What Is a Watershed and Why Is It Important?

A watershed is the land area surrounding a lake where all water—whether from rain, snowmelt, or runoff—flows, directly or indirectly, toward that lake. This area can extend over long distances and includes terrain with varying slopes. In all cases, a watershed functions like a natural funnel, with the lake as its final outlet.

Several practical measures can help protect lake water quality:

• One simple and low-cost action has consistently proven highly effective, delivering meaningful results across multiple levels.
• This action involves preserving trees and vegetation throughout the watershed—especially in mature forests and along lake shores and waterways, where vegetation should be restored if it has been removed.
• Best management practices recommend maintaining a fully vegetated strip of 10 to 15 metres from the high-water mark.
• This applies not only to lakes and rivers, but also to smaller waterways such as streams, which play a key role in carrying runoff toward lakes.
• All waterbodies should be protected from dissolved and suspended substances transported by runoff.
• The most effective natural solution is to maintain a sufficiently wide vegetated buffer that can slow, retain, and filter runoff, helping remove excess nutrients. With adequate vegetation in place, runoff water is naturally filtered before reaching the lake.

To better understand the importance of controlling nutrient inputs, we can look at the cyanobacteria outbreaks that affected Quebec in 2006.

• Cyanobacteria are naturally present in aquatic environments, but their rapid growth is triggered by high levels of nutrients—particularly nitrogen and phosphorus.

• Warm temperatures, calm water conditions, and strong solar radiation further promote their development.
• These conditions were common during the summer of 2006. However, elevated nutrient levels cannot be explained solely by human activity, especially in lakes with limited development.
• One widely accepted explanation links the high number of blooms observed that year to frequent and heavy rainfall. These events generated significant runoff, transporting large amounts of nutrients from the surrounding land into lakes.
• This relatively warm runoff water remained near the surface due to thermal stratification—a natural summer process in lakes—making nutrients immediately available and accelerating cyanobacteria growth.
• Poorly managed runoff may therefore have been a major contributing factor to the widespread blooms observed across Quebec that year.

This means that when a watershed loses its tree cover and vegetation—especially within riparian buffers—the risk of cyanobacteria outbreaks increases significantly, along with the ecological and recreational impacts that follow.

Source: Eau Secours

In Quebec, more than 92,000 trees were planted in watershed areas surrounding waterbodies in a single year. “In 2008 and 2009, two million trees and shrubs will be planted. The results will take time—typically between 5 and 10 years,” notes Nicolas Hamelin of the Regroupement des conseils de bassins versants du Québec.

Source: Bas Saint-Laurent