Large dams are disastrous for freshwater ecosystems. From preventing salmon migrations, to flooding niche river ecosystems, to blocking nutrient-rich sediment from getting downstream, hydropower is a catastrophe for wildlife. Yet currently, hydropower supplies more than half of our renewable energy. Isn’t cutting our dependence on fossil fuels worth the biodiversity cost of new dams?
The problem with that line of thinking is right there in the question. Dwindling biodiversity and rising emissions are not two separate issues: they are inextricably linked. The ecosystem degradation caused by large hydropower doesn’t just accelerate the global decline in freshwater biodiversity. It also displaces communities, undermines sustainable development, and contributes to climate change.
Green hydropower: fact or fiction?
Although often mis-labelled as renewable, hydropower is not actually net zero. In fact, many hydropower projects are responsible for more greenhouse gas emissions than an equivalent coal-power plant.
Dams transform fast-flowing rivers into large, stagnant reservoirs. They flood vast swathes of carbon-storing forests, wetlands, and peat swamps. When the flooded ecosystems rot, they release enormous quantities of methane gas—much as burning fossil fuels for energy releases greenhouse gasses once trapped in organic matter.
Beyond these direct greenhouse gas emissions, dams also prevent river sediment from reaching the ocean. When allowed to reach the ocean, this sediment not only nourishes highly biodiverse river delta ecosystems, but also traps 200 million tonnes of carbon annually. Again, biodiversity loss directly undermines hydropower’s net zero ambitions.
River deltas, like the Okavango delta pictured here, are some of the most biodiverse ecosystems on earth. Dams pose an existential threat to their survival. Image: © Michel Roggo.
Indeed, the United States and Western Europe are already lessening their dependence on hydropower. Along with its destructive impact on local ecosystems, large hydropower presents a number of logistical challenges that often make it more trouble than it’s worth.
On average, dam construction costs twice as much and takes 50 per cent longer than projections claim—meaning that in reality, it’s just not worth the price. Over time, river sediment clogs dams. The decreased reservoir capacity and damage to machinery severely reduces the amount of energy generated. Climate change and rising water demand means lower river volume, and hence shrinking hydropower capacity.
Inga 3: A case study in destructive hydropower
Yet the global North has proven less willing to apply this knowledge to hydropower investment in less wealthy nations. In the Democratic Republic of the Congo (DRC), the proposed Grand Inga Dam would be the largest hydropower project in the world. The project builds on the legacy of Inga I and II, two dams constructed in the 1970s and 80s. Today, Inga 1 and 2 run at less than 50 per cent capacity because of sediment build-up.