The assessment of flow characteristics at certain weirs in relation to the upstream movement of migratory salmonids

A major problem in relation to the rehabilitation of migratory salmonids into many industrial rivers in South Wales is the presence of large ion weirs which can, under certain flows, constrain the upstream migration of fish. A method is presented whereby the constraints at two weirs on the River Afan were assessed by relating the swimming capacities of different sizes of fish and the flows needed to stimulate upstream movement to the distribution of flows and current velocities. The ‘gateways’ for fish to ascend the weir are calculated, and their significance in relation to historical flow data and the requirement for remedial action are assessed.