Crater Lake is the result of a volcanic eruption 7,700 years ago. Mount Mazama, then 14,000 feet high, erupted for about a week spewing 12 cubic miles of magma across five future states and several Canadian provinces, an area of 500,000 square miles being covered by ash and other measurable debris. This was the largest ever volcanic eruption in the Cascade Mountain range, more than 100 times bigger than Mount St Helens. The previous biggest eruption in North America was at Yellowstone, 640,000 years ago, which I'll come back to another day. When the magma chamber was empty, the mountain collapsed straight down into the depressurised bubble and the 14,000 foot mountain lost approximately half its height in a matter of hours.
Over the next 200 years the resultant bowl filled with rain and snowfall, until the rate of precipitation matched the rate of water loss through seepage and evaporation. The water level has remained fairly constant ever since. The water is very clear and light reaches right down to the bottom of the lake, 1,943 feet below. Deep Rover, a one-man submersible, tested this in 1988-89, and discovered much about the ecology, bio-diversity and geology of the lake. It is the deepest freshwater lake in the US, and the 7th biggest in the world, though there is actually a pocket of saline water at the bottom, called Llao's Basin.
The caldera is roughly oval, 4 miles by 6 miles, so the whole lake are is about 20 square miles. There are 36 different geological features in and around the caldera, the most prominent being Wizard Island, the result of a later volcanic hiccup some 700 years after the main event. There is also a small rocky outcrop called the Phantom Ship close to one edge, small being relative as it is as big as a 16 story skyscraper.
This map shows the fallout from the Mount Mazama eruption, with Mount St Helens fallout shown by the dotted lines. Yellowstone National Park lies within the square to the southwest of Billings. Eugene to the Northwest of Mazama would not have caught any fallout, due to the prevailing winds from the west and south. Phew.