River Restoration At Continental Scale — Removing Dams And Freeing Water

River systems are among the most important ecological infrastructure on Earth. They move water from mountains to seas, transport nutrients from land to ocean, provide migration corridors for fish, amphibians, and invertebrates, create and maintain floodplain ecosystems that support some of the highest biodiversity on land, and regulate the chemistry and temperature of coastal marine environments. They are also the primary freshwater supply for most of humanity, the arteries of agricultural irrigation systems, and the backbone of cultures, religions, and civilizations that have organized themselves around flowing water for millennia.

The degree to which industrial civilization has disrupted river systems is extreme by any historical or ecological standard. Dams, channelization, water withdrawal, pollution, and riparian habitat destruction have transformed most major river systems into managed water delivery infrastructure with residual ecological function, rather than functioning ecosystems that also provide human services. The consequences include the collapse of freshwater fisheries (freshwater species have declined 83 percent since 1970 by Living Planet Index measures — faster than any other group of species), loss of coastal productivity as sediment and nutrient transport is interrupted, subsidence of river deltas as sediment supply is cut off, and the impoverishment of the cultures and food systems that depended on river productivity.

The Dam Legacy Problem

Dams were built with extraordinary optimism about their benefits and extraordinary inattention to their ecological costs. The engineering achievements were real — Grand Coulee, Hoover, Three Gorges, Aswan High, Itaipu — but the ecological and social costs were systematically underestimated at the time of construction and have accumulated since.

Upstream of dams, river valley ecosystems are permanently inundated. The Three Gorges Dam flooded approximately 1,000 villages and displaced 1.3 million people. The Aswan High Dam inundated the archeological sites of Nubia, permanently altered the Nile Delta's sediment supply (the delta is now subsiding and being eroded by the sea), and eliminated the annual flood that had fertilized Egyptian agriculture for 6,000 years. The ecological cost of large dam construction is now recognized in environmental economics as a genuine liability that should be included in project accounting — but rarely was.

Downstream of dams, river channels starved of sediment degrade. Without the sediment that rivers carry, channels incise (cut downward), losing the lateral connectivity with floodplains that creates riparian habitat. Riverbeds coarsen as fine sediments are winnowed out, degrading spawning habitat for fish that require specific substrate sizes. River deltas, deprived of their sediment supply, shrink and subside — a crisis facing every major delta system in the world, from the Nile to the Mekong to the Mississippi.

Fish passage is the most immediately visible ecological consequence. Dams without fish ladders — which remain the majority worldwide — are absolute barriers to migratory species. Atlantic salmon, Pacific salmon, American shad, lamprey, eel, and dozens of other commercially and ecologically important species require the ability to move between ocean and upriver spawning grounds across their life cycles. A single dam on a previously free-flowing river can eliminate a species from the entire upstream watershed.

The Dam Removal Revolution

Dam removal is now mainstream environmental management in the United States, and is growing rapidly in Europe. The American Rivers organization tracks dam removals annually; the acceleration since 2000 reflects the convergence of several forces: aging infrastructure with expired licenses that require expensive relicensing and safety upgrades, often more costly than removal; declining economic value of old dams as their power generation capacity becomes marginal relative to solar and wind; growing body of evidence on ecological recovery from removal; and successful advocacy by tribal nations, conservation organizations, and fishing communities.

The Elwha River case has become the gold standard for understanding dam removal at scale. Two dams — the Elwha Dam (108 feet) and Glines Canyon Dam (210 feet) — were removed between 2011 and 2014 on the Elwha River on the Olympic Peninsula of Washington State. The dams had blocked salmon runs since 1913, reducing salmon populations in the upper Elwha to zero and devastating the Elwha Klallam Tribe's traditional food system.

Post-removal monitoring has documented ecological recovery that exceeded even optimistic predictions. Within months, sediment that had been trapped behind the dams began moving downstream, rebuilding the river delta in the Strait of Juan de Fuca. Within a year, salmon and steelhead were detected in previously blocked habitat. By 2023, over 90 miles of salmon spawning habitat had been reopened, all five Pacific salmon species had returned to the river, and the Lower Elwha Klallam Tribe had resumed ceremonial salmon harvests for the first time in over a century. The river's recovery demonstrates that damaged ecosystems can restore themselves rapidly when the structural obstacle is removed — they do not need to be rebuilt piece by piece.

In Europe, the Dam Removal Europe initiative has documented growing momentum for dam removal across the continent. Spain has removed hundreds of small dams and weirs. France has removed dams on several rivers, most significantly on the Sélune, where two dams were removed between 2019 and 2022 in the largest European dam removal project to date. Portugal, which has one of the most heavily dammed river systems in Europe relative to its size, is facing intense pressure to remove or modify aging dams to restore Atlantic salmon and sea trout populations.

Continental-Scale River Restoration

Beyond individual dam removal projects lies the larger question: what would it mean to restore the ecological function of entire continental river systems?

The Mississippi River basin drains 40 percent of the continental United States, covering 1.2 million square miles. The system has been transformed beyond recognition since European settlement: 90 percent of the original wetlands have been drained, the river itself has been channelized and leveed to prevent flooding (and to maintain navigation), agricultural runoff has created a dead zone in the Gulf of Mexico covering thousands of square miles, and the river's delta — one of the most productive fisheries in North America historically — is subsiding at rates that will put New Orleans underwater within decades without massive intervention.

Restoration planning for the Mississippi at continental scale requires thinking about multiple interconnected interventions. Reconnecting floodplains — setting levees back from the river's edge in areas where the agricultural value of the floodplain is low relative to the ecological services of reconnection — would restore floodplain wetlands that naturally filter nutrients, store floodwater, and provide habitat. Reducing agricultural runoff of nitrogen and phosphorus — which requires changing farming practices across the corn belt — would reduce the Gulf of Mexico dead zone. Allowing more sediment to reach the delta would slow its subsidence. None of these interventions is simple, but together they would constitute a genuine continental-scale restoration effort.

The Colorado River is perhaps the most dramatic example of a river that has been fully consumed by human use. The Colorado runs approximately 1,450 miles from the Rocky Mountains to the Gulf of California. Since the 1960s, it has run dry before reaching the sea in all but the wettest years. The combined withdrawals of Los Angeles, Phoenix, Tucson, Las Vegas, Denver, and millions of acres of irrigated agriculture take every drop. The Colorado River Delta — historically one of the most biodiverse estuaries in North America, home to jaguar, cottonwood forests, and enormous fish populations — has been reduced to a fragment of its former extent.

A 2014 agreement between the United States and Mexico established a minimum environmental flow to the delta — enough water to sustain a remnant ecology, though a fraction of what the delta needs for full function. The cottonwood forests that were planted with the first pulse of water responded; birds returned; fish populations improved. The response demonstrated that even a severely degraded delta can recover with water. The planning question is how to reallocate enough of the Colorado's overcommitted water to restore the river system while managing the consequences for the cities and agricultural operations that have built their existence around water that was never sustainable to claim.

The Mekong and the Global South

In the Global South, the situation is the reverse of the United States: dam building is accelerating, driven by hydropower development and irrigation expansion, while the ecological consequences are beginning to manifest. The Mekong River — one of the most biodiverse river systems on Earth, supporting the largest inland fishery by volume in the world and the food security of 60 million people — has been radically altered by Chinese dams in its upper reaches and a cascade of Lao and Cambodian dams planned or under construction in its lower reaches.

The consequences are already visible. Sediment that previously flowed to the Mekong Delta in Vietnam — among the most productive agricultural regions in Asia — is being trapped behind Chinese dams. The delta is subsiding. Flood patterns have changed dramatically, disrupting the fish breeding cycles that depend on predictable seasonal flooding. Freshwater fish catches in the lower Mekong, which feed an estimated 40 million people, have declined sharply.

The political economy of Mekong dam removal is completely different from the U.S. context — China has no incentive to remove its dams, and the downstream nations have limited leverage. But the situation illustrates the continental-scale consequences of river fragmentation and the scale of what is being lost.

Planning Implications

Dam removal and river restoration belong in the category of civilizational-scale planning not because they are simple but because they are foundational. Rivers are the arteries of terrestrial ecosystems. Their restoration is a prerequisite for the recovery of freshwater biodiversity, coastal fisheries, delta agriculture, and the hydrological cycle that underpins much of terrestrial life.

The planning framework requires working across scales simultaneously. At the project scale, individual dam removal decisions require engineering assessment, ecological baseline, stakeholder engagement (particularly with tribal nations and fishing communities with deep relationships to river systems), and post-removal monitoring. At the watershed scale, dam removal must be integrated with agricultural runoff reduction, riparian habitat restoration, and floodplain reconnection to achieve meaningful ecological outcomes. At the continental scale, governance frameworks must coordinate decisions across state and national boundaries, recognizing that rivers are systems that cross political lines and that effective management requires treating them as systems.

The most important planning insight from the dam removal literature is the speed and completeness of ecological recovery when structural barriers are removed. The natural system wants to heal. It has the biological machinery — fish populations, seed banks, migratory species — waiting to recolonize restored habitat. The task of planning is to remove the obstacles and then to exercise the discipline not to re-install them.