Deep Tunnel has never reached its limit. After weekend's heavy rains, it’s almost full. What that means.

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.