Migratory fish like salmon and sea trout face many challenges on their journey upstream to spawn, and one of those challenges is man-made structures like weirs. On a tributary of the Afon Conwy, a weir built under a road bridge was causing a significant problem. With a head difference of around 1 metre (the change in height of water upstream compared to downstream of the barrier), fish were struggling to leap over it, effectively blocking their passage. This barrier was not only halting their natural migration but also disrupting the ecological balance of the river.

In-river barriers like weirs can have a profound impact on the energy reserves of migratory fish. These fish rely on stored energy to fuel their long journeys upstream to spawn. When faced with high barriers or turbulent water, they are forced to expend much more energy in attempts to leap over or navigate around the obstacle. This extra effort can leave them exhausted, reducing their chances of successfully reaching their spawning grounds.

While removing the weir entirely might seem like the easiest solution, it wasn’t an option in this case because the weir was integrated into the bridge structure. So, how do we allow fish to pass without compromising the bridge? That’s where some clever engineering and environmental science come in.

We considered several options, such as notching the weir to create a pathway for the fish. But after thorough consideration, we decided the best solution was to construct a rocky ramp.

What Is a Rocky Ramp?

A rocky ramp mimics the natural flow of a river by using strategically placed rocks and boulders to create a gradual slope instead of a vertical drop. The goal is to spread out the 1-metre head difference, reducing it to something more manageable for fish. Thanks to the rocky ramp we built, the head difference is now just 0.4 metres!

By slowing the water flow and breaking it into small, manageable steps, the ramp allows fish to swim over it rather than leap. It’s like turning a waterfall into a gentle stream, which significantly improves passability for a range of fish species. Rocky ramps also create resting pools between the rocks, where fish can conserve energy before continuing their journey upstream.

Why It Works

Fish, especially migratory species like salmon, are strong swimmers, but they need favourable conditions to overcome obstacles. Rocky ramps work because they mimic the natural riverbed, making it easier for fish to move upstream, even against the current. By reducing the height and intensity of the drop at the weir, the ramp gives fish a fighting chance to pass safely.

To ensure the changes we made are effective, we conducted SNIFFER in-river barrier assessments before and after construction. These assessments help quantify how passable the river is for different species. The results show a significant improvement, confirming that our efforts have made a real difference for the river's fish population.

By installing a rocky ramp, we’ve restored a crucial pathway for migratory fish on this tributary of the Afon Conwy, ensuring they can reach their spawning grounds and contribute to the health of the ecosystem. It’s a small change with a big impact, helping to bring the river back to life, one fish at a time.