Lohan has data indicating that there are more than 550,000 dams in the US. The Army Corps of Engineers has an inventory of 92,000, but it only includes relatively large dams or those whose failure would be catastrophic, thus omitting a huge number of low-head dams (1 to 15 feet high) with potentially serious safety and environmental impacts.
Although support for removal has been growing, only about 2,200 dams have been taken down, mostly in the past 25 years. Most of the book's treatment of issues is presented through case studies of major removal efforts on the Elwha, Snake, Klamath, Kennebec, and other rivers.
Dams' outsized environmental impact is on the migration, and therefore numbers, of fish. Salmon species are the fish most often discussed in the book, with a lot of attention to their remarkable river and ocean life cycle and a focus on how threatened they are by river barriers. Alewives, eels, and many other species also get their due, not to mention mussels. Lohan emphasizes how connected everything is. Summarizing the ecological consequences, she writes, "Dams have decimated migratory fish populations by blocking access to vital upstream habitat for spawning, feeding, and evading predators. The barriers also obstruct the downstream movement of sediment and nutrients, which depletes riverbanks and coastal beaches, hastening erosion and reducing riverside plant growth that feeds insects, birds, and other animals."
Meanwhile, behind the dam, slackwater can heat up and dissolved oxygen levels can drop, harming native species of fish and adversely affecting the aquatic food web. A surprising impact as well is methane and carbon dioxide emissions from reservoirs, produced when organic matter decomposes in the water—an issue that needs more research, Lohan says.
Removal advocates also cite human safety concerns ranging from low-head dam drownings to dam failures—1,600 dam failures since the mid-nineteenth century.
Lohan devotes whole chapters to "deadbeat dams" and to small dams. The deadbeats are thousands of dams that no longer serve the purpose for which they were built long ago. New England provides many examples where the industries supported are long gone but communities' attachment to the past remains. As for small dams, to a large extent they "slip under the radar of dam safety and wildlife agencies," Lohan says. These, with their strong hydraulics, often pose the greatest danger to people, and despite their modest size, often create significant ecological harm.
Workarounds for fish blocked by dams get a lot of attention in the book—fish ladders, elevators, bypasses, even transporting fish by vehicle. These compromises don't revive fish populations nearly to the extent that removals do, but in many cases they are the only measures that divided communities can come up with, or they are agreed on in combination with removal of some, but not all, dams.
Tearing down dams, where that result has been achieved, is hardly the end of the story. Lohan documents massive revegetation efforts that have followed some removals, with figures like 50 tons of native seeds spread and 118,000 trees, shrubs, and grass plugs planted in the wake of removals of four major dams on the Klamath.
"Dam removals work," Lohan writes on her final pages. "They restore rivers better and faster than other projects…. For those who feel like something was taken from them, I hope they can understand the significance of what has been returned to others."
"Undammed: Freeing Rivers and Bringing Communities to Life," by Tara Lohan, published Oct. 14, 2025, Island Press (now an imprint of Princeton University Press), 264 pages.