Recent legislative proposals in a number of states across the country have reignited the debate over how  ‘sustainable’ hydropower actually is,  and if it is truly emissions free. California’s Assembly Bill 1771, which was rejected in the state legislature this past April, would have allowed large hydropower facilities to contribute toward state Renewable Portfolio Standards (RPS). As a growing number of states establish increasingly ambitious targets for shares of energy production from renewable sources, there has been ongoing discussion about what types of hydropower should be included in these RPS schemes.

In the United States, state regulators divide hydro into two categories – small and large – depending on the facility’s installed generating capacity. For example, California considers any facility with at least 30 megawatts (MW) of capacity to be ‘large hydro’. Currently, utilities in most states can count only ‘small hydro’ toward RPS targets.

Zipingpu Dam in China's Sichuan Province

California’s Assembly Bill 1771 is neither the first nor the only proposal of its kind. As states that have implemented RPS programs scramble to reach their renewable energy targets, the movement to count large hydro towards these goals has gained momentum.  Similar bills have been proposed in California in the past, as well as in Minnesota. North Dakota currently counts all hydropower in its RPS, including power imported from Manitoba, but stipulates that large hydro facilities must have been placed in service on or after Dec. 31, 2010. Wisconsin will allow utilities to count hydropower from large facilities starting in 2015. 

Despite the ultimate defeat of Assembly Bill 1771, the proposal highlights the importance of understanding the distinct economic and ecological characteristics of large hydropower that distinguish it from sustainable renewable energy sources. These include the relative maturity of hydropower techonology, as well as its impacts on sensitive ecosystems, land use considerations, and negative effects on cultural and community resources.

A major reason that policymakers do not traditionally include large hydro in renewable energy targets is that RPS policies aim to encourage development of new renewable sources, including those – such as offshore wind and concentrated solar power (CSP)– which are still in developmental stages and need investment for economies of scale to kick in. Since many states already derive a large share of their energy from large hydro facilities that have existed for years, including this technology in an RPS would divert much-needed investment away from less mature technologies and diminish the intended impact of any RPS scheme.

Compared to most renewable energy sources, large hydropower is a mature and  financially viable technology; it has been used in various forms since antiquity, and modern usage for electricity generation dates back to the 19th century. Hydro accounts for approximately 18 percent of electricity generation globally, and at least 50 percent of the total electricity supply in more than 60 countries.  In the United States, about 9 percent of total electricity is generated by hydro, although this varies widely by region.  Hydropower provides around two-thirds of electricity generation in the Pacific Northwest and 17 percent of electricity generated in California. In addition to incubating emerging renewable energy markets, RPS policies also aim to achieve environmental and climate mitigation benefits by promoting cleaner and low-carbon energy sources. Although hydro is generally considered a carbon-neutral method of generating energy, research has shown that there is considerable uncertainty in assessing the carbon dioxide (CO2) emissions from hydropower facilities. Aside from the emissions attributed to the construction of the facilities, considerable greenhouse gases (GHGs) are emitted as a result of biomass decomposition from reservoir flooding to create hydro dams. The rate of decomposition is highly variable depending on the climate zone (i.e. tropical, boreal) as well as the specific characteristics of the flooded area (i.e. river area, wetlands, forest area), making it extremely difficult to determine the real rate of emissions. For example, the rate of decomposition is higher in tropical regions, where emissions from reservoir flooding are up to 20 times higher than in boreal areas. Post-flooding biomes and water columns also typically capture and store less carbon from the atmosphere, further contributing to net carbon emissions.

Soils, wetlands, forests, and other ecosystems play an integral role in the global carbon cycle, acting as sinks by absorbing vast amounts of carbon dioxide from the atmosphere. A study conducted by the United States Forest Service in 2011 estimated that the world’s forests absorb 2.4 billion tons of carbon dioxide every year, which accounts for roughly one third of the carbon annually emitted through the burning of fossil fuels. Because many hydroelectric reservoirs are located in areas close to forests and other green landscapes, flooding is extremely common,  preventing trees, plants, and soils from absorbing carbon. The creation of hydropower dams also contributes to bacterial accumulation on plant matter at the bottom of reservoirs, leading to accelerated decomposition of this organic matter which emits CO2 and methane, both greenhouse gases.

Recent studies have estimated that net hydropower GHG emissions can be up to two-thirds of those from a natural gas power plant – most of which occur during the first ten years following reservoir creation. GHG emissions from hydropower are comparable to those from nuclear and higher than those of most renewable options, with figures varying depending on site location and other considerations.

The Hoover Dam drastically changes the landscape of the Colorado River's Black Canyon.


In addition to climate impacts from GHG emissions, large hydropower has a long and controversial history of ecological disruption and human displacement. Reservoir flooding destroys ecosystems and local biodiversity. Hydropower facilities also have direct impacts on migratory routes for fish, natural river flow and intensity, and water temperature and composition. Many large hydro dams, such as the Three Gorges Dam in China, require forced relocation of local communities.

Regulators and policymakers should carefully weigh this range of issues when assessing hydropower facilities, determining what emissions or emissions reductions would have naturally occurred in the region in question, as well as the impact of such facilities on specific regions, landscapes, biomes, and human communities.

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