You'll read in various accounts that Tropical Storm Agnes, the great and hurtful deluge that struck the Chesapeake Bay 40 years ago this month, was the magnitude of storm that only strikes every two or three centuries on average — maybe even a 500-year storm.

But from the bay's standpoint, it was arguably unique, like nothing else in the thousands of years the estuary has existed. To this day, significant parts of the Chesapeake ecosystem have not regained their pre-Agnes health.

Agnes' winds were nothing much, seldom topping 50 mph after it made Florida landfall on June 20, when it was downgraded from hurricane status.

Agnes was all about historic rainfall, about never-before-recorded runoff, an estimated 25 cubic miles of water dumped in a few summer days into the James, Potomac and Susquehanna river basins. It would have raised the whole Chesapeake two feet had there been a dam at its mouth.

The ecological impact of so much water has a lot to do with the watershed it falls on. If there were other Agneses before the historical record began, they fell on a landscape cloaked in deep forest, blanketed with soft, spongy leaf duff, soggy with swamps and the ponded streams of a million beavers.

Agnes fell on a watershed paved and sewered, ditched and drained, fertilized and farmed by 12 million moderns.

Nutrients applied to farmland had doubled and tripled, even since the 1950s, as had concentrations of manure from livestock and poultry.

Sediment, about 20 million tons, trapped for decades by the big Conowingo hydro dam that plugged the Susquehanna in 1928, scoured into the bay.

Throw in sewage overflows, drums of chemicals washed from as far off as New York state, and farm animal carcasses from West Virginia — the upshot was a massive insult that literally could not have occurred in previous centuries and millennia.

All of this landscape alteration did not just start in 1972, of course; it had gone on for a long time. But in the decades before Agnes, the consequences of these changes had been minimized because those years had been as extraordinarily dry as Agnes was wet throughout the bay watershed, minimizing what ran off to the rivers.

From the early 1950s through the early 1970s, only a few years approached average river inflow to the bay — and several years in the 1960s were so droughty, there were fears that theWashington, D.C., region would run out of water to drink.

Agnes, in effect, pulled the trigger on sins that had been a-building.

Another thing made it uniquely destructive. It happened in June, the only one of the bay's historic storms of record to do so. It was a supremely vulnerable time for the bay's underwater grasses, then flowering and reproducing. They took a hit from the silt-choked, nutrient-laden water from which they have still not recovered. (This is not just because of Agnes; nutrient and sediment loads remain at unhealthy levels to this day).

Also unable to cope were bay oysters, which could not keep their shells closed in warm weather to survive the prolonged flows of freshwater that swept most of the way down the Chesapeake.

Called by federal emergency officials "the most massive flooding in the history of the eastern United States," Agnes also took a human toll, killing 122, including 19 in Maryland. It destroyed so many homes, bridges and businesses that it remained the most costly storm in U.S. history until Hurricane Andrew battered Florida in 1992.

More than one-sixth of downtown Richmond, Va., almost 200 blocks, flooded as the James peaked far above normal flows. Flash flooding drowned 16 along D.C.'s Rock Creek, where flows doubled anything ever recorded. A mother of three toddlers in Baltimore struggled to unbuckle her children from their car seats before the Jones Falls swept her into the limbs of trees downstream and drowned her family.

For a time, there were fears that the Conowingo Dam might not hold. The dam was evacuated and explosives installed to blow a section if needed. It held, but a crack had opened down one side, shutting U.S. Route 1 across the dam's top for months while it was re-anchored.

On a personal note, Agnes has been humbling. It was probably the biggest Chesapeake story of my 40-year career, and it happened when I had been reporting for The Baltimore Sun only about six weeks — so green, I didn't even get bylines on my front page stories.

It also reminds us how puny is the human lifespan for understanding the workings of nature. A single event, lasting for days, occurring on the order of centuries — millennia even — may have more impact than everything you might measure during years and decades.

Tom Horton, a former longtime writer for The Sun, is the author of six books about the Chesapeake Bay. This article is distributed by Bay Journal News Service.