For the first time in history, Chicago’s Deep Tunnel system is almost completely full after a weekend of heavy rain, leaving 11 suburban communities facing river flood threats and raising the possibility of sewer overflows into the Chicago River.
When the rain kept coming long after the weekend, Chicago’s underground flood shield finally hit a milestone it has never reached before: Deep Tunnel is now almost completely full.
The system—built over more than a half century—moves floodwaters from Chicago and nearby suburbs through more than 100 miles of tunnels into three reservoirs located just outside the city. The goal is to protect both the river and Lake Michigan. but after the latest stretch of storms. it’s clear the system is being tested in a way it hasn’t been.
Early Monday, the Thornton reservoir in South Holland stood at 94% capacity shortly after 10 a.m. The McCook reservoir in Bedford Park was at 98%. Together, the two reservoirs are holding almost 11 billion gallons of water.
Thornton has never filled up to more than about half its capacity; the most it reached before was almost 55% in June of 2019. The smaller McCook reservoir has gotten more filled before, including June 26.
The third reservoir, Majewski in Elk Grove Village, is considerably smaller than the other two sites. It holds less than 3% of the more than 13 billion gallons in the system.
Even with the reservoirs filling, officials warn that the danger doesn’t stay underground. Eleven suburban communities are facing river flooding threats: Blue Island, Calumet City, Dixmoor, Dolton, Glenwood, Harvey, Lansing, Phoenix, Riverdale, South Holland and Thornton.
In Chicago itself, sewer overflows into the Chicago River are also possible. The city’s sewers are a combined system of stormwater and wastewater, meaning they carry everything flushed down the toilet along with storm runoff.
The Metropolitan Water Reclamation District. which manages the tunnel and reservoir system. said the holding areas are full after more than 8 inches of rain over the past 30 days. It adds that more than 3 inches of rain fell in the Chicago area on July 3 and 4—enough to bring the total close to a month’s worth of rain in just two days.
Illinois State Climatologist Trent Ford said the cumulative effects are what matter now. “We have inundated soils and nearly full reservoirs, meaning that the region-wide flood risk is higher, even from rain that is not particularly heavy,” Ford said.
In recent years, the Chicago area has seen intense downpours of 8 or more inches over short periods of time, in part due to climate change. But this is different—steadier rainfall in recent months has left the region primed, making each additional storm more dangerous.
Deep Tunnel, experts say, can’t be relied on as the only answer. They argue that flooding prevention will require more greenspace and new types of water storage.
Margaret Frisbie, executive director of the advocacy organization Friends of the Chicago River, said natural buffers are part of the solution. “We need to blanket the region with natural areas,” Frisbie said. “This kind of rain is going to keep on coming.”
The system itself is still a work in progress. Despite being built over more than 50 years, Deep Tunnel is not complete. An expansion of the McCook reservoir is scheduled to be complete by the end of 2032.
Chicago Deep Tunnel flooding Thornton reservoir McCook reservoir Majewski reservoir Metropolitan Water Reclamation District combined sewer overflow Chicago River Illinois State Climatologist Trent Ford Friends of the Chicago River