And another migration is under way.
Photographers from across the region have flocked to the dam in the hope of capturing the perfect picture.
On a good day, there are more long lenses alongside the river than on the sidelines of a pro sports event. And the only thing louder than the shrill squawks of the eagles are the sounds of the camera shutters as the birds fight it out over a fish.
"I'll have dreams of bald eagles," said Ted Ellis, a physical therapist from Bryn Mawr, who first made the trip in 2007 and has been hooked ever since. He has traveled to the dam 18 times this fall.
"You sometimes can see six or seven in the air fighting for a fish," Ellis said, "and another 50 or 60 sitting on the rocks."
The eagle was taken off the endangered species list in June 2007, but remains protected under federal law and by many states.
The birds are drawn to the area by fish. Shad, catfish, bass and perch all swim in the waters above and below the dam, and the shallow waters make easy fishing. To see more eagles in one place would require a trip to the Chilkat River near Haines, Alaska.
"At this time of year, with binoculars or spotting scope, you can easily count over 200 eagles," said Fred Smith, shoreline specialist for Exelon Corp., which is licensed to run the hydroelectric project.
"In terms of being able to see foraging, numbers of birds, and the amount of activity, I don't think anything rivals Conowingo," said Bryan Watts, director of the Center for Conservation Biology (CCB) at the College of William and Mary. The group has been studying and banding the eagles at the dam since the late 1970s and has worked with Exelon.
Eric Esterle trained his Nikon D-4, with a 400mm 2.8 lens with a 2x converter, on a swooping eagle and banged off a series of shots. Not easy considering the bird can have a 90-inch wingspan and can dive at speeds up to 90 m.p.h.
"We are really lucky to have a place like this," said Esterle, 36, a photographer from Ellicott City, Md. "There are so many eagles in the air, I don't know which to follow."
The photographers have fostered a small but tight community. They show off photos, share tips, and even barbecue when eagle activity is slow.
A Facebook page, Conowingo Bald Eagles, created by photographer David Lychenheim, also of Ellicott City, has almost 500 viewers and keeps track of the number of eagles at the dam. It also boasts Lychenheim's photos.
"You get attached to the eagles," said Lychenheim, a retired aerospace engineer, who used to work on the Hubble Space Telescope.
"It is a changing population of birds that are using the site," said Watts.
As the lakes and rivers freeze up north, the eagles follow the southward migration of waterfowl. In the fall and spring, most of the birds commute from the Chesapeake Bay area, which has about 1,500 nests. From December to March, half the birds come from the northern areas - Canada, New England - and half are more regional. In the summer, about 20 percent of eagles at the dam come from Florida and parts south, Watts said.
The photographers help the biologists at the center with their research.
There have been two banding projects at Conowingo, Watts said. The first, a 10-year program, began in the late 1970s. The second, which commenced in 2002, is still in progress. When photographers capture an image with a band and number, the information is entered into a CCB database to track the bird's movement patterns and longevity.
"The photographers are really great about sharing," said Watts.
When it was built in 1928, the 4,648-foot-long, 94-foot-high concrete dam, which serves as the Route 1 bridge across the waterway, was the second-largest hydroelectric project, behind Niagara Falls.
Eagles have been congregating at the Maryland dam for decades, making the unlikely spot a perfect location for viewing. Visitors stand at the river's edge as the eagles fish in the middle of the river or roost on the Peco transmission towers on an island across from the parking lot. "I don't believe the people are disruptive," said Watts.
The eagles are not the only birds to frequent the dam. Peregrine falcons, a variety of owl, gulls, terns, and ducks can be seen hanging out, circling about, and fishing. Turkey vultures - known for munching on the black rubble, molding, and windshield wipers of cars in the parking lot - are in abundance.
Knowing the eagle's behavior is the key to photographic success, said Sue Houghton, 47, of Vernon, N.J., who could be heard calling out directions over the noise of shutters. "To the right. There he goes off the wing wall. He's got his landing gear down."
Houghton accompanies her husband, Herb, 48, a wildlife photographer who specializes in eagle photos.
"If you can't predict the behavior changes," she said, "the chances of getting a good shot are slim to none."
For those without long lenses, there are still opportunities for good photos if they are patient enough to wait for the birds to circle over the parking lot and land in the nearby trees, she said.
For many seasoned photographers, that most-sought-after shot is a full-frame, tack-sharp series showing an eagle swooping down with its talons outstretched parallel to the water, grabbing a fish, and maybe fighting off other birds trying to pirate the catch as it flies away.
"It is not always going to happen," said Herb Houghton, "so we keep coming back."
To see video of the eagles at Conowingo Dam, go to http://www.philly.com/conowingo
Contact Mari A. Schaefer
at 610-892-9149 or email@example.com, or follow on Twitter @MariSchaefer.