Scientists electronically chart Lake Mead

Monday, May 24, 1999 | 12:15 p.m.

Scientists from UNLV and Woods Hole, Mass., are spending May days on Lake Mead's surface, electronically mapping how the Las Vegas Wash and the Colorado River deposit sediments on its bottom.

UNLV Health Physics Director Mark Rudin with U.S. Geological Survey scientists David Twichell, VeeAnn Cross and Ken Parolski are using high resolution seismic and sonar equipment, the same technology used to find the Titanic, to understand how the sediments are distributed.

The work is important for two reasons, Rudin said.

First, it will help UNLV scientists find the best places to collect core samples of the sediment, which traps and tells the history of the lake's bottom.

The muck piling up under the lake records information ranging from where above-ground nuclear weapons fallout landed to which pesticides, toxins and other pollutants have swept from the Las Vegas Valley into the lake.

"We took a couple core samples last year," Rudin said as the houseboat left Lake Mead Marina's dock Friday. "They don't mean anything, because we did not have a clue as to the depth of the sediments. This is our scientific roadmap."

Second, the map will offer information on how much sediment the wash and the river deposit into the lake.

Lake Mead was flooded into existence in 1935 when Hoover Dam was completed. Water from the Colorado River was trapped in steep canyons carved by the river, and the lake's underwater ridges and mountains reflect its origin.

Since then the river has dragged tons of silt into the desert lake. As Las Vegas boomed, sewage and runoff added to the pile.

Southern Nevada squeezes extra water from its 300,000-acre-foot share of the river by returning treated wastewater to the lake. Learning how much sediment goes into the lake from the Las Vegas Valley -- as well as what's in it -- will help the scientists measure the health of the water.

Once the electronic map is in hand, Rudin and others plan to delve into the sediments for biological and chemical samples that will give them a precise measure of the state of the lake.

As part of their research, Rudin and Twichell hunted down the last map done by surveyors walking the dried riverbed in 1935. "I was stunned," Twichell said of the roads, mines and other archaeological wonders at the bottom of today's lake.

Indian trails dating back more than 5,000 years, a turquoise mine and the town of St. Thomas are part of the rich history captured in shadows as the sonar signals bounced off the lake's bottom.

The four scientists even have their underwater eyes and ears alert for signs of a B-52 bomber that crashed into the lake in 1945.

To draw an accurate map, Cross and Parolski wrestle a 300-pound probe, affectionately known as "Fish," over the end of the boat. When they combine the sounds from "Fish," the side scan sonar, with Global Positioning Satellite tracking, the lake's underwater profile emerges.

The GPS shows the boat moving as a red line across a computer screen. Purple lines from the boat's tracks during the past 10 days criss cross the screen.

Sophisticated imaging equipment set on every available counter, tabletop and shelf inside the sky-blue carpeted houseboat allow the scientists to see the 3-D images of the lake and its sediment fill computer screens aboard the boat.

"It's an underwater version of an aerial photograph," Twichell said as the peaks and valleys appear in stark black-and-white images.

Most of the silt filling the ancient river channel comes from the Colorado River, Rudin said.

"Actually, we don't anticipate that the Las Vegas Valley contributed all that much sediment," he said.

The mapping project is being conducted to launch a proposed institute. UNLV and the USGS contributed $75,000 each with the Southern Nevada Water Authority adding another $20,000 for the underwater map. If further funding becomes available, Rudin plans to open the Lake Mead/Mojave Environmental Research Institute to study pollution problems in local waters.