The Caño Cristales River in the Serranía de la Macarena mountains of Colombia

© Tom Till/Alamy

The Caño Cristales River in the Serranía de la Macarena mountains of Colombia

From July through November, visitors to the Serranía de la Macarena National Natural Park are likely there to see the riverweed bloom in the rushing waters of the Caño Cristales. When the water level is just right, the normally dull green plant that grows on the riverbed blooms in a fantastic burst of red, purple, orange, and gold. The park is an unusual meeting of three distinct ecosystems: the Andes Mountains, the Amazon Rainforest, and the Eastern Llanos. Recently, the Colombian government has limited access to the river, and now visitors must book guided tours—it’s an effort to preserve this strange scene for generations to come.