Standing 50 feet above the Susquehanna River, the view from a catwalk on the Conowingo Dam was a study in contrasts.

To the right, water roiled out from under the dam. After running off a 27,000-square-mile drainage basin that extends well into New York and western Pennsylvania, it had just pushed its way through a series of turbines, generating more than 500 megawatts of electricity in the process.

To the left, a slower flow of water poured over the dam and through a concrete channel into the river, creating a steady water flow aimed at luring migrating fish into an elevator. After hoisting them nearly 100 feet, the elevator releases the fish into another channel that allows them to pass over the dam.

To the right, it was water rushing downstream, trying to find its way to sea level, generating pollution-free power in the process.

To the left, it was an ages-old biological desire of anadromous fish to fight their way upstream, aided by a mechanical band-aid to help overcome an otherwise unsurmountable blockage.

The scene illustrates the trade-offs associated with Conowingo, the largest dam on the largest river on the East Coast. Figuring out how to reap the benefits of low-cost, pollution free, hydropower while mitigating the environmental impacts of the dam is the nub of negotiations between Exelon Generation, which owns the dam, and state and federal agencies. Exelon's 34-year license from the Federal Energy Regulatory Commission expires on Sept. 1, 2014, and the negotiations in the next several months will determine how environmental issues will be dealt with for the next three to four decades. (See "Relicensing of Conowingo Dam an opportunity for many Bay issues," January-February 2013.)

"Any type of power is going to have its drawbacks," noted Kenneth Poletti, general manager of the dam, during a recent tour of the facility.

Even "clean" power sources such as wind and solar have drawbacks — they don't produce power when the sun doesn't shine or the wind doesn't blow, require huge battery capacity to store power, and their installations can disrupt local landscapes and ecosystems.

Coal and oil have obvious pollution problems. Exelon officials note that a coal-fired plant would have to burn 2,100 tons of coal a day to produce the power generated by Conowingo — and it would be at a greater cost.

That's not to say the dam's power comes without impact. Concerns have been raised about the disruptions to downstream habitats caused by fluctuating water levels. Some would like to see improved recreational access for fishers and boaters on land owned by Exelon around the dam and the 14-mile-long reservoir behind it.

The reservoir itself is an issue because — after trapping sediment and keeping it out of the Bay for most of the dam's history — it's nearly filled. Now severe storms drive more of that stored sediment downstream, threatening Bay restoration efforts, scientists say.

Those and other issues need to be resolved before FERC can issue a new license. In addition, the state of Maryland has to issue a water quality certification for the dam's operation in conjunction with the new license.

The first issue on the table, though, is one of the most intractable — figuring out how to improve the passage of migratory fish up the river. Agencies have to make those recommendations to FERC by Sept. 30.

The Susquehanna River was once the largest East Coast spawning habitat for such species as American shad, river herring and American eel. Populations of all those are at historic lows, and river herring and American eels are being considered for protection under the Endangered Species Act.

Fish lifts & passages

It's not that Exelon, and the utilities that own three upstream dams have not tried. Over the last quarter century, they have poured tens of millions of dollars into fish lifts and other fish passages to get American shad and river herring upstream.

On a recent May morning, the Conowingo lift showed what it was capable of doing. A steady stream of water passed through the lift luring shad into a hopper which then hoisted the trapped fish nearly 100 feet above the river. There, the shad shoot out of a 3-foot pipe into a flume that they follow into the reservoir above the dam. En route, they were squeezed next to a large window.

On the other side of the window, Cody Werhel, a biologist with Normandeau Associates, stood inside a room, counting and identifying the fish, using a clicker to record what went past.

A lot of the silver fish looked the same, and it takes training to distinguish the species. "I grew up fishing so I was able to pick up in a couple of days," Werhel said.

American shad "look like a torpedo, is the way I think of it," he said, while gizzard shad look "more like a football."

On heavy days like this, the lift is operating at full capacity, lifting and releasing a load of fish into the flume about every 15 minutes. With each release, hundreds of fish start passing the window, and Werhel's clicking goes at a rapid-fire pace.

There's been a lot to click at: The day before, more than 75,000 fish passed the window.

"The fish that we are seeing are in really good shape," said Chris Avalos, also of Normandeau. "The females are looking really robust."

He credited the long, cool spring for a strong shad run, noting that it was nearly mid-May and nearby Lancaster County had a freeze warning the previous night.

"I still think this is going to be a good year. I've got a scarf on and it's May 13."

Still, the overwhelming majority of fish are common gizzard shad — though important for the food chain, including the bald eagles and osprey that thrive around the dam, they aren't much sought after by people.

The nonmigratory gizzard shad nonetheless pack the elevator, outnumbering American shad dozens to one. Last year, the lift carried 1.1 million fish over the dam, but most were gizzard shad; only 22,143 were American shad, the main target species.

So few American shad successfully complete the migration up the river that a group of scientists suggested in a recent paper that it "may be time to admit failure" for fish passage on the river unless dams are removed." (See "Scientists link restoration of Susquehanna's fish to removal of dams," April 2013.)

Figuring out the solution isn't easy. Shawn Seaman, of the Maryland Department of Natural Resources' Power Plant Research Program, who has been coordinating the state's review of the relicensing process, said gizzard shad may be crowding out American shad. But, he said, American shad also seem to be skittish about using the elevator — sometimes they swim into the hopper, then leave.

Seaman said biologists from the DNR, U.S. Fish and Wildlife Service and consultants hired by Exelon will recommend changes in fish lift operation, changes in the attraction flow and other modifications. "There is a whole bunch of stuff going on here and we are trying to find out the best solution for dealing with all of these different problems," Seaman said.

While those issues are dealt with, Exelon officials have proposed reviving a program to capture shad and truck them upstream past three other dams located within the next 45 miles to augment the fish lift.

"Truck and transport" took place from 1985 through 1996, but was discontinued after upstream fish passages were completed. But those passages never got as many fish upstream as did the trucks. In the best year, 2001, 16,200 shad got past all four dams, but in recent years only a few hundred typically get that far. In comparison, trucks hauled 56,674 around all the dams in 1995.

"No one wants to really truck fish," Seaman said. "That is not the goal of the agencies, but at some point you have to do what's successful," he said.

Still, trucking is not going to reach the overall goal of getting 2 million shad past all four dams — a goal some biologists doubt is attainable.

"I think the thing to start with is what is the right expectation for the number of fish to get to Harrisburg," said Vicky Will, vice president for environmental services with Exelon.

She also noted that dams are likely not the only factors affecting shad and herring populations. Some scientists and environmental groups contend that large numbers of those fish are being caught as bycatch in the ocean by other fisheries.

"We have to understand what is happening on the full East Coast to figure out what do you do to rebuild the shad population so they actually show up here to be lifted over the dam," Will said.

Getting fish upstream isn't just important for anglers. The inability to get one species upstream — American eels — may also be affecting the river's health. Research suggests the eels play an essential role in helping populations of a key freshwater mussel species reproduce. If those mussels, whose upstream populations have significantly declined, were as abundant as in the adjacent Delaware River, they would filter a huge amount of the Susquehanna's water.

Exelon is proposing to support a truck and transport program for eels, which can't use the fish lift. Getting large numbers of eels upstream is considerably easier than it is for shad: Shad migrate upstream as large adults to spawn, while eels move upstream as small juveniles, making them easier to capture and move.

Powering up the grid

Conowingo Dam, located just 10 miles upstream from the Chesapeake, was completed in 1928 by the Philadelphia Electric Company, effectively closing almost the entire Susquehanna to fish migration. It was named for the original town of Conowingo, which was inundated on the east side of the river as the 14-mile reservoir behind the dam was filled. A new town, the Village of Conowingo, was completed in 1928 on the opposite side of the river.

Conowingo was the second largest hydroelectric plant in the nation at the time of its completion. Its seven turbines could produce 252 megawatts of power. Four larger turbines were added in 1964, allowing it to produce more than 500 megawatts.

Driving all of those turbines at once, though, requires 80,000 cubic feet of water going through the turbines each second. During most of the year, the river isn't carrying that much water, so water stored in the 14-mile-long reservoir behind the dam is used to drive the turbines.

Then, the water in the reservoir is allowed to rebuild for several hours before it again operates at "full house." Water levels in the reservoir are not allowed to be drawn down by more than 5 feet so as not to impede upstream uses.

Conowingo is always generating power, but its full capacity is typically called upon twice a day, during peak power consumption in the morning and afternoon, to supplement power for PJM, the regional organization that coordinates the electric power grid in the mid-Atlantic.

The dam generates its own operating power. Two small turbines, called "house units," generate one megawatt each to supply the electricity that operates the facility. Because water passing through can always generate that power, Conowingo can play a unique role for the power grid if there's a regional power outage: It can provide the power needed to restart the grid. That happened with a regional outage in 1967. "A coal plant needs outside power to start," Poletti said.

The plant's power generation has been reduced slightly over time as the utility modified two turbines to help mix more oxygen into water leaving the dam to improve downstream water quality. It's one of many actions taken to improve habitats. And life around the dam is thriving — it is a hotspot for viewing bald eagles, drawing visitors from hours away.

But the ongoing negotiations over Conowingo's relicensing is a reminder that no power source comes without a trade-off.