Bob Ward had covered only half a mile on his walk along the historic C&O Canal towpath before he encountered a stream of gray water rushing toward the Potomac River.

“I’m well acquainted enough with that area to know that is not normal,” he said, remembering that Jan. 19 evening. He soon came upon “a raging flood of sewage.”

Ward was an early witness to what would soon become one of the largest wastewater spills in modern U.S. history. A pipe 6 feet in diameter collapsed that day and, in the next month, spilled hundreds of millions of gallons of untreated sewage onto National Park Service land and into the Potomac River.

In the Cabin John neighborhood, where Ward and about 2,000 others live, residents could not only see the wastewater but smell it, sometimes in their homes.

Responding to the collapse, crews from DC Water, the District government agency responsible for the pipeline, have contended with subfreezing temperatures, piles of snow and ice, and boulders that blocked access to the break.

They have also had to dig. After crews reached the collapsed portion of the pipe on Thursday, a month after it caved in, the agency estimated that initial repairs would be completed by mid-March.

The full extent of the environmental damage remains unclear. Water quality testing near the collapse site has revealed dangerous levels of E. coli.

In the past week, the catastrophe has become more of a spectacle after a series of incendiary social media posts from President Donald Trump, who accused Maryland Gov. Wes Moore and other local leaders of bungling the cleanup.

DC Water has not determined what caused the pipeline, known as the Potomac Interceptor, to collapse.

Here is how the environmental disaster unfolded.

A catastrophic collapse

In the weeks leading up to the collapse, DC Water was preparing for repairs near Cabin John as part of a 10-year project to upgrade vulnerable sections of the pipeline.

The work hadn’t begun, but agency representatives were meeting with contractors, including as recently as Jan. 16 — three days before the collapse.

In the hours after the collapse, a sinkhole formed, allowing wastewater to swell to the surface and flow into the Potomac. Upstream, the sudden backup forced sewage to shoot out of a manhole cover.

DC Water estimates that 244 million gallons of wastewater have escaped since the pipe collapsed.

But the Maryland Department of the Environment and groups that monitor the Potomac suggest a larger overflow — as many as 300 million gallons.

By the time Ward noticed the break, it appeared DC Water had too. A police car and a utility truck had pulled up close to the site.

“What could those guys do other than stand there and watch the water boiling up from underground?” he said.

A DC Water spokesperson said a security crew monitoring cameras at a nearby odor-control facility discovered wastewater escaping the pipeline.

In a news release that night, DC Water told the public their drinking water was safe because the collapse occurred downstream from the aqueduct that supplies it.

On social media and DC Water’s website, photos and video showed a dome of sewage, at least a foot high, near the site of the break.

For days, residents said, the smell of wastewater hung over Cabin John.

The historic C&O Canal provided DC Water with a short-term solution. A roughly half-mile-long bypass system would pump wastewater from the Potomac Interceptor into the previously dry canal, around the collapse and back into the pipeline.

But a storm was brewing.

A winter storm complication

The pumps began working Jan. 24, one day before a winter storm unleashed snow and ice on the area. The next day, when the storm hit, the agency reported it was “nearing full containment” of the overflow.

But the pumps needed constant attention. DC Water said its crews were working around the clock to clear the frequently blocked pumps of accumulating fats, grease and wipes that don’t decompose.

Each time a pump clogged, crews temporarily shut it down, reducing the bypass system’s capacity to handle wastewater and increasing the amount of sewage headed toward the Potomac.

Ten days after the break, DC Water reported the first 24-hour period without sewage overflows from the site.

With the bypass mostly working, crews began digging and clearing debris with industrial vacuums in hopes of reaching the collapsed pipe, 16 feet underground, so repairs could begin.

Big rocks and E. coli

By early February, DC Water’s daily progress reports began describing a blockage at the collapse site.

When the Interceptor broke, rocks covering the pipeline collapsed into it and flowed downstream, forming a dam.

Crews determined the boulders were so massive they would need an additional 4-6 weeks and heavy machinery to remove them.

But DC Water reported success, too. The agency said it had mostly prevented wastewater from spilling out of the broken pipeline and the C&O Canal bypass.

Then came the water test results.

DC Water hired a contractor that began sampling E. coli levels at six points along the river on Jan. 29.

The Environmental Protection Agency deems water unsafe for swimming if E. coli levels exceed 410 mpn per 100 milliliters. DC Water tests showed concentrations of E. coli from hundreds to more than 1,000 times that near the drainage site.

Environmental groups, including the nonprofit Potomac Riverkeeper Network, suggested DC Water shouldn’t be trusted.

Working with researchers from the University of Maryland, the Riverkeeper Network conducted its own testing and found E. coli levels higher than those reported by DC Water. It also discovered evidence of other bacteria near the collapse site, including a staph pathogen resistant to antibiotics.

“There should be a separate and independent response from the regulatory agencies and the public health agencies in D.C., as well as Maryland,” river keeper Dean Naujoks. “Unfortunately, everybody’s deferring to DC Water.”

State and local agencies issued health advisories starting Jan. 24.

What’s next

DC Water announced last week that its crews had reached the damaged section of the pipeline, bringing them a step closer to determining the cause of the collapse.

Matthew Brown, the agency’s chief operating officer, told Maryland lawmakers that hydrogen sulfide, a highly toxic wastewater byproduct, may have corroded the 60-year-old pipe.

Though the agency expects to have repaired the pipeline and ceased pumping into the C&O Canal by mid-March, it may take until early 2027 to complete upgrades around Cabin John.

DC Water said Friday it has stopped wastewater from flowing into the Potomac since Super Bowl Sunday, but the agency’s most recent tests show E. coli concentrations near the collapse site at unsafe levels.

On Saturday, Trump approved D.C. Mayor Muriel Bowser’s request for federal disaster assistance to help pay for the cleanup, which DC Water has estimated will cost at least $20 million.

Some say it was fortunate the collapse didn’t happen in the summer, when the river and its banks are buzzing with paddlers, hikers and picnickers.

But warming weather in the coming weeks will surely bring people to the Potomac, especially year-round kayakers, said Ed Gertler, an avid Potomac paddler.

“The specter of sewage out there will scare certain people off, but not hard-core paddlers,” he said.