Public support for the construction of large hydropower dams is diminishing in many regions due to their high social and environmental costs. However, there has been a remarkable surge in favour of the development of Small Hydropower Plants (SHPs; i.e. an umbrella term broadly referring to hydropower facilities that produce lower amounts of electricity and are designed to operate in smaller rivers). A recent compilation found that there are at least 82,891 SHPs across the world (91% of all the world’s hydropower facilities) and still potential for thousands of new constructions in the coming decades. SHPs are expanding rapidly in Brazil with over 1,007 SHPs currently in operation, and an additional 35 under construction and 156 approved but awaiting final licensing. This massive proliferation of SHPs is being fuelled by considerable political and economic incentives in recent decades under the pretences of supporting an environmentally sustainable technology, which includes environmental regulations that support little environmental oversight. However, very little scientific evidence is available to support the assumption that “small” equates to low environmental impacts, especially considering that the criterion used to differentiate “small” and “large” is largely arbitrary. In addition to that, there are many instances in which multiple SHPs are constructed in a single watershed as hydropower complexes, potentially leading to cumulative environmental impacts that can exceed the impacts of large dams (i.e. “a death by thousand cuts).

Given the massive number of current and future SHPs, and the limited knowledge regarding their ecological effects, there is an urgent need for ecological studies to inform conservation strategies and policy reforms. The science and regulations available are mostly limited to individual dams and small scales, and important knowledge gaps persist regarding the cumulative ecological impacts of SHPs in aggregate. This issue has been addressed in this research by exploring how a series of SHPs cumulatively modify river habitats and fish/invertebrate communities in the Chapecó River Basin, Brazil.

Currently, dozens of hydropower installations are operating in the Basin, and additional 16 SHPs are planned. This expansion is transforming the natural hydrological characteristics across the whole basin as a response of the conversion of free-flowing river sections and rapids to low flow sections and reservoirs. The indiscriminate proliferation of SHPs under pretences of sustainable energy is concerning and the results of this research will help to inform effective regulations and management practices for the SHP sector in expansion.