Patapsco Dam Removal Brings Herring Back Upstream
Biologists found river herring above the former Bloede Dam near Baltimore for the first time in decades after its 2018 removal.
Updated on Nov. 3, 2026 in Environmental
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NOAA and partners removed the Bloede Dam on the Patapsco River in 2018, ending a barrier that had blocked fish passage for over 100 years. Within three years, environmental DNA detections of herring upstream rose sharply, and biologists recorded two adult river herring above the former dam in 2021.
Why it matters
Dams and culverts block migrating fish from reaching their spawning habitat, and removing this structure eliminated a long-standing obstacle for Baltimore-area waterways. The project also removed a significant public safety threat posed by the aging dam.
Water sampling detected alewife eDNA upstream in 5 percent of samples and blueback herring eDNA in 13 percent within three years of removal, up from zero before. Whether these detections translate into a lasting population increase remains unknown.
The players
NOAA
The National Oceanic and Atmospheric Administration is a U.S. federal agency that led the Bloede Dam removal project.
U.S. Fish and Wildlife Service
A federal agency within the Department of the Interior that co-funded the Bloede Dam removal and monitoring.
National Fish and Wildlife Foundation
A congressionally chartered nonprofit that provided funding for the Bloede Dam removal and monitoring project.
The details
Scientists from the Smithsonian Environmental Research Center and the University of Maryland Center for Environmental Science used environmental DNA sampling to detect recent herring presence at the site. The removal and monitoring project cost more than $9.3 million and was funded by the National Fish and Wildlife Foundation and the U.S. Fish and Wildlife Service.
Timeline
Over 100 years: The Bloede Dam blocked fish passage on the Patapsco River.
2018: NOAA and partners removed the Bloede Dam.
Within three years after removal: Herring eDNA detections upstream increased from zero.
2021: Biologists recorded two adult river herring upstream of the former dam.
Roadmap
The Bloede Dam removal is one of many projects under NOAA's fish passage restoration program, which has reopened more than 4,400 miles of streams. The Patapsco result adds evidence on how quickly migratory herring respond to barrier removal in the Chesapeake Bay region.
Restored fish passage can rebuild river herring runs that feed larger fish and wildlife in the Chesapeake Bay. Removing aging dams also eliminates a public safety hazard along the river.
The takeaway
Removing an obsolete dam can reconnect fish to habitat they had lost for generations, though measuring the full benefit requires years of monitoring. Residents interested in river health can follow ongoing fish passage work through local watershed groups.
Further reading
Explore more on Environmental coverage for the broader context of river restoration efforts.
Source note: This article includes information reported by Hindustan Times.
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