Chicago's Deep Tunnel (TARP) Explained: Why Basements Still Flood

Chicago sits on some of the flattest ground of any major American city, and much of it drains through a combined sewer system that carries rainwater and household wastewater in the same pipes. To keep those pipes from overflowing into rivers and basements, the Metropolitan Water Reclamation District of Greater Chicago (MWRD) has spent decades building the Tunnel and Reservoir Plan, better known as TARP or the Deep Tunnel. It is one of the largest civil engineering projects in the country, and yet many Chicago and inner-suburban homeowners still find water on the basement floor after a hard summer storm. This guide explains how the Deep Tunnel actually works, where its limits are, and why a basement can flood even when the tunnel is doing its job.

What the Deep Tunnel Is and Why It Was Built

Chicago's older neighborhoods and many first-ring suburbs, such as Cicero, Berwyn, Oak Park, and Evanston, were built with combined sewers. When it rains, water from streets, roofs, and alleys flows into the same pipes that carry sinks, toilets, and laundry drains. In dry weather, everything flows to an MWRD water reclamation plant for treatment. In heavy rain, the volume overwhelms the pipes and the plants, and the excess has to go somewhere.

Historically, that excess had two outlets:

• Combined sewer overflows (CSOs) into the Chicago River, the Calumet River system, and the Des Plaines River, carrying untreated wastewater into local waterways.
• Backups into buildings, pushing a mix of stormwater and sewage up through the lowest drains, which in Chicago usually means a basement floor drain.

TARP was adopted by MWRD in the 1970s to reduce both problems. The idea is simple: instead of letting overflow spill into rivers and basements, capture it in giant underground storage, then pump it to treatment plants once the storm passes and capacity returns.

How TARP Works: Tunnels, Drop Shafts, and Reservoirs

The system has two main parts that work together.

Phase I: The Tunnels

The first phase is a network of more than 100 miles of large-diameter tunnels bored through bedrock far beneath the city and suburbs. The tunnels roughly follow the region's waterways, since that is where the old overflow outfalls are. When local sewers start to overflow, water spills over structures and falls down drop shafts into the tunnels instead of into the river.

The tunnels are primarily about conveyance and short-term storage. They hold a meaningful volume, but in a long, heavy storm they can fill quickly.

Phase II: The Reservoirs

The second phase adds huge reservoirs, mostly converted rock quarries, that take water from the tunnels and hold it until treatment plants can catch up:

1. Gloria Alitto Majewski Reservoir serves the northwest suburbs around the O'Hare area.
2. Thornton Composite Reservoir in the south suburbs serves the Calumet system, which drains much of the South Side and southern suburbs.
3. McCook Reservoir near the Stickney plant serves the large Mainstream and Des Plaines systems, which cover most of the city and the western suburbs. McCook is being brought online in stages, with its full capacity planned for the coming years.

After a storm, MWRD pumps the stored water out of the tunnels and reservoirs to plants such as the Stickney Water Reclamation Plant, one of the largest wastewater treatment plants in the world. That pumping can take days, which matters if a second storm arrives before the system has drained.

Why Basements Still Flood With the Deep Tunnel in Place

This is the part that surprises most homeowners. TARP is designed to catch overflow at the end of the sewer network, near the rivers. Your basement is at the beginning of that network. Several bottlenecks sit between your floor drain and the nearest drop shaft.

Your Neighborhood Sewer Has Its Own Limits

The pipe under your street and the main it feeds into were sized decades ago, often for a smaller population and less pavement. During an intense burst of rain, a local sewer can surcharge, meaning it fills completely and the water pressure rises, long before the Deep Tunnel is anywhere near full. When that happens, water looks for the lowest exit, and a basement floor drain several feet below street level is often it.

Intensity Matters More Than Total Rainfall

A slow, steady rain over a full day may never cause trouble. The same amount falling in an hour or two can overwhelm local pipes. Short, intense summer thunderstorms are the classic Chicago basement-flooding pattern because the local system simply cannot move water fast enough, regardless of how much room is left in the tunnels downstream.

The Tunnels Can Fill During Extreme Events

In the largest storms, or when back-to-back storms hit before pumping has emptied the system, the tunnels and reservoirs can reach capacity. At that point, overflows to the waterways resume, and in rare extreme events, the locks and gates that normally keep the river flowing away from Lake Michigan may be opened to relieve flooding. When the receiving waterways are high, local sewers drain more slowly, and backups in low-lying blocks become more likely.

Street Inlet Restrictors Hold Water on the Street on Purpose

The City of Chicago and some suburbs have installed flow restrictors in certain street catch basins. These deliberately slow how fast rainwater enters the sewer, letting water pond briefly on the street instead of rushing into the pipes and up into basements. Seeing water in the street after a storm can be a sign this strategy is working, not that the system has failed.

Your House Is Part of the Equation

Many Chicago bungalows, two-flats, and workers' cottages have:

• Floor drains and laundry tubs below street level connected directly to the sewer by gravity.
• Downspouts that historically tied into the sewer, adding roof water to an already busy pipe.
• Aging clay or cast-iron building sewers with cracks, root intrusion, or sags that slow drainage.
• Foundation drain tile that can collect groundwater and route it toward the same sewer connection.

Even with a perfectly functioning Deep Tunnel, these house-level details decide whether a surcharged street sewer pushes water into your basement.

A Hypothetical Example: Two Homes on One Block

The following scenario is fictional and uses invented names to illustrate how two houses can have very different outcomes in the same storm.

Renata Okafor, a fictional middle-school science teacher, owns a brick bungalow on a block in a combined-sewer neighborhood. Her basement has an original floor drain, a laundry tub, and a gravity connection to the street sewer. Next door, Tomasz Wierzbicki, a fictional CTA rail mechanic, owns a nearly identical bungalow, but a previous owner converted the basement to an overhead sewer system, where lower-level drains feed an ejector pit that pumps wastewater up and out above the level of the street sewer.

A fast thunderstorm drops heavy rain over about an hour. The street sewer surcharges. In Renata's house, water and diluted sewage rise through the floor drain and spread across the basement. In Tomasz's house, the sewer pressure has no low opening to escape through, so his basement stays dry. Deep Tunnel capacity was the same for both homes; the difference was entirely in the plumbing at the house.

The takeaway is not that one approach is always right, but that the house-side plumbing configuration often matters more than the regional system. The guide comparing an overhead sewer vs. a backwater valve walks through the tradeoffs.

What Homeowners Can Do to Reduce Backup Risk

TARP reduces regional flooding and river pollution, but protecting an individual basement usually requires work at the house. Common measures in the Chicago area include:

1. Backflow protection. A backwater valve or an overhead sewer conversion keeps surcharged sewer water from rising into the basement. Many municipalities require a permit and a licensed plumber for this work; local rules vary, so check with your village or the City of Chicago Department of Water Management. The backwater valve guide covers how these devices work and where they fit.
2. Disconnecting downspouts where your municipality allows it, directing roof water onto lawns or rain gardens away from the foundation instead of into the sewer.
3. Sewer line inspection with a camera to find roots, offsets, and sags in an older clay lateral.
4. Sump pump readiness, including a battery backup, if the home has drain tile and a sump pit. Power outages often coincide with the same storms that stress the sewers.
5. Storing belongings off the floor on shelving or in sealed bins, and avoiding finished living space below grade without backflow protection.
6. Keeping catch basins and alley inlets clear of leaves and debris near your property so rainwater can at least reach the sewer.

Part of the reason Chicago basements are so exposed is historical. When the city built its first sewer system in the 1850s and 1860s, the ground was too flat for gravity drainage, so sewers were laid near the old ground surface and street grades were raised over them. That is why many older blocks have streets and sidewalks noticeably higher than the front yards, and why basement floor drains often sit below the sewer under the street. Building sewers also run below the local frost depth, commonly cited as about 42 inches in the Chicago area, which keeps the house connection deep. Together, those facts make gravity drains in Chicago basements especially vulnerable to surcharge.

What to Expect During and After a Major Storm

Understanding the system helps set expectations when the forecast calls for severe weather:

• During the storm, avoid running washing machines, dishwashers, and showers in basement-level fixtures. Every gallon you send down the drain competes with stormwater in a pipe that may already be full.
• Watch the floor drain if it is safe to do so. Water rising from a floor drain usually means the street sewer is surcharging.
• After the storm, treat any backup water as contaminated. Combined sewer water contains sewage, so it calls for protective gear and proper disinfection, not just a mop.
• Expect delays in drainage if storms repeat over several days. The tunnels and reservoirs need time to be pumped down, and local sewers may stay full longer than you would expect.

For widespread storm events, the storm damage overview for Chicago explains what typically happens to homes across the city when wind, rain, and sewer backups combine.

The Bottom Line on TARP and Your Basement

The Deep Tunnel has substantially reduced combined sewer overflows into Chicago-area waterways and gives the regional system far more room to absorb storms than it once had. But it was never designed to guarantee a dry basement on every block. Local sewer capacity, rainfall intensity, back-to-back storms, and the plumbing inside your own house all play a larger role in whether water comes up through the floor drain. Knowing where your home fits into that chain is the first step toward making better decisions about backflow protection, drainage, and what to keep in the basement.