Riverside Geyser is a geyser in Yellowstone National Park in the U.S. state of Wyoming. The geyser is located on the Firehole River within the Upper Geyser Basin.
The geyser shoots steam and water to heights of 75 feet (23 m) in an arch over the river, sometimes causing brilliant rainbows. The eruptions occur every 51⁄2 to 7 hours.
Riverside is one of the most predictable geysers in the park, because it is not located near any other geysers that may disrupt the flow of underground water in the geyser's plumbing. Riverside is unique in its way that eruptions seldom occur at the average interval for any given time. Instead, Riverside is bimodal, meaning it usually erupts 20 minutes before or 25 minutes late for a duration of 20 minutes. The bimodality does not represent a problem for predictions if the previous erupt time is seen. If not, you are predicting on doubles and triples, which leads to a wide range for uncertainty. For an hour or two before the eruption, water pours over the edge of the cone.
The geyser was named by the Hayden Geological Survey of 1871. The geyser is one of the most reliable geysers in Yellowstone, despite the moniker and reputation of the better-known Old Faithful.