Water

Protecting Lake, river and stream water quality

Toronto is fortunate to be located on the shore of Lake Ontario, one of the Great Lakes, with a waterfront that extends 46 kilometers from the mouth of the Etobicoke Creek in the west to the Rouge River in the east. Improving beach water quality has been a priority for Toronto City Council for many years, and eight of the City's beaches that are supervised and safe for swimming have been awarded internationally recognized Blue Flag certification. In addition to the Blue Flag beaches, there are dozens of active boat clubs and marinas that are safe for recreational activities such as sailing, canoeing and kayaking.

For your safety, please do not swim or go in the water if it is not a designated, supervised swimming area.

Toronto Beach information: toronto.ca/health/beaches.

Why you should not swim during or after a rainfall, storm or flood

It is generally unsafe to go in the water during or at least 48 hours after a rainfall, storm or flood. Heavy rainfall can cause pipes in certain areas along Toronto's waterfront to discharge stormwater (rainwater that runs off the pavement into roadside catchbasins), and in certain areas to discharge combined sewer overflows directly into rivers, streams and Lake Ontario. These overflows may contain high levels of bacteria, making the water unsafe. Please note that Toronto Harbour is unsafe for swimming at any time.

Learn more about Lake Ontario water quality and Toronto's safe and clean Blue Flag Beaches at toronto.ca/health/beaches.

What is surface water pollution?

Surface water pollution is a complex issue and can come from many sources including: 

  • Various tributaries: There are many rivers and streams that flow into Lake Ontario (many that are north of City's boundaries) and each of these will pick up debris as part of the urban and rural stormwater runoff.
  • Illegal cross connections: These can occur when a homeowner or contractor incorrectly connects to a storm sewer rather than a sanitary sewer. In such cases, the discharge can flow directly into a stream, river or Lake Ontario. It is illegal and the City of Toronto actively searches out illegal cross connections.
  • Stormwater: As part of the natural water cycle, rain and melting snow wash over the surface of land, and what is not absorbed directly into the ground makes its way into watercourses. In a City like Toronto, this is compounded by our urban environment with many hard surfaces, where water has little or no place to be absorbed, so it makes its way into the nearest creek, river or storm sewer, picking up what is left behind, including what is on our roofs, roads, cars and sidewalks -- such as grease, bird/animal droppings, pet waste left on sidewalks, garbage, bacteria and other pollutants along the way before discharging into the nearest waterway.
  • Combined sewers: Some of the City's older areas, where the sewer system was built as long as a century ago, have combined sewers in which there is only one pipe that carries both sewage and stormwater -- a common sewer design used by many municipalities at the time. Most of the time, combined sewers carry all contents (rain, melted snow and sewage) to wastewater treatment plants for full treatment. During periods of intense, heavy rain fall however, the volume of stormwater that enters these combined sewers may exceed the system's capacity and some of the combined sewer flow (a mix of stormwater and sewage) must be diverted (or overflow) untreated, directly into creeks, rivers and the Lake. Combined sewer overflows (CSOs) were designed to act as a relief valve, preventing overloading which could lead to flooding of basements, roads and other public spaces. Many older North American municipalities with sewer systems built during or before the 1940s operate with some combined sewers.
  • Other: Surface water pollution can come from many other places that are not associated with sewer infrastructure or the natural flow of waterways. Litter left over from recreational use along the waterfront is just one example.

What is the City doing to improve water quality in local streams, rivers and Lake Ontario?

The City of Toronto is committed to protecting and restoring local waterways including Lake Ontario. The City has made significant progress in improving water quality in all of our waterways. Under the City's Wet Weather Flow Master Plan, adopted by City Council in 2003, we have already invested significantly in stormwater management projects including a City-wide mandatory downspout disconnection program; the construction of stormwater ponds; and a Basement Flooding Protection Program.  A number of other key projects are in development including the Don River and Central Waterfront Project, which will virtually eliminate combined sewer overflows – a very expensive, but important undertaking.

Wastewater Treatment Plants and Lake Ontario Water Quality

There are four wastewater treatment plants in Toronto that collect and treat wastewater, and three of these are located on Toronto's waterfront. Due to presence of combined sewers throughout the older parts of Toronto, two of the City's plants that service this area (Ashbridges Bay and Humber Wastewater treatment plants) occasionally may need to bypass some of the increased flows received during severe storm events. Learn more about wastewater treatment by downloading an illustrated explanation of the process.  [PDF]

What happens during a wastewater treatment plant bypass?

A bypass occurs when the volume of rainwater and sewage reaching a plant exceeds the volumes that can reasonably be processed in a short period of time. This only occurs due to a rain event. Bypasses are a necessity in a combined sewer system and serve the following functions:

  • Prevent the rainwater and sewage in the combined sewers from backing up and potentially causing basement and/or surface flooding;
  • Help protect the core biological plant process from damage and degradation; and
  • Help prevent the wastewater treatment plant from flooding, which can cause significant damage to mechanical and electrical equipment.


In very high flow situations – such as during a severe intense rainstorm – some of the incoming flow to the wastewater treatment plant is diverted around what is known as the aeration system (where oxygen is added) and rejoins the rest of the flow prior to the final disinfection process. This is referred to as a bypass.

Because bypassing of the aeration process happens during storm events, the water is mostly rain water (often a ratio of four or five parts rain to one part sewage) and the flow is very diluted. This increased storm-related flow is blended with fully treated flow and is then disinfected. The total flow discharged to Lake Ontario is similar to the normal effluent that is discharged as part of regulated wastewater plant operations, and is not a significant source of water pollution. Ultimately, the purpose of bypassing is to protect the treatment plant from becoming overwhelmed and causing possible flooding and very expensive repairs to equipment.

More: Frequently asked questions about wastewater treatment bypasses

Are there 'floatables' such as tampons or condoms in a bypass?

No, there are no floatables in a wastewater treatment plant bypass. All of these types of materials (condoms, tampons, sticks, plastics, etc.) are removed from the wastewater flow in the preliminary and primary treatment process and eventually end up in landfill.

In the wastewater treatment process, is any of the wastewater considered an 'untreated portion'?

No. All wastewater released from Toronto's wastewater treatment plants operate as per strict Provincial and Federal regulatory requirements, and wastewater must be treated even during a storm when the flow bypasses the aeration process. The City of Toronto continues to implement infrastructure upgrades and operational improvements to help reduce the volume and frequency of wastewater treatment plant bypasses, with the goal of eventually eliminating them all together.

Are wastewater treatment plant by-passes reported?

Yes. Toronto Water is required to report all sewer by-passes to the Ministry of Environment and Climate Change (MOECC) and Environment Canada.

Toronto Water publishes monthly wastewater treatment plant bypass reports on its website:


Learn more

Learn more about the City's infrastructure and its many programs and initiatives that work to improve the water quality of streams, rivers and Lake Ontario – including subsidies that help residents protect their homes from basement flooding:


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