Dam Removal Research Articles

Consequences of dam removal for reservoir carbon storage and emissions.

Abstract IntroductionBiodiversity loss is accelerating due to habitat destruction, economic expansion, and insufficient conservation efforts. Traditional mitigation strategies, which focus on minimizing harm rather than reversing damage, are inadequate for achieving net biodiversity gain.ObjectivesThis article introduces the restoration hierarchy, a framework prioritizing full ecosystem restoration over partial restoration and mitigation to implement ambitious restoration efforts along the widely used restoration continuum.MethodsThe methodology comprises a case study approach combined with a literature review on ecological restoration, economic cost–benefit analysis, and legal analysis into the foundational legal structures impeding restoration.ResultsUsing Finnish dam removals as a case study, we demonstrate that large‐scale, full restoration yields not only the greatest ecological benefits, but also the greatest social welfare in a cost–benefit analysis. Despite ecology and economics aligning on restoration, legal structures currently obstruct large‐scale restoration by prioritizing short‐term private economic interests, protecting existing land‐use rights, and limiting ambitious restoration efforts. We identify six key structural biases in law altogether, for instance, property rights, the relative permanence of resource permits, and the limited scope of application of restoration laws.ConclusionsThe article concludes that both ecological and economic perspectives support the consideration of full restoration at sufficient scale, rather than implementing fragmented restoration measures. Current legal structures in place, however, slow down or impede such ambitious approaches to restoration.

ABSTRACT Objective White Catfish Ameiurus catus has been introduced to coastal watersheds across the United States. In the Penobscot River, Maine, this species has become increasingly common in upstream habitats that have been made accessible by recent dam removals. We characterized the movements of White Catfish to understand the temporal variation in their movement patterns and contextualize these findings within the recent changes in watershed connectivity. Methods We captured and tagged 10 adult White Catfish (mean fork length = 271 mm) with acoustic transmitters in the lower Penobscot River in July 2022. The movements of the tagged fish were monitored through April 2023 with a large network of stationary receivers. Results The tagged catfish were detected up to 6 km upstream and 31 km downstream from the release site. The total distance that was traveled by individuals ranged from 0 to 154 km during the study. Fall and spring movements were associated with changes in river flow and water temperature, but fish were relatively stationary from December through March, when at least five individuals were assumed to have overwintered in lower river tributaries. Conclusion Our results show that individual White Catfish may move considerable distances within large river systems and that these movements are potentially facilitated by changing river conditions. Collectively, this study fills a long-standing knowledge gap about the movement ecology of this species, adds context to help explain a recent increase in observations within their introduced range, and shows how changes in river conditions may be used to predict when and where these fish will move within a tidal system.

The Sandusky River, Ohio, USA, has experienced more than a century of alterations, including dam implementation and removal, causing a cascade of habitat changes. The physical changes in the river led to establishment of several invasive species. Ten hoop-net sampling sites, spaced about 500 m apart were established in the river to monitor fish assemblage and their habitat preferences. Four 10-d sampling events were completed from April through October 2021. Ordination analyses were used to assess fish-assemblage structure seasonably, species-habitat relationships, and life-history strategies of 31 species. Generalized linear mixed-effects models were used to assess temporal factors that may drive diversity and community assemblage. Models indicated increased species richness after removal of the dam. Presence and proportion of catch data were compared to Ohio Environmental Protection Agency 2009 pre-dam-removal data to further assess changes in fish assemblage. Several species, especially catostomids, have begun to use the habitat downstream of the former dam, altering fish assemblage throughout the river. We expect shifts in assemblage structure to persist, making continued monitoring essential for understanding how non-native and recreationally important species continue to respond to dam removal.

ABSTRACTDemolition and removal of aging dam structures is increasingly common as a part of river restoration efforts, with anticipated benefits for re‐establishing connectivity among river biota and habitats. We studied fish movement behavior in the vicinity of a run‐of‐river, low‐head dam before and after its removal. Radio telemetry was used to track the movement of smallmouth bass and determine if movement past the dam site increased following removal. We also conducted a translocation experiment to confirm that habitat structure and flow conditions at the restored site did not impede passage. Movement of radio‐tagged smallmouth past the dam site was moderately different following dam removal, with a minor improvement in upstream passage. Results of translocation experiments indicated that the dam remnant site did not impede passage, as the return rate of fish translocated past the dam was comparable to homing behavior observed in previous studies. We hypothesize that technological constraints combined with ontogenetic patterns in smallmouth bass spatial ecology reduced our likelihood of observing individual dispersal. In systems lacking migratory (e.g., diadromous) species, research to document the effects of dam removal on fish movement and connectivity must consider how ontogenetic variation in movement behavior may limit our ability to observe and document ecological outcomes. A combination of approaches is likely to be most effective for studying non‐migratory species responses to dam removal. Experimental translocation can be effective for confirming short‐term improvements in fish passage, though other methods are more appropriate for documenting ecological benefits to fish populations and communities over longer timescales.

The increasing demand for esthetic restorations has driven advancements in adhesive dentistry, with rubber dam isolation emerging as a cornerstone for achieving long-lasting and predictable outcomes. While the benefits of the use of optical magnification when utilizing rubber dam have only been briefly described in previous literature, this article's main focus is on microscopically guided rubber dam integration (MGRI), a treatment protocol that highlights the precision of a dental operating microscope (DOM) during application of rubber dam isolation. By leveraging high magnification along with aligned illumination of the DOM, MGRI addresses challenges that clinicians commonly encounter during isolation procedures, thus enhancing accuracy, efficiency, and patient safety. The protocol emphasizes a step-by-step approach from clamp selection, clamp setting, soft-tissue coverage, and marginal inversion, to rubber dam removal, supported by four-handed dentistry for optimal execution. This minimally invasive method demonstrates how integration of the DOM can significantly elevate the standards of rubber dam application in restorative and esthetic dentistry.

Streams are vital components of ecosystems, supporting the survival of plants, animals, and humans. This study utilized an extensive review of literature from both online and offline sources to understand the causes and consequences of stream shrinkage and disappearance. Findings from international and local contexts, including Akwa Ibom State, revealed that both natural and human-induced factors contribute to this decline. Natural causes include climate change, erosion-induced siltation, changes in rainfall patterns, and prolonged droughts. Human activities such as urbanization, industrial expansion, poor waste disposal, pollution, over-irrigation, and dam construction further exacerbate stream depletion. The disappearance of streams has broad implications. Ecologically, it results in biodiversity loss, riparian zone degradation, and destruction of aquatic habitats. Economically, it raises production costs for agriculture and fisheries. Socially, it leads to community displacement, increased resource conflicts, reduced water supply, and loss of cultural and recreational benefits, along with heightened flood risk. To combat these issues, the review highlights the importance of mitigation strategies including reforestation, establishment of riparian buffer zones, effective pollution control, sustainable legal frameworks, and community participation. Successful global and local initiatives such as the Ganga Action Plan (India), Elwha River dam removal (USA), and sustainable water policies in the Hadjie Naguru Wetlands (Nigeria) underscore the effectiveness of integrated water management. The study emphasizes the urgent need for collaborative efforts from governments, NGOs, researchers, and communities. It recommends integrating these strategies with stakeholder involvement and encourages geographers to map vanished and existing streams using GIS for future reference.

Abstract Economists have long recognized that as incomes rise, values of ecosystem services will also rise relative to prices for market goods. Nevertheless, standard practices in benefit-cost analysis holds prices fixed over time, thereby creating a substantial bias against the environment. Recent contributions to theory and practice offer a practical framework for correcting this bias, including a simple rule for estimating future relative price changes for ecosystem services. However, equally important to future values of ecosystem service is the effect of population change. A rising population means that the value of both rival and non-rival ecosystem services will likely increase; and when both population and incomes are rising, the effects on ecosystem service values are compounded. The importance of correcting these biases is illustrated with the example of a long-proposed project to breach four dams on the Snake River in Washington State. A 1999 federally-mandated study without these bias corrections estimated an NPV for dam breaching of -$1.4B. When the impact of rising incomes on relative prices are accounted for, the NPV rises by $2.9B; and when both rising incomes and population are accounted for, the NPV rises by $6.5B. Strikingly, when the analysis is updated to 2023 and bias-corrected, the NPV is nearly $50B.

Dam removals and fish passage can enhance aquatic connectivity but may also promote upstream transport of legacy contaminants by migratory fish. This study assessed the potential for contaminant biotransport in Michigan's Boardman River following the planned removal of the Union Street Dam and installation of FishPass, a selective fish passage facility. We quantified polychlorinated biphenyls (PCBs), mercury (Hg), and organochlorine pesticides in carcass and egg samples from migratory species including Chinook and coho salmon, migratory rainbow trout, common white and longnose sucker, lake trout, walleye, and sea lamprey. Chinook salmon exhibited the highest PCB concentrations in both carcasses and eggs, exceeding those of rainbow trout and native suckers. Similarly, Chinook salmon were predicted to deposit up to 2,200 mg of PCBs upstream under a high run size scenario-over 80 and 100 times greater than rainbow trout and native suckers. While suckers had lower individual contaminant burdens, their relatively large run sizes contributed moderately to potential contaminant biotransport compared to rainbow trout indicating an interaction between abundance and spawner contaminant burden. Stream-resident brook and brown trout in reaches open to migratory fish had higher PCBs and lower Hg concentrations than in closed reaches, likely reflecting dietary exposure to eggs and growth dilution. These results demonstrate that the potential for contaminant biotransport varies widely among migratory species and highlights the need for managers to consider both contaminant burden and run size when making fish passage decisions to balance ecological restoration with contaminant exposure risk.

ABSTRACTRiparian ecosystems harbor diverse biota and provide numerous services to rivers including mitigating sediment run‐off, regulating temperature, and contributing food‐web subsidies. Dams are a major threat to riparian communities, but dam removal and riparian restoration efforts have the potential to restore these communities and the services they provide. The goal of this study was to understand how the removal of small run‐of‐river dams and habitat restoration may change riparian community composition both taxonomically and functionally. We sampled the riparian vegetation and arthropod communities of three sites with dams, two sites where dams were removed, and one free‐flowing reference site along the Kalamazoo River in Michigan, USA. Considering the difference in river conditions created upstream and downstream of dams, we sampled reaches both within the reservoir/former reservoir and below the dam/former dam location. Ordinations revealed that dam sites were characterized by more facultative upland trees, while dam‐removed sites and the reference site had more facultative wetland forbs and grasses. Vegetation community composition at the species level was not different among sites. Riparian arthropod communities did not differ between dam and dam‐removed sites, but reaches within the reservoir/former reservoir were characterized by significantly more detritivorous isopods than below dam/former dam reaches. Moreover, vegetation richness, percent cover of non‐native species, canopy cover, and riparian zone width predicted arthropod richness and Shannon diversity, regardless of site classification. We conclude that dam removal results in subtle changes in riparian functional composition, but long‐term monitoring is essential in evaluating the full extent of stream restoration on these habitats.

ABSTRACTThe size structure of populations is crucial for predator–prey dynamics and ecosystem function. Anthropogenic pressures such as habitat alteration may affect the demography of many species. We investigated the size structure of European perch (Perca fluviatilis) populations in lakes before and after dam removal by using the planned removals of multiple dams in the outlets of boreal lakes in central Sweden as a natural experiment. In five lakes from which we had obtained data both before and after dam removal, removal resulted in a reduced abundance of small perch and an increase in body size of large, piscivorous perch, but there was no major effect on perch biomass. In a second comparison of 22 lakes where we only had access to data from either before or after removal, the effects were not as evident. We suggest that a plausible mechanism explaining a potential effect on perch demography was alterations in the predator–prey dynamics caused by changes in refuge habitat area for small fish. Specifically, dam removal caused a reduction of aquatic plant coverage. Large piscivorous fish are known to play a crucial regulatory role in controlling lake ecosystem function. Thus, we suggest that lake outlet dam removal may induce processes leading to positive effects on fish populations and ecosystem state. Moreover, our study emphasises the importance of before versus after studies to evaluate restoration measures.

Global restoration and conservation of freshwater biodiversity are represented in practice by works such as the Klamath River Renewal Project (KRRP), the largest dam removal and river restoration in the United States, which has reconnected 640 river kilometers. With dam removals, many biological outcomes remain understudied due to a lack of pre-impact data and complex ecosystem recovery timeframes. To avoid this, we created the KRRP molecular library, an environmental specimen bank, for long-term curation of environmental nucleic acids collected from the restoration project. We used these initial samples, environmental DNA metabarcoding, and generalized linear mixed-effects models to evaluate patterns of pre-dam removal fish richness and diversity. Demonstrating the suitability to resolve biological differences, the baseline shows that tributary and mainstem streams had greater native fish diversity and 2.3–10.7 times greater native fish species richness than reservoirs. These and future sampling efforts should, at a minimum, allow tracking of fish community response to ecosystem restoration. Anticipating the acceleration of omics innovation, we preserved samples for long-term storage and identified requisite phases for sustained function and adaptation of the molecular library: securing a physical storage facility for genetic material, establishing a governance structure, and confirming support for archive management.

Abstract Dams disrupt river networks by interrupting longitudinal transport of sediment and nutrients and obstructing the movement of aquatic organisms. Increasingly, water resource managers are looking for dam removal as a solution to restore connectivity and improve aquatic habitats, water quality, and fish passage. Empirical studies on small dams (<7.5 m) that incorporate both ecological and geomorphic monitoring over longer time periods (3 year+ post‐removal) are rare, limiting the data available to restoration stakeholders to inform barrier removal prioritization decisions. To help address this gap, we implemented a suite of geomorphic, biological, and water quality monitoring efforts to assess the effect of a small dam (3.7 m) removal project in the Hudson River Estuary watershed, New York State (USA). We monitored the site prior to removal and continued observations for three years post‐removal to assess differences in ecological conditions between the upstream impoundment and downstream tail‐reach before and after dam removal. Instream sediment composition and mean particle size were highly disparate between upstream impoundment and downstream tail‐reach areas prior to the dam removal but became more uniform and of higher habitat quality across the study site within two years after removal. Functional diversity, taxonomic diversity, and taxa richness of the macroinvertebrate community improved dramatically in upstream habitats within one year of the dam removal, and differences between the upstream and downstream reaches disappeared by the third year after removal, suggesting rapid recovery of stream conditions in the previously dammed upstream reach. Upstream aquatic habitat designations improved within two years from being “moderately impacted” to “slightly impacted,” rising above the biological impairment threshold according to New York State's Biological Assessment Profile score. This allowed both New York and the Environmental Protection Agency to document water quality improvements as a Type‐3 nonpoint source success story. Combined, results from this temperate watershed show that dam removals may provide aquatic ecosystem recovery in relatively short time frames.

Decadal-scale effects of a dam removal on channel geomorphology, sediment and large wood on the Elwha River, Washington, USA

The Valencia region exemplifies the intricate interplay of climate, urbanization, and human interventions in managing hydrological systems amidst increasing environmental challenges. This study explores the escalating risks posed by flood events, emphasizing how anthropogenic factors—such as urban expansion, sediment exploitation, and inadequate land use—amplify the vulnerabilities to extreme weather patterns driven by abnormal Greenhouse Gas (GHG) concentration. Nature-based solutions (NBS) like floodplain restoration and dam removal are analyzed for their benefits in enhancing ecosystem resilience and biodiversity but are critiqued for unintended consequences, including accelerated river flow and sedimentation issues. This study further examines the impacts of forest fires, exacerbated by land abandonment and insufficient management practices, on soil erosion and runoff. A critical evaluation of global policies like the Sustainable Development Goals (SDGs) reveals the tension between aspirational targets and practical, locally-driven implementations. By advocating historical insights, ecological restoration practices, and community engagement, the findings highlight the importance of adaptive strategies to harmonize global frameworks with local realities through modeling and scaling simulations, offering a replicable model for sustainable flood mitigation and resilience building in Mediterranean contexts and beyond.

Increasing energy demand propels the construction of river dams and reservoirs for hydropower, raising concerns about environmental and societal ramifications. Ecological effects like river fragmentation, habitat loss, biodiversity decline, and disruptions of biogeochemical cycles have been addressed for several decades. The impact of water impoundment on submerged soils, particularly carbon stocks, is of growing interest. Studies reveal both increases and decreases of carbon stocks in submerged soils, depending on factors such as substrate resilience, water level fluctuations, soil type and submergence duration. This study examines the effects of 24 years of water impoundment on properties of organic and mineral constituents in Andosols under the Blöndulón hydroelectric reservoir in Iceland´s highlands. Submerged soils show higher carbon stocks than reference soils but are depleted in pedogenic minerals ferrihydrite and allophane. Unlike reference soils, where carbon declines with depth, submerged soils display rather uniform carbon distribution. This is likely due to movement of organic material from upper to lower horizons, and carbon additions from decaying vegetation in the years after the impoundment. Importantly, the apparent carbon enrichment of the submerged soils raises concerns about its long-term stability. The depletion of pedogenic minerals ferrihydrite and allophane may render the carbon sensitive to oxidation in the coming decades, particularly when soils are exposed during water level fluctuations. In short, the carbon enrichment of the drowned soils may not be permanent. Assessments of the consequences of water level fluctuations or potential future dam removal need to take the vulnerability of the exposed soils into account and consider the risk of increased carbon emissions from these soils.

ABSTRACTThe global proliferation of dams has altered flow and sediment regimes in rivers, presenting a major threat to freshwater biodiversity. Diadromous species, such as fishes, decapod crustaceans and gastropods, are particularly susceptible to fragmentation because dams obstruct their breeding migrations between coastal waters and rivers. Although dams have contributed to significant declines in abundance of some commercially important diadromous fishes (salmonids and anguillids) and Macrobrachium shrimps, understanding of the impacts of fragmentation on the majority of diadromous animals is limited. Moreover, the number of species known to have diadromous life cycles has risen substantially during the last four decades, from ~250 to more than 800. This synthesis aims to consolidate the global impacts of fragmentation on diadromous animals and highlight potential knowledge gaps. We identified 338 publications documenting the impacts of dams on diadromous fishes and decapods, but this was reduced to 65 publications after application of our strict selection criteria. Specifically, we only included studies that compared unfragmented (e.g. undammed) or restored (e.g. dams with fish passes) with fragmented (e.g. site above dams) rivers. To assess statistical significance, the results of studies that were replicated sufficiently to enable calculation of standardised effect sizes were also subject to meta‐analysis focusing on three topics: impacts of dam‐induced fragmentation; efficacy of fish passes; and the mitigative potential of dam removal. Study outcomes were evaluated from five key variables: abundance; species richness; assemblage composition; population genetic diversity; and population genetic structure. We found that fragmentation led to net negative effects across all key variables for diadromous fishes. Fishes with limited jumping or climbing ability and obligate diadromous migrants that cannot persist as landlocked populations were more threatened by fragmentation. However, fishes that were capable climbers or jumpers and facultatively diadromous were nonetheless susceptible to impacts, particularly in their abundance and gene flow between fragmented populations. Installation of fish passes did not lead to positive outcomes, whereas dam removal was effective in restoring connectivity for fishes, suggesting that it is a more effective, albeit potentially contentious, approach (e.g. the dam may serve an important societal need), for restoring habitat connectivity. A smaller number of publications investigated diadromous decapods (seven versus 61 on fishes), and our synthesis of their findings suggests that decapods were vulnerable to habitat alteration by dams, but were less sensitive to their barrier effects because they were better climbers than fishes. Gastropods were the least studied diadromous taxon, and none met our criteria for systematic review or meta‐analysis. The imbalance in information about diadromous taxa was compounded by a scarcity of studies from the tropics, particularly in South America, Africa, South Asia, Southeast Asia, and East Asia. These regions support diverse aquatic assemblages so the impacts of dams may be underestimated given existing knowledge gaps. The conservation of diadromous migrants would be best served by avoiding the construction of dams while improving mitigation strategies, such as fish passage design, to limit the most damaging effects of river fragmentation.

ABSTRACT Low‐head dams are widespread in river ecosystems around the globe, but due to their effects, removals have become a major mechanism for river restoration. Investigations of fish responses to dam removals have been mostly conducted in the short term and have shown mixed assemblage responses (e.g., increased or decreased species richness following removal). Here, we examined the effects of low‐head dam removals on taxonomic composition and functional diversity at a relatively long term (6 years post dam removal). We hypothesised that differential responses of fish assemblages to dam removal are driven by suites of guild and morphological traits and that removal impacts functional complexity, with free‐flowing sites having greater distributions of functional diversity than impounded sites. We conducted our study in the West Fork River, West Virginia, where three low‐head dams were removed in 2016. Boat electrofishing surveys were conducted 1 year before the removal (2015) focusing on species of recreational fisheries importance. Then, we conducted surveys in the same sites 6 years after the removal (between September and October 2022). These electrofishing surveys were conducted at sites that were previously impounded but are now free‐flowing and those that currently remain impounded by low‐head dams. Results from our univariate and multivariate analyses indicated that current impounded sites and free‐flowing sites have statistically distinct taxonomic and functional fish assemblages. Free‐flowing sites were associated with overall higher taxonomic richness and greater biomass of some recreationally important species, including muskellunge, smallmouth bass and channel catfish. Conversely, currently impounded sites were associated with lower taxonomic richness and greater biomass of some recreationally important fishes, including green sunfish, bluegill and largemouth bass. Furthermore, functional diversity indices showed high functional richness (FRic) at free‐flowing sites, while less evident changes were observed for functional evenness (FEve) and functional divergence (FDiv). Our findings suggest that long‐term changes in the fish assemblages following the dam removals led to greater taxonomic and functional diversity while maintaining fisheries of high recreational importance. While some previous short‐term assessments suggested decreased functional complexity shortly after removals, our results suggest increases in functional complexity in the long term. Though dam removals may have high upfront socioeconomic costs, the long‐term effects in this study appeared to lead to greater ecosystem function and enhanced recreational opportunities that may outweigh initial costs.

ABSTRACT Objective With dam removal and the reexpansion of eel populations back into headwater streams of the northeastern United States, there are concerns over the potential for competition and predation between Brook Trout Salvelinus fontinalis and American Eel Anguilla rostrata. Methods This case study directly assessed predation and competition between these top predators through an analysis of stomach contents, preferred prey, and diet overlap throughout the summer in reaches of stream above and below Crabtree Falls in Nelson County, Virginia. Brook Trout are present above and below the waterfall, but American Eel are only present below the falls, providing an ideal setting to assess the effect of American Eel on Brook Trout. Results This study was the first to estimate the efficiency for gastric lavage on American Eel, which was determined to be 89% by number of prey items; however, this result was size dependent and no prey items that exceeded 10% of the eel’s length were extracted by lavage. Brook Trout abundance, size, and body condition (for adults and young of the year) did not differ significantly between the sample reaches, suggesting that the presence of American Eel did not reduce the abundance or size of Brook Trout. An analysis of similarity indicated that Brook Trout preferred terrestrial invertebrates, with no significant difference in diet where they co-occurred with American Eel (R = 0.07, P = 0.50). American Eel preferred crayfish and no Brook Trout were observed in the diet samples from American Eel, so there was no evidence for predation. Conclusions As conservation efforts increase the co-occurrence of trout and eel, there appears to be limited potential for predation or competition for prey, as demonstrated by the results of this regionally novel case study.

Short-term geomorphic response of a mountain stream channel to dam removal and a major flood