crater lake is a caldera lake in the u.s. state of oregon. it is the main attraction of crater lake national park. it is famous for its blue color and clear water. the lake partly fills a nearly deep caldera. the volcanic crater was formed by the collapse of the volcano, mount mazama, about 7,500 years ago. there are no rivers flowing into or out of the lake. the evaporation is compensated for by rain and snowfall at a rate such that the total amount of water is replaced every 250 years.