ROCK CREEK IS now D.C.’s “dirtiest” waterway and, in a sense, that’s good news. The city’s rivers are getting cleaner, and some areas are often free enough of bacteria for residents to swim in them safely for the first time in many years. Though they can’t do it legally, yet.

Well into the 20th century, people could float idly in the calmer parts of the Potomac. Back in the 1920s, children dove and did front flips off platforms at a bathing beach by the Tidal Basin. But the water got more polluted, and a surge in environmentalism in the early 1970s brought awareness that D.C. waters were dangerously sewer-like. The city banned swimming, and until recently it hasn’t looked back.

The primary culprit behind the bacterial contamination of D.C.’s waterways is pipes that combine storm water and other runoff from the streets with waste coming from homes and offices. When there’s a lot of rain, the city’s treatment facility overflows, and water that would usually get filtered before being released into rivers, creeks and streams comes pouring out still filthy.

Thankfully, the same Environmental Protection Agency that pressured the District to close its rivers to swimmers has pressured it to clean them up. A 2005 mandate sparked a massive tunnel project to hold excess wastewater until there’s room to treat it. The first portion servicing the Anacostia is complete and has contained billions of gallons of sewage. The next stretch is scheduled to launch in 2023, and the Potomac will get tunnels of its own in the coming decade. The campaign also involves creating more green infrastructure: miniature parks in the medians of roads, and roofs seeded to soak in what the sky sends down.