Computer Mapping Gives Accurate New View of Crater Lake’s Bottom

While a small boat cruised the sunlit blue surface of Crater Lake pounding sonar pulses deep into the water, Jim Gardner sat in the dark of a boathouse on Wizard Island, “flying” through the latest computer images the soundings produced.

“We’re flying up over the plateau--the plateau Wizard Island is on,” the U.S. Geological Survey scientist said, his body twisting and his head bobbing as he used a joystick to move through the violet, red, blue, green and yellow bands of the 3-D image of the lake bottom on the computer screen. “Then we fly down into the South Basin.”

Gardner and eight other scientists camped out for a week this summer on Wizard Island, a cinder cone inside the blown-out volcano that forms the nation’s deepest and clearest lake. For all but a few hours each day they generated the most accurate maps ever of the lake bottom. But they also dined on Cajun cuisine, enjoyed hot showers and did a little fly-fishing.

“The neat thing about this is that it is exploration, still,” said Larry Mayer, who directed the data gathering and is director of the Center for Coastal and Ocean Mapping at the University of New Hampshire. “You feel the excitement of the old surveyors exploring new territory.”

The $153,500 project was a partnership between the Geological Survey, the National Park Service and the University of New Hampshire.

It’s a far cry from the first crude mapping in 1886, when William Steel, the father of Crater Lake National Park, 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.

The remains of the boat used by Steel, the Cleetwood, lie on the lake bottom near Wizard Island, a victim of the harsh winter.

Gardner’s team used high-resolution multi-beam echo-sounding sonar mounted on a 26-foot boat that was flown by helicopter into the lake to generate millions of depth soundings, their positions plotted by a differential Global Positioning System.

“We’re taking the water out of Crater Lake,” Gardner said of the images. “We have the technology to do it, so we do it. Every time we map an area, we find something we didn’t expect. We’re going to see things that are surprising to us.”

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 obliquely as if you were flying through the caldera, the bowl left when the volcano collapsed, with all the water taken out.

Each day’s data was stored on a disc and walked up the side of the caldera, then driven to park headquarters to be posted on the Internet.

The most obvious finding was that Crater Lake is actually a little deeper than anyone thought: 1,958 feet, compared with the official 1,932 feet from a 1959 survey. That’s enough to maintain Crater Lake’s title as the deepest lake in the United States, but not enough to surpass the Great Slave Lake in Canada as deepest in North America at 2,015 feet.

There’s a lot more to it all than depth.

“The scientific information derived from this will be phenomenal in helping us and other scientists understand how the lake was formed,” said Mac Brock, chief of natural resources at Crater Lake National Park.

While most people think of the national parks as beautiful places to see on vacation, they also serve as benchmarks while an exploding population and changing technology alter the rest of the world, Brock added.

“That makes them excellent places for scientists to go to,” he said. “They are huge natural laboratories. They are a classroom.”

Crater Lake was formed about 7,700 years ago when Mt. Mazama, a 12,000-foot volcano in the Cascade Range, went through a massive eruption that blew ash as far as Alberta, Canada.

Over the next couple of hundred years, smaller volcanoes grew within the caldera. Wizard Island is one, rising 764 feet above the water. The other is Mirriam Cone, hidden underneath the surface. Their lava flows sealed the bottom of the caldera, and rain and snow filled it.

Local Indians, who saw the lake as a source of power, guarded the secret of its location, but a party of gold miners found it in 1853. Steel first visited the lake in 1885 and campaigned to have it protected as a national park, which happened in 1902.

The lake is about 6 miles across, and its extreme depth and clear water give it an incredible blue color. Though the human eye can see about 120 feet into the water, that is not enough to make out more than the shallows along the shoreline.

A 1959 survey using early echo-sounding technology generated about 4,000 data points around the lake, which are the basis for current maps and models.

The new survey generated some 6 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 on 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.

After taking two turns around the shoreline, the boat ran about 200 times back and forth across the lake to cover the bottom completely.

Besides measuring depth, the equipment also measured something called backscatter, which tells the scientists whether the bottom is rock, mud or sand. They hope to locate earthquake faults as well as more of the hydrothermal vents discovered with a one-person submarine in 1988.

The scientists were pretty comfortable living in the boathouses on Wizard Island, where research vessels and excursion boats are stored for the winter. They slept on folding cots. Fresh groceries were delivered every day. Their meals were prepared by a Cajun chef sent along by C&C; Technologies of Lafayette, La., which owns the echo-sounding equipment.

There is a composting outhouse on the island, and the scientists brought along a battery-powered portable shower unit so they wouldn’t have to take baths in the cold lake water.

“If you think we came down here to eat out of a can, you’re crazy,” said Gardner. “That way we can concentrate on the work and not struggle with survival.”

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On the Net:

U.S. Geological Survey: https://tahoe.usgs.gov/craterlake/

Crater Lake National Park: https://www.nps.gov/crla/home.htm