Mimicking existing natural features to mitigate impacts on fish passage
The Cowichan River on Vancouver Island in British Columbia is important for its fishery and freshwater resources. Climate change is driving longer, hotter, drier summers and there is less water discharging from Cowichan Lake into the river. Our team was hired as prime consultant for the design of a new, larger weir that could store more water at the lake to offset the risk of late-summer drought.
The new weir’s impacts on fish passage were mitigated using a three-part strategy. First, a nature-like step-pool fishway serves as the primary fishway and comprises a rock and gravel area set in a concrete frame for flood resiliency when overtopped. Gated ports control discharge in the fishway over the range of lake levels. Second, stakeholders relayed that the dense vegetation on the island provides fish with velocity refugia and cover from predation while passing when the existing weir is overtopped in high flows. Our solution? To add a vegetated rock and earth ramp downstream of the weir to maintain passage at that location. Finally, the existing vertical slot fishway was preserved and modified to serve as redundancy should the operator choose to use it.
With the project now complete, our innovative, nature-based solutions ensure that this stakeholder-driven project will help mitigate risks from climate change and can be implemented with minimal risk of impact to this important fishery.
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