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.2

Sudden, violent events like earthquakes, volcanic activity and coastal landslides have all been known to trigger turbidity currents. But the currents also can have less dramatic causes, like the gradual buildup of excess sediment at the edge of the continental shelf.

Scientists believe these powerful currents have been one of the most important forces shaping and reshaping the seafloor.

Studying them in the ocean environment, however, can be tricky.

“Turbidity currents are very hard to observe,” Pratson says, “because they’re episodic in nature.”

Despite years of research, scientists still can’t predict with certainty where or when a turbidity current will occur, except in the relatively rare case when scientists know that a flooding river carries a concentration of suspended sediment that exceeds the concentration of sediment suspended in the basin into which it flows. Nor can scientists yet predict how big a current will be, how fast it will move, or where it will eventually deposit its sediment. Unless a research vessel or moored instrument happens to be in the right place at the right time, the best researchers can do is collect sediment samples from the seafloor after the event has occurred.

These samples provide a snapshot of what’s happened at the site, but their usefulness is limited by variables beyond researchers’ control, Pratson says.

“It’s difficult to discern how much of the sediment you’ve collected from the ocean floor comes from turbidity currents and how much comes from other events in the marine environment,” he explains.

To reduce variables, scientists can simulate small-scale turbidity currents in the controlled environment of the laboratory using a special tank of water called a flume tank. But the clearest picture, Pratson says, comes from observing and tracking naturalscale turbidity currents and the sediment strata they form in the field.

And that’s what brings him to Lake Powell.

The lake, a ribbon of flat, blue water woven through a moonlike landscape of eroded mesas and jagged rock spires, was created in 1964 when the controversial Glen Canyon Dam was constructed on the Colorado River, just south of the Utah- Arizona state line.

Located about 15 miles upstream from Glen Canyon’s more famous neighbor, the Grand Canyon, the new dam was built to provide a fixed supply of water to towns and farms downstream, and to regulate the flow of the erratic Colorado, whose raging spring flow often slowed to a barely anklehigh trickle by summer.

Fed by the upper Colorado and the San Juan rivers, Lake Powell gradually submerged the entire length of Glen Canyon north of the dam, as well as more than 50 side canyons.

With a mean depth of 132 feet and a maximum depth of 560 feet, the lake at full capacity could hold more than 21.5 million acre-feet of water, experts estimate, enough to meet the water needs of the rain-starved region for about three years.

Over time, however, sediment from the muddy waters of the Colorado and San Juan began to fill in the bottom of the lake.

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