Emerald Pool is the name of several locations.
Emerald Pool is a small, shallow lake, with an area of less than one acre. It is located about 80 meters above Vernal Fall in Yosemite National Park. It is named for its deep green color, which is caused by algae living on the rocks at the bottom of the pool. In the summertime during diminished water flow, the Silver Apron (a smooth granite slope over which the Merced River flows into the Emerald Pool) is frequently (albeit illegally) used by hikers as a water slide. Swimming or wading in the Emerald Pool or entering the Silver Apron is prohibited by the National Park Service since waders or swimmers have been swept over Vernal Fall and killed, and people sliding down Silver Apron risk collision with hidden rocks at its lower end. This prohibition is clearly marked with signs.
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There also is an Emerald Pool in Northern California along the Yuba River. The Yuba River flows down a canyon then falls down a water fall into a sheer rock canyon around 60 feet. The walls around the first pool measure from around 30 feet to 80 feet in height with a length of around 50 meters and a width of around 30 meters, the depth is 27 feet near the middle of the pool, the water is crystal clear and many people jump off the cliffs for enjoyment. There is a second pool nearby, not as big, but close. It is popular with young adults from Reno and Grass Valley areas. It is a popular swimming spot although the water is cool.
Emerald Pool, a hot spring named for its emerald green color, is one of the main attractions at Black Sand Basin in Yellowstone National Park. Its temperature is 154.6 °F and its dimensions are 27x38 feet with a depth of 25 feet. The color is due to growth of yellow bacteria and algae. Cooling, the result of objects thrown into the pool and natural debris, has affected the growth of the bacteria and algae, making the pool appear orange and brown around the edges.