Driving from the capital, San Jose, out to the Caribbean coast you cross a bridge which passes over a river named Rio Sucio. Translated, Rio Sucio means "dirty river" and it is where two rivers meet and connect. The Rio Sucio runs down from the Irazu volcano and is really murky. It intersects with Rio Hondura, which is a clear mountain stream.
The color and clarity difference between these two rivers is striking. They run along side each other a short ways, then cross under the bridge where the waters mix.
Looks like oil and water.