What is a watershed?

Lakes are only one part of a larger system called a watershed.  A watershed consists of two major components, water and land, and is made up of many watercourses, including the land surrounding them, all of which drain into a specific point at a lower elevation (see Figure 1). 

The relationship between a lake and its watershed is a vital one and it is important to know that the results of our activities anywhere in a watershed, including on the land, can have a negative impact on the health of that watershed’s lakes and rivers.

We all have an environmental responsibility for the care of our watersheds.  When we do not take the necessary precautions, our lakes and other bodies of water may suffer from problems such as widespread aquatic plant growth, algal blooms and the loss of aquatic life and habitat.  These problems are important ones because all of them can impact the health of the body of water and ultimately the uses it can support in the watershed.  Our watersheds are important ecosystems and their health can have a big impact on our way of life.

Outlined below are some watershed characteristics that have the potential to influence the condition of a lake.

• Watershed size: A lake having a large drainage area (see Figure 2) will be subject to more surface run off into the lake which can influence the rate of which water in that water body changes or is renewed.  A larger watershed also has a greater potential to transport sediment and nutrients to a downstream lake.
  

• Topography: Steep slopes result in rain and surface water running off the land at a faster rate than from gradual slopes which allow time for more water to enter the ground and slowly be released to watercourses.  The surface run off therefore has the potential to move contaminants quickly or more slowly to lakes depending on topography.
• Wetlands: Wetlands adjacent to a lake or within the watershed of a lake function to filter and remove contaminants, slow the flow of water and sustain flows to the lake during dry periods of time.
• Soil type: Fine soils composed of clays, silts and sands can be more easily washed into nearby lakes than soils composed of coarser materials.  Soils carry nutrients and potential contaminants into lakes if eroded. Soil covered in vegetation is much more stable and less likely to be carried into a lake or watercourse.
• Geology: As water passes through soils and around rocks into streams and groundwater, the minerals and chemicals contained in the rock can be dissolved gradually and influence the water quality.
• Land cover: Rural areas with low densities of human activity have the potential to influence lake water quality much less than areas which have intensive uses such as urban, agriculture or forest harvesting.  Where lands have become paved, cleared, built upon or used for waste disposal, the water passing over, under or through these lands can pick-up contaminants and transport them to watercourses and lakes.