Dam and reservoir removal projects: a mix of social-ecological trends and cost-cutting attitudes

Rivers have historically supported human settlement and development, but extensive regulation, fragmentation, and dam construction have degraded ecological continuity and affected human well-being. Modern management increasingly weighs the ecological, social, and economic trade-offs of maintaining versus removing dams. Against this backdrop, the study aims to provide an overview of dam and reservoir removal projects, summarize national and regional implications and constraints, and present case studies from the US and Europe. The main objectives are to identify key stakeholders in dam removal debates, catalog their arguments and attitudes, and compare regionalized approaches between Europe and the US to elucidate the mix of social-ecological trends and fiscal considerations. A further goal is to distinguish differences between small/low barriers with single functions and large, multifunctional dams to inform decision-making processes.

Global databases indicate pervasive river regulation with millions of dams and reservoirs worldwide. The GRanD database shows the highest counts in the US, followed by Russia, India, and China. An estimated ~16.7 million dams/reservoirs (>0.01 ha) exist globally, with at least 172 new dams constructed between 2011 and 2019. Only 37% of rivers >1000 km are free-flowing along their entire length and 23% reach the ocean unhindered; more than 50% of large rivers have lost hydromorphological and ecological continuity, potentially rising to 93% when planned dams are considered. Europe is estimated to have 0.6–1.8 million barriers, with approximately 230,000 dams in 13 countries; the US has 91,468–91,486 dams higher than 7.5 m. Recent trends in North America and Western Europe show more dams being removed than built, driven by shifting priorities toward restoration amid ongoing demands for water, energy, and food. The EU Water Framework Directive (WFD) has accelerated restoration and barrier removal to improve ecological status, with national strategies (e.g., Spain 2006) and ministerial programs (France, Sweden) reinforcing these actions. However, data systems are fragmented and often incomplete, varying by country and agency, which complicates comparative assessments.

The authors conducted a review of dam removal studies published up to February 28, 2020, using scientific databases (journal platforms, Google Scholar, ResearchGate) and, critically, governmental and nongovernmental datasets. Data sources included: US datasets (NABD, NID, USGS DRIP, FEMA, ASDSO); European national agencies (EA-England, SEPA-Scotland, NRW-Wales, SMHI-Sweden, MAPAMA-Spain, NVE-Norway, FNRRC-France, OTKZ-Poland, PGW WP-Poland, FSETNOR-Russia); nonprofit portals (American Rivers, Dam Removal Europe, European River Network); and scientific projects (GRanD, AMBER). The team compiled and harmonized records to analyze counts of removed dams, cumulative removals by year, and height distributions, and to assess temporal trends and intensity of removals. They also identified determinants of implementation (e.g., legal frameworks, ownership, safety, ecological goals) and synthesized social, economic, and environmental impacts from peer-reviewed literature and technical reports. Regional case inventories (US, multiple European countries, Russia) were assembled, noting data gaps (e.g., missing heights or removal dates in some national lists).

- United States removals and characteristics (1968–2019): A total of 1654 dams were dismantled; 1250 had defined heights. Approximately 86% were low barriers ≤7.5 m: 43.0% ≤2.5 m, 42.7% 2.5–7.5 m, 10.9% 7.5–15 m, and <1% >15 m. Six removed dams exceeded 30 m. Uses of removed dams: 28% hydropower, 22% recreation, 14% freshwater supply, 13% mining, 7% mills/sawmills, 16% other. Removal intensified from the 1980s onward, especially for small dams, with a consistently increasing trend. Projections suggest 4000–36,000 total removals by 2050, including 2000–10,000 dams >7.5 m (registered). Age profile of removed dams: 28% pre-1900, 50% built 1900–1940, 22% post-1940; oldest removed in 2015 dated to 1750. Only 30% of records list reasons; safety, liability, and restoration recur. Safety concerns are significant: 241 fatalities and 98 injuries associated with low-head dam incidents (2000–2015, 42 states; ASDSO). Notable projects include the Elwha River (two large dams, 32 m and 64 m, removed 2011–2014) and San Clemente (36 m, 2015). - Europe removals and characteristics (1996–2019): 342 objects dismantled; ~95% were low barriers: 54.7% ≤2.5 m, 40.6% 2.5–7.5 m, 2.3% 7.5–15 m, and 2.0% >15 m. Only one removed dam exceeded 30 m by 2019 (Vezins, 36 m, Sélune River; demolition began 2019); others planned/ongoing. Removal intensity increased around 2006–2014, coinciding with EU WFD implementation and funding instruments; trends for larger dams remained relatively flat. Data completeness varies; many records list locations but lack heights/removal dates (e.g., Sweden, Finland, UK). - Country highlights: UK maintains separate jurisdictional registers; many actions reduce reservoir volumes below regulatory thresholds rather than full dam removal. Wales: decommissioning dates from 1986–2017; dam heights ~2–20 m; safety often primary reason. Scotland: several small reservoirs discontinued (1.2–3.0 m). Sweden: of 5280 dams, 557 dismantled; among 190 with heights, only 2 >7.5 m; most removals are low dams, driven by safety, policy, economy, ecology. Norway: 4758 registered dams; 61 decommissioned/removed/modernized by 2019; larger dams often modified or sunk rather than fully removed. France: >90,000 obstacles (~70,000 weirs); first major removals 1996–1998 (Saint-Étienne-du-Vigan 12 m, Maisons-Rouges 3.8–4 m, Kernansquillec 14–15 m). Poland: 32,972 registered dams; limited removal data; Wilkówka (10 m) prepared for demolition after structural failure. Russia: 6816 abandoned low dams documented in 2008; 319–945 removed annually during 2010–2014; removals limited to low dams (<10 m). - Stakeholders and decision processes: Key stakeholders include public administration, politicians, scientific experts, business, environmental organizations, indigenous peoples, and local communities. In the US, all stakeholder groups are typically engaged; in many European countries, public administration dominates and participation is limited. Main triggers for formal discussions: loss of primary function/utility, cost-cutting and high modernization costs, poor technical condition and safety, and ecological restoration goals. The WFD significantly catalyzed European low-barrier removals; in the US, aging privately owned dams and safety/liability concerns are major drivers. - Costs and economics: Removal can be substantially cheaper than repair for small dams (e.g., Gray Reservoir, NY: repair USD 1.5M vs removal USD 0.3M). French cases (Saint-Étienne-du-Vigan, Maisons-Rouges, Kernansquillec) totaled ~€5.3M; loss of local tax revenue can occur (e.g., 7.5% of a city’s tax base). US-wide estimated total removal cost by 2050: USD 50.5M–25.1B (mean USD 10.5B; median USD 416.5M); removing large dams may be 10–30 times cheaper than sustaining repair/maintenance. - Environmental effects: Dam removal creates distinct upstream/downstream transitions. Key concerns include reservoir sediment erosion rates, downstream deposition, groundwater level changes, invasive species spread, and demolition-related impacts. Many negative impacts are short-lived. Fish recolonization can be rapid (within a year) in many cases, increasing migratory species (e.g., trout, salmon), though not universal (e.g., Wolf River, WI). Macroinvertebrate responses vary, with short-term declines and subsequent recovery observed in some systems. Sediment responses range from rapid evacuation (e.g., Clearwater River dams) to very slow emptying (e.g., Newaygo, possibly 50–80 years). Elwha case: ~65% of stored sediment (15.5–19.3 Mt) released in first five years; negative sedimentation impacts have subsided. - Regional contrasts: US vs Europe differ in ownership models (more private in US; more public or PPP in Europe), policy drivers (WFD in EU), historical/cultural context of reservoirs, and tempo of removals (steady rise in US; EU surge 2006–2014). Indigenous peoples have supported removals citing cultural, ecological, and economic benefits (e.g., Elwha, Klamath; Sámi in Nordic countries). Opposition arguments include loss of cultural heritage, landscape change, potential pollution from contaminated sediments (e.g., Enobieta), flood risk perceptions, and property value concerns.

The study addresses the central question of how social, economic, and ecological factors shape dam and reservoir removal across regions. Findings show that removals predominantly target small, low barriers with diminished utility, where safety and cost considerations align with ecological restoration benefits. The US exhibits a sustained, bottom-up trajectory driven by aging private infrastructure, liability, and multi-stakeholder processes; Europe’s activity accelerated in response to the WFD and funding availability, with public administrations exerting stronger control and participation often limited. Environmental outcomes generally support connectivity restoration and native fish recovery, though system-specific risks (e.g., invasive species spread, contaminated sediments) necessitate case-by-case planning. Economic analyses indicate that, particularly for obsolete small dams, removal can be more cost-effective than repair, while for large, multifunctional dams decisions are more complex due to broader services and socio-cultural values. The results highlight the need for consistent, holistic decision frameworks that integrate technical condition, functional utility, socio-cultural heritage, ecological goals, and long-term costs, and that engage stakeholders early to mitigate conflicts and improve project outcomes.

Comprehensive, comparable statistics on dam removals are lacking, with national datasets often incomplete and inconsistent. Many European cases involve decommissioning (capacity reduction) rather than full removal, and abandoned or low-standard dams remain in registers. In Russia and some EU states, removals largely concern small structures, while programs to build or refurbish dams continue in places like Poland and Russia. In the US, only 14 large dams have been removed among 91,486 registered; in Europe, about 12 large dams have been removed, with larger deconstructions mainly targeting worn-out facilities. Effective decision-making should identify stakeholder groups, apply multi-criteria analyses of social needs and alternatives, and use decision-support tools grounded in thorough understanding of natural and anthropogenic contexts, structure types and functions, and geographic variability. Holistic, system-scale approaches that consider connectivity, invasive species, sediment management, and downstream infrastructure are recommended to optimize trade-offs. Dam removal will remain a key challenge and instrument in future river basin management plans.

- Data incompleteness and inconsistency across countries and agencies (e.g., missing dam heights, removal dates; disparate definitions of removal vs decommissioning). - Regional databases (e.g., DRE) mix culverts, small barriers, and dams, complicating analyses and potentially biasing perceptions of removal trends. - Limited ecological and geomorphic assessments accompanying removals: for 1100 US removals before 2016, only 130 had assessments, and fewer included before–after studies, constraining inference on ecological outcomes. - Potential discrepancies in national counts (e.g., US NID totals) and classification thresholds hinder precise cross-regional comparisons. - Case-specific environmental responses (e.g., sediment dynamics, biotic recovery) limit generalizability; local context and management strategies strongly influence outcomes.