Water from rain or melting snow can evaporate, filter into the ground, be taken up by plants, or form runoff. This runoff is called wet weather flow or stormwater. In a natural area, most stormwater filters into the ground or is taken up by vegetation. However, in cities much of the surface area is pavement; stormwater cannot filter into the ground but instead runs off the ground and roofs, emptying into stormwater grates.

Brick Sewer c.1915 (City of Toronto Archives - Sewer 414)

Brick Sewer c.1915 (City of Toronto Archives - Sewer 414)

All the neighbourhoods in the Garrison Creek watershed are served by combined sewers.1 These sewers carry both stormwater and sanitary sewage from our homes to be cleaned and then discharged. During heavy storms, the sudden increase in water volume overloads the sewers and treatment plants. The system can overflow, spilling its untreated contents into rivers, streams, and Lake Ontario.

This overflow is a problem because it carries oil, pesticides, sewage, and other pollutants. The City of Toronto is trying to address this problem in their 25-year Wet Weather Flow Management Master Plan (WWFMMP) which includes: public education; monitoring; shoreline management; stream restoration; and source, conveyance, and end-of-pipe controls.2


To keep our rivers and lakes healthy for nature and for our own use, we need to prevent stormwater pollution.

You can help by:


1. City of Toronto Works and Emergency Services,

2. City of Toronto Public Consultation Presentation, “Overview of City-Wide 25 Years Plan” (2002),

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