What is stormwater?

Stormwater is something that affects all of us, particularly those of us living in urban areas, but it’s not something we think about very often. It is a topic that is sometimes clouded with jargon, or complicated terminology, but at its core it is a simple concept.

To understand stormwater we first need to understand what water does in a natural setting.



Photo courtesy of http://www.state.nj.us/drbc/library/images/hydrocycle2.jpg

When it rains or snows, the precipitation falls onto the ground and can do a few different things. It can soak into the ground, where it recharges our groundwater sources, which fills our wells as well as our lakes and streams. Many municipalities rely solely on groundwater for municipal water. The falling precipitation can also land on the surface of the earth and travel across it until it reaches a river, lake, or the ocean.

Once the precipitation reaches the earth, it can be evaporated directly from water bodies, or it can be used by plants, which then breathe it back into the air. Once the water vapour ends up back in the atmosphere it cools and condenses, forming clouds, which then return the water to the surface of the earth through rain. It’s all one big cycle!


Water flows downhill. The whole surface of the Earth can be divided up based on the direction water will flow, into areas we call watersheds. Any drops of rain that fall in a given watershed will eventually run downhill to reach a common spot. Watersheds are separated by hills or ridges, so the rain falling on one side of a hill may end up in a different watershed than rain falling on the opposite side.

Because all water is flowing downhill, ultimately toward the ocean, but more locally toward rivers and streams, our activities can have an impact on everything that happens downhill of us.


The story is a little different when we talk about urban environments. In the urban world we have hard surfaces, like roads, parking lots, buildings and sidewalks. Precipitation hitting these hard, impermeable surfaces can’t soak into the earth like it would in a natural setting, resulting in rain and snowmelt that runs across the surface. This surface flow is called stormwater runoff.


By U.S. Environmental Protection Agency, Washington, D.C. [Public domain or Public domain], via Wikimedia Commons

As stormwater runoff moves over these hard surfaces it picks up pollutants and contaminants, including oil, road salt, lawn chemicals and dirt. Because there is nothing to slow down this runoff, it also moves quickly, eroding roadsides and stream beds.

In most urban areas we deal with stormwater runoff by funnelling it as quickly as possible into a storm drain system. This is a network of underground pipes and culverts that carries the stormwater to a water treatment facility, or directly to a water body like a lake or the ocean. All of the pollutants, dirt and chemicals carried by the stormwater are also deposited into the water body, causing considerable damage.

By not allowing the stormwater to soak into the ground we also interrupt the cycle that replenishes our groundwater supplies. We rely on groundwater for drinking, recreation and agriculture, and changes to this system can have dramatic impacts.



Schematic of a typical combined sewer system that discharges directly to surface waters during wet weather. From U.S. EPA. 2004. Report to Congress: Impacts and Control of CSOs and SSOs. EPA 833-R-04-001.

Sewer systems are the most common way of dealing with stormwater runoff in urban settings. In many cities, we have a single system for dealing with both sewage and stormwater. This one system collects waste and runoff and carries it to a sewage treatment plant (or Publically Owned Treatment Works, POTW), where it is all treated and discharged.

In wet weather, or during spring thaws, the amount of water flowing through the combined sewer system can overwhelm the systems. When this happens the excess water, a mix of runoff and sewage, leaves the sewer system through emergency exits, which usually lead directly into rivers or the ocean.

So on a bad day, we end up dumping untreated sewage directly into our environment. But even on a good day, with a light rain, we are treating the stormwater runoff as though it were sewage – an expensive way to deal with much cleaner water!


Fortunately, we can make easy changes to the urban environment to mimic what would happen in a natural environment. We want to slow the water down, to give it time to sink into the earth. We want to hold back water during big storm events, to reduce the load on our sewer systems. We want to keep the runoff clean, to reduce the impact on our water resources.

Small changes can have a huge cumulative impact! Learn more about what you can do as a property owner, an concerned citizen, or as a municipal decision-maker:

Tools for property owners

Tools for municipalities and communities

Tools for businesses