Standing on a dam, many people see a technological marvel and a clean, renewable energy source. However, behind all the engineering and carbon-free energy lies a hidden side: the startling environmental impacts of dams.
Dams have become an essential part of the world we live in, providing hydropower that accounts for 5.7% of total energy produced in the U.S., according to the U.S. Energy Information Administration (EIA).
According to the U.S. Department of Energy, fewer than 2,700 of the 90,000 dams built generate hydropower. The rest of the dams are thousands of smaller ones that have a significant impact on the surrounding environment, especially on migrating fish.
The ecosystem
“Dams are essentially a giant chunk of concrete and steel in a river. Any time you put something like that into a natural system, it changes the way that system operates,” said Kelly Catlett, the Hydropower Reform director at American Rivers, a nonprofit organization dedicated to protecting rivers in America, along with the habitats and wildlife around them.
One direct effect that dams have on their environment is the disruption of river flow, which can lead to problems both upstream and downstream of the river. Upstream, the dam creates a reservoir, a man-made lake where water is stored before being released by the dam. In these reservoirs, water does not move frequently, which can cause it to heat up and lead to problems downstream.
“Sometimes, when you have a reservoir that heats up water, and you send that downstream, that can change the temperature of the whole stream,” Catlett said.
The increase in temperature can cause problems for migrating fish, as salmon cannot survive well at temperatures above 68 degrees Fahrenheit. Dams have also been shown to generally increase downstream river temperatures. In a study conducted by the U.S. Department of Agriculture, small dams were found to increase the water temperature downstream on average, with the dams sometimes heating temperatures by 5.25 degrees Celsius, or 9.45 degrees Fahrenheit. Overall, the introduction of a dam into an ecosystem can disrupt the natural cycles that were previously present.
“There are a lot of things like bugs, fish, and wildlife that have built their life cycles around the cues that they get from how much water is in the river and at what time,” Catlett said. “When you disrupt that, you disrupt that system.”
Fish Migration
In the Pacific Northwest, dams are often built on rivers that anadromous fish use for spawning and reproduction. Anadromous fish, like salmon and steelhead, are fish that migrate from the saltwater seas to freshwater rivers to spawn. These fish rely on spawning areas in rivers to reproduce, and some dams built on the rivers in this region block off a large portion of the spawning areas.
“The series of dams that we constructed in the 1940s, 50s, and 60s along the foothills that surround the Central Valley blocked most upstream fish migration and restricted those fish to spawning in the valley floor, which is pretty inhospitable and not what they are used to doing,” said Andy Fecko, the Placer County Water Agency’s water manager, when talking about dams in California.
The reduced spawning area led to a decline in the fish population, and new methods were implemented to help the fish traverse over the dams.
“When you have the dam blocking the route for migrating fish, the fish can’t return, and over time, their population is going to fall,” said Jaziel Salómon, an Advanced Placement (AP) Environmental Science teacher at Carlmont. “Fish ladders are one of the solutions. A fish ladder is a series of artificial waterfalls that a fish can jump up.”
Other types of fish ladders function in different ways, but in general, fish ladders provide routes for migrating fish to travel upstream.
Unfortunately, they are not particularly effective. A study by Henry Hershey, published in Fish and Fisheries, that synthesized the findings of 75 studies on the effectiveness of fish passageways found that fish ladders had an average efficiency of only 60%. Furthermore, fish ladders are only effective on smaller dams; on larger dams, trap-and-haul methods must be employed.
Trap and haul procedures, also known as collect and transport methods, are more effective ways to transfer fish between the two sides of a dam, but they also cost much more. The process requires fish to be captured and transported by truck to the other side of the dam. This process must also be repeated for the fish travelling back downstream, and, in total, it costs between $1 million and $12 million per dam.
“It’s a very expensive endeavor, but it’s one that California water managers believe could help the salmon and steelhead populations, which are at historically low levels right now,” Fecko said.
Removing dams
In certain cases, if a dam is old enough and has a significant negative impact on the environment and the surrounding community, it may be considered for removal.
Most of the dams in the U.S. were built during the early 20th century. According to the National Inventory of Dams, almost 70% of dams were built before 1973. These older dams were typically built with less consideration for the environment and have also deteriorated over time. Once these dams no longer benefit society as a whole, they can be removed.
One example is the Klamath River, a large river that runs through northern California and Oregon, with dams built on it starting in 1918. In total, six dams were built on the river, with the last one being built in the 1960s. These dams were all owned by Pacificorp, and the four dams located in the lower parts of the river were constructed for hydropower generation.
The Klamath is also the third-largest anadromous fish-producing river on the West Coast. As a result, the dams on the river began to cause numerous problems. The fish were forced to breed in the lower part of the river, and many died off as a result. In 2002, an estimated 33,000 fish died while migrating due to a parasite outbreak, which was caused by a combination of low river levels that forced the fish close to each other and increased water temperatures, likely resulting from the dams.
This prompted protests for change, led by Native Americans who relied on the fish for food, as well as others, including Catlett.
“I was there on behalf of Friends of the River, advocating for environmental protection measures to address the impacts of the project’s operations,” Catlett said.
Pacificorp and the group reached a decision in 2009 to remove the dams, and they agreed to spend $500 million on the removal. They decided to remove the dams because keeping the dams running would not have generated a profit.
“It became clear that the cost of keeping the dams was going to be more than the cost of removing them,” Catlett said.
The removal of the dams was finally completed in November 2024; however, the impact of the removal on fish populations has yet to be observed. It has been predicted that it will take another 16-25 years before the fish populations return to their original state on the river.
The importance of dams: How we move on
Along with electricity, dams also provide flood control, water supply, and drinking water. This makes them essential in many places, including the Central Valley of California.
“Without dams, we wouldn’t have a water supply in the summer in California,” Fecko said. “We wouldn’t have hydropower generation, and we wouldn’t have, maybe most importantly, flood control.”
Additionally, with the number of dams that exist in the world, removing most of them is too expensive and not worth the benefits that would be lost. The dams can also benefit the environment if they are conscious of their surroundings.
“There are a lot of benefits that society gets from a well-maintained, well-operated hydropower project,” Catlett said. “Dams are going to continue to be a part of our society, a part of our world, and a part of our river systems.”
Instead of focusing on completely removing dams, Catlett suggests that we should focus on mitigating the effects of dams.
“The goal really needs to be, in most instances, to bring the impacts of dams down to a manageable level,” Catlett said.
