Rio Celeste (Turquoise River) is a beautiful river in Costa Rica with an interesting story to explain its color. The hike through Tenorio Volcano National Park is nearly 6.5 kilometers long, with some 121 meters of elevation gain (though the total gain is about twice that much, considering the walk down to the falls and back up). There is abundant wildlife in the area as well as impressive flora. Note that the trail can be quite muddy, and sometimes after heavy rain the river can be muddy and brown as well, so you may want to check the weather before going.
El Pilon station is located about a 90-minute drive from La Fortuna. There is an entrance fee of $10 for foreigners, and the park is open from 8 a.m. to 4 p.m. There is ample parking at the trailhead, where there are also restrooms, a cafe, and rubber boots for rent. Note that there is a checkpoint there; rangers do not allow bread and other similar foods, as coatis can be quite intent on going after those. The Misterios del Tenorio (The Mysteries of Tenorio) trail is well marked, and it can be busy at times. A guide is not necessary, but someone with knowledge of the fauna, flora and geology can add much to the experience. The trail proceeds through the forest, presenting plenty of opportunities to observe wildlife. As in other parts of Costa Rica, there are venomous snakes, including different types of viper, thus you may want to refrain from touching plants as you walk (snakes camouflage very effectively). You can also see dart frogs between the leaves, which are poisonous, so do not touch them.
The hike is steep in places, and the muddy and rocky trail makes the hike fairly strenuous. Make sure to wear sturdy shoes, insect repellent, and bring a raincoat or umbrella. Approximately a mile from the trailhead you will find steps to go down to the waterfall. It is over 200 steps, but it is definitely worth it, as it is a spectacle to see the blue river crashing down the mountain. A pair of black eagles have used the rocks above the waterfall as a nesting site for a few years, thus you may have the opportunity to see eaglets in the nest.
The trail continues up to the Blue Lagoon, a particularly blue section of the river just downstream from hot springs (swimming on the river is prohibited, though). There are a couple of small hanging bridges that offer good views of the river. Some 2 miles from the trailhead you will arrive at the Teñideros (or dyers), where Rio Celeste is formed by the confluence of two smaller rivers. The two rivers are clear, yet as soon as they come together they form this larger, perfectly turquoise river. The reason is similar to why the rivers carrying glacial powder look blue: Sunlight is scattered as it reflects on millions of particles suspended water. However, it takes a bit more to understand why the color changes as the two river come together. One of the rivers carries small particles of aluminum silicate. The other is highly acidic as a result of volcanic activity in the area. The acidity in the water makes the aluminum silicate particles aggregate into larger particles, which scatter sunlight as described above.
After the hike, you may want to stop by some of the local restaurants on the way back to La Fortuna.