The Positive Environmental and Safety Impacts of Dam Removal: Many Old Low Head Dams Remain to Be Removed

  

     The positive impacts of dam removal are well documented, as are the negative impacts of dam construction. While many dams do provide positive benefits such as flood control and electric power, there are negative effects such as disruption to fish spawning, especially for species like salmon.

     The group Rewilding Europe has begun removing dams on the continent and noting immediate positive impacts. The goal to remove unnecessary river blockages began in 2024 with the removal of five small dams on the Giovenco River in Italy. Since then, the group has restored about 70 miles of rivers to be free of flow restrictions. In Sweden, many dams were constructed long ago to float timber, and those dams no longer serve that function.

     According to an article in The Cool Down:

“Free-flowing waters are cooler, creating optimal conditions for native species to thrive. That helps combat the effects of climate change by encouraging native species to remain in their home habitats rather than migrate to regions where they could become invasive.”

"Removing dams, which really choke rivers, is critical," said one team leader with Rewilding Apennines. "Demolishing these barriers allows rivers to breathe again and better support nature and people. In the face of climate change, we must treat free-flowing freshwater as a precious resource to be cherished."

     The dam removals in Europe were accompanied by releases of native fish.

     In Washington and Oregon, dam removal projects encountered known dams and, unexpectedly, many undocumented dams. Some were found to be buried by vegetation and/or sediment.

     According to an article in Planet Sage:

“Scientists expected changes to unfold over months or years. Instead, rivers began reshaping themselves within hours. Channels widened, sediment shifted, and water pathways reformed as if the rivers remembered their original courses. The speed of this transformation surprised even experienced researchers.”

     One effect of dams is that they hold back sediment, which often accumulates at the dam where it is trapped. That sediment formerly flowed with the river, in some cases to coastal areas where it was deposited and helped to combat the effects of coastal erosion. While sediment flow was restored, there was also a temporary increase in water cloudiness. The response of fish moving into new areas also occurred faster than predicted. Other results included both increased riverbank erosion and increased sedimentation along the banks.  

“Removing old dams often caused water in certain areas to deepen as flow accelerated. Areas that previously resembled shallow ponds transformed into swiftly moving channels again. This deepening helped restore faster-flowing environments that many species depend on.”

“The shift also altered the distribution of nutrients and oxygen within the river. Scientists observed how these deeper channels played a crucial role in reestablishing natural river dynamics.”

     Vegetation along riverbanks also changed, reflecting changing water depths. New pools, riffles, and gravel bars were created, which also created new habitats with associated increases in biodiversity. This also happened very quickly.

“The speed and magnitude of the changes forced scientists to reconsider their assumptions about river recovery. Earlier models assumed slow, gradual adaptation, but the real-world changes were far more dramatic and immediate.”

“This realization may influence future restoration planning. Understanding that rivers can react so quickly helps improve forecasts and encourages more accurate, flexible management strategies.”

     In Oregon, hydroelectric dam operators and biodiversity advocates have long been at odds over water levels. According to The Cool Down:

“U.S. District Judge Michael Simon recently ruled that Oregon hydroelectric dams keep their current reservoir levels and increase the spill at eight dams to allow salmon to pass over dams rather than through their turbines, which puts the animals at risk.”

Industry groups argued that more spill dissolves more nitrogen in the water, which can be harmful to fish downstream.

"The order increases the risk of harm to infrastructure, listed species, and public safety while failing to demonstrate that there will be benefits to listed salmon and steelhead," said the Inland Ports and Navigation Group.

     They also argued that the injunction negatively affects river transportation, which pollutes less than trucking.

     As noted, dams can have positive effects as well, such as flood control and conserving water for irrigation. These effects, however, are countered by the negative impacts on ecosystems.

     Sediment flow of the Leitzaran River affected by the Olloki Dam in the Basque region of Spain was studied recently by a group of fluvial geomorphology geographers. According to another article in The Cool Down:

“The research group monitored 1,800 stones in the Leitzaran River between 2016 and 2022, embedding a code inside the stone samples. They distributed them at three points in the river: upstream, downstream, and at a control site. They then scanned the river from bank to bank with a detector.”

"When the device detects a code, it informs us which stone it is and gives us the GPS coordinates," Ibisate explained.

“The process enables researchers to understand the function of rivers and their sediment flow, providing a better prediction of the demolition of dams. The researchers discovered that some stones in the river traveled over 5 miles.”

"We didn't think the sediments would move so much," Ibisate stated, "and the other experts in the field were also surprised."

     The scale of outdated low-head dams in the U.S. is huge. I know here in Ohio that the Ohio EPA has a low-head dam removal program, and that is likely the case all over the country.

“There are roughly 500,000 dams in the United States, and 85% are over 50 years old and no longer serve a purpose. However, around 900 dams were removed by 2015, and an additional 50 to 60 per year thereafter.”

     Progress is being made, and dams that cause the most damage should be high graded for removal. At the current rate of dam removal, it would take over 7500 years to remove all of the dams that are no longer necessary.

 

Dam Removal Can Sometimes Improve Safety

     There is a place here close to where I live, along the Hocking River, where there used to be a historic mill, where the water flow was altered for the mill, which creates effects similar to a low-head dam. A handful of kayakers drowned while going over the mill site as they became trapped by undercurrents. One fatality happened just a few years ago. Low-head dams are known to be hazardous because they create a recirculating current downstream. This current can trap people and debris, leading to accidents and fatalities. That is what happened at the mill site and also what happened at a low-head dam along the Little River in Tennessee. The site is also an old mill site, and is known as Peery’s Mill Dam, with four drownings over the past two decades. Others trapped were able to be rescued.

“The U.S. Army Corps of Engineers' National Inventory of Dams explained that many low-head dams have "outlived their initial purpose" and their original owners, and noted that nearly 800 fatalities have been linked with the obsolete structures.”

"For this reason, low-head dams have also been called 'drowning machines,'" the Army Corps acknowledged.”

“Moreover, the barriers impede the local ecosystem from thriving. The Army Corps also stated that dam removal was necessary to restore the river and protect endangered aquatic wildlife.”

“Outdated dams withhold nutrients from native wildlife and prevent fish from migrating. Removal projects across the country have yielded promising results, with animal populations rebounding.”

 

References:

 

Officials celebrate incredible recovery of crucial waterways: 'Those days are long gone'. Kate Saxton. The Cool Down. December 7, 2025. Officials celebrate incredible recovery of crucial waterways: 'Those days are long gone'

Nobody Knew These Dams Existed. Removing Them Changed Entire Rivers Overnight. Barrie Davenport. Planet Sage. December 3, 2025. Nobody Knew These Dams Existed. Removing Them Changed Entire Rivers Overnight

Tennessee residents celebrate state's plans to remove river dam: '[They are] drowning machines'. Daysia Tolentino. Newsbreak. Tennessee residents celebrate state's plans to remove river dam: '[They are] drowning machines' - NewsBreak

Officials spark backlash with concerning pla

ns for US dams: 'Increases the risk of harm'. Simon Sage. The Cool Down. March 10, 2026. Officials spark backlash with concerning plans for US dams: 'Increases the risk of harm'

Scientists point to unexpected side effects of knocking down river dams. Nicole Westhoff. The Cool Down. April 1, 2026. Scientists point to unexpected side effects of knocking down river dams