At first, the sight of even one or two juvenile American eels wriggling about in the holding tank atop the Daniels Dam near Ellicott City was encouraging, said William Harbold, a biologist with the Maryland Department of Natural Resources.
Although they were few and far between, their presence meant that the new eel ladder attached to the defunct hydroelectric dam was working. In each of the ladder’s first four years, an average of 28 young eels used it during their journey to feeding grounds up the Patapsco River.
But then came a shock to the system.
In 2018, a contractor demolished Bloede Dam — a more-than-century-old obsolete dam downstream of Daniels on the Patapsco, which separates Baltimore and Howard counties and becomes Baltimore’s harbor. Up swam the eels.
By 2020, 361 eels climbed the eel ladder. The next year, the number jumped past 3,000.
But the eel count for 2022 stunned even the most optimistic observers — 36,594.
“We started getting kind of excited about it. And then this year, it’s exploded,” Harbold said.
The removal of the Bloede Dam is likely to bolster other river species, too, including river herring. But none have responded more quickly — or more resoundingly — than the American eel.
North America’s only freshwater eel, the creatures spawn in the Atlantic Ocean’s Sargasso Sea. After they hatch, baby eels are carried by ocean currents to coastlines as far north as Greenland and as far south as Venezuela. From there, the young eels make their way up estuaries and rivers to live and grow, before they eventually make their own one-time pilgrimage to spawn in the Sargasso Sea and perish.
Given that the eels are so well-traveled, any number of factors could be contributing to their resurgence in the Patapsco, including the whims of ocean currents, Harbold said. But one thing is clear. With the Bloede Dam gone, the eels have taken the river back.
Elsewhere in Maryland, eel numbers have also been strong in recent years. In 2021, the Conowingo Dam’s elevator on the Susquehanna River captured by far the most eels in its 14-year history — over 600,000, according to the U.S. Fish and Wildlife Service. But this year, the numbers have returned to normal levels, as the eel count at the Daniels Dam continued to surge.
The removal of Bloede Dam, coupled with the eel ladder, means the mysterious species has more real estate on the Patapsco, a rare success story in an age where development frequently swallows habitat — never to be reclaimed, said Jessie Thomas-Blate, director of river restoration for American Rivers, a nonprofit that pushed for the dam’s removal.
“The fact that all of these eels are actually able to make it farther upstream and access more habitat means that they can spread out, they can have better access to food,” she said, “which will likely make them hardier when they ultimately go back downstream to spawn.”
Improving fish passage in the rivers and streams flowing into the Chesapeake has been a rare bright spot for the Chesapeake Bay agreement, said Matt Ogburn, a marine ecologist for the Smithsonian Environmental Research Center. That agreement has made headlines recently as states failed to meet promised limits for the flow of harmful nutrients into the estuary by 2025.
In 2016, the bay states achieved a 2025 goal to open 1,000 stream miles to fish passage, so they pledged to open 132 additional miles every two years.
“It’s more than nutrient and sediment, although those are a big deal,” Ogburn said of the bay plan. “There are other parts that have been really successful, and this is an example.”
Determined to save money on the project, Harbold and his co-workers built the Daniels Dam eel ladder themselves. To form the ladder, which runs from the base of the dam to the top, they fastened together a number of metal boxes originally designed to house fiber optic cables. The bottom of each box is covered in mesh, and the baby eels (which look like squirming spaghetti noodles, Harbold said) climb up the structure themselves.
Once they reach the top, the eels land in a large bag inside a water-filled trough, which is covered by a plywood lid. The biologists then dump the eels upstream of the dam. But not before they’re counted, which became an arduous task once larger numbers of eels made the trip up the ladder.
“There was one day where we had 823 at one time, and I wound up counting all of those,” Harbold said. “And I was like: ‘No, this is not a good way to continue.’”
Starting at the end of the 2021 season, the biologists found themselves weighing the bag full of eels, and using the average weight of a young eel [less than 2 grams] to calculate how many were inside.
Sometimes, the biologists discovered unexpected visitors climbing the eel ladder, including a snapping turtle, a frog and even a juvenile rabbit.
Over the summer, checking the bag becomes extra challenging, Harbold said, for reasons wholly unrelated to its wormlike captives.
The dam, located in Patapsco Valley State Park, has become an incredibly popular spot for recreation, partially because there’s no fee to enter, unlike other areas of the park nearby. On the weekend, cars line the narrow Daniels Road beside the dam, and visitors routinely ignore the numerous signs warning against scaling the structure and jumping from the top, Harbold said.
“I would — in summertime — try at all costs to avoid going in the afternoon, because it would be challenging to find a place to park, there would be people following me past the fence to try to jump off the top of the dam,” Harbold said.
Defunct dams can pose a safety threat, adding volume to calls for their removal. The Bloede Dam, also located in the state park, was the site of several drowning deaths over the years.
The makeshift eel ladder on the Daniels is rusting away in a few places, but that hasn’t stopped the eels. And repairing it may not be worthwhile, since the Daniels Dam could meet the same fate as the Bloede soon.
The Department of Natural Resources is conducting a feasibility study on demolishing the dam, said spokesman Gregg Bortz.
The early 19th century dam was built to power the now-abandoned mill town of Daniels that once stood nearby.
Environmentalists say demolishing it could be a smart move. Though the dam is a popular attraction for visitors, including kayakers who frequent the still water upstream, its status as an ecological barrier means it ought to be removed. Granted, some questions remain, like what lies in the sediment trapped behind the dam.
For the Bloede Dam, scientists determined that sediment could be released passively after the demolition, Thomas-Blate said. But the situation could be different at the Daniels, partially because it lies upstream of Old Ellicott City, where historic floods have devastated the community in recent years, so officials are proceeding cautiously, she said.
The dam was last inspected by the state in 2019. The Maryland Department of the Environment determined the dam had a “low hazard potential,” but noted that a 2009 study had documented deterioration and recommended repairs that were never made. MDE called for a new structural study, but that hasn’t been commissioned yet because of the ongoing study focused on removing the dam, Bortz said.
Removing inoperative hydroelectric dams has become a popular strategy for river restoration groups. In November, U.S. regulators approved the largest dam removal project in the world, which would eliminate four dams on Oregon’s Klamath River, freeing the river’s flow for salmon.
There’s a great deal of research focused on this section of the Patapsco, including Ogburn’s work to monitor the presence of other species besides American eel. In particular, Ogburn is studying the response of two types of river herring to the dam removal: the blueback herring and the alewife.
Ogburn and his team have set up sonar cameras that can detect passing fish, and pulled water samples from the river to test for the fish’s DNA.
So far, the results have been encouraging, Ogburn said, but a herring population boom resulting from the Bloede Dam removal would take time.
That’s because the herring have a completely different life cycle than the eels. They are born in the river, but spend most of their lives in the ocean, before returning to their natal rivers to spawn every four to five years.
So, this year’s was the first crop of herring that could have been born upstream of the old Bloede Dam site, and therefore returned there to spawn.
“If the dam removal does have a really big effect on improving the population, I would expect it to take more in the five-to-10-year range before we really see that, once we start to get a couple of generations of fish coming back,” he said. “So, we’re just in the early stages.”
This article has been updated to reflect that a contractor demolished the Bloede Dam. An earlier version incorrectly identified who did the work. The Sun regrets the error.