The machines arrived on the Brandywine in Wilmington, Delaware, and began taking apart a wall nearly two centuries old.
The wall is stone, timber and concrete, about eight feet tall, and for most of its life it held water back for a string of mills.
The mills are long gone.
But the wall stayed, and the fish never came back past it.
So crews took out its middle, piece by piece. How fast can the shad follow?
What a shad cannot do that a salmon can
The gap between a shad and a salmon sounds small until you watch one try to climb. Shad are poor jumpers by nature, and a low wall stretched bank to bank is enough to stop a whole run. That is what happened here. Once mill dams went up near the mouth, shad that had spawned in this river for as long as anyone had watched it ran out of river and turned back.
Eleven dams eventually crossed the Delaware portion of the Brandywine, packed into roughly twelve miles between downtown Wilmington and Rockland upstream. Each took another slice of river away from the fish. And so the run that once defined the valley ended.
The reach that proved the idea
The lowest barrier went first: a low concrete water main encasement near the Brandywine Zoo, about 115 years old and long past doing anything useful. The water above it had held no shad at all, and surveys taken before the work confirmed that.
The river answered quickly. The following summer, researchers netting the newly opened reach found 159 juvenile shad and three adults above the old dam site, the first sign of spawning there in more than a century. “There was no shad past Dam 1 prior to its removal,” said Jim Shanahan, executive director of the Brandywine River Restoration Trust, which leads the effort.
What actually came out of the water
Dam 6, known as the DuPont Experimental Station Dam, was built in 1839 to power a cluster of powder mills. It is made of stone, wood and concrete, stands about eight feet tall, and slopes some 40 feet along the riverbed. Long defunct, it kept doing what dams do, including contributing to periodic flooding after heavy rain at the research campus beside it.
Crews moved slowly to avoid a large sediment release that could travel downstream and cause problems for the city’s water supply. The silt pooled behind the structure over 186 years of standing is not something anyone wants arriving at a drinking water intake at once.
Only the center section came out. Roughly a third of the dam was left on the banks as a public exhibit of the river’s industrial history, and the job cost close to a million dollars, including studies and engineering plans, with the chemical manufacturer that co owns the dam covering about a third of it.
Eleven dams, one fish, one river at a time
Dam 6 is only the second barrier to come down on the Delaware stretch, and the plan covers all of them. Some will be removed outright. Others cannot be: the dam just above downtown feeds the city’s drinking water intake, and several upstream sit inside a national historic landmark district, so those are slated for fish ladders or rock ramps shaped to let a shad through without a leap.
That is why the shad are not here yet. Four dams still stand between the freed water and the lowest reach the fish can currently use, and the trust is working upstream in order, with the Broom Street dam next. Its stated goal is to reconnect an 11-mile stretch for migratory fish for the first time in nearly 300 years.
On the Klamath, Chinook salmon returned to habitat above four former dams soon after they came out, and this effort is betting on a similar response at a far smaller scale.
What the river carries when the shad return
American shad were a staple for the Lenape long before the mills, and federal accounts credit them with the first recorded request to take a dam out of this river, in the 1700s. Then the mills went up, the dams followed, and the fish vanished from most of the water above the city.
Yet living things at both ends of the food chain gain when a run comes back. Freshwater mussel larvae hitch a ride on shad gills and are carried upstream, where they settle and filter the water as adults. Meanwhile ospreys and bass feed on the young fish heading down. A single returning species threads itself through the whole system.
The evidence from the first removal is not just anecdotal. A telemetry study in the journal Marine and Coastal Fisheries reported that three quarters of acoustic-tagged shad used historic spawning grounds opened by that project, which added only about eight tenths of a mile of river.
Dam 6’s center is gone now and the river runs through the gap, quietly moving sediment downstream. What nobody can say yet is how many shad will arrive once the lower barriers are solved, since each opened reach only counts when it connects to the one below it. That same patience shapes every slow recovery where what was left behind takes far longer to clear than it did to put there.
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