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Mapping the Bottom of a Lake

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Eight U.S. Geological Survey (USGS) scientists camped out for a week on Wizard Island in Oregon, a cinder cone inside the blown-out volcano that forms Crater Lake, the nation’s deepest and clearest lake. They generated the most accurate maps ever of the lake bottom for a $153,000 partnering project among the USGS, the National Park Service and the University of New Hampshire.

Using high-resolution multibeam echo-sounding sonar mounted on a 26-foot boat flown by a helicopter into the lake, the scientists generated millions of depth soundings, their positions plotted by a differential GPS. The data was run through computer software used by oil companies to map the sea floor for pipelines and exploration. It produced color 3-D images that can be viewed from above as a map or sideways as if you were flying through the caldera, the bowl left when the volcano collapsed with all the water taken out.

A 1959 survey using early echo-sounding technology generated about 4,000 data points around the lake, which is the basis for current maps and models. The new survey generated some six million data points. It is far more accurate, thanks to GPS navigation and equipment that measures the temperature (low 60s in the summertime at the surface, just above freezing year-round at the bottom) and chemical composition of the water to factor into the soundings. The GPS system, used in Lake Tahoe in 1998, is accurate to within 50 centimeters, or about a foot and a half.

A transducer in the boat sends out a fan-shaped pulse of sound made up of 111 beams, which bounce back from the lake bottom and are recorded by a network of receivers. In the shallows, the fan of sound covers about 50 feet, while in the deep it spreads to 800 feet.

The survey confirmed that Crater Lake, being only six miles across, remains the deepest lake in the United States at 1,985 feet (it was measured as 1,932 feet in 1959). That’s not enough to surpass the Great Slave Lake in Canada as the deepest in North America at 2,015 feet.

Crater Lake was formed about 7,700 years ago when Mount Mazama, a 12,000-foot volcano in the Cascade Range, went through a massive eruption that blew ash as far as Alberta, Canada. Wizard Island is one of the small volcanoes that grew within the caldera.

William Steel, the father of Crater Lake National Park, performed the lake’s first crude mapping expedition in 1886. Steele used a weight attached to piano wire mounted on a rowboat to measure more than 100 depths around the lake and triangulated the positions using conventional surveying techniques of the time. The remains of the boat Steele used, the Cleetwood, lie on the lake bottom near Wizard Island.

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