For thousands of years, people have been harnessing the energy embodied in flowing water. Originally, this energy was used mechanically to grind grains, saw timber and facilitate other labor-intensive tasks. Over the past century, increasingly large hydroelectric dams have been providing massive amounts of electricity around the world, and today hydropower makes up about 7% of the electricity generated in the United States. Though supporters tout hydropower as a clean and sustainable way forward, large dams do present serious environmental concerns. While a range of technologies exists to harness hydropower – each with a very different profile of benefits and impacts – this post focuses primarily on large hydroelectric dams.
In 1936, the Hoover Dam began generating electricity, becoming the largest hydroelectric facility in the world. With a nameplate capacity of 1,345 megawatts (MW), it can generate as much electricity as a large coal-fired power plant today. In 1942, the behemoth Grand Coulee Dam took the title of the world’s largest electricity generating facility with a capacity over five times greater than Hoover’s (about 6,800 MW). The trend continues to this day, and in 2008 China’s Three Gorges Dam opened with an unimaginable capacity of 22,500 MW. That’s about the same as fifteen average nuclear power plants. It’s clear that hydroelectric dams can generate enormous amounts of electricity without consuming any fossil fuels, but it’s important to consider the substantial impacts associated with their development.
Put simply, dams are built to block a river’s flow, allowing water to pool upstream of the dam, forming a reservoir. The water in the reservoir is then let through the dam in a controlled way, where it pushes turbines that generate electricity. The potential energy of the reservoir water is converted into the mechanical energy of the rotating turbine, and finally into the electricity that powers our homes.
On the scale needed to generate thousands of megawatt hours of electricity, creating these artificial reservoirs floods immense areas of land upstream. In fact, the reservoir created by the Three Gorges Dam is so massive that it flooded over 240 square miles of land, prompting the forced relocation of 1.4 million people. And the dams don’t just affect human populations; blocking free passage downstream affects migratory routes, such as those of salmon and trout, and causes fragmentation of entire aquatic ecosystems.
The reservoirs also release more greenhouse gases (GHGs) than uninterrupted rivers. Although hydroelectric power is considered renewable and doesn’t release any “tailpipe” emissions, large dams are responsible for increased GHG emissions from the flooded areas upstream. The emissions spike is due to the flooding that occurs when these reservoirs are created. All the vegetation in these areas decomposes quite rapidly once it becomes submerged, and for many dams the affected areas are not just fields or swamps but are often large portions of vibrant forests. Measuring emissions from decomposing organic matter over vast landscapes is notoriously difficult. While many believe these emissions are an unfortunate cost of an otherwise beneficial technology, some are beginning to question whether the GHGs that result from certain large dams rival those of fossil fuel-based facilities. The data are far from conclusive, but it appears clear that the type of landscape that’s flooded is the biggest determinant in the volume of emissions. Despite the uncertainty, add to this the significant emissions associated with construction, including pouring thousands of tons of concrete, and hydropower’s environmental profile becomes even less rosy.
Still, hydropower currently provides thousands of MW of renewable generating capacity. One of the main focuses for increasing hydropower generation in US is on upgrading non-powered dams. In a report from April, 2012, the Department of Energy found existing dams provide the potential for up to 12,000 MW of new capacity. And since this would all be installed on existing dams, developing it would avoid several environmental impacts (or, more accurately, the harm to the environment would have already been committed). Other future development will likely utilize run-of-the-river technology, which does not require a sizeable reservoir. These upgrades and newer technologies, combined with the fact that the US is reaching its limit for viable large hydroelectric dam development, suggest that the landscape of hydropower is changing.
The Verdict: Hydropower provides enormous amounts of electricity by taking advantage of a renewable resource. Although the environmental impacts of these facilities are significant, so is the potential for growth in ways that can avoid many of these issues. Where the damage has already been done, we can take advantage of opportunities to harness more energy. Where pristine rivers and aquatic ecosystems remain, we have the technology to consider alternatives to the concrete giants of our past.