For the first time in history, Deep Tunnel, Chicago’s massive system of underground tunnels and reservoirs designed to hold flood water, is almost completely full after a weekend of heavy rain.
Built over more than a half-century, Deep Tunnel sends floodwaters from Chicago and nearby suburbs through more than 100 miles of tunnels into three reservoirs located just outside of the city. It was designed to help protect the river as well as Lake Michigan.
The Thornton reservoir in South Holland was at 94% capacity shortly after 10 a.m. on Monday. The McCook reservoir in Bedford Park was at 98%. Between the two, they are holding almost 11 billion gallons of water.
The Thornton reservoir has never filled up much more than half its capacity and that was at almost 55% in June of 2019. The smaller McCook reservoir did fill up on June 26.
The third reservoir, Majewski in Elk Grove Village, is considerably smaller than the other two sites, holding less than 3% of the more than 13 billion gallons in the system.
As a result, 11 suburban communities face river flooding threats. They are Blue Island, Calumet City, Dixmoor, Dolton, Glenwood, Harvey, Lansing, Phoenix, Riverdale, South Holland and Thornton, according to officials.
In Chicago, sewer overflows into the Chicago River are also possible. The city’s sewers are a combined system of stormwater and wastewater, including everything flushed down the toilet.
According to the Metropolitan Water Reclamation District, which manages the tunnel and reservoir system, the giant holding areas are full after more than 8 inches of rain over the past 30 days. With more than 3 inches of rain in the Chicago area July 3 and 4, the total is almost a month’s rain in two days.
While the Chicago area has seen intense downpours of 8 or more inches of rain over short periods of time in recent years in part due to climate change, the steady rainfall in recent months has created a cumulative effect.
“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,” said Illinois State Climatologist Trent Ford.
Experts say that Deep Tunnel isn’t enough to prevent flooding and that more greenspace and new types of water storage need to be developed.
“We need to blanket the region with natural areas,” said Margaret Frisbie, executive director of the advocacy organization Friends of the Chicago River. “This kind of rain is going to keep on coming.”
Even though it’s been built over more than 50 years, Deep Tunnel is still not complete. An expansion of the McCook reservoir is scheduled to be complete by the end of 2032.