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.4
Housed in a four-foot-long, 400- pound yellow
plastic body, the $500,000 piece of high-tech hardware was
lowered in and out of the water “very carefully” using a chain
winch, Pratson says.
“We were a little nervous at first that it would
hit the side of the boat or smack a submerged rock and be
damaged its first time out,” he says. “But the only run-ins
it had were with waterlogged tumbleweeds that kept getting
tangled in it. That’s one of the unusual aspects of using
oceanographic equipment in a desert environment.”
Freed of its tumbleweeds and towed behind the
boat at a depth of about 10 feet, the profiler performed perfectly.
It collected two types of data: sidescan sonar images that
could be pieced together to map the lake bottom, and sub-bottom
reflection profiles that provided acoustically derived cross-sections
of the stacking patterns of sediment strata found there. These
crosssections allowed the scientists to determine the type
of sediment and its thickness.
Pratson and Gerber are now analyzing data from
their survey and comparing it to data from past geological
surveys made before the Glen Canyon Dam was constructed and
about once a decade following its completion. (Pratson himself
conducted one of these surveys—covering the lake’s Colorado
River arm, or about half its total area—in 2001.)
By comparing the data, they can document changes
that have occurred over the years on the lake bottom as a
result of turbidity currents, and get a better idea of the
rate at which Lake Powell is filling in.
“The rate of infill determines the reservoir’s
useful lifespan,” Pratson explains. With the drought lowering
the lake’s water level at the same time sediment is filling
in the lake floor, “Lake Powell is being squeezed from both
top and bottom,” he says.
A 1986 survey, headed by scientists from the
Bureau of Reclamation, estimated the reservoir had 100 years
of useful life left if the rate of infill remained the same.
Preliminary findings from Pratson’s team’s May
2004 survey, however, suggest that the rapidly dropping water
levels caused by the drought may be exacerbating erosion in
the lake’s deltas and causing more sediment than in past years
to be carried into the lake.
The bottom slopes of the eroded deltas have
grown so steep that turbidity currents can now quickly build
up the speed, volume and velocity they need to carry sediment,
and any contaminants the sediment may contain, farther and
farther into the lake.
“There are contaminants that have been introduced
into the Colorado arm of the lake and we’re watching to see
if sediment transfer carries them farther downstream, where
they could more broadly affect the lake’s ecosystem,” Pratson
notes.
He can’t yet pinpoint how far down the lake
turbidity currents have reached, in part because the underwater
canyon walls are so steep in places that they interfere with
accurate sonar measurements. But data from the 2004 survey
is providing some interesting insights.
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