Industry Market Trends

The Damage Done, Part 6 -- How Green Is Hydro Power?

Jan 10, 2012

Nurek Hydropower plant, KyrgyzstanWhat could be more environmentally friendly than generating electricity from flowing water? Water has to run downhill anyway, so why not let it turn the wheels of a turbine while it's on the way down and generate some juice? No noxious air pollution, no greenhouse gases -- green and clean, right? (Photo: Nurek Hydropower plant, Kyrgyzstan. Credit: Oleg1975, CC BY 2.0.)

But here at ThomasNet, we hate to take things for granted. That's why we've undertaken the "Damage Done" series examining the environmental effects of energy sources. Each week, we're examining one of the sources of electric power -- both "green" and conventional -- to try to make an apples-to-apples comparison of their environmental impacts. To see how we're doing so far, take a look at our articles on coal, natural gas, and nuclear.

We now turn to hydro as the first of the supposedly "green" energy sources. It's easy to claim that hydroelectric generation is obviously greener than coal or gas or maybe nuclear. But how do we know for sure? What are the life-cycle environmental effects of a kilowatt hour (kWh) of hydro-generated electricity, and how can you measure them?

The clean-energy group Renewable Energy Policy Network for the 21st Century (REN21), in its "Renewables 2011 Global Status Report," says that hydropower production represented about 16 percent of electricity production worldwide in 2010, with global hydro capacity at 1,010 gigawatts (GW). According to U.S. Energy Information Administration data, the U.S. gets 6 percent of its electricity from hydro.

Percentage of US power generation by source

The Life Cycle of Hydropower

In an article for Renewable Energy ("Dynamic life cycle assessment (LCA) of renewable energy technologies," Renewable Energy, 2006), Martin Pehnt of the Institute for Energy and Environmental Research in Heidelberg, Germany, puts together dynamic life cycle assessments for a number of renewable energy technologies, including hydropower.

Pehnt says that Life Cycle Assessment (LCA) "investigates environmental impacts of e.g. systems or products from cradle to grave throughout the full life cycle, from the exploration and supply of materials and fuels, to the production and operation of the investigated objects, to their disposal/recycling." Increasing concerns about the environmental effects of energy production call for greater focus on the upstream and downstream impacts of energy conversion systems, says Pehnt.

Pehnt's study takes Germany as geographic reference and 2010 as time reference. The study assesses several environmental impacts:

  • Energy resource consumption (that is, consumption of non-renewable energy somewhere in the chain to produce renewables)
  • Non-energy resource consumption
  • GHG emissions contributing to global warming
  • Eutrophication (i.e., excessive algae growth in water, or blooms, due to deposition of chemicals such as nitrates and phosphates)
  • Acidification (i.e., decrease in pH and increase in formation of acid in water, particularly in seawater)

The following chart sets out some of the measurements cited by Pehnt, showing how the environmental impacts of renewable sources compare to those of a conventional mix of energy sources.

Environmental Impact Conven-tional Energy Mix impact

Hydro impact

/kWh (large facility)
Wind (on-shore) impact

PV Solar impact

Geo-thermal impact

Non-Renewable Energy Demand in megajoules (MJ)






Iron Ore in grams (Proxy for non-energy resource consumption)






Global Warming (g CO2 equiv.)






Acidification (mg SO2 equiv., or sulfur dioxide equivalents)






Eutrophication (mg PO43- equiv., or phosphate equivalents)







You can see that hydropower compares favorably to conventional energy in all respects. The value chain for hydropower uses significantly less non-renewable energy and results in considerably less pollution in CO2 equivalents, in acidification, and in eutrophication. In resource usage, hydropower consumes somewhat less iron ore than conventional energy, though not a lot less. As a key mineral resource, iron ore appears in Pehnt's study as kind of a proxy for resource consumption in general.

Does Hydropower Contribute to Climate Change?

Hoover Dam hydroelectric generators, 1991While the Pehnt study does include global warming potential in its life cycle assessment of hydro power and other renewables, another study more specifically addresses the carbon footprint of electricity production.  (Photo: Hoover Dam hydroelectric generators, 1991. Credit: Brian Snelson, CC BY 2.0.)

An article in Energy Policy by Benjamin K. Sovacool of the Vermont Law School includes a comparison of the life cycle carbon footprints of many sources of electrical energy (see "Valuing the greenhouse gas emissions from nuclear power: A critical survey," Energy Policy, 2008.)

Carbon footprint is normally measured in "carbon equivalents," a term that wraps all of the greenhouse gases (GHGs) into one metric, to account for an emission source's contribution to climate change. Carbon footprint is expressed as "gCO2eq/kWh," short for "grams of CO2-equivalent per kWh."

The following chart based on Sovacool's comparison shows the life cycle carbon footprints of several key energy sources. Note that hydropower has one of the lowest footprints, just slightly above wind.

Source/Technology Lifecycle CO2 Equivalents (gCO2eq/kWh)
Coal 960 to 1,050
Natural gas 443
Nuclear 66
Solar Photovoltaic (PV) 32
Hydroelectric 10 to 13
Wind 9

But What About Those Dams?

Hydroelectric dam in UkraineThe Pehnt and Sovacool assessments paint a fairly rosy picture of hydro as an environmentally-friendly energy technology. However, many researchers have expressed concern about the environmental effects of damming up rivers and creating large new bodies of water upstream. This changes or destroys ecosystems and habitats, not to mention making areas of fertile land unusable for agriculture. Silt builds up upstream of the dam, and silt that would normally be deposited downstream never makes it there. Photo: Hydroelectric dam in Ukraine. Credit: Giorgio Monteforti, CC BY 2.0.)

In an article for Energy, Daniel Weisser of the International Atomic Energy Agency investigated life cycle GHG emissions from electricity production, including renewable sources. (See "A guide to life-cycle greenhouse gas (GHG) emissions from electric supply technologies," Daniel Weisser, Energy, September 2007.) As with other studies, Weisser finds that hydro compares favorably to other generation technologies, especially fossil-fuel.

In his section on hydroelectric generation, though, Weisser writes that

[I]n some cases hydro power plants that use reservoirs can emit significant quantities of GHGs that easily surpass all other GHG emissions in the energy chain, due to land-clearance prior to construction but especially due to flooding of biomass and soil. For example, flooded biomass decays aerobically -- producing carbon dioxide -- and anaerobically -- producing both carbon dioxide and methane. The amount of GHG release depends on reservoir size, type and amount of flooded vegetation cover, soil type, water depth, and climate.

In his 2001 book Silenced Rivers: The Ecology and Politics of Large Dams, Patrick McCully, then executive director of International Rivers and now executive director of Black Rock Solar, wrote,

The two main categories of environmental impacts of dams are those which are inherent to dam construction and those which are due to the specific mode of operation of each dam. The most significant consequence of this myriad of complex and interconnected environmental disruptions is that they tend to fragment the riverine ecosystem, isolating populations of species living up and downstream of the dam and cutting off migrations and other species movements. Because almost all dams reduce normal flooding, they also fragment ecosystems by isolating the river from its floodplain, turning what fish biologists term a 'floodplain river' into a 'reservoir river'. The elimination of the benefits provided by natural flooding may be the single most ecologically damaging impact of a dam. This fragmentation of river ecosystems has undoubtedly resulted in a massive reduction in the number of species in the world's watersheds.

Artificial lake in BulgariaHydro might look good if all you consider are studies like Pendt's and Weisser's. But all energy sources have environmental effects that are hard to quantify -- the toxic wastes deposited by coal ash, the effects on water supplies by hydraulic fracturing (fracking) to extract natural gas, the radiation emitted for tens of thousands of years by radioactive waste. (Photo: Artificial lake in Bulgaria. Credit: Klearchos Kapoutsis, CC BY 2.0.)

So, with hydropower, how do you put numbers to the ecosystem damage described by McCully? I don't think anybody has done that, but I did find an article in Science by Christer Nilsson and colleagues ("Fragmentation and Flow Regulation of the World's Large River Systems," Christer Nilsson, Catherine A. Reidy, Mats Dynesius, and Carmen Revenga, Science, April 15, 2005) that sets out a number of interesting figures, including the fact that out of the world's 292 large river systems (LRSs) 172 are affected by dams. They find that there are in the world more than 45,000 dams above 15 meters high. Here they describe some of the environmental effects of dams:

Inundation destroys terrestrial ecosystems and eliminates turbulent reaches, disfavoring lotic biota. It can cause anoxia, greenhouse gas emission, sedimentation, and an upsurge of nutrient release in new reservoirs. Resettlement associated with inundation can result in adverse human health effects and substantial changes in land use patterns. Flow manipulations hinder channel development, drain floodplain wetlands, reduce floodplain productivity, decrease dynamism of deltas, and may cause extensive modification of aquatic communities. Dams obstruct the dispersal and migration of organisms, and these and other effects have been directly linked to loss of populations and entire species of freshwater fish.

Harnessing the kinetic energy of water flowing downhill might sound like an obvious method for generating electricity. But 100 percent green and clean? Maybe not.