WILMINGTON, Del. — When migratory fish follow their ancestral instinct to swim up Delaware’s Brandywine Creek during this spring’s spawning season, they will find, for the first time in more than 200 years, that their route is not blocked by a dam.
The fish — American shad, hickory shad and striped bass — have been unable to return to their traditional spawning grounds in the Pennsylvania section of the creek about 25 miles to the north since a series of dams was built across the creek by early American settlers, starting in the mid-18th century.
This year, the fish will be able to swim past the site of a dam that was demolished by the city of Wilmington last fall, allowing them to move as far as the next barrier, Dam 2, about three-quarters of a mile upstream, where large numbers are expected to create a sudden bonanza for anglers.
Beginning next month, “there will be thousands of American shad sitting here,” said Jerry Kauffman, a University of Delaware professor. “This area will be full of fishermen because it will be a big fish magnet. It’s going to be like Christmas.”
Dr. Kauffman, who leads the university’s Water Resources Center, is part of Brandywine Shad 2020, a nonprofit that hopes to remove or modify all 10 remaining dams on the 23-mile Delaware section of the creek over the next three years.
Dam 2 won’t immediately be removed because that project would be bigger and more expensive. Dr. Kauffman and his associates want to remove or modify four other dams this year, kicking off one of the nation’s biggest dam-removal programs across a single watershed.
“This one is probably the most dams that have been targeted on a single river,” said Laura Craig, director of river restoration at American Rivers, a conservationist group. “The main thing that distinguishes it is the watershed approach, looking at a set of dams at the same time for removal. This is going to be an example that others will continue to follow.”
Removing dams reconnects fish habitat, lowers water temperature, speeds water flow, increases dissolved oxygen — an important indicator of a river’s ability to support fish populations — and improves water quality for cities like Wilmington, Dr. Craig said.
The return of shad and other migratory fish to the creek will not only restore an ancient natural rhythm but will also nurture other wildlife like bald eagles that prey on the fish, while their decomposed bodies, post-spawning, add nutrients to the waterway.
Supporters of the project also argue that dam removal can reduce flooding upstream by allowing water to flow unimpeded rather than pooling behind a dam wall. The need for flood control is becoming more urgent with the bigger and more frequent storms that are forecast with climate change, they say.
“We would like to get this dam out by the next big storm,” Dr. Kauffman said of Dam 3, a 135-foot structure that is already damaged.
The dams, originally built to provide water power for many grist mills along the creek over the last two centuries, have long been obsolete, and some have already been partially demolished by storms and two centuries of water pressure. Three dams are National Historic Landmarks, and will remain in place, along with two other historic dams, after others have been partially or totally removed.
The landmark structures will be modified with different technologies that are designed to allow the anadromous fish — which spend most of their lives at sea but spawn in fresh water — to swim over or around them and continue their journey upstream.
The techniques include a bypass, a gradual slope that allows the fish to swim around one end of a dam; a “rock ramp,” which builds up the full width of the creek bed on the downstream side of a dam so that fish can swim over it; and a fish “ladder,” a concrete channel that’s designed for fish to avoid the dam and reach the upstream side.
The modifications can enable 50 to 80 percent of fish to get past the dams, Dr. Kauffman said. In 2015, a University of Delaware study calculated that removal or modification of all the dams would produce some 26,000 shad — whose annual upstream “run” is prized in the Delaware Valley for its abundance.
Other dam-removal projects have found that fish return quickly to their upstream spawning grounds after dams are removed. In the Musconetcong River in northwestern New Jersey, the local watershed association has removed five dams since 2008, allowing migratory fish to access about six miles of the waterway above its confluence with the Delaware River for the first time in about 200 years. And in another part of northern Delaware, fish are now swimming past the site of an 18th-century dam that was removed from the White Clay Creek in 2014.
The pattern is being repeated across the United States as nonprofit organizations and some state governments recognize that removing aging dams helps the environment. Fish have returned to the Penobscot River in Maine after the demolition of two dams in 2013 and 2014. In Oregon and California, four dams on the Klamath River are due for removal by 2022. And in New York State, where there are an estimated 2,000 dams on the Hudson River between New York City and Albany, the state has committed $5 million for dam removal.
While dam modifications can help fish return to their spawning grounds, the techniques are less successful than outright removal, and so can be seen as an imperfect solution to the challenge of holistic river restoration, said Dr. Craig of American Rivers.
“Bypasses and rock ramps are less effective than removing the barrier completely,” she said. “There are places where those approaches have been successful in helping to reconnect habitat but you lose all of the other benefits to the river ecology.
“It’s suitable for fish if fish are the only thing you care about, but American Rivers tries to look deeply into the feasibility of removing the barrier even it’s more challenging and more expensive because of that whole-system benefit,” she continued.
The Brandywine is ready for the return of migratory fish, Dr. Kauffman said, because its water quality has improved markedly since the federal Clean Water Act of 1972, which regulated the discharge of pollutants into waterways. Dissolved oxygen has risen as industrial effluent has fallen, while phosphorus, bacteria and sediment have all declined. The main water-quality challenge remains high levels of nitrogen from fertilizer used by upstream farms, although those too are declining.
Migratory fish won’t return to polluted waters even if there are no dams to block their way, Dr. Craig said. The Delaware River, she said, is an example where industrial waste caused such poor water quality in the 1950s and ’60s that shad and other species would not migrate north of Philadelphia. Now, improved water quality has allowed the fish to move much farther up the river bordering northern New Jersey.
But supporters fear that the improvements could be set back by the Trump administration’s recently finalized rollback of the Waters of the U.S. rule, which will remove federal protections for intermittent streams and some freshwater wetlands, making it easier for landowners to build on those areas.
Still, the Brandywine dam project is pressing ahead, with the goal of removing or modifying Dams 3, 4, 6 and 11 this year, depending on the availability of funding from federal, state and private sources. This year’s projects will require an estimated $680,000, of which $410,000 would pay for the removal of Dam 4, a 150-foot structure that spans the creek between a state park and the site of an 18th-century textile mill where condominiums are now under construction.
Later, construction of a bypass channel for Dam 2 — which must be preserved because it diverts water to a Wilmington treatment plant — is expected to cost $1.4 million.
Funding so far has included $241,000 from the federal government as a result of the Delaware River Basin Conservation Act of 2016, which helps local conservation projects, and a matching grant from the State of Delaware, which owns three of the dams. Dr. Kauffman is hoping for a similar amount from the same federal source under a new round of funding early this year.
But even before any more dams have been removed, he’s confident that this year’s fish run — enabled by last year’s removal of Dam 1 — will show that the fish are ready to return.
“They will literally hit their heads on Dam 2,” he said. “They will just keep swimming until they can’t swim any more.”