Oceanography Among the Tumbleweeds in Utah

Lincoln Pratson Looks to the Desert's Lake Powell to Shed Light on One of the Deep Sea's Murkiest Processes p.3

Both rivers, Pratson says, carry unusually high volumes of suspended sediment. Near where the San Juan enters Lake Powell, he’s collected samples showing it contains more than 10 percent sediment by volume. By comparison, he says, wet concrete is only about 30 percent sediment.

“The mud and sand that used to flow downriver into the Grand Canyon, where it renourished beaches and sandbars on the canyon floor, is getting trapped in Lake Powell,” Pratson says. “The water flowing into the lake is brown. The water being discharged out of it is clear and cold.”

These conditions, coupled with the lack of marine events and phenomena to muddy the issue, make Lake Powell an ideal setting for studying the dynamics of turbidity currents.

“When the Colorado and San Juan rivers enter the lake, the density of their water is many times greater than that of the lake water, so it plunges and creates a turbidity current down the bottom slope,” Pratson says. “It’s a manmade but naturalscale laboratory where complications can be minimized and conditions like water level and sediment input are well known. I can make observations here that I can’t do in a flume tank or the ocean.”

To document changes on the lake bottom caused by turbidity currents, Pratson spent five days last May mapping the lake floor, assisted by Nicholas School doctoral student Thomas Gerber and National Park Service aquatic ecologist Mark Anderson.

They mapped the lake by day, doing their best to steer clear of the lake’s two chief dangers: submerged rocks and tourists’ houseboats. Lake Powell attracts more than two million visitors a year, many of whom tour the lake aboard rented houseboats.

“Since it was May, we didn’t see as many houseboats, jet skis and speedboats as we would have seen in summer,” Pratson says. “It could get busy at times, but we had some parts of the lake virtually to ourselves.”

At night, they would beach their boat and set up camp on the lake’s desolate shore, dwarfed by the eroded rock formations and towering canyon walls that ring Lake Powell. “It’s so remote there—at night you don’t hear a sound, you don’t see a light,” he recalls. “It’s just you and the stars. The feeling of solitude is incredible.”

A 32-foot metal-hulled National Park Service boat served as their research platform. Its shallow draft allowed them to enter twisting side canyons and other waters where boats with deeper drafts would have been grounded—an important practical consideration following six years of drought that has dropped the lake’s water level 117 feet.

“On the canyon walls you can see a clearly delineated white line about 117 feet high, marking where the water level used to be. It’s like a giant bathtub ring,” Pratson says. “When a houseboat passes by and you see how small it looks compared to the height of the watermark, you realize the scope of the drought’s impact.”

To map the lake’s floor and analyze its sediment strata, the team used a newly acquired chirp acoustic sonar profiler, purchased with support from the U.S. Office of Naval Research, the National Science Foundation and the Nicholas School.

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