Removing Weirs to Improve Aquatic Life

A stream flowing over a concrete weir.

Improving the Health of the Credit River Watershed

Weirs are a topic of conversation within our Aquatic and Wetland Restoration team at Credit Valley Conservation. That’s because their removal presents an opportunity to improve the health of aquatic ecosystems.

What is a Weir?

A weir is a human-made, small barrier built across a stream or river to raise the water level on the upstream side. A weir allows a small amount of water to flow over the top. It’s different from a dam which lets out a controlled amount of water by gates or spillways. Water rarely flows over the top of a dam.

Stream with a concrete weir.
A weir holding back water creating a pond upstream.

Where are Weirs Located?

In the 1960s, many weirs were installed throughout the Credit River Watershed to create ponds for agriculture and aesthetic purposes. Most are in the upper and middle parts of the watershed and found on both private and public properties. There are less weirs in the lower watershed due to urbanization and flood prevention initiatives. Weirs can be made out of concrete, metal, or wood boards with T-bars.

How Do Weirs Impact the Credit River Watershed?

A weir of any size can negatively impact rivers and streams by creating barriers. This is called fragmentation. Fragmentation reduces the ability of some fish species such as brook trout and other aquatic wildlife to move up and downstream. This movement is important so fish can migrate to spawning grounds and find resting habitat.

Deteriorating concrete weir on a private landowner property creating a barrier for fish passage.

Weirs also have upstream and downstream impacts. Upstream water slows and can become more like a pond. Sediment with gravel and cobble are not able to move downstream. That means, downstream, there is less spawning gravel and cobble available for fish. Weirs also negatively impact our fish populations and biodiversity within watercourses by:

  • Increasing water temperatures, impacting cold water fish species like brook trout
  • Reducing the amount of dissolved oxygen available to plants and fish
  • Exacerbating erosion downstream, reducing spawning habitat
Fish in hands
In proper conditions, brook trout have an average life span of five to seven years.

The impact a weir has on a watercourse depends on its location. For example, a weir on the main Credit River could have a major impact on the watercourse compared to a weir on a small tributary. Our team uses resources within Credit Valley Conservation to prioritize weir and dam mitigation projects.

Currently, we’re working on two weir removal projects. The goals of these projects are to:

  • Improve brook trout habitat
  • Improve natural channel processes
  • Provide climate change resiliency

Our Aquatic and Wetland Restoration Team are always looking for new opportunities to improve the health and beauty of wetlands, streams and ponds. If you have questions about making environmental improvements on your property, connect with us.

By Krystal Lomas, Technician, Aquatic Restoration

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