SCIENCE sciencemag.orgAS THE DEBATE swirled, Merritts and
Walter decided to put their ideas into prac-
tice. During their research, they had met
Joe Sweeney, a farmer who owned land that
encompassed Big Spring Run, and Ward
Oberholtzer, an engineer at LandStudies, a
river restoration firm. Sweeney had hired
Oberholtzer to examine why trees he had
planted on Big Spring Run’s high banks to
prevent erosion were dying. The conclusion:
Their roots couldn’t reach the groundwater;
trenches dug by Merritts, Walter, and their
students suggested several meters of legacy
sediment caked over the site. To restore such
connections, the team proposed re-creating
the kind of languid wetland that Walter and
Merritts believed had once existed on the
spot. But first they would monitor it for sev-
eral years, to establish a baseline that could
be used to evaluate any post-
restoration changes.
In 2011, after more than
2 years of planning and assis-
tance from the Pennsylvania
Department of Environmental
Protection, the National Science
Foundation, the Environmental
Protection Agency (EPA), USGS,
and others, bulldozers began to
remove 22,000 tons of legacy
sediment along 4 square kilo-
meters of the valley. (The silt
was trucked to F&M and used as
fill beneath a new building.) A
layer of rich, black, precolonial
soil emerged from beneath the
legacy sediment. In it, research-
ers found seeds that provided
an archive of the wetland plants
that had once grown along the
stream. Although federal regu-
lations required the restora-
tion team to carve a single new
channel, they built low banks
and installed stumps and other
obstacles that would encourage
high waters to jump the banks,
transforming the stream into a
multithreaded wetland.
Within 1 year, the banks bloomed with
sedges like a Chia pet. Today, bog turtles scut-
tle and geese nest in thick native vegetation
that has put down roots that hold sediment
in place. There’s room for floodwaters to slow
down and spread out, instead of sweeping
away bankside trees and plants. “The biology
does not have to re-establish itself ” after ev-
ery severe storm, Oberholtzer says.
Monitoring shows the restoration has also
altered the stream’s biogeochemistry. Storage
of organic carbon tripled in the restored area
and levels of nitrate, a key pollutant, dropped
sharply, soaked up by the wetland plants. The
load of sediment swept downstream from therestored area declined drastically, by 85%, ac-
cording to a USGS report published this year.
Polluting phosphorus, which hitches a ride
on silt particles, dropped 79%. Ken Forshay,
a research ecologist with EPA based in Ada,
Oklahoma, says he was skeptical he’d see such
improvements. But the data have “turned a
nonbeliever into a believer,” he says.
Even before all the results were in, the Big
Spring Run project prompted similar resto-
rations in Pennsylvania and Maryland, with
20 now completed and 10 more underway.
It’s simple to see why: Though the project
would have cost $1 million in today’s dol-
lars to restore its 800 meters, it was at least
16 times more cost effective at reducing pol-
lution than other techniques, found Patrick
Fleming, an agricultural economist at F&M.
“This practice blew the other ones away.”TWELVE YEARS AFTER their Science paper
appeared, a clearer picture is emerging of
how far beyond Big Spring Run the ideas
floated by Merritts and Walter can be ap-
plied. Evidence that precolonial streams of-
ten resembled wetlands has popped up in
more places—in Kentucky, for example, says
Arthur Parola, a stream scientist at the Uni-
versity of Louisville. “The more we look, the
more we’re finding,” he says. “These wetland
systems were maybe the common types of
streams in the eastern United States.”
In New England, however, Merritts and
Walter found a different picture when they
surveyed streams with Snyder. Although co-lonial dams did trap sediment, they found,
the glaciated landscapes provided far less
grist than those farther south. The thick
beds of legacy sediment seen in the mid-
Atlantic are “not going to be seen everywhere,”
Merritts says. And “not every place had that
many milldams.”
Noe found similar variation in a massive
study of 68 river sites in the mid-Atlantic,
now nearing publication. “There’s more
nuance now,” Noe says. “Milldams are very
important” in understanding sediment in
some watersheds, he says, “but they’re not
necessarily the causative factor everywhere.”
Noe’s study will also provide the first de-
tailed, large-scale accounting of sediment
sources and sinks for the region. The good
news is that, at nearly all the rivers his
team studied, the floodplains downstream
were capturing as much sedi-
ment as was eroding upstream,
potentially curbing pollution.
The floodplains are acting as
kidneys, he says, and are “water
quality superheroes.”
But Noe adds that if those
floodplains weren’t busy cap-
turing colonial silt, they could
instead be a greater sink for the
sediment runoff from farms and
cities. And the further removal of
dams, as many states are pursu-
ing, will only free up new slugs
of mud. So legacy sediment prob-
lems aren’t going away, says Karl
Wegmann, a geomorphologist at
North Carolina State University.
“It’s like Chernobyl. We’re going
to be living with it for centuries.”
The question now is what to
do about it. The Chesapeake Bay
Commission, which leads the
cleanup of the bay, is evaluating
how to credit legacy sediment
restorations for their pollution re-
ductions, based on long-term data
from project like Big Spring Run.
It’s “been tremendously valuable,”
says David Wood of the Chesa-
peake Stormwater Network, a nonprofit
that coordinates restoration practices. This
is “the type of research that is needed else-
where across the watershed,” he says.
For their part, Merritts and Walter are
pragmatic, not environmental romantics.
They may have revealed a prehuman base-
line for many waterways, but they know
change is a constant of geology. Many riv-
ers are so drowned in silt that they can-
not be redeemed. The world is not pristine.
But only by acknowledging and accounting
for the legacy of the past, they say, can we
take a first step toward solving the prob-
PHOTOS: (TOP TO BOTTOM) ROBERT WALTER AND DOROTHY MERRITTS/FRANKLIN & MARSHALL COLLEGE; LANDSTUDIES lems of today. j
Meters of mud had buried the rich, black soil that typified Big Spring Run before
Europeans arrived (top). It took heavy machines to remove the legacy sediment.21 AUGUST 2020 • VOL 369 ISSUE 6506 901
Published by AAAS