In the 1950s, scientists believed high-banked rivers
throughout the eastern United States,
a signature of the landscape,
resulted from repeated floods, which piled sediment
onto the sides of rivers.
Decades later, researchers Dorothy Merritts and Robert
Walter, presented a different theory for how these
steep-banks came to be—human activity.
In 2008, they published a paper in Science,
suggesting that when Europeans arrived in
the U.S. 500 years ago, rivers on the east coast were
marshier and had far lower banks.
To this day, their findings call into question
the best way to spend the millions of dollars
funneled toward improving the rivers every year.
Merritts and Walters' study began in the Little Conestoga Creek,
when a student showed Merritts a photo of 6-meter high walls of sediment.
Merritts doubted that floodwaters could have
deposited enough silt to make a bank that tall.
Her colleague, Robert Walter, agreed—
and suggested that they might have formed from
still, not moving, water.
It didn’t take long for them to find the cause of the
standing water—a dam downstream from the site.
These milldams were used to power flour and textile mills.
After digging through historical maps
and conducting field work,
they found milldams were abundant throughout Pennsylvania.
Further research also showed they cropped
up in other parts of the eastern United States.
Merritts and Walter concluded the thick banks
were made up of “legacy sediment,”
soil that eroded from the intense land transformation
of colonial-era logging and farming.
They were concerned that these large packages of sediment, still laden with pollutants,
could negatively impact water quality as it
eroded into the river channel.
When too many nutrients, like phosphorus and
nitrogen, end up in waterways, it can upset
the delicate balance of ecosystems by causing
massive algal blooms that gobble up oxygen.
They believed restoration efforts that didn’t
address legacy sediment could run into problems
with pollution down the road.
Not all researchers agreed with their conclusions.
Some pointed to areas without dams with lots of legacy sediment,
while others argued that the old 1950s framework still stood true.
Amidst all the debate, state and federal agencies
weren’t sure how to account for legacy sediments
in future projects.
The duo decided to apply their findings on a smaller scale—
a tributary of the Little Conestoga Creek,
which borders the sprawling farm fields of Lancaster, Pennsylvania.
In 2011, over 22,000 tons of legacy sediment
were removed from the banks of Big Spring Run,
transforming the steep, meandering river
to a braided, gently-sloping system.
Today, Big Spring Run is almost unrecognizable—
wetlands have emerged, and native species of animals
and plants flourish.
The wetlands soak in floodwaters,
while the vegetation helps hold sediment in place,
preventing large amounts from moving downriver.
A report published this year shows just how
effective this rehabilitation was:
85% less sediment and 79% less phosphorus
make their way downstream.
Removing thousands of tons of sediment is costly up
front—about $1150 per 1 meter of stream length.
But, it’s far more effective at reducing
sediment and nutrient pollution than other,
more well-known management practices,
like planting vegetation on the banks of rivers
to filter sediment before it enters the water.
Its success has spurred similar projects,
and more are underway.
As the Chesapeake Bay, the estuary that Big Spring Run
feeds into, grapples with how to reduce incoming
nutrient and sediment loads, the contributions of legacy
sediment are still unsettled.
Continuing to understand how humans have
transformed landscapes will be vital to ensuring that
the money and time invested in re-sculpting
rivers is ultimately helping, not harming,
the environment.
