Crater Lake occupies the old caldura of Mt. Mazama. About 7,700 years ago, Mt. Mazama had a huge eruption. It blew and it blew, so much volcanic ash that much of the Pacific Northwest was blanketed in the silty fine stuff. Native Americans were living at that time and the stories of the eruption, that they have passed down, are surprisingly accurate after all those thousands of years. See more at bottom of page.
The eruption was so massive that it exhausted all the hot material in the caldura under the mountain. This caused the massive mountain to fall in on itself leaving a huge hole in the ground.
This hole is now filled with the deepest lake in North America; Crater Lake. Its water is very pure and all preserved in a national park. The lake has no visible outlet, from the surface, but water leaks out into underground springs. The springs feed the north fork of the Umpqua River.