What is a Watershed

Watersheds: The Basics

Simply put, a watershed is an area of land that catches precipitation and moves it through soils, tributaries, rivers, wetlands, and smaller lakes to a larger receiving body, which is most commonly a larger lake. There are two types of watersheds.

Open watersheds

Water from these types of watersheds eventually makes its way into an ocean. Most of Canada’s watersheds are open.

Closed watersheds

Water from closed watersheds does not make its way into an ocean, but rather evaporates or seeps into the ground. 

Subwatersheds and Ocean Watersheds

Subwatersheds are smaller watersheds operating at a more local level. Their boundaries are delineated by a number of factors like the contours of the land they are a part of as well as the soil, climate, and vegetation of the area. Anywhere in the land where one water droplet would fall and still end up in the larger receiving body is a part of that watershed.

Ocean watersheds are water’s final destination on land before reaching the sea. Water from open watersheds will end up here. Canada has five ocean watersheds: The Arctic, Atlantic, Hudson Bay, Pacific, and Gulf of Mexico.

Components of a Watershed


Photo courtesy of Long Tom Watershed Council

Click through the below watershed components to learn more!

Water in Eastern Ontario

In Eastern Ontario, water destined for the sea flows into the Atlantic Ocean via either the Great Lakes Basin or the St. Lawrence River Watershed, the latter of which is one of the most populated watersheds with over 15 million Canadians calling it home. Land use, which increases as population size increases, has a major impact on both the quality and movement of water through a watershed. Human actions like cutting down trees, filling wetland habitats and hardening natural surfaces through pavement and housing construction, building golf courses, and using land for agricultural purposes can reduce the amount of water that goes into the soil. This often reduces natural water filtration while increasing flooding. In this instance, water ends up flowing faster and over the land, carrying excess nutrients and contaminants with it until it eventually ends up in a lake. As a result, the lake water may reach harmful levels of bacteria and nutrients, deeming it a hazard to both wildlife and humans. Conversely, removing water from a watershed to water gardens, lawns or agricultural fields, or draining wetlands can exacerbate drought conditions or low water situations across the entire watershed area.

Because of their interconnectedness through watersheds, what we do on the land impacts our lakes and what we do to our lakes has the potential to impact water health on a much larger scale. Our lakes are part of one big system! For this reason, it is vital that all of us take an active role in ensuring that our water is and remains healthy and intact.

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