As aquatic ecologists, we have been very intrigued that amidst evolving conversations over the environmental sustainability of large hydropower, a remarkable surge in favor of Small Hydropower Plants (SHPs) has emerged. Broadly referring to facilities that differ from large hydropower plants by producing less electricity and operating in smaller rivers, we admit that the modifier “small” has long puzzled us.

Let’s be honest, the words “small” and “large” are quite ambiguous. Based on our years of field adventures ranging from North to South America, and well beyond, we have seen SHPs that include an incredible diversity of dam sizes, operation modes, and flow control structures. So, just how small are SHPs with respect to their global number, physical attributes, and potential environmental impacts? In a recent paper published in the journal Frontiers in Ecology and the Environment, we sought to answer these questions.


After scouring data from numerous articles, reports and policy documents, we were astonished to reveal an estimated 82,891 SHPs currently in operation or under construction in 150 countries across the world. China appears to be the global leader with more 47,000 SHPs in operation, largely sparked by private investments, technology leadership and rural electrification programmes. Europe is currently home to around 27,000 SHPs, where many nations have a long history of hydropower development and have witnessed a recent resurgence to meet international agreements promoting clean energy. Similarly, many other countries have initiated aggressive energy policies in recent decades that continue to fuel rapid SHP growth. Overall, we found that there are 11 SHPs for every large hydropower plant in the world.

Our assessment also revealed disparities in how countries classify hydropower plants as “small”. Remarkably, almost one-fifth of all countries that currently operate SHPs do not have a formal (or widely accessible) national definition. For the remaining countries there are striking differences in maximum generation capacity, ranging from facilities generating less than 1MW for Germany and Burundi to a value of 50MW for Canada, China, and Pakistan. About three-quarters of countries with formal definitions classify SHPs as installations with less than 10MW; an international standard being increasingly recognised. The large diversity of definitions across countries – based exclusively on capacity – includes schemes that have substantially different dam sizes, reservoir areas and storage, outlet structures, and operating procedures. In one example, two SHPs in Brazil (São Sebastião, Braço Norte II) have the same capacity (10MW) but differ 30-fold with respect to their reservoir areas (0.2 and 6.0 km2, respectively).

According to a global policy compilation, we found that at least 44 countries require a formal environmental licensing process to construct and operate SHPs; leaving potentially upwards of over two-thirds of the world’s countries without recognised environmental requirements. This concerns us because emerging research suggests that SHPs do cause environmental impacts, including those associated with the dam construction and land inundation, as well post-construction alteration to flow regimes, water quality, and the loss of habitat connectivity. Perhaps most under-appreciated is that the environmental impacts of SHPs may be manifested across multiple installations, and therefore should be considered collectively and not just in isolation. For example, the presence of multiple obstacles can exacerbate the difficulties of freshwater fish and other organisms to disperse.

Management challenges

Moving forward, our research points to three primary management challenges associated with SHPs. First, we advocate that environmental impact assessments should be compulsory during the licensing process for all SHP projects due to their enormous diversity of sizes, operations, and geographic locations. A hydropower project being labeled as “small” should not automatically lead to a faster and less comprehensive licensing process.

Second, environmental assessments must additionally go beyond individual SHP projects by considering the broader watershed context before new construction is approved. The continued proliferation of SHPs necessitates a systematic evaluation of cumulative effects of current and planned installations, an issue that is overlooked by most present-day environmental regulations.

Third, as aquatic ecologists we believe more science is needed to support management strategies and policy reforms that attempt to minimize the environmental impacts of SHPs while continuing to recognize their role in contribution to a more sustainable energy future.


Julian Olden, Professor, School of Aquatic and Fishery Sciences, University of Washington, Seattle, Washington USA; Email:

Thiago Couto, PhD Student, School of Aquatic and Fishery Sciences, University of Washington, Seattle, Washington USA; Email:



Couto, T.B.A., and J.D. Olden. 2018. Global proliferation of small hydropower plants – science and policy. Frontiers in Ecology and the Environment, doi:10.1002/fee.1746.