Dams reshape rivers and the landscapes around them in ways that ripple across entire ecosystems. Globally, more than 41,000 large dams block rivers, and their reservoirs have flooded roughly 305,000 square kilometers of land, an area larger than Italy. The environmental costs range from collapsing fish populations to accelerating coastal erosion, and many of these effects compound over decades.
Blocking Fish Migration
Rivers are highways for migratory fish, and dams are walls across those highways. Species like salmon, sturgeon, and shad depend on moving upstream to spawn and downstream to reach the ocean. Even when dams include fish passage systems like ladders, bypasses, or spillways, the success rates are far from complete. A USGS study tracking Atlantic salmon smolts found that only 65% of fish present at a dam’s forebay managed to pass it at all, despite repeated attempts. Of those that did get through, just 33% used the bypass, 18% passed over the spillway, and 15% went through the turbines, a route that often injures or kills them.
The problem works in both directions. Adult fish struggling upstream face exhaustion and delays that reduce spawning success. Young fish heading downstream get disoriented in the slow, warm water of reservoirs, which bears no resemblance to the fast-flowing current they evolved to navigate. Over time, blocked migration routes can collapse entire fish populations, eliminating species that once defined a river’s ecology and supported commercial fisheries.
Trapping Sediment and Starving Deltas
Rivers naturally carry enormous loads of sand, silt, and clay downstream. This sediment replenishes riverbanks, nourishes floodplains, and builds the deltas where rivers meet the sea. Dams stop this process. Sediment settles behind the dam wall instead of traveling downstream, and the effects are dramatic on both sides.
The Mekong River offers a stark example. A cascade of mega-dams on the upper Mekong was built without sediment bypass tunnels or low sluicing gates. The result: two of the dams, Manwan and Dachaoshan, have already lost 52% to 90% of their reservoir capacity to accumulated sediment. Meanwhile, the Mekong Delta, home to roughly 17 million people and some of Southeast Asia’s most productive farmland, is sinking and eroding because it no longer receives the sediment it needs. Even under the most optimistic scenario for flushing sediment through the dams, researchers estimate the delta would receive only about 62 million tonnes per year, well below its historical level of 100 to 160 million tonnes. Without major engineering retrofits, the dams will continue trapping sediment for at least 170 years.
Downstream of dams, “hungry water” stripped of its sediment load erodes riverbeds and banks more aggressively, undercutting bridges, destabilizing habitats, and deepening channels in ways that disconnect rivers from their floodplains.
Flooding Forests and Displacing Wildlife
When a reservoir fills, every hectare of land behind the dam disappears underwater. In tropical regions, this often means losing irreplaceable forest. A study of 12 proposed mega-dams in northern Borneo estimated they would cause at least 2,425 square kilometers of direct forest loss. Just three of those dams would together flood 1,354 square kilometers.
The biodiversity toll is severe. In the Borneo dam region alone, researchers identified 331 bird species affected, including two endangered species (Storm’s stork and the Bornean peacock-pheasant) and 14 classified as vulnerable. Among mammals, the area supports one critically endangered species, the Sunda pangolin, along with six endangered species including the Bornean bay cat, the grey gibbon, and the flat-headed cat. These animals don’t simply relocate when their habitat floods. They’re compressed into smaller, fragmented patches of forest where competition for food intensifies and genetic isolation increases.
Disrupting Natural Flow Patterns
Healthy river ecosystems depend on variability. Seasonal floods pulse water across floodplains, triggering breeding cycles, dispersing seeds, and recharging wetlands. Dry periods concentrate nutrients and create distinct habitat niches. Dams flatten this natural rhythm, releasing water on schedules dictated by electricity demand or irrigation needs rather than ecology.
Australia’s Murrumbidgee River illustrates what happens when flow patterns are homogenized. Dams and water diversions have driven extensive drying of the Lowbidgee Floodplain, a 3,250 square kilometer wetland ecosystem that depends on “boom-and-bust” flooding cycles. The loss of flow variability has reduced floodplain connectivity, degraded riverside habitats, and helped invasive species spread at the expense of native biodiversity. Similar patterns play out on dammed rivers worldwide, from the Colorado to the Nile.
Altering Water Temperature and Oxygen
Reservoirs stratify into layers: warm water on top, cold water on the bottom. The layer a dam releases determines what happens downstream. Water drawn from the bottom of a reservoir is often much colder than what the river would naturally carry in summer, which can shock or kill warm-water species adapted to seasonal temperature swings. This cold water also tends to be depleted of dissolved oxygen because organic matter decaying in the deep reservoir consumes it.
Conversely, water released from the surface may be unnaturally warm and, while better aerated by turbulence over the spillway, can reduce the oxygen-carrying capacity of the downstream river during hot months. Either way, the tailwater below a dam often bears little resemblance to the river that existed before, and the aquatic communities that once thrived there are replaced by a narrower set of species tolerant of the altered conditions.
Mercury Contamination in Reservoirs
One of the less obvious consequences of dam construction is a spike in mercury contamination. Mercury occurs naturally in soil, and when land is flooded to create a reservoir, bacteria in the newly submerged sediment convert that mercury into methylmercury, a highly toxic form that accumulates in fish tissue. The process is driven by sulfate-reducing bacteria that thrive at the boundary between oxygen-rich and oxygen-poor layers in sediment.
The contamination can be dramatic. Canadian studies documented five- to twenty-fold increases in mercury concentrations in fish after reservoir creation. Newly deposited mercury is methylated at five to seven times the rate of mercury that has been in sediment for a long time, which means the years immediately after a reservoir fills are the most dangerous. The contamination works its way up the food chain, concentrating in predatory fish species and ultimately posing health risks to people and wildlife that eat them. This effect has raised concerns about China’s Three Gorges Dam, where flooding vast areas of previously dry land is expected to produce intense methylmercury formation.
Greenhouse Gas Emissions From Reservoirs
Dams are often promoted as clean energy, but their reservoirs produce significant greenhouse gases. When vegetation and organic matter are submerged, decomposition by microorganisms generates both carbon dioxide and methane. Methane is the bigger concern because it traps roughly 80 times more heat than carbon dioxide over a 20-year period.
Researchers estimate that the world’s hydropower reservoirs emit approximately 2.8 teragrams of carbon per year as methane from the water surface itself, plus another 11 teragrams of carbon per year from downstream degassing, the release of dissolved methane when water passes through turbines and spillways. Tropical reservoirs tend to produce more methane because warmer temperatures accelerate decomposition and because they receive more organic carbon from surrounding landscapes. Some tropical reservoirs produce per-kilowatt-hour emissions comparable to fossil fuel power plants, undermining the climate case for hydropower in those regions.
Helping Invasive Species Spread
The still, warm water of a reservoir is a fundamentally different environment from a flowing river, and that shift favors different species. Native river fish adapted to current, gravel beds, and seasonal flow pulses lose their competitive edge. Invasive species that prefer calm, lake-like conditions thrive.
In Brazil’s Upper Paraguay River Basin, which includes the Pantanal wetland, hydroelectric reservoirs have provided favorable habitat for invasive cichlids including Nile tilapia and peacock bass. These aggressive, adaptable species were introduced through aquaculture and stocking programs, but it is the reservoir habitat that lets them establish and expand. Peacock bass in particular are voracious predators of native fish. Climate projections suggest one species could expand its suitable range by 248% in coming decades, with reservoirs serving as stepping stones for colonization into new watersheds.
The problem extends beyond fish. Altered flow regimes downstream of dams reduce the natural disturbance cycles that keep invasive plants and invertebrates in check, allowing them to outcompete native species along riverbanks and in floodplain wetlands.