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Combined sewer overflows

To see if there is any combined sewer overflow activity in your area, visit MyUtilityInfo.

Combined sewer systems are sewers that are designed to collect both land drainage (rainwater and snowmelt) and wastewater (sewage from homes and businesses) in the same pipe. Most of the time, combined sewer systems transport all of the land drainage and wastewater to a sewage treatment plant, where it is treated and then discharged to the river. However, during periods of heavy rainfall or snowmelt, the additional volume in combined sewers system can exceed the capacity of the sewer system. During these occasions, combined sewer systems are designed to overflow and discharge the excess volumes directly to the river without reaching the sewage treatment plant. These overflows, called combined sewer overflows (CSOs), contain not only land drainage, but also wastewater and debris.

Why do we have CSOs?

Historically, combined sewers discharged directly to rivers. In the 1930s, diversion weirs were put in place to divert flows to the North End sewage treatment plant. The outfalls were retained to offer a level of protection against large wet weather events that could overload the system and result in basement flooding.

Combined sewers carry all of the wastewater flow to the sewage treatment plants during dry weather conditions. During periods of heavy rainfall or snowmelt, there is the potential for basement flooding due to high volumes of land drainage entering the sewer system. CSOs protect basements from flooding by relieving excess flow to the river.

Why should we reduce CSOs?

About a third of the City's sewer system contains combined sewers. Typically, they were built before the 1960s and serve older areas of the city. Every time there is a CSO, there is a temporary discharge to the river of bacteria, floating debris, and organic material (nutrients). By reducing CSOs, we can do our part to help protect the health of our rivers and lakes.

Doing Our Part

The City is committed to protecting the long term health of our environment by working towards a plan to reduce the effects of CSOs on our rivers and lakes in an environmentally sound, sustainable and in a cost-effective manner.

How the city is reducing CSOs

The City of Winnipeg has been working to reduce CSOs since 2002 in order to minimize the impacts they have on our waterways. By reducing CSOs, we aim to decrease the amount of bacteria, nutrients and floatables in our rivers and lakes.

Past projects

Prior to 2013, it is estimated that the City invested over $300 million in CSO relief. Some of these projects include:

  • Completing a CSO Management Strategy in 2002; a study that looked at river monitoring data between 1994 and 2001 and estimated total nutrient contributions from different sources to Lake Winnipeg
  • Implemented a Monitoring Program from 2009 to 2015 to identify and quantify CSO events; 39 outfall locations have been instrumented for CSO monitoring between 2009 to 2015
  • Completing infrastructure upgrades to eliminate CSO events such as sewer separation works, storm relief works, sustainable drainage system, latent storage dewatering stations and outfall chamber upgrades
Recent projects

Since 2013, the City invested over $90 million in systems and infrastructure in the combined sewer system. The work and associated projects are listed as follows:

  • Cockburn/Calrossie Sewer Separation Project
    The City is constructing new land drainage sewers in the Cockburn & Calrossie Combined Sewer District.
  • Jefferson East Sewer Separation Project
    The City is constructing new land drainage sewers in the Jefferson East Combined Sewer District.
  • Ferry Road and Riverbend Sewer Separation Project
    The City is constructing new land drainage sewers in the Ferry Road and Riverbend Combined Sewer District.
  • Sewer Permanent Instrumentation
    The City has permanent flow monitoring instrumentation at 39 primary outfalls and will look to increase the number of permanent instrumentation in the collection systems to better understand the sewer operation.
  • Interceptor and District Flow Monitoring
    The City maintains a number of flow monitoring instruments that can be moved to any location in the system. The monitors are used to collect observed system flow data and can be used to assess the operation of the system and for calibration of the City's hydraulic models.
  • Hydraulic Sewer System Model
    The hydraulic model that provides estimated volumes at all the CSO outfalls based on simulated rainfall and river levels.
  • CSO Public Notification
    An internet-based public notification system for reporting all discharges from combined sewers overflow points.
  • North East Exchange Sustainable Drainage
    Strata cells, underground soil storage, were installed beneath the roadway on John Hirsch Place to provide the un-compacted soil volume necessary for tree growth in the public right of way. Excess water during severe storms will enter the strata cells via the combined sewer, where it will have a chance to be cleaned in the soil strata and absorbed by plantings. More information on the North East Exchange District Engineering Study can be found here.
  • Mission 2018 Sewer Cleaning
    Project consisted of the cleaning and inspection of various wastewater and combined sewer mains and manholes in the Mission Sewer District.
  • Latent Storage Dewatering Stations
    Latent storage dewatering stations were installed as part of the outfall gate chamber upgrades on McDermot Avenue and River Avenue.
Future projects

On or before August 31, 2019, the City will be submitting the CSO Master Plan to Manitoba Sustainable Development, which provides a roadmap for achieving our goal of reducing CSOs by 85 per cent in a Representative Year by 2045.

The CSO Master Plan provides different types of projects that will help reduce the impact of CSOs on our waterways, including:

  • Grey Infrastructure
    Grey infrastructure projects are conventional, large scale infrastructure projects, such as sewer separation projects and building large underground storage tanks.
  • Green Infrastructure
    Green infrastructure projects use natural processes to keep rainwater out of the sewer systems. Examples of green infrastructure include green roofs, permeable paving and bioswales.
  • Real-time Control
    Installing flow monitors and flow control structures in the sewer system will optimize the capacity during wet weather events, reducing the volume of CSO discharge to the rivers, while still protecting against basement flooding.
  • Floatable Management
    Floatables management projects aim to reduce the volume of debris reaching our waterways by using methods such as source control to prevent floatables from entering the combined sewer system, bar screening at discharge points such as outfalls or sewer separation.

The CSO Master Plan has an estimated capital cost of $2.3 billion in 2019 dollars. More information about the CSO Master Plan can be found here .


The City of Winnipeg supports initiatives to improve water quality in our rivers and lakes. The Water and Waste Department has partnered with the following community groups that promote green technologies and education:

  • Fort Whyte Alive
  • Netley-Lebau Marsh
  • Manitoba Eco-Network
  • Save our Seine
  • Lake Winnipeg Research Consortium
  • Rivers West
  • International Institute of Sustainability

How you can reduce CSOs

Even when CSO solutions are implemented, land drainage from our city will continue to flow to streams and rivers during wet weather events. You can help to reduce CSOs and their effects on lakes and rivers by:

  • Using rain barrels to capture rain from downspouts, reusing it to water gardens and lawns
  • Planting trees, shrubs, and other vegetation; the roots help absorb excess water and the leaves help to delay rainwater from reaching the ground
  • Improving how water is absorbed and filtered into the ground by maximizing green space, using soil amendments like compost
  • Introducing rain gardens or bioswales (green infrastructure) in low-lying areas
  • Consider planting structurally safe green roofs
  • Using paving stones instead of concrete in your yard to improve drainage
  • Ensure that all downspouts and sump pumps drain into a rain barrel or an area where the water can be absorbed by the ground, instead of going directly into a street drain
  • Installing backwater valves and sump pit drainage systems on homes built before 1990
  • Disposing of household chemicals and used oil properly, and not pouring them down the drain or in storm sewers on the street
  • Picking up your pet waste
  • Fixing fluid leaks from your vehicles
  • Applying lawn chemicals in a way that minimizes runoff to storm sewers
  • Supporting community groups who promote green technologies and education

Frequently Asked Questions

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Combined sewer overflows occur, on average, 22 times during the year at each outfall. To check the current probability of an overflow, visit our sewer overflow information system.

No. Many North American cities, including others in Manitoba, have similar sewer systems. Hundreds of communities built combined sewers because they were a cost-effective way to provide sewer service and improve drainage. Combined sewers in our city date from 1880. They were the first sewer infrastructure.

There were 79 combined sewer outfalls or outlets to the river system in 2013, but that was reduced to 76 in 2018.

No. The large amounts of suspended soils give the Winnipeg rivers their natural murky brown appearance.

Yes. The Chief Provincial Public Health Officer with the Winnipeg Regional Health Authority advises:

  • Never drink river water, whether overflows are occurring or not
  • Do not swim in the river system at any time because of fast currents, cloudy water, and slippery, muddy banks
  • Wash your hands if they come in contact with river water, particularly before you touch food

There can be debris floating in the rivers when an overflow occurs. However, most of the time, litter from streets is carried into sewers (separate and combined) during a rainstorm. Litter dumped along the river banks can also contribute to the debris floating in the rivers during a rainstorm. Residents can help reduce floating debris by keeping their yards clean and not putting garbage down their household drains or toilets.

Foam on the river:

  • Is not harmful to the environment
  • Occurs naturally
  • Is caused by materials such as pollen and algae
  • Is sometimes brown in colour
  • Is similar to the foam you often see when waves crash on a beach

Yes, as long as they are cooked thoroughly.

In 2002, a long term report* looked at river monitoring data between 1994 and 2001, estimating total nutrient contributions from different sources to Lake Winnipeg. The approximate contributions from the City of Winnipeg to Lake Winnipeg were 5.7 percent TN and 6.7 percent TP, which includes the three wastewater treatment plants, land drainage and CSO discharges. The report estimated that Winnipeg CSOs only make up 79 tons a year or 0.1 percent of TN, and 16 tons a year or 0.3 percent of TP.

November 2002

Lake Winnipeg has a very large watershed that receives drainage from Alberta, Saskatchewan, Manitoba, north west Ontario, North Dakota, South Dakota, Montana and north west Minnesota. The drainage area exceeds 982,000 km2 (approximately 1.5 times the size of Manitoba), where much of it is intensively cultivated agricultural lands. Many cities (e.g. Calgary, Edmonton, Regina, Brandon, Kenora, Grand Forks, and Fargo) drain to the watershed, before it makes its way to Lake Winnipeg.

High levels of nutrients in Lake Winnipeg, like phosphorus and nitrogen, cause excessive algae and weed growth. This lowers water quality, harms fish and other aquatic life as a result of lower oxygen levels, and affects the appearance and recreational enjoyment of our rivers and lakes.

Phosphorus and nitrogen are essential components of a healthy ecosystem. Virtually all human activities can introduce new sources of nutrients to aquatic systems. Lake Winnipeg was estimated on average to receive 63,207 tons a year of total nitrogen (TN) and 5,838 tons a year of total phosphorus (TP)*.

For further information on nutrient loading to Lake Winnipeg, please visit the Province of Manitoba's website.

As part of the CSO Master Plan water quality analysis work was completed and documented in the preliminary proposal**. It agreed with the 2002 provincial study findings and was determined that the relatively small nutrient pollutant loadings from CSO discharges do not have much impact on Lake Winnipeg, but because of the severity of the lake's condition, all sources of contamination should be considered.

Phosphorus is considered to be the limiting nutrient and therefore of the most concern for the lake. The baseline (2013) loading from CSOs for the representative year was estimated to be 15,000 kilograms (kg) per year. Implementing the 85% capture in a representative year alternative would reduce this by about half, and the higher performing alternatives would essentially eliminate it.

The CSO Master Plan estimated the total phosphorus (TP) loading to Lake Winnipeg from CSOs for baseline conditions was estimated to be 0.26% of the total lake loading reported in the 2002 preliminary estimates, which would reduce to 0.14% with the 85% capture in a representative year alternative. From a practical perspective it is unlikely that this small of a change could be detected in the lake.

Treatment plants produce much higher nutrient loads than CSOs, and therefore the potential for reduction is much greater. Nutrient loadings will continue to decrease as the City continues to invest in the current Sewage Treatment Program (STP) upgrades. The total contributions of total phosphorus (TP) and total nitrogen (TN) in terms of lake loadings from the City's STPs are estimated to decrease from 6.4% to 2.1% for TP and from 5.6% to 2.6% for TN.

Similar type reductions are not possible through the CSO program, since the loadings are so much less, and they would not be cost effective.

The second CSO consideration for Lake Winnipeg was with bacteria surviving the journey to the lake. Even with the hostile river environment, it was predicted that bacteria discharges from CSOs would not be more than from 100 to 1,000 most probable number (MPN) per 100 mL at the mouth of the lake, based on normal decay rates. The potential for longer bacterial survival times through shielding methods was not assessed. After reaching the lake, the bacteria levels would continue to reduce through die-off and dispersion, and would not be expected to cause concern at any of the beaches.

For further information on CSO Master Plan analysis of nutrient loading to Lake Winnipeg, please see the CSO Master Plan Preliminary Proposal.

The annual nutrient loading estimates from the treatment plants can be found here .

* A Preliminary Estimate of Total Nitrogen (TN) and Total Phosphorous (TP) Loading to Streams in Manitoba, 2002
** CSO Master Plan Preliminary Proposal, 2015

Yes. The City is regulated by the Province of Manitoba through Environment Act Licence No. 3042.

Data values on this page are representative of the 2013 Baseline Sewer Network.

Combined Sewer Overflow Animation

Last updated: July 27, 2022

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