Aniakchak Caldera was once partly filled with a large lake, similar to Crater Lake in Oregon. Eventually, a weaker portion of the caldera rim collapsed. The resulting outflow eroded the rim to create The Gates through which the Aniakchak River now flows. Surprise Lake, located in the northeast part of the caldera, is all that remains of the once larger lake. Warm springs, melting snow, and glaciers feed Surprise Lake, which in turn gives rise to the Aniakchak River.
Last updated: April 2, 2021