The Potomac River swelled to twice its normal width, and much of DC’s waterfront flooded. Abandoned cars float in floodwaters near Baltimore. Photograph by Walter M. McCardell/Baltimore Sun.

Boats torn from moorings littered the shores for miles. Trees, roots and all, spun in the current. It would turn out that more than 200,000 trees had been ripped from the C&O Canal National Historic Park alone. With 66 miles of towpath severely damaged, the park would take years to repair.

Much of DC’s waterfront flooded, including parking lots at National Airport and the Kennedy Center. Only the coincidence of the Potomac’s crest with low tide prevented the river from breaking the flood of 1936’s record.

To the south, a sixth of Richmond—some 200 blocks of the city’s downtown—was inundated. Flows peaked in the James River on June 23 at 70 times normal. (They were 45 times normal in the Potomac.) Floodwater crested well above a seawall that had been constructed to protect the Virginia capital after the 1936 flood.

In Maryland, the Patapsco River ran 12 feet high down Ellicott City’s historic main street. A warehouse of barrels containing unknown chemicals and roofing products washed into one of Baltimore’s major drinking-water reservoirs, mobilizing a days long “fishing” expedition with every boat that Maryland’s Department of Natural Resources could muster.

Most attention was riveted, however—and Agnes’s awful power was most apparent—near the northern Maryland town of Conowingo. There, for only the second time since it was built in 1928, all 53 floodgates had been opened at the mammoth, 116-foot hydro dam plugging the Chesapeake’s mightiest river, the Susquehanna. I had seen water cascading through Conowingo’s floodgates during lesser storms, but now they disgorged into the river massive, graceless chunks—“projectile vomiting,” my notes from that week say.

In normal times, the 464-mile-long Susquehanna, draining lands as far away as Cooperstown, New York, sends almost as much freshwater to the Chesapeake as do the rest of the bay’s rivers combined. (The Potomac is a distant second.) Agnes struck harder in Pennsylvania’s sprawling Susquehanna basin than anywhere else—the whole state was a federal disaster area.

On Friday, June 23, a bulletin came from the dam’s operators: The river was reaching a point where Conowingo’s stability “cannot be controlled.” The dam was evacuated, with explosives installed to blow out a section if necessary. People emptied from the Maryland towns of Port Deposit and Havre de Grace, downstream from the mile-long lake of water held in check by Conowingo Dam.

Eventual flows peaked above anything seen before or since, but the dam, anchored in bedrock and thick enough to carry two lanes of US Route 1 nearly a mile across the river, held. Subsequent surveys showed that Agnes actually had moved the dam a quarter inch on one side. Route 1 was shut down for months while it was reanchored.

Expert observers at Conowingo had noted that the water thundering through the floodgates was darkening beyond the solid brown coming from upstream. That signified a rare occurrence: The river’s force was so great that it was scouring deep into long-buried, deoxygenated sediments that had been trapped behind Conowingo Dam for decades. The scour sent an estimated 20 million tons of smothering, polluting sediment hurtling downstream—in addition to another 14 million tons washed from upstream. No other storm has come close.

It was impossible to predict what the impact of such an event would be on the Chesapeake environment. Such a storm was statistically unlikely to recur for centuries. With no assurance of funding, scientists from Maryland and Virginia, along with Johns Hopkins, launched research cruises in the wake of Agnes, eventually making it the only tropical storm with its own 600-page book (The Effects of Tropical Storm Agnes on the Chesapeake Bay Estuarine System,Johns Hopkins Press, 1977).

The researchers knew that, among the world’s coastal ecosystems, the Chesapeake was almost uniquely sensitive to what occurred across the lands of its watershed. The reason was partly the sheer size of the watershed, more than 20 times the acreage of the bay it drained into. What’s more, there was surprisingly little water in the Chesapeake to dilute what its rivers sent down. The bay looks big—about a million feet long and up to a million feet wide—but it’s thin, with an average depth of about 22 feet.

This shallowness is key to the bay’s historic productivity, letting enough light penetrate to sustain hundreds of thousands of acres of rooted aquatic seagrass meadows. These lush grass beds are among the richest, most productive ecosystems on earth, on a par with coral reefs, mangroves, rainforests, and wetlands.

The grasses had been torn up by other hurricanes such as Hazel, Connie, and Diane in the 1950s and had come back quickly. Agnes, whose unprecedented clouds of sediment shut off vital light during the critical part of the growing season, devastated the grasses. Six years later, their baywide acreage was still down by nearly two-thirds. Only in the last decade or so have they rebounded in the upper Chesapeake and parts of the Potomac. But across the bay as a whole, they’ve never approached pre-Agnes levels.

Another Chesapeake icon that Agnes tipped over the edge was the oyster, whose once-extensive reefs were both a key habitat and a pollution filter for the estuary. Oysters were already in decline from overharvesting, but catches in the Potomac River alone still exceeded 600,000 bushels annually before Agnes struck.

It was an unlikely-sounding “pollutant,” the massive thrust of freshwater from the storm that wiped out many of the Potomac’s rich shellfish bottoms, according to A.C. Carpenter, executive secretary of the Potomac River Fisheries Commission. To live, oysters need about five parts salt per thousand parts water, Carpenter says. Freshwater is zero parts salt, ocean water about 32.

The Chesapeake, a mixture of ocean and riverflow, normally maintains tolerable salinities throughout the oysters’ range in the estuary. But Agnes’s torrents literally shoved the Atlantic Ocean back dozens of miles, apparently blocking any normal rebound of salt up the Potomac for several days. In colder weather, the oysters might have clamped their shells shut and effectively hibernated, but they couldn’t do that in the warmer June waters.