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Hydropower: The Good, the Bad and the Ugly

Welcome back to yet another Friday morning ClimateRoots newsletter where, as promised, we will be discussing some of the pros and cons of hydropower. The often heated debate around damming of rivers for use as both an energy production method and water source in the United States goes back to the early 1900’s (Forest Service - History). At that time Gifford Pinchot (the first chief of the US Forest Service) and John Muir (founder of the Sierra Club) were engaged in a very public debate surrounding the proposed damming of Hetch Hetchy Valley in today’s Yosemite National Park. More on that later; for now, let’s dive into the good and the bad of hydropower.

Many of the benefits of hydropower were discussed in last week’s education session (click here if you missed it), but we’ll recap a few of them here as well as introduce a few that are new! For starters, hydropower is not only a clean and renewable energy source, but it is extremely flexible. Dams are able to release a precise amount of water to produce power when we need it most. In this regard, it mimics the fossil fuels that currently provide energy to our energy grid; something that wind and solar power are unable to do. Not only does hydropower easily match our energy demand, but it is also extremely efficient. The precise control of energy output leads to minimum wasted energy while production itself boasts 90% efficiency, compared to solar, wind and coal whose efficiencies are all in the mid 30%'s (Pros and Cons of Hydroelectric Energy - Kiwi Energy). An inevitable side effect of dams are the reservoirs that they create. These reservoirs provide a great outlet for outdoor recreation in the communities they serve, including swimming, fishing and boating. In the same vein, the reservoirs held behind dams help with flood control and irrigation support while providing clean drinking water to millions of people. For example, nearly every state in the Western United States relies on dams for farm irrigation and drinking water. The use of a natural resource (i.e. a river) for human benefit in a sustainable fashion (i.e. dams) is generally referred to as conservation.

When studying the negative consequences brought on by the use of hydropower, it is important to start at the beginning of the damming process: the construction itself. Dams are made almost entirely out of concrete, and a whole lot of it at that! The production of concrete is an extremely carbon intensive process; one ton of CO2 is released for every ton of cement (the main ingredient in concrete) produced (“Emissions from the Cement Industry”). For reference, the Hoover Dam used over 8 million tons of concrete for the main and supporting structure. Construction projects as large as these are also bound to cost loads of money upfront, even though the operation costs of dams are market competitive (Pros and Cons of Hydroelectric Energy - Kiwi Energy). Some of the more obvious reasons hydropower can be harmful is the drastic impact on the surrounding ecosystems. This would primarily include the upriver and downriver ecosystems. The reservoir impedes river flow flooding large areas, destroying natural habitats and killing vegetation upriver of the dam. Dams will also reduce the oxygen content in a river, harming aquatic species downriver. Salmon, in particular, have been nearly wiped out because they can no longer return to their breeding grounds due to dams clogging up rivers. The natural erosion properties of a river (which carries sediment downstream) are drastically slowed since the sediment builds up behind the dam. Recreation activities downstream from dams, including rafting and fishing, are also impacted. Lastly, for a hydropower plant to produce power it needs a source of energy: water! This might sound obvious, but reservoir levels in areas that rely on hydropower the most are decreasing at an alarming rate due to drought.


  • Hydropower is clean, renewable, and flexible! We can control exactly how much energy is produced when we need it.

  • This is not something we can do with wind and solar without batteries or other storage options.

  • It’s efficient! Hydropower boasts a whopping 90% efficiency, compared to the 33% average efficiency of solar, 35% average efficiency of wind, and 37% average efficiency of coal (Pros and Cons of Hydroelectric Energy - Kiwi Energy)

  • Reservoirs create great opportunities for outdoor community recreation such as swimming, fishing, and boating.

  • Dams help with flood control, irrigation support, and can even provide clean drinking water.

  • The American West relies heavily on dams to provide farming irrigation and water.


  • The emissions impact of dam construction is massive. Building dams calls for roads and power lines being built and maintained, on top of the concrete emissions itself (“Emissions from the Cement Industry”).

  • Construction is expensive! Challenging terrain combined with massive scale is a recipe for delays and overspending. Although the cost of operation is very reasonable, the upfront cost is huge!

  • The reservoir created behind the dam can flood a large area, destroying natural habitats and killing vegetation upriver of the dam. Decreasing flow can decrease oxygen content in the water, harming fish and other aquatic life downriver.

  • Dams can also impede recreational activities such as rafting and have a huge impact on the communities down and upriver from them.

  • This past summer has shown us just how bad drought conditions are getting in the western US. It might sound obvious, but hydropower doesn’t work without water!

To provide some context of this heated debate, let’s dive into two examples of highly contested dams in the United States; the O’Shaughnessy Dam at Hetch Hetchy Valley in Yosemite National Park and the Glen Canyon Dam on the Colorado River in The Glen Canyon National Recreation Area. The O’Shaughnessy Dam was built after a long and heated debate between preservationist John Muir and conservationist Gifford Pinchot in the early 1900’s. The basic principle was that the dam would be (and still remains to this day) the principal water source for the city of San Francisco (Us). In Pinchot’s mind, this would provide the greatest good for the greatest number. Muir, however, would not stand for it. He argued that flooding Hetch Hetchy Valley, an area of Yosemite National Park that rivalled Yosemite Valley itself in beauty, would forever ruin the pristine wilderness and impede the wild ecosystem of the area. The Glen Canyon Dam found itself in the middle of a similar debate in the mid 1900’s when the flooding of the not well known, yet stunning Glen Canyon was proposed for the dam. The dam would flood the Colorado River at Glen Canyon, submerging many historic Native American sites and impede the flow of the river just upstream of the Grand Canyon itself. The construction of this dam provided power and water that helped grow the American West, but in the act all but destroyed the downriver ecosystem of the Grand Canyon and submerged Glen Canyon into the history books. There are still environmentalists today who push for the destruction of the dam to revive what was lost.

Hetch Hetchy Valley, before and after flooding / Image courtesy of San Francisco Chronicle

The bottom line is that all dams will have their pros and cons. The debate will never be resolved; rather the key to hydropower is in which dams are constituted as good and which are considered harmful. There are roughly 91,000 dams in the United States today (The Problem America Has Neglected for Too Long: Deteriorating Dams). This is a huge number impeding thousands of miles of river. The vast majority of these dams were completed during the Dam Building Era in the mid 1900’s, meaning they are decades old today and in many cases have out served their purpose. With limited resources to maintain all of these dams, many have begun to be removed, leading to extraordinary results for river ecosystems. Removal may make sense in many cases, but in others it may cause more harm than good. Many have claimed that we have now entered the dam removal era in the US, but in deciding which dams stay and which dams go, the dam debate will be here to stay!



  1. “Emissions from the Cement Industry.” State of the Planet, 9 May 2012,

  2. Forest Service - History.!ut/p/z0/04_Sj9CPykssy0xPLMnMz0vMAfIjo8zijQwgwNHCwN_DI8zPwBcqYKBfkO2oCADIwpjI/?pname=Forest%20Service%20-%20History%20&navtype=BROWSEBYSUBJECT&ss=110507&pnavid=150000000000000&navid=150140000000000&ttype=main&cid=null. Accessed 18 Nov. 2021.

  3. Pros and Cons of Hydroelectric Energy - Kiwi Energy. Accessed 18 Nov. 2021.

  4. The Problem America Has Neglected for Too Long: Deteriorating Dams. Accessed 18 Nov. 2021.

  5. Us, Phone: 209/372-0200 Contact. Hetch Hetchy - Yosemite National Park (U.S. National Park Service). Accessed 18 Nov. 2021.

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