Winding through the verdant rainforest of Tenorio Volcano National Park of Costa Rica, is a bright blue river called Rio Celeste. The river is formed by the confluence of two smaller rivers called the Sour Creek and the Good View River. At precisely the point where the waters of the two completely transparent rivers meet, that the blue color starts. For that reason, this point is known as El Teñidor, which means “The Dyer” in English.

Until very recently, scientist were unable to fully explain why Río Celeste has such a distinctive turquoise coloration. Many hypotheses were put forward such as the water contained copper or calcium carbonate and sulfur, or that the river’s proximity to the Tenorio Volcano caused it have the blue color. It is now known that the blue color arises due to a physical phenomenon known as Mie scattering triggered by the presence of certain minerals in the river’s water that causes sunlight to reflect in such a manner that it gives the water its incredible hue.

Photo credit: The Rohit/Flickr

As already mentioned, Río Celeste is fed by two rivers — Sour Creek and Good View River. The Good View River carries significant quantities of a type of whitish mineral known as aluminosilicate, that is composed of aluminum, silicon and oxygen. This is the mineral that is responsible for reflecting the blue color in sunlight. But if the mineral is also contained in the Good View River, why doesn’t it appear blue like Río Celeste? The answer lies in the size of the particles.

Researchers found that Good View River particles have a size of 184 nanometers, while in the Río Celeste the particles are much larger at 566 nm.

“This increase in size is what causes the scattering of sunlight, such that it occurs principally in the blue region of the visible spectrum. So that’s why we have that spectacular light blue color of the Rio Celeste” said Dr. Max Chavarría Vargas, lead investigator.

But why are the aluminosilicate particles in Río Celeste bigger than those in Good View River, when the particles itself came from the Good View River? It so happens that Sour Creek, the second river to join Good View River, is highly acidity due to volcanic activity (which is why it’s called Sour Creek). When these two streams mix to form Río Celeste, the drop in pH causes the aluminosilicate particles to aggregate and enlarge producing Mie scattering which gives the river a strong turquoise color.

“It’s one of those quirks of nature where one of the rivers provides mineral material with one size and the other river provides the acidic environment so that those particles grow,” said Dr. Max Chavarría Vargas.

Photo credit: Steve Corey/Flickr

Photo credit: The Rohit/Flickr

The point where the two rivers mix. See how the color changes abruptly. Photo credit: François Bianco/Flickr

Photo credit: Efrain Gonzalez Buitrago/Flickr

Photo credit: The Rohit/Flickr

Photo credit: Bruce Thomson/Flickr

Sources: The Costa Rica Star / Wikipedia