How environmentally-damaging are run-of-river hydropower systems?

Posted on June 1, 2016

Hydropower generation typically involves the creation of large dams for the storage and release of water, which can have serious adverse effects on riverine environments. However, growing pressures on governments to meet renewable energy targets have stimulated interest in small-scale hydropower systems, especially run-of-river schemes, which use the flow within a river channel and operate without water storage. In these systems, some of the river’s flow is diverted along a secondary channel to a turbine and returned to the main channel downstream. Although run-of-river systems are thought to be less environmentally damaging than large-scale hydropower systems, evidence is urgently needed to test this assumption. High-head run-of-river schemes usually rely on natural waterfalls in steep upland streams, require the construction of an in-channel barrier, and divert water over relatively long distances (0.1–1 km). Low-head systems are placed in lower reaches, are often retrofitted to existing structures, and have shorter diversion distances (<0.05 km). Diversion depletes the main stream flow, which can reduce the availability of in-stream habitat, food resources and living space, and prompt the dispersal of biota downstream. Barriers in the stream restrict the natural movements of sediment, organic matter, nutrients and aquatic species. Upstream of a barrier, the “weir pond” effect reduces flow variation and turbulence and encourages the deposition of fine sediment. Ways of mitigating the negative environmental effects of run-of-river systems include setting minimum flow requirements for the depleted stretch, ceasing scheme operation during sensitive times of year (e.g., key stages of fish life cycles), installing fish passes (e.g., rock ramps, natural bypass channels) and screening the diversion channel. Although it’s been suggested that a large number of potential run-of-river hydropower sites lend themselves to ecological improvement as well as energy generation, there’s a pressing need for a lot more research and modelling work on the impacts of small-scale hydropower developments, especially the cumulative effects of installing multiple run-of-river systems in the same stream.

Reference: Anderson, D. et al. 2015. The impacts of ‘run-of-river’ hydropower on the physical and ecological condition of rivers. Water and Environment Journal 29, 268–276.