How many fish swim 13 miles in from the Atlantic, get to the edge of Horn Pond, and never make it in?
The Mystic River Watershed Association has a 2025 answer. According to the Massachusetts Division of Marine Fisheries estimate it reports, more than 815,000 river herring passed through the fish ladder at the Mystic Lakes Dam in Medford in 2025. About 6,900 of them made it up the spillway into Horn Pond. Volunteers counted hundreds more crowded in the pools at the bottom of Scalley Dam, unable to reach the spawning habitat they came for.
If you've walked past the Scalley Dam end of the pond since late August, you've seen the construction equipment. It's there to fix that gap. A record run has been stopping at Woburn's doorstep, and the project now underway is designed to bring it the rest of the way into the pond, along with a new public spot for watching it happen.
A run that grew four times over, then stopped at one dam
Horn Pond's place in this story comes from what happened downstream first. MyRWA volunteers started monitoring at the Mystic Lakes Dam in 2012, and the group estimates that first year's run at about 200,000 herring. Two dams got fish passages, and the numbers followed.
| Barrier on the route | What changed | What it opened |
|---|---|---|
| Mystic Lakes Dam, Medford | Fish ladder built in 2012 | Run jumped in 2015 as new breeding habitat came into use |
| Center Falls Dam | Fish passage built in the decade after Mystic Lakes | With Mystic Lakes, reconnected more than 200 acres of spawning habitat |
| Scalley Dam, Woburn | Pool-and-weir passage under construction since August 2026 | 100+ acres of spawning habitat in Horn Pond |
MyRWA's results page puts the run at about 640,000 fish in 2024 and about 815,000 in 2025. The group says 2025 was the highest estimate since monitoring began, and for the second year in a row the largest herring migration recorded in the state. The Division of Marine Fisheries biologist Joseph Holbeche told MyRWA for that report:
"The story of the recovery for River herring in the Mystic River is one of the greatest restoration success stories for Massachusetts in the last 20 years."
So the fish make it from Boston Harbor all the way up to Scalley Dam. That's where the recovery has stalled. In 2025, roughly one fish in a hundred that cleared the Mystic Lakes Dam got into Horn Pond.
Why a pile of rocks has been doing the work of a fish ladder
Right now herring use the rocky Scalley Dam spillway to get into Horn Pond. MyRWA explains that how well fish can work their way through it depends heavily on water levels, and that Division of Marine Fisheries staff often have to adjust the rocks to keep the route usable. MyRWA's report includes Weston & Sampson photos of the same spillway in wet conditions and in dry ones, when it has no water running over it at all.
The water-level problem is visible this month. On October 9, 2026, the City of Woburn declared a Level 2 significant drought and asked residents and businesses to cut water use and follow the restrictions in place. Whether a herring gets into the pond in a given year shouldn't hinge on rainfall and somebody moving rocks by hand. That's the case for an engineered passage.
What's actually going in at Scalley Dam
The City's project page describes the new passage as a pool-and-weir system, a series of pools that lets herring climb from the base of Scalley Dam into Horn Pond a step at a time. Three planned features matter most to someone walking the pond:
- An underwater viewing window, which the City calls state-of-the-art, where people will be able to watch herring finish the trip in real time
- An ADA-accessible pathway to that window, plus new educational signs
- More viewpoints along the passage itself
These are planned features of a project still under construction. None of them is open yet.
The City of Woburn, MyRWA, the Division of Marine Fisheries and herring monitor volunteers held the official groundbreaking on Wednesday, August 19. The City said construction would start that month, with a goal of finishing before the river herring migration begins in spring 2027. Mayor Michael Concannon, City Engineer Alex Rozycki, David Elmer of Weston & Sampson, Joseph Holbeche of the Division of Marine Fisheries, and representatives of the Natural Resource Damages Trustee Council and the Woburn Conservation Commission were in the ceremony photo, along with Ward Three Councilor Jeffrey R. Dillon and Ward 6 Councilor Lou Dimambro.
Funding came from several sources. They include settlement funds from the Industri-Plex Natural Resource Damages Trustee Council, a North American Wetlands Conservation Act grant, and a $1 million gift from an anonymous foundation to MyRWA. In August the state added $147,785 through its Municipal and Tribal Technical Assistance program. As the Woburn Daily Times Chronicle reported, that money pays for construction observation by Weston & Sampson.
Why the parking lot is part of the same project
The work by the Lake Avenue lot is connected to the fish passage. The City expects more people to visit Horn Pond once the passage opens, and its plan adds about 30 parking spaces, more handicap spaces, new sidewalks and ADA ramps. The plan also includes a rain garden. Curb cuts along the north edge of the lot will send runoff into a sediment forebay and a shallow planted basin, where the water filters through before reaching Horn Pond, so the expanded lot doesn't push more sediment and phosphorus into the pond.
The City's Public Works notice said the Lake Avenue lot expansion would start September 18 and run for up to three weeks, with fewer spaces available while the work was underway. The project schedule from March listed lot paving and the rain garden for fall 2026. If you drive to the pond for a weekend walk this fall, expect the lot to look like a work zone for a while.
What spring 2027 could look like from the pond's edge
The Division of Marine Fisheries estimates that with the new passage, the Mystic River herring run could top one million fish. MyRWA Executive Director Patrick Herron described the project in the City's groundbreaking announcement as helping herring "clear this final barrier to their ancestral freshwater spawning habitat." Mayor Concannon said in the same release that kids and families from Woburn and beyond would be able to watch the herring migrate.
You don't have to wait for the viewing window to get involved. Volunteer counts at Horn Pond are already part of the run's record. Here's how MyRWA's herring monitoring program works:
- March: MyRWA recruits volunteer monitors.
- April through June: Volunteers do weekly 10-minute counts in person at Horn Pond and the Mystic Lakes.
- Any time during the run: People can count fish online using MyRWA's fish camera.
MyRWA hasn't published a 2026 run estimate yet, so the 2025 figure is the latest. That makes this spring the clearest comparison. If the passage is finished on schedule, the volunteers at Scalley Dam will be counting fish going into Horn Pond through the new pools for the first time, and the 6,900 from 2025 becomes the number to measure against.
If you've been walking past the Scalley Dam construction and wondering how all this work by the water fits into your own home's story, the team at Kim Covino & Co follows changes like this across Woburn and the towns around it. Request a complimentary home strategy, and we'll show you how what's happening near your street figures into it.
