Globally, that imbalance between the number of small plants and their overall output is similar. Small plants represent an estimated 91% of all hydropower installations, but only make up 11% of hydropower output. And yet, they have seen a worldwide boom in recent years – as well as a growing body of evidence from around the world suggesting their impact is more profound than was previously thought.
“The biggest problem associated with small hydropower concerns their sheer numbers,” says Thiago Couto, an aquatic ecologist and postdoctoral associate at the University of Miami. He has studied the effects of small hydropower plants around the world, including in countries such as Brazil. “Today, for every large hydropower dam operating there are other 11 ‘small’ ones, and their numbers are expected to continue rising into the future. The cumulative impact of all these small dams on hydrology, fish migration and water quality is an issue of high concern, especially in rivers that accommodate multiple dams.”
Small hydropower dams may for example play a disproportionate role in interrupting the natural flow of rivers, known as river fragmentation, research by Couto and others shows. This can prevent migratory fish from travelling. Environmental activists have described the impact of such an accumulation of small, disruptive plants on rivers as “death by a thousand cuts“.
From an environmental perspective, “small” is not a particularly meaningful term, Couto notes, since it usually just refers to generation capacity – rather than to factors that actually matter for the plant’s ecological impact, such as how much water is diverted, and how much remains in the stream bed. And while small hydropower is often marketed as a good solution for unconnected communities, globally, “the proliferation of small hydropower is not being primarily propelled by rural electrification,” he says, but rather, by incentives and subsidies that make it a lucrative investment for businesses.
Nor are remote communities necessarily in favour of small hydropower. Sámi reindeer herders in northern Norway have opposed small hydropower development in the past, out of concern over its impact on reindeer pastures.
Scientists have also pointed to the knowledge gap around small hydro, since they are only beginning to fully understand how different factors affect a plant’s overall impact.
A team of scientists conducting a long-term ecological research project in the Matschertal valley (Val di Mazia), led by the Institute for Alpine Environment at the Eurac Research Center in South Tyrol, made a surprising discovery. Their project’s main aim has been to study the impact of climate change on an Alpine river ecosystem. But a few years into the study, a small run-of-river diversion hydropower plant with a weir was added to the steep, glacier-fed, fast-flowing stream they were monitoring. They used the opportunity to make a before-and-after comparison of the plant’s impact on benthic macroinvertebrates – small aquatic animals such as stonefly larvae that live in waterbodies, and are commonly used as indicators of their ecological condition. They expected the plant to have a significant impact on the macroinvertebrates – but those expectations were not confirmed.
Instead, the results of the five-year study showed “no significant variation in the benthic macroinvertebrate communities stemming from the activity of the hydropower plant”. A second study looking specifically at the functional traits of the macroinvertebrates before and after the plant’s installation – meaning, how they interacted with their environment and with other species – and did not find any significant differences, either.
“One really important point is that there are no fish in this particular part of the river,” says Roberta Bottarin, a limnologist and the vice head of the Institute for Alpine Environment at Eurac, who co-authored the study. “If there had been fish, we would probably have had very different results.” In her view, other factors, including the relatively large amount of water left in the original stream, may also have helped buffer the impact of the plant.
Still, she cautions against interpreting the results as a green light for small hydropower in the high mountains, emphasising that the team only measured the impact on the macroinvertebrates, not on the wider ecosystem. “From an ecological point of view, these high mountain streams are very sensitive. And if you build a plant there, you risk disrupting that ecological continuity and disturbing the natural balance. It’s really something you have to weigh very carefully.”