When the Hoover Dam was constructed it was the largest dam in the world. Photo: Andrew Parnell via flickr.
It's about 6% of the electricity in the United States, 15% in China (and climbing), 41% in Switzerland, 80% in Colombia, and 96% in Ethiopia. It doesn't involved sooty smokestacks or radioactive waste. Powered by flowing water, it's often lumped together with wind power, solar power, and biomass as renewable energy sources. But should hydropower really be considered a clean power source? The simplest answer is 'sometimes'. Here's why:There are three broad types of hydropower we'll consider here, differentiated widely in terms of scale and environmental impact.
Three Gorges Dam, Yangtze River, photo: Le Grand Portage via flickr.
This is what most people think of when you mention hydropower: The Hoover Dam, the Three Gorges Dam, the Sardar Sarovar Dam on the Narmada River in india. Big, lots of concrete, lots of land submerged behind them, lots of politicians like to stand in front of them, muscle-flexing TV shows are made about them. Often multi-gigawatt in size, lots of electricity is generated but in terms of environmental impact these are the most disruptive of the lot.
Chief Joseph Dam on the Columbia River in Washington, one of the largest run of river projects. Photo: US Army Corps of Engineers via Wikipedia
Run Of River
Instead of using a huge storage reservoir behind a towering dam, though still often damming the full width of the waterway, run of river hydropower projects rely on the natural flow of the water and the natural drop of the river to generate electricity. These generally have lesser environmental impact than big dams in terms of lesser displacement of people and lesser habitat disruption away from the river, but even though downstream flow is not affected as much, aquatic habitat can still be disrupted.
Though sometimes termed small-hydro--which compared to massive dams I suppose is true--run of river projects of multi-gigawatt capacity have been developed.
Mini & Micro HydropowerWhen hydropower gets scaled down past the community level you come into mini hydropower and micro hydropower--under 1,000 kilowatts and under 100 kilowatts respectively. Smaller still are pico hydro projects, under 5 kilowatts in size and capably of powering just a few lights or electric devices.
The Tungu-Kabri micro hydro project in Kenya (pictured at left), by Practical Action is a good example of these sort of projects. Others don't employ a small weir as this one does, instead relying on pipes to channel water past turbines.
On to the environmental issues...
Habitat Loss & DisruptionThe most visible and immediate impact of large-scale hydropower is from the reservoir behind the dam. There's no getting around the fact that you're drowning vast areas of land that was habitat for animals, and was likely a storehouse of biodiversity, sequestering decent amounts of carbon. You've also fragmented the habitat that remains.
That's just on land; building the dam (and this is true for both large-scale and run of river projects) disrupts the aquatic ecosystem as well, both upstream and downstream. There are ways of mitigating this, in some cases (not so much with large dams) in regards to wildlife, but some level of disruption is assured. Downstream, the changes in water flow that result from the water passing through the turbines, even if total volume is maintained, can lead to erosion, differences in oxygen levels and water warmth affecting animal populations.
This is all hard to quantify with a single statistic, in part because the conditions vary from project to project, but also because there are just so many areas of the ecosystem impacted. As you can imagine though, these sort of problems are greater with large-scale projects that community-level ones.
photo: Bert K via flickr.
Greenhouse Gas Emissions
While it is true that the actual production of electricity from hydropower doesn't release any greenhouse gases, when it comes to large dams that flood large areas of countryside that's decidedly not the case. Studies going back two decades now have demonstrated this.
An article from New Scientist lays it out:
This is because large amounts of carbon tied up in trees and other plants are released when the reservoir is initially flooded and the plants rot. Then after this first pulse of decay, plant matter settling on the reservoir's bottom decomposes without oxygen, resulting in a build-up of dissolved methane. This is released into the atmosphere when water passes through the dam's turbines.
Seasonal changes in water depth mean there is a continuous supply of decaying material. In the dry season plants colonize the banks of the reservoir only to be engulfed when the water level rises. For shallow-shelving reservoirs these "drawdown" regions can account for several thousand square kilometers. In effect man-made reservoirs convert carbon dioxide in the atmosphere into methane.
The exact greenhouse gas emissions from one of these hydropower reservoirs is going to vary from project to project, but in one example in Peru cited in the same article linked above emissions from electricity generation were three and a half times those if the electricity had been generated from oil (which is only slightly better than using coal). This is at one extreme, in areas with less biomass being submerged emissions are generally considered to be 2-8% of those from fossil fuels.
As you can see, each project is going to vary in this regard, but to assume that large-scale hydropower is emissions-free isn't entirely accurate. And that's not even taking into account the massive embedded emissions from constructing them.
Since run of river projects don't use reservoirs on the scale of the large dams, you've really only got the embedded emissions from construction to pay back before you're producing clean power. With small-scale hydro and its even smaller variations, for all practical purposes you've got a clean, renewable electricity source from the start.
International Rivers estimates that some 500,000 people downstream from Ethiopia's Gibe 3 dam on the Omo River will have their lives dramatically disrupted by the nation's push to further developed hydropower, much of the electricity from which will be exported to neighboring countries. The effects of the project may extend all the way to Lake Turkana in Kenya. Image: International Rivers.
In addition to these issues, particularly with large-scale projects and bigger run of river projects, the disruptions to the environment include impacts on people large enough that they become issues of environmental justice.
An estimated 40-80 million people have been displaced from ancestral homes because of hydropower projects. The problems with relocating people after the construction of the Three Gorges Dam in China are well-known, but this happens nearly every time a large reservoir is constructed. Then there's the effect on people of ecosystem changes downstream, which can certainly be significant.
Even if adequately compensated for the physical relocation, there unquantifiable disruptions to cultural patterns and traditions which can occur to the people all along the watercourse around the hydropower project. All of which is made worse when the power generated by the project is carried by transmission lines to places far away, with only a small amount of the electricity used locally.
As with the problems with habitat disruption and potential greenhouse gas emissions, the amount of the disruption to communities around the project goes up dramatically as the size of the project increases.
photo: Nikki via flickr
So Is Hydropower Clean or Not?
Back to that 'sometimes' from the intro...
As hopefully you've grasped, scale is everything when it comes to environmental impact of hydropower (as it is with most environmental issues). Large-scale hydropower dams certainly can provide a lot of power and are a renewable resource--assuming the water source doesn't decline, an real concern with climate change but something beyond the scope of this piece. Run of river projects generally have lower impact, but there are still serious environmental issues to be considered. Small, micro- and pico-hydro projects have even lower environmental impact and in places without grid access can be a great (if inherently limited) source of electricity. Ultimately, all these factors have to balanced against the impact of generating the same amount of power from other sources.
In the worst-case (large dams flooding forested areas) hydropower's greenhouse gas emissions and effects on biodiversity can be as bad as any fossil fuel. More usually, the greenhouse gas emissions, even for large projects, are a fraction of any fossil fuel alternative. The impact on ecosystems and the communities around the project have to be taken on a case by case basis.
The way to exploit hydropower electricity generating potential with the least-environmental impact? At small-to-medium scale and community level, without reservoirs, and in conjunction with other renewable energy sources such as decentralized solar power, wind power, or sustainably-scaled biomass.
More on Hydropower:
Hydropower Not Likely Under New Climate Future
Run Your Home on Small-Scale Hydropower
This is Big: US Could Double Hydropower Capacity With Minimal Impact, Says Steven Chu
Micro-Hydro Power: Small-Scale, Large Impact
China Mulls Building Dams in Southern Tibet
China Builds Dam on Indus, Doesn't Tell Pakistan