CHICAGO (WBBM NEWSRADIO) — While some roads remain flooded from Tuesday’s rain and melting snow, billions of gallons of water did not wind up on roads or in basements.

3.5 billion gallons of water ended up in the brand new McCook Reservoir, which opened in early December.

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The 300-foot deep old rock quarry is designed to protect more than 3 million residents of Chicago and 36 suburbs from flooding. The reservoir collects water, and eventually sends it to a treatment plant; it is then released into an actual waterway.

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“MWRD (Metropolitan Water Reclamation District) facilities are handling a large volume of water. This infrastructure is protecting our waterways and basements throughout the Chicago region,” said Allison Fore, Public and Intergovernmental Affairs Officer. “We have had some CSOs (combined sewer overflows) but the volumes are significantly reduced. Overall, with the size of this rain event combined with the snow melt, the system performance has been outstanding.”

A second phase of the McCook Reservoir is being built; it’s scheduled to be completed in 2029 and provide capacity for another 6.5 billion gallons of rain water.

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MWRD estimates the reservoir will provide $114 million in flood reduction protection every year.