About 13 billion gallons of combined storm and waste water flowed into two Cook County reservoirs this past weekend, part of the region’s network of stormwater management infrastructure.

That’s enough to fill nearly 20,000 Olympic-sized swimming pools, although that comparison might not be much easier to comprehend. 

As of Thursday afternoon, the Thornton Reservoir was 88% full with 6.9 billion gallons of water, while the McCook Reservoir stood at 66% full with 2.3 billion gallons. The third reservoir, Majewski Reservoir in Northwest Cook County, was empty. 

Flooding is a persistent threat across the region. Aging local sewer systems can be overwhelmed by heavy rains, which are growing more frequent. And impermeable surfaces that fail to catch water where it falls — common in industrial corridors and urban centers — only exacerbate the flood-prone nature of the Chicago area. 

But the regional Tunnel and Reservoir Plan (TARP), also known as the “deep tunnel,” can provide a critical backstop against flood threats.

“We’re still in a ‘near-capacity situation,’ but we’re starting to pump down the reservoirs,” said John Murray, executive director of the Metropolitan Water Reclamation District of Greater Chicago (MWRD). “And so as each day goes by without any rain, we’ll get closer and closer to emptying the reservoirs.”

The MWRD is currently draining 600-700 million gallons per day out of the reservoir system. And while it will take weeks of dry conditions to fully empty the reservoirs at that rate, each bit adds capacity to handle the next storm. 

Chicago, along with many of the 128 municipalities served by TARP, uses combined sewer systems. That means stormwater and wastewater flow through the same sewers to get to treatment plants.

MWRD is able to divert billions of gallons to three reservoirs around Cook County, where the water is held until sewers regain capacity. But if more rain comes while reservoirs are near capacity, the risk of combined sewer overflows (CSOs) increases. 

In that case, sewer water would flow back into the area waterways.

“It is hazardous for humans that recreate on the water — people who kayak, canoe, go fishing. You’re at a higher risk of receiving any impact from pathogens that could be in the water,” said Tessa Murray, watershed policy manager at Friends of the Chicago River, and of no relation to John Murray. 

“It’s also really impactful for our aquatic habitats,” Tessa Murray said. 

Prior to the construction of TARP, hundreds of combined sewer overflows would happen every year. Now, that number has been cut ten-fold. But Friends of the Chicago River still aims for zero.

According to Tessa Murray, a combined sewer overflow at Bubbly Creek at the end of June killed around 100 fish. Ironically, the improved health of the river over time partially contributed to the die-off.

“We’re seeing a lot more fish than we have in years past, because we’re getting better with the health of the river system,” Tessa Murray said. “But these combined sewer overflows can really set us back.” 

The overflows are more likely to occur in summer, but they’re not unheard of in winter either. Rapid warming that melted snow on the ground, combined with rain, caused an overflow in January of this year. 

When the risk of overflows is high, Friends of the Chicago River and MWRD may issue overflow action alert days, advising people to cut back on water temporarily. But while individuals can help, it’s just one of many layers of protection MWRD uses to manage stormwater.  

“One of the greatest challenges for the district is that none of this infrastructure exists anywhere else in the world,” John Murray said. “And so we’re learning, too, how to deal with some of these issues also.”

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