Clean Rivers Project · 18 miles of tunnel
The tunnels worked. We can show you by how much.
DC Water is spending billions boring storage tunnels under the city to stop sewage reaching the rivers. The Anacostia's are finished, so for the first time there is enough data to say whether the idea works.
How a storage tunnel stops an overflow
A combined sewer overflows because rain arrives faster than the pipes can carry it to Blue Plains. You can fix that by making the pipes bigger, by keeping rain out of them, or by holding the water somewhere until the storm passes. The Clean Rivers Project mostly does the third.
The tunnels are enormous — 23 feet in diameter, up to 160 feet underground. During a storm, combined flow that would have spilled into the river drops into the tunnel instead. Afterwards, pumps lift it back out and send it to Blue Plains for treatment at a rate the plant can handle.
Nothing is treated in the tunnel. It is storage, not treatment. The benefit is entirely about timing: it converts a burst the system cannot handle into a steady flow it can.
What happened to the Anacostia
The Anacostia tunnels came online in stages, the largest — Northeast Boundary — in September 2023. The effect is not subtle.
- Anacostia
- Potomac
- Rock Creek
The Anacostia has already beaten the target it was given for 2030 — and beaten it by an order of magnitude, five years early.
Was it just dry weather?
A fair question, and the one that decides whether the money worked. Overflow volume swings with rainfall, so a run of quiet years could produce the same picture.
It was not the weather. Measured as gallons spilled per inch of storm rain — the measure that best predicts overflows — the Anacostia is down 90%, essentially identical to the raw figure. Rainfall across the two eras was almost the same: 41.1 inches a year before, 42.3 after.
In the sharpest form: in 2005 the Anacostia received 76 million gallons of overflow for every inch of rain. In 2025 it received 0.15.
Why the Potomac has not changed
Because its tunnel is not built yet. The Potomac River Tunnel is under construction with a deadline of March 2030. Until then the Potomac's combined sewers behave much as they did in 2005.
That has an awkward consequence. As the Anacostia improved, the Potomac did not — so the share of DC's remaining sewage overflow that goes into the Potomac has risen from 31% to about 75%. The city's overflow problem did not so much shrink as relocate.
The 2030 date has already moved once. The original decree required everything finished by 2025. A 2015 amendment pushed it to 2030, on affordability grounds, over the objection of the environmental groups who brought the original lawsuit. Full timeline →
Where the money went
| Component | Watershed | Status |
|---|---|---|
| Blue Plains Tunnel | Anacostia / Potomac | in service |
| Anacostia River Tunnel | Anacostia | in service 2018 |
| Northeast Boundary Tunnel | Anacostia | in service Sept 2023 |
| Poplar Point Pumping Station | Anacostia | in service Mar 2018 |
| Piney Branch improvements | Rock Creek | in service May 2018 |
| Rock Creek green infrastructure | Rock Creek | in construction |
| Piney Branch Tunnel | Rock Creek | in construction |
| Potomac River Tunnel | Potomac | due 2030 |
The 2015 amendment also changed the method for the Potomac and Rock Creek, allowing green infrastructure — rain gardens, permeable paving, anything that keeps rain out of the combined sewer in the first place — to substitute for part of the tunnel capacity.
Status dates from DC Water's quarterly consent decree status reports, 2005–2026. Overflow figures from the quarterly CSO operations reports. All sources and datasets →