Science - USA (2020-08-21)

sciencemag.org SCIENCE

PHOTO: NICHOLAS HERTZLER

C

enturies ago, parts of the east-

ern United States were drowned

in mud. Now, Robert Walter

was dancing in it. The geo-

chemist stood calf deep in this

small stream 100 kilometers

west of Philadelphia, thick curli-

cues of chocolate sediment flow-

ing around his legs. Walter did a

little jig as his colleague and spouse, geo-

morphologist Dorothy Merritts, watched.

More mud stirred, heading downstream.

Brown water might not hold much in-

terest for many researchers. But a dozen

years ago, it catapulted Merritts and Walter

to scientific prominence. The pair, profes-

sors at Franklin & Marshall College (F&M),

showed that Big Spring Run and many other

meandering, high-banked streams in the

eastern United States look nothing like the

low-banked, marshy waterways that existed

when European explorers first arrived nearly

500 years ago. The original streams, Merritts

and Walter argued in an influential 2008 pa-

per published in Science, are now buried be-

neath millions of tons of “legacy sediment”

that was released by colonial-era farming

and logging, and then trapped behind count-

less dams built to power flour, timber, and

textile mills. “We realized,” Walter says, “that

the [streams] had been completely manufac-

tured and altered.”

The finding challenged decades of conven-

tional scientific wisdom and sparked push-

back from researchers who said the pair had

overstated its case. It called into question

expensive efforts to restore rivers by using

heavy equipment to resculpt them into what

practitioners believed had been their natu-

ral shapes. And the work raised concerns

that a massive, multibillion-dollar effort to

clean up the nearby Chesapeake Bay would

fail if planners didn’t figure out how to pre-

vent massive slugs of legacy sediment, which

also carries harmful nutrients, from sloshing

down the bay’s many tributaries. “It was un-

comfortable,” Merritts says, “because I knew

that my colleagues had other ideas.”

Now, a dozen years later, new research is

settling many of the debates that Merritts’s

and Walter’s paper touched off. Although

dams are not solely to blame for legacy sedi-

ment, it’s now clear colonial-era erosion did

dramatically alter streams in much of the

continent’s tectonically quiet eastern half,

says Ellen Wohl, a geomorphologist at Colo-

rado State University, Fort Collins. “There’s

been an accelerated recognition of how ubiq-

uitous this sediment is,” she says. And that

recognition has been driven by Walter and

Merritts, says Noah Snyder, a geomorpho-

logist at Boston College. Their study is “one

of the most influential papers I’ve seen.”

Now, the duo is hoping to inspire a new

approach to stream restoration by turning

back the clock at Big Spring Run. By remov-

ing centuries of mud, they have returned the

stream to its marshy, precolonial glory, and

are now demonstrating the environmental

payoff such strategies can deliver.

MERRITTS AND WALTER weren’t the first to

realize that erosion has clogged many U.S.

stream valleys with sediment. In 1917, Grove

Karl Gilbert, a storied geologist who studied

western North America, revealed that gold

mining in the late 1800s had caused sedi-

ment to fill and reshape deep river valleys in

the California Sierra Nevadas. In the 1940s,

Stafford Happ, a soil scientist at the U.S. De-

partment of Agriculture, documented how

silt eroded over centuries had buried and

transformed Wisconsin waterways. In the

following decades, two other researchers—

geomorphologist Jim Knox and hydrologist

Stanley Trimble—documented thick beds

of legacy sediments beneath waterways in

Georgia and the Upper Midwest.

“Agricultural erosion in parts of this coun-

try was far more severe” than many geo-

logists realized, says Trimble, who recently

retired from the University of California,

Los Angeles. “We are talking about buried

farms and villages.” Beaver, a small town in

Minnesota, had been smothered by nearly

5 meters of eroded silt from uphill farms

that reached the second floors of homes.

Port Tobacco, Maryland, once a boomtown,

faded after its wharfs silted up. But these

A dozen years after two scientists showed how

centuries-old mud has smothered many

U.S. streams, their ideas are guiding restoration efforts

By Pa u l Vo ose n, on Big Spring Run in Pennsylvania

NEWS | FEATURES | MUD

A MUDDY

LEGACY

898 21 AUGUST 2020 • VOL 369 ISSUE 6506

Published by AAAS