Most lakes are made by rivers, glaciers or dams. Crater Lake was made by a mountain falling into itself. It is the deepest lake in the United States, and one of the clearest anywhere, and its story starts with an eruption about 7,700 years ago.
The mountain that was
Before the lake, there was Mount Mazama, a volcano in the Cascades of southern Oregon that stood around 12,000 feet tall. When it erupted, it emptied so much of its own interior that the summit collapsed, leaving a caldera five to six miles across and more than half a mile deep.
Filled from the sky
Rain and snow did the rest. Over several hundred years, precipitation filled the caldera until the water lost to evaporation and seepage balanced what fell. That balance still holds the lake at a depth of 1,943 feet, with about 4.9 trillion gallons of water in it. Roughly 34 billion gallons come in and go out again every year.
And it snows a lot. The National Park Service puts the average yearly snowfall at about 42 feet, down from 51 feet in the 1930s.
Why the water is so clear
Fed by rain and snow rather than rivers, Crater Lake gets very little silt, and it stays remarkably clear. Clarity averages a depth of 102 feet, which is why the blue looks almost unreal from the rim. The lake rarely freezes over in winter.
A shoreline that drops straight to the bottom: on the map, the caldera walls show up as a tight band of layers right off the rim.
The volcano inside the volcano
The caldera was not quiet for long. Eruptions from new vents built Wizard Island, the cinder cone near the western shore, along with three more volcanoes that formed underwater. The last eruption happened about 4,800 years ago.
Crater Lake in birch
On a depth map, most lakes ease into deep water over a shelf. Crater Lake does not. The layers stack tight against the rim and then the bottom opens up, the way the real caldera does. The Crater Lake map is 16 × 20 inches with the full rim, Wizard Island and the deep basin engraved.
Its Pacific Northwest neighbor, Lake Chelan, is the third deepest lake in the country.
Keep reading: The 4 Deepest Lakes in the United States · Lake Tahoe by the Numbers · What Is a Bathymetric Map?
Source: National Park Service, Crater Lake National Park, “Crater Lake.”