NEWS | FEATURES | MUD
sciencemag.org SCIENCEThese hazards prompted her, in 2002, to
look for a safer project closer to home. She
had heard concerns about silt eroding from
the banks of rivers flowing through farms in
Pennsylvania, so she and her students began
to survey local waterways. They appeared
to behave in the ways that Leopold and
Wolman had laid out. But the traditional
model of river evolution couldn’t fully ex-
plain a picture that a student showed Mer-
ritts one day; it displayed a nearly vertical,
6-meter-high wall of layered sediments
along the Little Conestoga River.
As it happened, Walter, who had recently
arrived at F&M, was visiting as Merritts and
the student discussed the photo. Now 69,
Walter was born and raised in Lancaster,
Pennsylvania, where F&M is located, and
had spent days fishing nearby streams, but
landscape evolution was not his focus. A spe-
cialist in the chemistry of volcanic rocks, he
began his career dating the terrain surround-
ing the skeleton known as Lucy, the famed
human ancestor discovered in Ethiopia. Still,
one look at the student’s photo was enough
to persuade him that the layers of sediment
it showed had been deposited in still—not
moving—water. “There has to be a dam
there,” he said. There’s only one way to get
that kind of deposit, Merritts adds. “A lake.”
CURIOUS, the next day the two research-
ers journeyed to the Little Conestoga.
Sure enough, just downstream from the
towering bank of finely laminated mud
they found the remains of a colonial-era
milldam. That’s when Walter made a leap.
“These are everywhere,” he said. “I bet all
these streams come from these old dams.”
Merritts was doubtful. “I thought it was
kind of crazy that you could [make that]
leap from one outcrop,” she recalls. But
subsequent trips to Lancaster’s historical
society, along with reviews of other records,
confirmed the dams had, indeed, been seem-
ingly everywhere. On some rivers, settlers
had built one every few kilometers. “It was,”
Merritt says, “just astonishing.”
It was also disconcerting. The ubiqui-
tous dams could mean many of the riv-
ers that Leopold and Wolman had used to
draw their conclusions had this unrecog-
nized backstory, and so sat atop far moreanthropogenic sediment than realized. It
suggested that efforts to restore streams to
meandering, high-banked single channels
were misguided. And it implied that mas-
sive blankets of stored sediment could be
a major source of nutrient pollution that
would run downriver for decades to come.
The duo spent the next several years
building its case, driving to dam sites and
documenting and dating sediments. The col-
laboration also became a courtship, as the
two scientists found they made both a scien-
tific and personal match. They were married
next to an old mill in 2004.
Together, Merritts and Walter make a for-midable team, say those who know them.
Merritts is meticulous, Wohl says, “just
thorough and detailed and comprehensive.”
Walter is more of a provocateur and disci-
pline jumper. Their qualities are comple-
mentary, says Kathy Boomer, a river scientist
at the Foundation for Food and Agriculture
Research. “They’re the most collaborative
and open-minded scientists I know.”
In January 2008, Merritts and Walter un-
veiled their ideas in Science. “The modern,
incised, meandering stream is an artifact of
the rise and fall of mid-Atlantic streams in
response to human manipulation of stream
valleys for water power,” they wrote. Ulti-
mately, they concluded, the findings “imply
the need to reconsider current procedures
for stream restoration” that rest on “the
assumption that eroding channel banks
are natural and replenishable.” The paper
quickly became the most influential of their
careers, with some 750 citations.NOT ALL THE ATTENTION was positive. “What
surprised me was the resistance they met,”
Wohl says. “People really had a hard time ac-
cepting this.”
In critiques later published in Science
and elsewhere, some researchers faulted
Walter and Merritts for implying that their
findings, based largely on rivers in eastern
Pennsylvania, where colonial mill dams were
common, could be applied widely through-
out the eastern United States. “I thought
the conclusions far exceeded the evidence,”
Trimble recalls. Other research, he and oth-
ers noted, had found that legacy sediments
had piled up even along river reaches that
didn’t have dams. But until the couple’s pa-
per came out, those studies had failed to gain
broad traction.
Other scientists were irked by the sug-
gestion that Leopold and Wolman’s iconic
theoretical framework was flawed. “To say
channel morphology is dependent on his-
toric milldams is incorrect,” says Martin
Doyle, a river ecologist at Duke University.
“The classic understanding of how rivers
work is still true.”
The real-world implications raised the
stakes. River restoration specialists risked
wasting heaps of cash on projects that might
be quickly undone if floods pushed piles of
old sediment into newly carved streams.
State and federal agencies had to decide how
to account for legacy sediments as they set
water quality guidelines and environmental
cleanup goals. And efforts to curb the sup-
ply of silt washing into the Chesapeake Bay
might have to contend with far more of it
than planners had counted on. “This is the
900-pound gorilla for how we restore our
streams,” says Gregory Noe, a USGS ecologist
who studies mid-Atlantic streams. PHOTO: SAMUEL FEIBELDorothy Merritts and Robert Walter put
their research on rivers to a real-world
test by helping restore Big Spring Run.900 21 AUGUST 2020 • VOL 369 ISSUE 6506
Published by AAAS