What is a Watershed?

In simple terms, a watershed is an area of land that encompasses all the water that flows from the tops of the mountains and into a drainage area, such as a lake or the ocean. Water either flows over the surface of the land in streams and rivers or seeps into the ground to flow underground and recharge groundwater aquifers.

Water flows originating at the mountain tops define the watershed’s upper boundaries. At their base, watershed boundaries are defined by the drainage area they flow into (usually the ocean but it can also be a lake) and include all water sources that drain into this area.

Groundwater aquifers are part of the watershed area, however their boundaries may not coincide directly with the surface water flows. Established watersheds are separated from one another by the height of the land; these are known as "drainage divides".

These boundaries do not typically align with political boundaries, which makes for a complicated and complex structure when it comes to watershed management.

Watersheds are nested systems of large and small streams and aquifers that ebb and flow, combine and separate through-out the watershed, essentially nourishing and connecting ecosystems within.

The geology of the watershed, combined with several other characteristics such as the nature of the soils, types of vegetation within the watershed, and land-use features, make each watershed unique.

When watersheds are healthy they supply water, habitat, and food sources for native plants and animals. By moving sediment downstream they cycle nutrients and convert them into forms that living organisms can use. They even influence air quality by absorbing pollutants. This is why it’s vitally important that we keep our watersheds healthy.

Chapman Lake (Photograph by Val Lebresque)
Chapman Lake (Photograph by Val Lebresque)

Glossary of Terms

Aquifer: An underground layer of permeable rock, soil, and/or sand that allows water to move through.

Drainage Area: The area of land into which all water drains from upstream sources

Drainage Basins / Catchments: Smaller drainage areas that feed into sub-watersheds, may be referred to as drainage basins or catchments.

Drainage Divides: Established watersheds are separated from one another by the height of the land

Ecosystem: An ecosystem consists of the plants, animals, and other organisms, along with weather and landscapes, which work together to form a biological community of life in a particular a geographic area.

Groundwater: Water that is held underground in the cracks and spaces in soil, sand and rock. It is stored in and moves slowly through geologic formations of soil, sand and rocks called aquifers.

Permeability: The ability of a material to allow the passage of a liquid, such as water. Permeable materials, such as gravel and sand, allow water to move quickly through them, whereas impermeable materials, such as clay, don't allow water to flow freely.



BC Tomorrow Society. (2017, April 25).
British Columbia Watersheds

Government of Canada. (2020, January 24).
Understanding Watersheds

Ministry of Forests. (2018, January 2).
Community Watersheds - Province of British Columbia

National Geographic. (no date).

The Groundwater Foundation. (2023, July 28).
What is Groundwater?

Capital Regional District. (2013, December 23).
Watershed Basics:  The Water Cycle

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