Water seeping into the conduit of the Rincon de La Veija Volcano trickles down until it makes contact with the heated magma layer below the Earth's crust. The water is filtered and purified and the returned to the surface as a hot river of water. Blue River Resort tours in Rincon de la Vieja park beside the volcano include a dip in a warm water river. The minerals under the ground that make contact with the volcanic activity is what form the process for what creates hot springs and blue rivers. These blue rivers found on Rincon de la Vieja only flow down the north side of the volcano by Blue River Resort & Hot Springs (in Guanacaste). There are also other Costa Rica National Parks with blue rivers. Hot springs appear where these underground rivers break through to the surface of the land and the filtered volcanic water creates natural pools. Closer to the source, the water will be extremely hot. As it flows away from the heat source it cools, becoming pleasantly warm and therapeutic throughout Costa Rica.
On February 25, 2014 at about noon, there was a significant phreatic eruption (water, hot gases and lake sediments) in Laguna Caliente on Poas Volcano, Below is a picture: