While the effects of large hydro projects (200 MW) have been well known and documented for over a century, small (up to 10 MW) and medium sized (10 MW to 200 MW) hydroelectric projects involve many of the same impacts per unit of power generated and, cumulatively, the environmental degradation can exceed that of large hydro projects.  Small and medium sized hydro projects are situated on smaller and often more sensitive riverine ecosystems; however, like large hydro projects, will also alter the river’s flow regime and can have significant impacts on the aquatic environment, as flow is a major determinant of a river’s ecological characteristics and its aquatic biodiversity.

A recent study examined scaled hydropower impacts in the Nu River basin of southwestern China, where the researchers calculated impact per MW of capacity across 14 metrics between small and large hydropower projects (with small being below 50 MW as defined in Chinese policy).  They found that small hydropower dams had greater impact per MW for 9 of the 14 metrics, including length of river channel affected and impact on habitat designated as conservation priorities.

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