Where does all the water go after torrential rain in Chicago? The Tunnel and Reservoir Plan

Chicago Scene

Have you ever wondered where all that water goes after a big rain? Even when you flush the toilet or run a shower? It all goes somewhere.

Many Chicagoans have heard of the The Tunnel and Reservoir Plan, or TARP. It’s also known as the “Deep Tunnel.” But not many know what it does exactly.

It first went online in 1981. It is a system of deep, large-diameter tunnels and vast reservoirs designed to reduce flooding, improve water quality in Chicago area waterways. It also works to protect Lake Michigan from pollution caused by sewer overflows.

n aerial view of McCook Reservoir Stage I shows how the MWRD captures up to 3.5 billion gallons of stormwater and sewage during heavy rains to protect the Chicago Sanitary and Ship Canal (left) and Des Plaines River (right) before the MWRD can pump the water to Stickney Water Reclamation Plant for treatment.

Chicago is an old city with old sewers. It’s a city with combined-sewer overflow sewers. Meaning, all of our water, from the toilets you flush to the water in the street after it rains, all flows into one system, to be taken away from our houses and streets. Both sanitary and storm flow are conveyed through the TARP system, sent to the largest wastewater treatment plant in the world.

TARP captures and stores a combined stormwater and sewage that would otherwise overflow from sewers into waterways in rainy weather. This stored water is pumped from TARP to water reclamation plants to be cleaned before being released to waterways.

With a massive tunnel system of four tunnels, TARP has a total of 109 miles of tunnels, 8 to 33 feet in diameter and 150 to 300 feet underground. The four tunnels capture all of the Chicago region’s water, combining the sewage and stormwater to the Majweski Reservoir and McCook Resivor.

The entire system will have a combined sewage capacity of 17.5 billion gallons when Stage 2 of McCook Reservoir is completed – slated for 2029. That’s 4,666 gallons for each person in its service area.

Fun facts about the McCook Reservoir featured in this story:

  1. McCook Reservoir Stage 1 was completed in 2017 and provides 3.5 BG of storage.
  2. Stage 2 will be completed in 2029 and provide 6.5 BG of storage.
  3. The McCook Reservoir will provide more than $143 million per year in flood damage reduction benefits to 3.1 million people in 37 communities, along with a total capacity of 10 BG.

To find out more on TARP, click here.

McCook Reservoir Stage II is currently being excavated to store an additional 6.5 billion gallons of water to protect waterways from pollution and mitigate flooding for area homes and streets.

To see more Chicago Scene fun, follow @tomwgnchicago on Instagram.

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