The cheapest path to decarbonization relies on firm and clean electricity to support variable renewable energy. As I’ve written previously, geothermal could be one such clean energy technology, but hydroelectricity is the largest source of renewable firm generation in both the US and the world today and has been for half a century. But the dams required to make hydroelectricity are controversial — so much so that a diverse coalition of environmentalists, outdoors enthusiasts, tribes, and agricultural interests have all called for some level of dam removal. If met, their desire could have serious climate implications. Just how far back would bringing down American hydroelectric dams set climate progress?
I analyzed the problem and found that dam removal and meeting clean energy goals are not necessarily at odds — that the apparent tradeoff between hydro's climate benefits and costs is less than what it might seem. While the most hazardous US dams make up most of our hydroelectric capacity, many of them are quite small, and their removal wouldn’t substantially threaten clean energy goals.
Hydroelectricity is a major part of the modern energy system, but it comes at a great cost. Dam construction has displaced millions of people globally, often from disadvantaged communities and often in unjust ways. Dams also create very modern risks and serious ecological disruption, both of which could be exacerbated by climate stressors. When poorly maintained dams fail or are threatened by flooding, as detailed recently in Michigan and China, thousands more can be displaced or at risk of death.
Only 3% of the nearly 80,000 dams on US rivers actually produce electricity — most are for irrigation, flood control, recreation, or other purposes. Those electricity-producing dams have a nameplate capacity of over 103GW and produce 20% of our emissions-free electricity, about as much as wind energy.
The Army Corps of Engineers collects data on most of these power-producing dams in the National Inventory of Dams (NID). If they are large or dangerous enough, they are listed in the NID and ranked by hazard level — high, significant, and low. Around half of the 2600 power-producing dams are deemed high hazard (shown in the figure below).
The hazard status indicates what types of damage would occur if the dam were to fail — a “high” hazard status means the loss of human life in addition to environmental and economic damage (the condition for significant hazard) is likely to occur upon dam failure. This high hazard is not to be confused with the condition of the dam. The “loss of human life” standard means a dam could be deemed highly hazardous simply by its size or proximity to towns or other buildings, even if its condition is pristine. It also means that there is likely a wide range of hazard — some dams might threaten a single structure or dwelling, others may threaten thousands.
The bad news is that the highest-hazard hydroelectricity plants generate a substantial portion of US electricity. By cross-referencing NID dams with Energy Information Administration (EIA) data, I found that high-hazard dams make up over 70% of the total US hydroelectric capacity. After resolving inconsistencies and repeated values in the NID and EIA datasets, breaking down the remaining 600 high-hazard dams by size reveals that the average nameplate capacity of these dams is fairly large at 119MW. That means a single dam could power a small city (~100k homes). This fact is unsurprising given how hazard is defined — the dams that produce the most electricity will tend to be larger, and therefore more likely to cause enormous impacts on failure. They also could be considered a shortlist of the potentially most environmentally and ecologically destructive dams that produce electricity.
This is the apparent challenge. Removing high-hazard dams makes sense, but they also make up the majority of firm and emissions-free hydroelectricity.
So is dam removal a nonstarter for climate action? Not really. Going deeper into the data shows that this average and the total capacity of the high-hazard dams is misleading and that the vast majority are actually quite small.
In fact, half of all high-hazard dams in the dataset are smaller than 20MW, totaling less than 2GW of capacity. Almost a quarter were smaller than 5MW, making up a mere 0.5% of the total high-hazard dam capacity. And, though less than 20% of the total capacity, dams smaller than the simple average (119MW) are 80% of the total dams in the dataset. Conversely, the 16 dams over 1GW in capacity represent one-third of the total high-hazard dam capacity (26GW).
When charted (shown below), a clear trend emerges: as dam size goes up, their frequency goes down while their share of the total capacity goes up.
This analysis has an obvious takeaway — even if the US dismantled a significant number of dams, its clean energy supply would be only marginally impacted.
Perhaps, just as importantly, the removal of every small, high-hazard dam could be compensated for with newer, less hazardous, and less ecologically damaging hydroelectricity. The Department of Energy (DOE) predicts that it is possible to have as much as 50GW of new hydroelectricity by 2050. Though any new dams are likely to face strong opposition and difficult financial prospects, solutions as simple as making existing hydroelectric dams more efficient or adding ways to make electricity from the ~77,000 dams that don’t produce any are available. With these steps alone, we could squeeze 12GW from already existing dams, an increase of 12% over the existing 103GW of US hydroelectric capacity. When adding in new pumped storage or innovative solutions like generating electricity from the water flowing in pipes underneath cities and towns or smaller and lower impact “standard modular hydropower,” we could create more than 50GW of new hydroelectricity.
Though removing many high-hazard dams wouldn’t significantly jeopardize climate goals, we should reckon with the potential non-energy tradeoffs of dam removal, since communities adapt to and sometimes depend on the altered environment created by the dam.
Consider the high-hazard, 1.3GW, Glen Canyon Dam (GCD) in Page, Arizona. GCD has long been despised by river conservation and restoration groups for causing the Colorado River to run dryer than it would undisturbed (among other environmental impacts). Its history is also highly problematic from a cultural perspective. After settlers and the US government violently displaced the Native American communities in the region, the GCD’s construction ignored their cultural ties to the region and made traditional livelihoods more challenging.
But the GCD is also a major economic force for the surrounding northern Arizona, southern Utah, and Navajo Nation communities. In one form or another, Lake Powell tourism, Colorado River rafting, casinos, and numerous other businesses all benefit from the dam’s cheap electricity or controlled water flow. Removing it would disrupt thousands of lives who have adapted or grown around the dam’s existence, including the descendants of those unjustly displaced in the first place.
The benefits of removing the Glen Canyon Dam probably outweigh the costs. Nonetheless, it illustrates how complicated dam removal can be. Even if not strictly “necessary” from a clean energy perspective, removal must be weighed against other human impacts beyond just decarbonization.
But the analysis demonstrates there are many dams that are even less beneficial than the GCD. Though megadams like GCD are an obvious target, progress on dam removal can be made while also minimizing the hit to clean energy. And, though unlikely to play a significant role in the US, hydroelectricity’s flexibility — sometimes operating as baseload, sometimes as peaker, sometimes as storage — will allow easier integration of variable renewables and make it a useful part of the energy system for decades to come. From a climate perspective, bringing down all dams would not be wise, but bringing down some of the smaller, more hazardous, dams would not really be that risky.
Editors Note: A previous version of this article referred to the risk level of dams instead of their hazard level.