Staff and dip nets

What’s in a weir?

Neys Provincial Park recently removed an obsolete weir as part of its work to restore and maintain ecological integrity. Superintendent Allison Dennis has the story…

The term “weir” piqued my curiosity following my first review of the Neys Provincial Park Management Plan.

Turns out that a weir is a barrier constructed across the width of a river or stream which raises the water level on the upstream side to a specified height. Unlike a dam, which redirects excess water using spillways, a weir allows excess water to flow over the top of the structure and continue downstream.

So what does this have to do with a provincial park?

Time to update Neys’ water source

Original Neys weir

Here at Neys Provincial Park, we have a concrete weir located on the stream that formally served as the park’s primary water source. This concrete weir allowed the stream to pool water. This water was then used to supply the park’s original pump house.

In 2003, our new water treatment plant was built to comply with updated standards for safe drinking water. This new system provides a better water supply for the park, and made the old weir obsolete.

It was time for us to decommission the weir.

Why remove the weir?

Baby brook trout

Weirs — even small ones — can result in habitats becoming fragmented. This means the movement and migration of fish and other stream dwellers can become restricted.

Coldwater streams, like this one, contain fish habitat and serve as important spawning and nursery grounds for salmonid species.

Removing this weir was an important step in restoring ecological integrity at Neys. The removal improves stream connectivity and habitat, as well as reducing the possible impacts of sedimentation. It can make access to the habitats and food sources fish need to survive a challenge!

Weirs can also result in changes to a stream’s upstream and downstream landscape. Upstream of a weir, stream-like habitats begin to pool and become more lake-like. This can result in a loss of natural features (like riffles) which are important spawning and nursery habitats.

Downstream, spawning habitat (and the diversity of that habitat) can also be affected through a reduction in the amount of gravel able to move freely downstream.

Across the board, human-made structures like weirs can also impact stream temperatures, water velocity, oxygen content, and erosion. Together, these changes have the potential to stress a stream’s ecosystem, and negatively impact stream biodiversity and fish populations.

Clearly it was worthwhile to get that weir out of here!

How did we do it?

Staff and dip nets

The removal project began with fisheries monitoring work to determine what species were present, and support the environmental assessment screening process.

Since removing the old weir would require in-stream work, mitigation strategies were also needed in order to avoid causing harm to the fishery. It was important to confirm the presence of fish both above and below the weir prior to developing our plans.

In the spring of 2017, Ontario Parks staff conducted dip netting at various points along the stream. This helped identify the types of fish species that called this stream home, and to establish baseline data for the site. It also allowed us to track the benefits that the removal of the weir would have on the fishery over time.

Our investigation confirmed the presence of Brook Trout above the weir site, and Brook Trout, Coho Salmon, Sculpin, and Longnose Dace below the weir at the stream’s outflow.

With fisheries monitoring and environmental assessments completed, we were ready to remove the weir.

We took several steps to do this safely. We installed sediment screening prior to commencing work. We avoided working in the water during certain key life cycle periods of the fish and their habitat. Lastly, we reseeded the stream bank post-project, providing erosion control.

All these actions contributed to ensuring fisheries compliance and restorative success.

After a few days of hard work, the weir was successfully removed. We’re happy to say our habitat restoration was successful!

The removal of the weir helped both support aquatic ecosystem resilience, and contribute to Ontario Parks’ goal of promoting ecological integrity.

The weir post removal

To learn more about Ontario Parks’ commitment to ecological integrity, click here.