A sudden, intense storm whipped up over Philadelphia on Labor Day. It dumped less than a quarter-inch of water. But it made a big impact.

Frankford Creek, which normally flows at a lazy 10 cubic feet per second on its way to the Delaware River, roared to 130, according to a gauge in Juniata Park.

Water carrying trash from city streets mixed with diluted raw sewage from nearby homes as stormwater blew past the treatment capacity of the city’s aging sewer systems. Clear water turned muddy brown as it carried human waste, plastic bottles, and other debris along.

Water quality plummeted as microbes began to feed on fecal bacteria and other contaminants, sending life-giving oxygen levels plummeting. Short as the storm was, it took days for the water to recover.