What is a Watershed?
Each of us lives in a watershed, regardless of how far we are from a body of water. A watershed is the area of land that drains rainfall, snowmelt, sediment and dissolved materials to a particular water body, such as a stream, river, lake, reservoir or marine harbour. Watershed boundaries can be drawn on topographical maps by linking all the surrounding high points in the land, as shown in the diagram below; the dotted lines represent the watershed boundaries or “divides.” All watersheds, regardless of size, consist of the basin within these boundaries and the surface water body (or bodies). The physical characteristics of a watershed – the geology, soil, vegetation and slope, as well as human land uses – influence the quality and quanity of the water that flows through it.
Components of Watersheds
- upland areas such as forests and meadows
- streams and rivers
- marine harbours and shorelines
- riparian zones, areas of saturated soils and water-loving vegetation that surround water bodies
Watersheds may be big or small, depending on the scale at which they are designated. For example, the watershed for the Fraser River is approximately 234,000 square kilometres in size and comprises about a quarter of the total land area of British Columbia (see diagram). Within that area are smaller watersheds belonging to tributaries of the Fraser.
The watershed for Portage Inlet and the Gorge (orange area):
Functions of Watersheds
Watersheds and their components perform many important services for people and wildlife. For example:
- Rivers and streams, sometimes called the “arteries of the land,” nourish and connect ecosystems throughout the watershed
- Wetlands and lakes help to store and filter water, and provide habitat for fish and wildlife
- Upland forests and meadows provide wildlife habitat, nutrients for aquatic ecosystems, and encourage infiltration of rainwater into the ground
- The various components of watersheds provide people with opportunities for recreation, tourism, education and aesthetic appreciation
- Watersheds supply us with clean water for drinking and irrigation, from sources such as reservoirs and groundwater
- Watersheds provide habitat for fish (including the Pacific salmon), birds, mammals, as well as insects and other invertebrates
Interconnections Between Watersheds
As part of the water cycle, rainwater may take different routes across and through the land of the watershed as it flows downhill toward the ocean. Substances present on the land and in the air – either naturally or due to human activities – can be picked up by rainwater runoff and transported into natural water bodies. Minerals, metals, sediment, gases, bacteria and synthetic compounds are just a few of the substances that can be transferred from the air and land into water bodies. The connection between the land and water in a watershed is an important concept that helps us to link processes and activities that might not be immediately obvious. For example:
- pesticides applied to a home garden may end up in a lake or harbour many kilometres away, where they can harm fish and other wildlife
- livestock manure, if stored uncovered and exposed to rain, can cause bacterial contamination of marine shellfish
- oil and gasoline, dripped onto roadways from cars and eventually flushed into the ocean by the rain, contain many compounds that are toxic to marine life
- activities that affect the quantity and timing of water flows can also affect ecosystems and human property in watersheds
By recognizing these connections, we can reduce human impacts on watersheds. Watersheds usually span political and jurisdictional boundaries, such as parks, municipalities, and private property. This presents both a challenge and an opportunity for coordinated planning across these boundaries. The more we understand and respect the natural processes at work in the watershed, the more sustainably we can live within that system.