Weighing the benefits of hydropower against its damming consequences

At first glance, hydropower seems to have encountered some rough patches in recent years. Having gradually fallen out of favour amongst most developed Western countries – with France and Switzerland being prominent exceptions – hydropower development has hit another hurdle earlier this year even in the Global South, given Cambodia’s 10-year suspension of hydropower dams in the lower Mekong River.

Despite these bleak prospects, hydropower remains the largest renewable energy source by far, stemming from its tendency to gravitate towards mega-projects with vast economies of scale in power generation. Consequently, hydropower has been able to beat fossil-fuels in price-competitiveness for at least a decade, a stark contrast against some of its renewable energy counterparts which are still struggling to attain price-parity today. Hydropower also has comparatively low life-cycle CO2 emissions, allowing it to aid the fight against global warming.

Source: IRENA, 2020

Source: IRENA, 2017: 34

Why is hydropower so controversial then? For starters, hydropower is notorious for its devastating impacts on wildlife, notably through destruction of terrestrial habitats, interference with biogeochemical cycles and ecological fragmentation. By flooding sizable plots of land during their construction and operation, hydropower dams not only deprive terrestrial species of living spaces, but also set the stage for nutrients like nitrogen and phosphorous to be released into freshwaters following decomposition of the submerged organic matter, as demonstrated in the upper Mekong River. Subsequent onset of eutrophication – broadly defined as the excessive growth of algae and aquatic plants due to elevated nutrient levels – has been harmful to other aquatic species, by decreasing availability of dissolved oxygen and even poisoning the surrounding water on occasion.

Meanwhile, hydropower dams also impede the migration of freshwater fishes, which severely disrupts their reproductive cycles and reduces diversity across ecological gradients. Though fish-passages have been proposed as a mitigating measure, studies suggest that these range from being ineffective to outright detrimental, indicating that the root problem remains at-large. The video below also outlines other costs of hydropower, which must be addressed for it to gain widespread acceptance.

Furthermore, even its purported benefits of reducing CO2 emissions do not accurately represent the contribution of hydropower towards global warming, as this neglects the impact of other greenhouse gases (GHG). Notably, significant volumes of methane (CH4) – a greenhouse gas with 25 times the potency of CO2 – is known to be emitted from large dams in tropical regions, as they contain reservoirs for biomass to accumulate under warm temperatures. Methanogenic bacteria can then break down this waste material in the absence of oxygen to release CH4 gas. Accounting for both CO2 and CH4 production over a 100-year lifecycle causes 18% of hydropower reservoirs examined in the Mekong River Basin to have GHG emissions comparable with that in fossil-fuel power stations. While most hydropower dams there have GHG emission intensities in line with other renewable energy sources, this case-study highlights the importance of conducting site-specific environmental impact assessments instead of assuming all hydropower is equally clean, as inadequate due-diligence may result in highly detrimental outcomes.

Source: Demarty & Bastien, 2011

My opinion of hydropower is far less positive than solar or wind energy. While generally supportive of the latter 2 technologies aside from specific circumstances, I adopt the opposite stance here – that its costs outweigh the benefits in most situations, since measures taken to mitigate its detrimental ecological effects have been largely unsuccessful. However, I believe there is much potential in exploring smaller-scale hydropower technologies like in-stream energy converters, which may prove less destructive for their surrounding ecosystems. But until I learn of any revolutionary transformation in hydropower technologies, I will continue approaching them with a healthy dose of skepticism!