PORT DEPOSIT - As 1,200 gallons of water per second thundered over the Conowingo Dam, scores of shad at the bottom of the wall flashed back and forth in the roiling froth like greenish-blue torpedoes.
The fish were blocked in their springtime pilgrimage up the Susquehanna River to spawn. But then they nosed into an underwater trap, and were suddenly hoisted in a steel box 100 feet into the air and poured to the other side of the dam.
This $15 million fish elevator - the largest in North America, and one of only a handful in the world - is credited with helping to revive the shad, a species that played a mythical role in American Colonial history before being nearly wiped out.
Given the elevator's success over the past 14 years, scientists are puzzled about why the number of shad hitching a ride has mysteriously fallen this spring. The lift transported about 70,000 fish during the monthlong season that is ending about now, down from an average of 130,000 to 140,000 the past four years.
Richard St. Pierre, biologist with the U.S. Fish and Wildlife Service, said he hopes the decline doesn't suggest a permanent setback for a restoration program that has gained international recognition. He guessed that the drop might be a fluke caused by cool weather conditions, shifting ocean currents or perhaps a bad reproduction season four years ago, when this generation of shad was hatched.
"We were expecting a great abundance of shad this year, but we didn't see that, and it has confused us," St. Pierre said. "These fish populations can be highly unpredictable from year to year. Hopefully, maybe next year we'll have 300,000 shad passing over the dam."
Shad were once the second-most popular fish for eating in America and one of the Chesapeake region's most precious commodities.
The fish are a species of herring that grow to about 20 inches in length and weigh up to six pounds, with deeply forked tails and a line of black dots down their silvery sides. They spend most of their lives at sea, swimming as far as 2,000 miles each year as they migrate in schools up and down the Atlantic coast. But they return to the freshwater streams where they were born to lay their eggs.
When the water in the Susquehanna River and other waterways warms to 60 degrees, the fish charge upstream in pods of thousands. Each female drops up to 300,000 eggs in the water, then usually dies.
Pulitzer Prize-winning author John McPhee, in his 2002 book The Founding Fish, describes why the shad played a legendary role in American Colonial history as the fish that saved the republic.
In the winter and spring of 1777-1778, George Washington camped his 12,000-soldier army on the banks of the Schuylkill in Valley Forge. The patriots were starving and near despair. But then, a massive school of "savior shad" swarmed up the river and were devoured by the hungry soldiers, several authors have reported.
McPhee's investigation concluded that Washington - who worked as a commercial shad fisherman in Virginia - and his soldiers probably caught and dined on shad during their encampment in Valley Forge. But this was likely in May, after other food had arrived and the threat of starvation had ended, McPhee wrote.
Nevertheless, the arrival of the shad in Valley Forge is remembered as "the deliverance of embryonic America, the finest hour of the founding fish," McPhee wrote.
After serving as a mainstay of the American diet for decades - often salted or pickled and packed in barrels - overfishing in the late 19th and early 20th centuries nearly eliminated the bony but savory fish.
"People would stretch nets clear across the Susquehanna in the spring and catch 10,000 to 20,000 shad at a time, and you really couldn't sustain that," St. Pierre said.
Also conspiring against the shad were a series of four dams built across the Susquehanna between 1904 and 1932, including the 95-foot-tall Conowingo hydroelectric dam in 1928, which is the southernmost, lying 10 miles north of the Chesapeake Bay.
In an attempt to revive the species, Pennsylvania officials in 1976 began to stock the Susquehanna with millions of artificially bred shad fry.
But the dams remained as barriers. So the U.S. Fish and Wildlife Service in the early 1980s sued the owner of the Conowingo, the Philadelphia Electric Co., in federal court to force the company to build passageways for fish.
Across the country, dam owners had built hundreds of fish ladders, in which water pours down a gradual series of pools, allowing salmon to jump from one level to the next, said Boyd Kynard, biologist with the U.S. Geological Survey in Massachusetts.
"But a fish ladder wouldn't work with the shad on the Susquehanna River, because salmon jump for a living, but shad don't jump, so they can't go up a ladder," Kynard said.
As an alternative, wildlife officials argued that the Conowingo's owner should construct an enormous contraption to trap the shad underwater and lift them over the top of the dam. A fish elevator of this size had never been attempted, although a much smaller one was built in 1959 over the Holyoke Dam north of Hartford, Conn., Kynard said.
The company resisted, claiming it would cost too much and never work. And many fish experts were skeptical, saying that the shad could never be restored on the Susquehanna because only a few hundred remained.
But after years of fighting, the Conowingo owners agreed to pay $15 million to build in 1991 what was then the largest fish elevator in the world. Over the next decade, the other three dams upriver spent another $48 million to build lifts.
After some tinkering, the elevators proved successful, with the numbers of fish passing over the Conowingo surging from 37,516 in 1996 to 193,574 in 2001, St. Pierre said.
The tallies are kept by researchers who sit in a darkened booth, watching through an underwater window as the shad pour out of the elevator and down a chute to the other side of the dam.
Mike Martinek, a fisheries biologist, clicks a handheld counter every time a shad zips past the window. Gazing up at the greenish light of his own private aquarium, he has become highly attuned to the fish.
"Maybe we had fewer this spring because there was not much rain going into the river," Martinek said. "So it's a little shallower. ... The shad might be afraid to come up the Susquehanna."