Overflow Prevention

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Like a kitchen sink, on a dry day there is capacity in sewers to manage the flow of water. But during a rain storm, the excess stormwater combined with the typical daily flow can overwhelm collection systems and can pollute area waterways, similar to the overflow at a kitchen sink. In order to prevent overflow, conserve as much water at home as possible.


During rainy days, the MWRD promotes Overflow Action Days. Launched in 2016 with our partners at Friends of the Chicago River, this initiative urges area residents to use less water at home when weather forecasts predict significant rain. This allows sewers more capacity to handle increased volumes of water. Based on the concept of Ozone Action Days, Overflow Action Days includes a dissemination of alerts that indicate the potential for combined sewer overflows (CSOs) that occur when the MWRD’s intercepting sewers and water reclamation plants (WRPs) reach capacity. The heavy rain overwhelms local sewers, and like a backed-up sink, a CSO will drain and spill out to a waterway. These alerts are used to remind Chicago area residents to conserve water before and during rainstorms. Actions such as delaying showers or reducing their duration, flushing less, and waiting to run the dishwasher or laundry can help reduce the amount of water in the sewer system.

To take the Overflow Action Days pledge, visit here.

MWRD staff works 24/7 to protect the water environment, mitigating flooding, managing waterway elevations, and keeping operations moving at its seven WRPs. The MWRD treats an average of 1.47 billion gallons of water per day, but that number can soar above 2 billion with intensive rainstorms. On those days, water can surpass local sewer systems for local waterways and never reach the MWRD’s collection systems. Because many of these sewer systems were designed and built generations ago before city development, that water instead overflows into local waterways, polluting our environment. After consecutive days of rain, there is less capacity for the MWRD to hold and treat the additional water.

The MWRD’s 109 miles of tunnels and three large reservoir systems, which comprise of the Tunnel and Reservoir Plan (TARP), provide an outlet for billions of gallons of floodwaters. This allows MWRD water reclamation plants more time to treat water before it overflows into local waterways, helping improve water quality in waterways and protecting Lake Michigan.
One inch of rain across Cook County can yield approximately 16 billion gallons of water, so every drop of water conservation helps during storms.

Less water, more capacity: Our water environment depends on it.

Studies have shown that the average household uses about 300 gallons of water per day for drinking, cooking, bathing, washing dishes and clothes, flushing toilets, watering lawns and gardens, and maintaining pools. In the Chicago metropolitan area, Friends of the Chicago River projects that we consume nearly 1.4 billion gallons of water a day through residential, commercial and industrial uses—enough to fill the Willis Tower three times over.

Here are some tips to help conserve water:

  • The MWRD recommends replacing old and leaky faucets, toilets and showerheads with the new products approved by EPA WaterSense.
  • Consider mitigating flooding, protecting local water quality and eliminating run off by investing in green infrastructure at home. Disconnecting a downspout or installing a rain barrel can reduce the load on the local sewer system. Check out the MWRD’s Green Neighbor Guide for advice.
  • Prioritize native plants. Join the MWRD’s mission to save the monarch population by planting native plants like milkweed. With extensive root systems, native plants can help reduce flooding and help improve local water quality.
  • For more tips, check out Friends’ “Overflow Action Days: A Simple Guide to Water Conservation.”