The Bronx River — A Story of Return

01/12The Problem

What a dam looks like to a fish.

The first thing I had to figure out was what problem anyone was actually trying to solve. It turns out the problem is four hundred years old: since the 1600s, dams built for mills and waterworks have severed the Bronx River's spawning highways. Until very recently, any herring that swam up from the Atlantic hit a wall at the first downstream dam and went no further.

River herring — alewife and blueback herring — live most of their lives in the Atlantic, from Newfoundland to North Carolina.⁴ Each spring, they return to the freshwater streams where they were born, to spawn. It is one of the great rhythms of the Atlantic seaboard, and it is in trouble.

Populations along the Northeast coast have collapsed from their historic highs. Migration barriers are a major reason why.⁴ Every blocked stream is a closed door — a freshwater nursery cut off from the ocean that feeds it.

The Bronx River once carried that migration. Its lower eight miles are now a designated National Water Trail. But three dams stand in the way: the East 182nd Street Dam, the Bronx Zoo Double Dam, and the Stone Mill Dam.³ Each was built to power industry or shape the landscape of the great northeastern parks. Each blocks the fish.

Removing these blockages does more than help one species. River herring are prey for striped bass, bluefish, tuna, ospreys, herons, seals, and dolphins.⁴ Their migration moves ocean nutrients upstream and freshwater energy back down. Restore the herring, and you restore a chain that stretches from a Bronx mill pond to the open sea.

0 dams

Block upstream fish migration on the lower Bronx River — within NYC Parks' jurisdiction.³

0 acres

Of freshwater spawning habitat opened by the first fish passage, built in 2014.²

0 feet

The height of the East 182nd Street Dam — first built to power a mill, renovated with boulders in the 1910s to look like a waterfall.²

0 → 0 fish

Fish per year passing at the 182nd St. fishway, from its opening in 2015 to its peak count in 2018.⁸

03/12The Three Barriers

The dams are short. The decisions are not.

One thing that struck me reading about these dams: they don't look like the engineering problem they are. They look like small waterfalls. That's part of what makes this project hard — the barriers are, for many park-goers, beloved landmarks. Each sits in a different landscape (a public park, a world-famous zoo, a botanical garden), and each demands a different solution.

Fishway completed · 2014

East 182nd Street Dam

A 17.7-foot stone masonry dam, originally built to power a mill and transformed in the 1910s with boulders to mimic a waterfall. It was the first downstream barrier on the river — the wall at which alewife had been stopping for four centuries.

Between 2010 and 2014, NYC Parks and Milone & MacBroom designed and installed an Alaskan SteepPass fishway: a series of trough-like aluminum ramps with baffles that slow the water enough for fish to swim up. The whole thing was encased in reinforced concrete — both to resist vandalism and to double as a canoe portage.²

Construction was not tidy. A five-inch rainstorm in May 2014 sent 1,700 cubic feet per second through the site and breached a cofferdam the day after concrete work finished. When the upstream side was dewatered, the crew discovered the dam's old low-level intake didn't match the 1958 plans at all.² Designs were revised in meetings with state dam-safety engineers. The fishway opened to water in July 2014 and to fish in March 2015.²

Dam height

17.7 ft

Habitat opened

~12 acres

Construction cost

$1.39M

Design

Alaskan SteepPass

Learn more: NYC Parks Anadromous Fish Restoration · 2015 Final Report (PDF) · Fishway opening press release

Feasibility study · 2021

Bronx Zoo Double Dam

The second barrier sits on the grounds of the Bronx Zoo, split into two spillways around a central island. The original dam appears on an 1896 map; a western spillway was added by 1901. The structure is now showing its age: erosion on the central island in 2020 raised the real possibility of an uncontrolled breach and sediment release.

Unlike 182nd Street, the team is seriously considering dam removal rather than another fishway. A partial removal concept would demolish the western earthen section, leaving the eastern spillway intact as a historic marker and cutting a new, naturalized flowpath through the impoundment.

Beginning in March 2022, NYC Parks partnered with the U.S. Army Corps of Engineers' Hudson-Raritan Estuary project to evaluate the alternatives in full. The dam is already undersized for 50- and 100-year flood events, and climate change is making those events more frequent.

Habitat gained

+0.6 mi

Fishway est.

$4–6M

Removal est.

$10–13M

Flood capacity

Undersized

Learn more: NYC Parks Capital Project Tracker · Bronx Zoo (WCS) · Bronx River Alliance: Restoring the Ecology

Future project

Stone Mill Dam

The third and furthest-upstream dam sits on the grounds of the New York Botanical Garden. Beyond it lie roughly seven more miles of the Bronx River flowing through Westchester — a significant expansion of potential spawning habitat.

Designs for a fishway here were completed and approved in 2014 alongside those for the Double Dam. But construction at Stone Mill is being sequenced deliberately: it will wait until fish migration is successfully observed at the Double Dam, which in turn depends on success at 182nd Street. Each step up the river has to prove itself before the next one is built.

NYC Parks is now engaging with the Botanical Garden to consider dam removal at Stone Mill as well — an option that would restore seven river miles of connectivity for a wide range of species, not just river herring.

Habitat gained

~7 miles

Owner

NYBG

Designs

Completed 2014

Start

Sequenced

Learn more: NYBG Forest Research · New York Botanical Garden · About the Bronx River

04/12The Geography

The lower 1.2 miles, where everything happens.

The lower Bronx River runs roughly three miles from where it enters Bronx Park down to the East River. Along that short stretch, three dams stand between migrating fish and their spawning grounds upstream.

The lower river

The three barriers are clustered within about 1.2 river miles of each other, all within city parkland. The southernmost dam, at East 182nd Street, now has a functioning fishway. The two upstream dams — Bronx Zoo Double Dam and Stone Mill — remain barriers, though both are under active restoration planning as of 2022.

Opening passage at all three would restore approximately seven miles of upstream spawning habitat, reaching well into Westchester County.

East 182nd Street Dam

DOWNSTREAM

Fishway opened 2015. Alaskan SteepPass design. First river herring passed in April 2017 — the first in 400 years.

The schematic is a simplification. Here's the actual river.

The map above is a diagram — clean edges, labeled barriers, strategic omissions. Below is the same 1.2-mile stretch as Google's satellite sees it: the tree canopy of Bronx Park, the Bronx Zoo's footprint, the New York Botanical Garden's forest. The three dams disappear into the landscape. That is, in some sense, the point — they look like waterfalls, not walls.

Google Maps satellite imagery, centered near the East 182nd Street Dam (40.848°N, 73.879°W). Zoom in to see the fishway boulders at 182nd Street, the two spillways of the Double Dam, and the Stone Mill waterfall at NYBG. Open in Google Maps for full interactivity, or compare with NYC Parks' own Bronx River Wetlands project map.

05/12The Why

This is an argument about a whole coastline.

For a while I assumed the argument for restoring a fishway was self-evident. But the more I read, the clearer it became that this project is doing two kinds of work at once. First, it's reconnecting a single urban watershed to a migration corridor that stretches from Newfoundland to North Carolina — which matters for dozens of species, a thousand miles of coastline, and an entire ocean food web. Second, it's restoring a river that runs through one of the most park-starved, environmental-justice-burdened boroughs in New York. Both cases are worth making. Here they are.

A migration that crosses borders.

River herring are diadromous — they use both marine and freshwater habitats during their life cycle. Adults spend most of the year at sea, primarily in the cold, productive waters of the Gulf of Maine and the Bay of Fundy off the Canadian coast. Each spring, they return to the rivers where they were born to spawn.⁴

The Bronx River is one node in a migration corridor that spans from Newfoundland to North Carolina. A fishway built in the Bronx connects a single local watershed back to that entire Atlantic network.

American eels run in the opposite direction — catadromous. Adults in the Bronx River, some of them decades old, travel to the Sargasso Sea (east of Florida, off Bermuda) to spawn and die. Their tiny "glass eel" offspring drift back on Gulf Stream currents, climbing the eel ladder at 182nd St. to begin their freshwater lives.

Anadromous — river herring spring spawning migration, from Bay of Fundy / Gulf of Maine rearing grounds back to natal rivers

Catadromous — American eel adult spawning migration, freshwater to the Sargasso Sea, thousands of miles

Bronx River — one of hundreds of natal rivers along the Atlantic coast

Key prey in streams & wetlands.

Across their freshwater lives — as spawning adults, eggs, fry, and juveniles — river herring and young eels feed the birds and mammals that define healthy riparian ecosystems.⁴

Riparian predators Birds and mammals of the banks, wetlands, and inshore waters

Osprey

Pandion haliaetus

Riverine raptor

Great Blue Heron

Ardea herodias

Wading bird

River Otter

Lontra canadensis

Semi-aquatic mammal

Harbor Seal

Phoca vitulina

Estuarine mammal

Fuel for the open ocean.

At sea, adult river herring are forage fish — a link between plankton and the large predators that drive entire fisheries. Their collapse along the Northeast coast is felt across the food web.⁴

Estuarine + pelagic predators Fish and marine mammals whose diets depend on forage species like river herring

White Perch

Morone americana

Estuarine fish

Striped Bass

Morone saxatilis

Inshore piscivore

Bluefish

Pomatomus saltatrix

Pelagic predator

Atlantic Tuna

Thunnus thynnus

Open-ocean predator

Bottlenose Dolphin

Tursiops truncatus

Marine mammal

The second case

The restoration is also for the Bronx.

This is the part that doesn't always show up in fish-counter data. The Bronx has historically been one of the most park-starved boroughs in New York, and the Bronx River corridor runs through neighborhoods that have long been on the receiving end of the city's environmental exports — highways, power plants, sewage overflows, a century of industrial discharge into the water itself.⁴

Restoring the river isn't a side benefit of the fishway; for many of the project's partners, it's the point. The 2014 construction included an improved canoe portage alongside the fishway — because the Bronx River is also part of a designated National Water Trail that local residents paddle. The Greenway now connects previously disconnected neighborhoods along its banks. The Bronx River Alliance runs school programs, volunteer stewardship days, and community science projects. Over ten years, one volunteer told the American Museum of Natural History, they've "pulled out literally tons of car parts."

The volunteer hours, the canoe access, the stewardship programs — those aren't side benefits. They're how an urban restoration actually holds together over twenty years of uneven funding, political cycles, and changing staff.

"Restoring our ecological resources is as important in The Bronx as it is anywhere else around the country."

Ruben Diaz Jr. · Former Bronx Borough President, 2015

Residents of the Bronx — about 1 in 6 New Yorkers, more than the entire population of San Diego.

0 mi

NYC's only freshwater river, flowing through 10+ neighborhoods from Westchester to the East River.

Volunteer hours on knotweed removal alone, 2010–12 — roughly one full-time staffer for a year, donated.⁵

“

Restore the herring, and you restore a chain that stretches from a Bronx mill pond to the open sea.

A fishway at 182nd Street is not, in the end, a local intervention. It is a regional one — reconnecting a single New York watershed to a migration corridor that reaches from Newfoundland to North Carolina, from the Gulf of Maine to the Sargasso Sea. The fish that climb the ladder are eaten by striped bass in the harbor, ospreys in the park, bluefish off Long Island, tuna in the Atlantic. The health of one urban river influences thousands of miles of coastline.

This is the argument for restoration.

06/12The Work, by Year

Permits, storms, cofferdam breaches, fish counters.

I had assumed restoration projects moved faster than this one did. Fifteen years, from the first alewife stocking in 2006 to fishway monitoring results in 2021, is the answer to "what does urban ecological restoration actually look like, start to finish?"

2002

NOAA asks the question

The National Oceanic and Atmospheric Administration suggests that NYC Parks consider fish passage on the Bronx River dams. This was new territory for the department.

2003–04

The feasibility study

NRG and Lehman College study whether a herring run could exist in the river at all. Water quality, flow, and channel conditions all check out — the obstacle is the dams themselves.

2006–07

First stocking

Partnering with Connecticut DEEP, adult alewife are transplanted upstream of the 182nd Street Dam. Juveniles appear the following summer — proof that the river can rear them.

2009–10

Fish at the wall

Monitoring at the base of the 182nd Street Dam confirms adult alewife are swimming up to the barrier each spring, waiting for a way around.

2010–12

Design, permit, permit, permit

Designs advance from 50% through final. Permits come in from the NYSDEC, the USACE, the NYC Public Design Commission, and the State Historic Preservation Office. Tectonic Engineering studies the retaining walls.

2014

Construction — and a cofferdam breach

Trocom Construction breaks ground. In May, a five-inch storm drops 1,700 cfs into the river and breaches the cofferdam, one day after concrete pours finish. In July, water flows through the fishway for the first time. In December, the contract closes.

2015

The fishway opens

The River Park Fish Passage goes live for the migration season. 143 eels climb the accompanying eel ladder in year one.

2017

First herring pass

On April 16, the first documented alewife swims up the fishway — 19 of them in all that year. Stocking of adult herring resumes to build a self-sustaining run.

2018

Peak use

267 fish pass the ladder. 544 eels climb the eel ladder. The highest counts on record — so far.

2021

A troubling dip

Only 8 river herring counted at the fishway. Annual variation is expected, but the drop is sharp — possibly tied to climate, ocean conditions, or bycatch in the commercial fishery.

2022

Partnering on dam removal

NYC Parks partners with the U.S. Army Corps of Engineers' Hudson-Raritan Estuary project to evaluate engineered fishway vs. partial dam removal at the Bronx Zoo Double Dam. Target construction: 2025–27.

08/12The Debate

The next dam is the hard one.

When NYC Parks' team turned to the next dam upstream, a bigger question surfaced: is the goal simply to get fish past a wall, or to give the river its own shape back? Their 2021 alternatives analysis compared the two paths.⁸ The summary comparison is below; the full scoring matrix (environmental, social, cost) is in the appendix.

Engineered Fishway

Keep the dam · add a ladder

Cost

$4–6M

Construction

12 months

Lifespan

~25 years

Trees removed

Additional habitat favors alewife spawning
Shorter, lower-cost construction
No change to wetlands, water quality, or aesthetics
Passage for target species only; fish must find entrance
Requires annual monitoring and repair
Dam still stressed by more intense climate-driven storms
Little to no improvement for non-fish wildlife

Recommended

Partial Dam Removal

Cut a new channel · keep the east spillway as heritage

Cost

$10–13M

Construction

12–18 months

Lifespan

Permanent

Trees removed

46 (replanted)

Connectivity for a full suite of species — eels, turtles, frogs, waterbirds
Restores natural stream processes, sediment transport, aeration
Resilient to climate-driven flood events
Eliminates failure risk and uncontrolled sediment release
Preserves eastern spillway as historic resource; new cultural interpretation
Improved recreational canoe access
Higher upfront cost and short-term disturbance

"In the future, review of fish passage design alternatives on the Bronx River should include a thorough review of the dam removal alternative."

— NYC Parks, 2015 Final Report on Fish Passage Construction²

09/12The Banks

The plant that's taking over the banks.

The second half of the story is about the banks themselves. A healthy migration needs a healthy riparian zone — shade, leaf litter, insects, root systems that hold the bank together. Along most of the Bronx River, that zone is dominated by a single plant — one that outcompetes just about everything else that used to grow there.

Japanese knotweed (Reynoutria japonica) was introduced to the United States from Japan in the 1870s as an ornamental plant, prized at the time for its quick growth and bamboo-like stems.⁵ A century and a half later, along the Bronx River, it has become the dominant plant on the riverbanks. Its stems grow over five meters tall. Its rhizomes reach ten meters long and plunge a meter deep. A buried fragment can resprout an entire colony. On the Bronx River it has hybridized with a relative (Reynoutria × bohemica) to form dense stands that leave little room, light, or soil for the native plants that used to hold these banks together.¹

Because herbicide use is tightly restricted on public land and around volunteer labor, NYC Parks and its partners tested two mechanical removal strategies between 2010 and 2012 in 120 plots across the Bronx River Forest and the New York Botanical Garden.⁵

The finding, published in Ecological Restoration in 2017, mattered for how cities manage difficult plants on a shoestring: repeated cutting alone worked about as well as the much more laborious rhizome removal.⁵ Digging out roots — painful, slow, hours of crew time — did not buy significantly better control of knotweed cover or density than cutting stems three times a season.

The experiment, briefly

Cut

All above-ground stems cut to the ground in May, July, and September with pruners and loppers. Applied each year 2010–2012.

Remove

Cut once in May, then dig out all roots and rhizomes to at least 15 cm deep in July and September. Same three-year cycle.

Both treatments reduced knotweed height by at least 60% compared to controls. Neither eradicated it. Pin oak (Quercus palustris) saplings grew in both treatment types. The native herbaceous layer mostly did not recover — the surviving species were disturbance-adapted colonizers, not the forest-floor community the project hoped to restore.

2,100+

Volunteer hours

devoted to mechanical knotweed management on the Bronx River between 2010 and 2012, supplementing 530 hours of paid staff time.⁵

Why this matters beyond the Bronx

Urban natural resource managers operate with limited budgets and rely heavily on volunteer labor. A study showing that the simpler, accessible method performs as well as the exhausting one has direct budget implications for parks departments across the country facing knotweed invasions.

But the corollary is also honest: on a three-year timescale, neither method eradicated knotweed or restored the native herbaceous layer. The research's own authors note that long-term impacts are still unknown.

Learn more: Haight et al. 2017 (Ecological Restoration) · NYBG Forest Program Research · NYC Parks Invasive Species Removal

11/12Appendix

The full scoring matrix, for reference.

The summary comparison in §08 The Debate is a distillation. Behind it sits NYC Parks' full 2021 alternatives analysis: three scoring tables covering environmental effects, social and experiential effects, and cost and constructability. If you arrived here directly, you may want to read §08 first — these numbers are arguing about something. Short-term effects refer to the construction phase; long-term effects refer to post-restoration conditions.

The full alternatives analysis

The above summary simplifies a much more detailed comparison. NYC Parks' 2021 analysis scored each alternative across four categories — ecological factors, construction & resiliency, recreation & aesthetics, and wildlife impact. Here are the full tables.

I · Ecological Factors

Ecosystem Component	Engineered Fishway	Score	Partial Dam Removal	Score
River Herring Spawning Habitat	Additional habitat favors alewife spawning Juveniles subject to predation	+	Additional habitat favors blueback herring spawning but still allows alewife Less predation risk	+
Upstream Fish Passage Efficiency	Passage for target species only Fish must locate entrance Potential migratory delay	+	Passage for more species Entire channel available for passage Less migratory delay	++
Other Wildlife	Little to no improvement	0	Improves habitat connectivity Restores natural stream processes for American eel, turtles, frogs, salamanders, and water birds	++
Wetlands	No change	0	Preserves wetlands at eastern spillway Increases extent and diversity Restoration with native species	+
Tree Canopy	10 trees removed (4 native) No change in long-term	− short 0 long	46 trees to be removed (32 native) Increase of trees in long-term with native tree planting along riparian corridor	−− short + long
Water Quality	No change	0	Increased flow velocity, less ponded water More aeration and cooler water temperature	+

II · Construction, Maintenance & Resiliency

Factor	Engineered Fishway	Score	Partial Dam Removal	Score
Construction Duration	12 months	+	12–18 months	−
Construction Cost	$4–6 million	+	$10–13 million	−
Construction Access	Site access from Jungle World Road Disruptions to Bison Loop for heavy equipment	−	Site access from overflow parking lot Heavy equipment crossing riverbank, channel, and islands; less impact to Bison exhibit	−
Dam Maintenance	Monitoring 2–3 days/week, April–June Routine inspections and future repair likely needed for aging dam	−	None for removed western spillway Inspections and repair needed for eastern spillway only	+
Lifespan	Estimated 25 years or less Contingent upon lifespan of the dam, which shows signs of deterioration	−	Permanent for stream channel	+
Public Safety & Liability	No change	0	Reduces liability of damage in event of failure	++
Resiliency	Stress on dam will increase with more frequent, high-intensity storms as a result of climate change	−	Dam removal and controlled release of sediment provides more resilient option	++

III · Recreation, Heritage & Aesthetics

Factor	Engineered Fishway	Score	Partial Dam Removal	Score
Recreational Access: Canoe Portage	No improvement in current design Could be added to fishway, but would increase maintenance and visibility Could be added to earthen dam, but would require dam repair	0	Can be designed to accommodate canoe passage or improve portage Opportunity to expand interpretive features	+
Historical Resources	No change	0	Retains a portion of the dam as historic resource Restores river to historic flowpath and offers opportunity for cultural interpretation	+
Aesthetics	No change	0	Impoundment would be replaced by a natural river channel Floating trash would no longer accumulate at spillway	+
Riparian Vegetation Management	Relatively small area of disturbance during construction No significant change from present	+ short 0 long	Larger construction area requires replanting and maintenance during native plant establishment Greater extent results in higher potential for long-term improvement No long-term change in management needed	− short + long

Scoring key: ++ strong positive, + positive, 0 neutral, − negative, −− strong negative. Some factors split into short-term (construction-phase) and long-term (post-restoration) scores. Source: NYC Parks, 2022 — "Bronx River Connectivity and Restoration," presentation to Columbia EEEB 4135.⁸

12/12Credits

The people who actually did the work.

This work has taken more than two decades and required a wide coalition of city, state, and federal agencies, research institutions, and community-based groups. The list below is partial — behind each institutional name are hundreds of individual staff, scientists, and volunteers who actually showed up with gloves, loppers, and canoes.

$500K

Bronx Borough President

$460.8K

NY State Dept. of State

$430K

WCS – NOAA

$150K

Nat'l Fish & Wildlife Fdn

$85K

Harbor Estuary Program

The Bronx River is one ladder shorter.

Twelve sections, ~25 minutes end to end.Skim the table of contents below, or jump in anywhere.

What a dam looks like to a fish.

Three species, two directions, one river.

Alewife

Blueback Herring

American Eel

The dams are short. The decisions are not.

East 182nd Street Dam

Bronx Zoo Double Dam

Stone Mill Dam

The lower 1.2 miles, where everything happens.

The lower river

The schematic is a simplification. Here's the actual river.

This is an argument about a whole coastline.

A migration that crosses borders.

Key prey in streams & wetlands.

Osprey

Great Blue Heron

River Otter

Harbor Seal

Fuel for the open ocean.

White Perch

Striped Bass

Bluefish

Atlantic Tuna

Bottlenose Dolphin

The restoration is also for the Bronx.

Permits, storms, cofferdam breaches, fish counters.

NOAA asks the question

The feasibility study

First stocking

Fish at the wall

Design, permit, permit, permit

Construction — and a cofferdam breach

The fishway opens

First herring pass

Peak use

A troubling dip

Partnering on dam removal

The fish came. Then they didn't.

Cumulative counts, 2015–20218

The next dam is the hard one.

Engineered Fishway

Partial Dam Removal

The plant that's taking over the banks.

The experiment, briefly

Cut

Remove

Why this matters beyond the Bronx

Sources, videos, places to keep reading.

The restoration, in motion.

Tools to keep looking.

iNaturalist — Bronx Park

Bronx River Alliance — Resources

NYC Parks Capital Tracker

2015 Final Report on Fish Passage

Haight et al. 2017

NYBG Forest Program

The full scoring matrix, for reference.

The full alternatives analysis

I · Ecological Factors

II · Construction, Maintenance & Resiliency

III · Recreation, Heritage & Aesthetics

The people who actually did the work.

Twelve sections, ~25 minutes end to end.
Skim the table of contents below, or jump in anywhere.

Cumulative counts, 2015–2021⁸