Conclusion

As the use of rock weirs as a component of natural channel design in small-scale watercourses (i.e., < 2 – 10 m channel width, low gradient) becomes more common, it is necessary that appropriate monitoring techniques are established to evaluate fish passage effectiveness. Additionally, where rock weirs are constructed, determining the critical design and structural components, including placement, materials and geometry, that enhance fish passage effectiveness is important for informing future designs. The modified rock weir system at Weslie Creek was assessed for fish passage effectiveness based on gap and over-weir flow under different water level conditions. Fish passability was most effective (100% fish passability through 9/10 rock weirs) under low water level conditions, which was representative of the system during the majority of the field season. Further, fish passability was least effective under intermediate and high-water level conditions, likely due to the asymmetrical nature of the rock weir structures and the low ratio of over-weir flow pathways to gap flow pathways with more water in the system. These conditions typically lasted for approximately three days, and observations from adjacent pools suggest adequate refuge habitat while passage through the weirs were limited.
The structural components of the rock weirs were compared to fish passage effectiveness and critical design components were identified based on gradient, keystone characteristics (i.e., number and size), distance between rock weirs, embeddedness, and orifices. Critical design components for effective fish passage include:
Design considerations for effective fish passage should not undermine the conditions required to provide channel stability through the reach. These findings should be applied in future river restoration works, where natural channel design is used to address fish passage and stability. Incorporating critical design components for rock weirs will enhance fish passage efficiency, provide bed and bank stability, and limit the requirements for post-construction monitoring.