Weirs and dams disrupt the natural processes within river systems, with perhaps the most well-known disruption being the impact that such structures have on fish migration. In addition to the impact on biological and ecological processes, in-stream barriers also impact the abiotic processes of a river, including the movement and transportation of sediment.
In the case of large dams, the downstream movement of sediment can be almost completely stopped and the so-called ‘sediment balance’ downstream of the dam is disrupted. Over time, this can result in the loss of sediment in this section of the river as it is slowly washed downstream, while inputs of sediment from upstream are severely diminished, or even stopped altogether. The impact on the river’s fauna can be profound, particularly in the case of species that rely on certain sediments for particular stages of their life-cycle, for example salmon and trout spawning on gravel beds.
These impacts are seen on some rivers in the UK where a large dam has been built. To mitigate the impacts of the dam built on the River Elan in Wales, the Wye and Usk Foundation have recently introduced gravel into the river downstream of the large dam – the first new input of gravel into this section of river for a century. You can see a great video of it HERE.