2013. http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0075165

Evans, Marcel. “Investigation Reveals Why the Rio Celeste is Light Blue.” The Costa Rica Star. September 13, 2013. https://news.co.cr/investigation-reveals-why-the-rio-celeste-is-light-blue/27007/ Accessed August 2, 2017.

Parker, Jenn. “Where Does Rio Celeste Get Its Magical Blue Color From?” Culture Trip. July 19, 2017.https://theculturetrip.com/central-america/costa-rica/articles/where-does-rio-celeste-get-its-magical-blue-color-from/Accessed August 2, 2017.