Yet unseen, on the other side of the dam, millions upon millions of tons of sediment and nutrient pollution are slowly building up that could wreak havoc on the bay if they get through.
"It's an invisible problem," said Michael Helfrich, the Lower Susquehanna Riverkeeper, as he watches the fishermen. But to Helfrich, it represents the single largest threat in the region to the future health of the bay.
The Susquehanna is the bay's largest tributary, and a major source of the nutrients and sediment fouling its waters. For years, two-thirds of the mud washed downstream, mostly from Pennsylvania and New York, has been settling in the calm water behind the dam rather than flowing into the bay; the same is true of about 40 percent of the phosphorus from sewage plants, farm fields and lawns upriver.
That's lulled government officials and even scientists into thinking the massive hydroelectric facility could be counted on to help protect the bay for years to come. Only last fall officials predicted the dam could keep trapping sediment for another 15 or 20 years.
But it appears the end may be nearer than most thought, as federal scientists now say Conowingo's days as a pollution control structure are coming to a close more rapidly.
That poses a new, pressing challenge for the bay restoration effort, as state and local governments throughout the region stretch to meet a stringent federal "pollution diet." The remedies are likely to be costly or politically unpopular, or both. But if something isn't done to reduce or hold onto the sediment and pollution now being captured by the dam, it could undermine much of what's being done or planned to clean up the Chesapeake's water.
"The picture isn't pretty," said Robert M. Hirsch, a research hydrologist with the U.S. Geological Surveyin Reston, Va. He's been reassessing the dam's trapping ability in the wake of last summer's Tropical Storm Lee, and said he's found that more sediment and nutrients are getting through than before — or than previously believed. "The evidence [is that] things have changed."
Hirsch said he's not able to provide details yet, but he'll be briefing policy makers in coming weeks. He's scheduled to meet Tuesday with a task force that's been mulling for more than a year what to do about Conowingo.
"Let's face it," said Herbert M. Sachs, special projects coordinator for the Maryland Department of the Environment. "The whole [pollution diet] we have for the bay is based on Conowingo holding back these sediments."
Sediment can cloud the water, blocking out sunlight needed by the underwater grasses that provide food and shelter for fish and crabs. It also can smother oysters and other shellfish. Additionally, phosphorus, which binds to the sediment particles, can feed the algae blooms that cause a massive "dead zone" to form in the bay every summer.
Officials have known for years that the dam's reservoir, which stretches 14 miles up the Susquehanna, would eventually "fill up" with sediment. But many believed the dam wouldn't lose its ability to filter out pollution for another decade or two, and would keep trapping two million of the three million tons of sediment estimated to be washing down the river every year.
High river flows caused by big storms, unusually heavy rains or snow melt can scour some of that sediment from the river bottom and flush it through the dam. Those weather events can create some extra room for sediment to accumulate behind Conowingo, but the surges can hurt the bay's water quality.
Scientists say Tropical Storm Agnes, the worst storm to hit the region in decades, washed an estimated 20 million tons of sediment into the Chesapeake in 1972, either triggering or contributing to a mass die-off of underwater grasses from which the bay has yet to recover.
Scientists believed that the river's flow had to reach a certain threshold before it would begin to stir up much of the bottom muck. Through the 1970s, '80s and '90s, that's only happened every five years on average, and never to the degree it did with Agnes.
But according to a presentation Hirsch made earlier this year, the past decade has seen more extreme weather events making the river surge more often. And it doesn't take as much of a storm to start scouring out the accumulated sediment, either.
"It takes a smaller flow today to kick up the sediment than it did, say, 10 or 20 years ago, simply because the reservoir's filled in," Hirsch said. He estimated the reservoir, once thought capable of holding 200 million tons of sediment, is 85 percent or more full.
If the dam stops trapping altogether, the amount of sediment washing into the bay from the Susquehanna would more than double, while the amount of phosphorus could climb 70 percent, scientists have estimated.
Even before the reservoir fills up, the dam's declining trapping ability poses problems for the bay cleanup effort. Officials figured they wouldn't have to deal with an increase in pollution from Conowingo until after the 2025 deadline set by the Environmental Protection Agency for placing enough controls on sewage plants, farms and urban runoff to restore the Chesapeake's water quality.
"We anticipated that it was going to be further down the road — a decade or so — that we wouldn't have to deal directly with this," said Richard Batiuk, associate director of the EPA's Chesapeake Bay office in Annapolis. Now, he said, "it's in front of us."
A task force with representatives from the federal and state governments, the Susquehanna River Basin Commission and environmental groups has been studying what to do about the Conowingo for about a year. Preliminary findings won't be ready until next year, according to Claire D. O'Neill, project manager for the Army Corps of Engineers Baltimore District office, who is coordinating the effort.
One option — dredging sediment from the reservoir — has been explored before, and dropped because of the costs and logistical challenges. Studies figured it would require dredging around the clock for most of the year just to prevent any additional sediment buildup, and cost nearly $48 million annually. That didn't factor in costs for transportation or disposal.
Who would pay for that, or any remedy, remains to be worked out. Mary Helen Marsh, director of environmental programs for Exelon Power, which owns the dam, said the company is happy to help but pointed out that what's collecting behind Conowingo washed off lands upriver in Pennsylvania and New York.
"The sediment is a watershed issue, and it has to be addressed as a watershed issue," she said.
Whatever remedy is ultimately chosen, one thing seems clearer: There's not much time left for pondering how to defuse what some have called the bay's "ticking time bomb."
"It's not blowing up in our face, necessarily," Batiuk said. "But it's certainly in our face. We've got to deal with it."