Getting to the Bottom of Crater Lake
Tom McDonough

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Ever wonder what's on the bottom of Crater Lake? Scientists do, and last summer they took a close look without even removing any of the water. Using the latest echo-sounding technology, researchers unveiled details on the bottom of Crater Lake never before seen, and discovered that our lake is even deeper than previously believed.

The lake partially fills a 3,900 foot deep caldera that formed 7,700 years ago following a major eruption of Mount Mazama. About 12 cubic miles of mountaintop disappeared as the summit of the volcano collapsed into its magma chamber. During the following centuries smaller eruptions occurred above the collapsed mountain even as water began to accumulate.

The base of Wizard Island and a series of overlapping lava flows immediately to the east, called the Central Platform, appear to have formed at the same time. North of the Central Platform is a volcano slightly larger than Wizard Island called Merriam Cone. Except for Wizard Island, all of these volcanoes have been concealed by hundreds of feet of water for at least 7,400 years.

Last summer, scientists from the U.S. Geological Survey and the University of New Hampshire, cooperating with the National Park Service, used the most accurate sounding equipment available to look at the bottom of the lake. A high-resolution multi-beam sonar scanner was attached to the underside of a research boat and flown by a helicopter to the lake. As the boat traveled across the surface of the lake, sound waves were transmitted from it. A computer converted the sound echoes into images of the bottom. More than 16 million soundings were used to construct the most accurate picture yet of the lake basin.

Underwater features have been explored before. During the summers of 1988 and 1989, scientists used a small submarine to visit the bottom of the lake. Before this, core samples of sediment had been extracted. But last summer's exploration was the most comprehensive examination so far of the lake bottom. Not only was the entire basin pictured but objects as small as three feet across could be identified.

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In the pictures on this page, you can distinguish deeper regions of the lake (colored blue) from shallower areas (colored orange). The Central Platform is to the east of Wizard Island. On its northeast side is a depression that looks like an eroded crater. Lava channels are clearly visible radiating from the depression. Where these channels meet the ancient shoreline surrounding the Central Platform, the lava flow fractured and rolled downward into the deepening lake. This has left the Platform, and the plateau beneath Wizard Island, with very steep sides.

Several lava shelves appear along the slope of the Wizard Island plateau. Here again, lava entering the lake marked the ancient lakeshore. The volcanoes were growing as the lake depth was rapidly increasing. The vertical distance between each of these shelves probably marks breaks in the volcanic activity.

Merriam Cone likely formed a little later than the other volcanoes, its lava cooling completely underwater. Visual evidence comes from the absence of a summit crater and its perfectly symmetrical shape.

High-resolution sonar recorded the results of a massive avalanche at Chaski Bay. The uneven topography extending northward from the south lake basin are blocks of lava that fell into the lake long ago. Some of these boulders are 650 feet long. Similar aprons of rock debris can be seen at other locations like Danger Bay and Cloudcap Bay on the east side of the lake.

The sonar equipment was also able to re-measure the lake's depth. The high-resolution scanner collected 16 million soundings and was accurate to within 4 feet. Based on these measurements, the official depth of Crater Lake is now 1,947 feet.

Reporting lake depth can be a confusing exercise. With evaporation and seepage, the depth of any lake can change from season to season. Don't be surprised if you see a variety of lake depths reported in park folders and interpretive signs. We expect it will take a while before the new number becomes official everywhere. The important point is that Crater Lake is still the nation's deepest lake, the seventh deepest in the world.

This latest sonar sounding continues to reveal new information about the mysterious past of this beautiful lake. Results are still being analyzed but already scientists are gaining a better understanding of how events in the past unfolded. By essentially "removing" the water in the caldera, researchers have revealed more of the hidden details that contribute to the already amazing story of Crater Lake.