Stormwater - Frequently asked questions

Stormwater runoff is rain or melting snow that collects on the surface or flows into sewer pipes instead of seeping into the ground.

Runoff will either:

• Soak into the ground
• Get absorbed by plants
• Evaporate
• Flow along the surface to the nearest sewer or waterway

In urban areas, we have a greater amount of runoff since we have decreased our natural ability to soak up water. When fields and forests are replaced with roads, parking lots and buildings, less rainfall can soak back into the ground and more water collects on hard surfaces. This increased runoff quickly drains via paved surfaces and storm sewers to local streams and rivers.

When we replace plants with pavement, we decrease the city’s natural ability to soak up stormwater.  This means that when it rains, less water soaks into the ground and more water collects on the surface.  A small storm can cause a surge in the levels and flows of rivers and streams which can lead to flooding and erosion.   

In addition to increased volume and speed, the runoff picks up pollutants like dirt, oil, and animal waste as it flows over the surface and decreases the quality of our local waterways.  Eventually, all stormwater reaches the Ottawa River which is the drinking water source for Ottawa and many downstream communities. Polluted stormwater runoff also impacts the water quality at beaches and can lead to beach closures. With extreme weather events becoming more frequent, managing stormwater is even more important to reduce flooding and protect water quality.

Rain or melted snow falls on roofs, driveways, parking lots and roads and travels as runoff to reach a waterway. This runoff collects various pollutants (dirt, phosphorous, metals, nitrogen, animal waste) and debris (grass-clippings, garbage, cigarette butts) as it flows over these surfaces, resulting in polluted water.

In Ottawa, we have three types of sewers:

1. Storm sewers carry rainfall and runoff directly to either a stormwater pond or the nearest creek, stream or river, generally without treatment.
2. Sanitary sewers gather wastewater from homes, businesses and industrial sites, and transport the waste through a network of sewers to the Robert O. Pickard Environmental Centre for treatment.
3. Combined sewers carry both wastewater and runoff to the Robert O. Pickard Environmental Centre for treatment. These sewers are located in the downtown core and can overflow into local waterways if stormwater volume exceeds their capacity.

Storm sewer grates are often marked with a fish symbol as a reminder that they  drain to local rivers and streams. The fish symbol is there to remind the public that what goes down the drain can harm our rivers and streams. Remember: Only rain goes down the drain.

Stormwater management has been applied to new urban developments in Ottawa since the 1980s. Over the last few decades it has evolved to include improving water quality and reducing the total amount of runoff by designing sites to allow more rainfall to soak back into the ground. Stormwater facilities such as man-made ponds, culverts and catch basins are built to temporarily hold water.

Ponds also provide water quality treatment via filtration to remove pollutants or and debris. Cleaner water is then slowly released back to the natural waterway, mimicking the natural runoff rate and quality. This helps prevent flooding and stream bank erosion.

In older areas of the city, stormwater runoff drains directly to creeks and rivers with no treatment or control of any kind. The continued discharge of uncontrolled stormwater runoff to the Ottawa River and its tributaries has been recognized as an important issue for the City to address. The City launched the Ottawa River Action Plan (ORAP) in 2010 to improve and protect the health of the Ottawa River and its tributaries.

In rural areas, more stormwater runoff infiltrates through a greater amount of permeable surfaces. However, runoff can be subject to agricultural by-products such as excess fertilizer, pesticides and manure. Runoff is also channelled along ditches and through culverts to the nearest watercourse. The runoff from agricultural operations can have an impact on surface water sources such as groundwater, streams, lakes, and rivers. The Ottawa Rural Clean Water Program provides funding to improve surface water and groundwater quality.