Undamming Rivers a Review of the Ecological Impacts of Dam Removal by Angela Bednarek 2001
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Abstract
The prolonged history of industrialization, inundation control, and hydropower production has led to the structure of fourscore,000 dams across the U.S. generating significant hydrologic, ecological, and social adjustments. With the increased ecological attention on re-establishing riverine connectivity, dam removal is condign an important part of large-calibration river restoration nationally, especially in New England, due to its early on European settlement and history of waterpower-based industry. To capture the broader dimensions of dam removal, we constructed a GIS database of all inventoried dams in New England irrespective of size and reservoir volume to certificate the magnitude of fragmentation. We compared the characteristics of these existing dams to the attributes of all removed dams over the last ∼25 years. Our results reveal that the National Inventory of Dams significantly underestimates the bodily number of dams (four,000 compared to >14,000). To combat the effects of these ecological barriers, dam removal in New England has been robust with 127 dams having been removed between ca. 1990–2013. These removed dams range in size, with the largest number (30%) ranging between 2–iv m high, just 22% of the removed dams were betwixt 4–half-dozen 1000. They are non isolated to modest drainage basins: well-nigh drained watersheds between 100–1,000 km2. Regionally, dam removal has re-connected ∼3% (3,770 river km) of the regional river network although primarily through a few select dams where arable barrier-free river lengths occur, suggesting that a more strategic removal approach has the opportunity to enhance the magnitude and rate of river re-connection. Given the regional-scale restoration of forest embrace and water quality over the past century, dam removal offers a significant opportunity to capitalize on these efforts, providing watershed scale restoration and enhancing watershed resilience in the face up of significant regional and global anthropogenic changes.
Introduction
1 of the pressing challenges facing biophysical scientists, policy makers, environmental managers, and environmental advocates is how to rehabilitate ecological systems that are increasingly characterized by long-term, significant, and complex anthropogenic changes. There is a growing consensus—representing fields as diverse as restoration ecology, conservation biology, sustainability science, political environmental, and a host of others—that research seeking to understand and enhance the sustainability of human-environment systems within the context of the Anthropocene must embrace trans-disciplinary perspectives (Jerneck et al., 2011; Steffen et al., 2011; Van Andel and Aronson, 2012; Seidl et al., 2013; Olsson et al., 2014) and think and act across multiple social-ecological scales from the local to the transboundary (Ogden et al., 2013). Nowhere are these challenges of the Anthropocene more important than in the case of efforts to sympathize, govern, and manage h2o systems (Sivapalan et al., 2014), wherein decades of human alteration through dams and other infrastructure have profoundly affected a host of hydrological and ecological processes.
According to the U.S. Army Corps of Engineers National Inventory of Dams (NID), more 80,000 dams be in the US, with virtually of them occurring in the eastern United states (Graf, 1999). This greatly understates the truthful number of dams, however, considering NID's dam height and reservoir book criteria neglect to include the tens of thousands of historical manufacturing plant dams scattered throughout the nation (cf. Smith et al., 2002; Walter and Merritts, 2008). This aging infrastructure, in combination with new ecology concerns regarding river and watershed restoration (Doyle et al., 2008; Doyle and Havlick, 2009), has prompted novel environmental, economical and social concerns surrounding their fate and led different stakeholders to argue for and against dam removal. Over the past several decades, more than i,100 dams have been removed nationally (American Rivers, 2014; O'Connor et al., 2015) due to increasing public business organisation over their safety, an unwillingness to invest scarce resources in infrastructure repair, and a growing interest in restoring degraded ecosystems. Contempo estimates indicate that more than 60 dams are being removed per yr (Service, 2011a) with well-nigh of these being pocket-size run-of-river structures located primarily in Pennsylvania, Wisconsin, and Michigan (Pohl, 2002; Service, 2011a) – although large dams such equally the Elwha (WA), Condit (MT) and Veazie (ME) have been recently removed (Wilcox et al., 2014; Pess et al., 2014; Magirl et al., 2015; East et al., 2015; Warrick et al., 2015; Randle et al., 2015).
Because dam removal tin minimize habitat fragmentation and re-establish longitudinal and lateral connectivity (Bednarek, 2001; Hart et al., 2002), many ecologists and environmentalists embrace dam removal as a key component of river restoration. This perspective, however, encounters ii broad challenges. Outset, recent thinking and enquiry on the Anthropocene make information technology clear that restoration efforts are greatly complicated every bit watersheds and broader ecological assemblages are in effect "novel ecosystems" (Hobbs et al., 2009). Some argue that such novel ecosystems—because of the lack of any baseline ecological knowledge on which to peg restoration objectives—imply an arroyo termed "intervention environmental" that reflects a more than thoughtful and experimental approach to the adaptive management of highly altered ecosystems (Hobbs et al., 2011). Dam removal may indeed be cast in this lite. Second, this initial presentation of dam removal as a wide panacea of watershed restoration has encountered resistance from both the scientific and policy community, in part due to uncertainties regarding the issue of released sediment – some of which may be contaminated – on downstream geomorphic and ecological processes (Grant, 2001; Pizzuto, 2002; Doyle et al., 2003; Stanley and Doyle, 2003; Doyle et al., 2005). Despite the potential environmental costs, dam removal offers meaning environmental benefits by re-connecting upstream-downstream sediment and geochemical fluxes, providing aqueduct-floodplain exchanges, and allowing greater opportunity for fish passage (Bednarek, 2001; Bushaw-Newton et al., 2002; Doyle et al., 2005; Hogg et al., 2013; Pess et al., 2014; Vedachalam and Riha, 2014; Kornis et al., 2014; Hogg et al., 2015; Magilligan et al., 2016).
The impacts of dams in New England are especially acute as it possesses one of the highest densities of dams in the U.s. (Graf, 1999), with the NID documenting more than 4,000 regional dams (Table 1), some of which have been in place for ca. 2 hundred years. Environmental organizations and state agencies in New England perceive dam removal as part of a broader integrated strategy to restore aquatic ecosystems and associated wetlands and riparian areas, and dam removal is increasingly becoming a crucial component of the river restoration toolkit (Nislow et al., 2010). These efforts are informed by: (1) the age and small-scale size of many structures, (2) the associated risks and costs of rubber and maintenance, and (3) the express utility in terms of power generation and alluvion storage – thus allowing dam removal to reach both conservation and human infrastructure benefits. The overall ecology context of the region also underscores the potential importance of dam removal equally a conservation strategy. Native diadromous fishes (such equally Atlantic salmon, river herring, sturgeon, and American eel) comprise a substantial proportion of historical native fish biodiversity (∼30% of species in coastal rivers) and historically provided major economic and ecosystem benefits, but have experienced precipitous declines in distribution and affluence since European settlement (Saunders et al., 2006). Barriers to passage by dams are straight linked to loss of diadromous stocks, and restoration of fish passage has been a major justification for dam removal efforts. For freshwater resident fishes, furnishings of dams in New England may be less apparent, only a number of recent studies bespeak the importance of within-river movements (Kanno et al., 2014; Letcher et al., 2007; Nislow et al., 2011) in allowing admission to critical habitats, particularly in the context of the not-stationary thermal and menstruation regimes feature of the Anthropocene (Hodgkins et al., 2003; Isaak et al., 2012). Further, dams reduce the quality of fluvial habitat for both resident and migratory species via effects on sediment regimes and geomorphic processes (Kondolf and Wilcock, 1996). Finally, dam removal may exist specially valuable in the New England region given some of the major mural-scale improvements in ecology quality over the last century. Woods cover, which had been substantially reduced post-obit European settlement, has recovered in full general to its pre-settlement extent throughout the region (Foster et al., 2010). This restoration, forth with more recent regulation and mitigation of both betoken and non-signal sources of h2o pollution, has resulted in major improvements in water quality (Mullaney, 2004). As a result, dam removal efforts are likely to yield admission and connection to generally high-quality and resilient habitats.
Our chief goal in this analysis is to provide a regional cess of dam removals in New England and to present the attendant ecological and hydrologic benefits at the watershed scale, particularly the proceeds in fish passage associated with re-connecting costless-flowing mainstem and tributary reaches. However, dam removal does not occur within an institutional and social vacuum. For example, because dams are nested within watersheds, their removal may have effects extending well across the removal site (Grant and Lewis, 2015) or may generate unintended consequences (Doyle et al., 2005; Sethi et al., 2004). Dam removal – if done "right" – thus presumes a coordinated regulatory and institutional vision and/or an extremely well-organized and well-funded grassroots mobilization to see desired aims. In contrast, removal may simply reflect an advertizing hoc process lacking formalized top-downwardly or bottom-up structures (Trick et al., 2016). We are thus mindful of how the goals of restoration advocates are shaped and modified past the institutional structures for regulating the social and ecological utilise of watersheds. Our specific research questions address: (1) what is the spatial distribution of removed dams and how does this pattern relate to stated management goals of restoring critical habitat for native and diadromous fish; (2) what are the structural or management commonalities in dam types that have been removed; (three) what has been the incremental addition of free-flowing river length achieved in terms of ecological changes; and (iv) what policy or management lessons tin can be derived from the expected and unexpected biophysical benefits of dam removal? Our results nowadays the ecological achievements at the watershed scale associated with dam removal as an on-going and hereafter river restoration machinery – a direction choice that may further serve to heighten the resilience of watersheds. Given contempo discussions in the literature regarding watershed resilience (McCluney et al., 2014; Nemec et al., 2014; Waldman et al., 2016), our results point to the ancillary benefits of dam removal as a ways of increasing the resilience of social-ecological systems (Walker et al., 2004).
Methods
To best document the geomorphic, hydrologic, and potential ecological furnishings of dam removal at a regional level, nosotros accept compiled a database from state and federal agencies of all inventoried dams in each country in New England and compared the attributes of these existing dams to the population of removed dams compiled by NGOs (east.chiliad. American Rivers) and state agencies, where available. Dissimilar the NID, in that location are minimal to no height or storage restrictions, and each state, whether for liability or environmental reasons, maintains a record of its dams, frequently including information about its type, function, and characteristics (due east.g. height, length, etc.). For each existing and removed dam having geospatial information, we "snapped" its specific location direct to the river in ArcMap using the one:100,000 hydrography layer. To ensure that nosotros snapped correctly, nosotros visually inspected the snapped location for each of the removed dams to exist certain information technology was associated with the right waterbody. Snapping thus permitted calculating watershed attributes such as basin size, distance to next upstream barrier, and number of free-flowing river km opened upwardly past the removed dam. Well-nigh, if non all, of the removals lacked geomorphic attributes, such equally sediment characteristics (bedload vs. suspended load, local or reach slope, etc.). To best represent these geomorphic parameters, we measured the distance and top alter between the onetime dam and the headwater divide and used this basin-derived slope (also known as relief ratio) every bit a proxy of sediment type, based on established relationships between grain size and slope (Dade and Friend, 1998; Snyder et al., 2013).
For each removed dam we also calculated watershed attributes (e.g. percent of watershed adult, urbanized, or forested) using the National Land Cover Database (Jin et al., 2013). Moreover, the EPA (https://www.epa.gov/midweek/pages/ecoregions/na_eco.htm) divides New England into five major ecoregions (Acadian Plains and Hills, Atlantic Coastal Pine Barrens, Eastern Smashing Lakes Lowlands, Northeastern Coastal Zone, and the Northeastern Highlands) differing in channel habitat, riparian and watershed vegetation, aquatic biodiversity and fish assemblages and potential for river restoration. Our GIS assessment groups the existing and removed dams occurring in each of the ecoregions. This linking to ecoregions provides an opportunity to ascertain which ecological types and settings are currently under- or overrepresented by dam removal efforts and the extent to which dam removal currently contributes to the ecological integrity of river systems at a regional calibration. Nosotros also compare watershed and structural attributes of removed dams to the general population of dams, to define the extent to which removals reverberate the broad array of dam types and settings in the region. Our institutional analysis is based on the findings of an ongoing assessment of the social dimensions of dam removal in New England (Trick et al., 2016) consisting of semi-structured interviews with land and non-state stakeholders, participant observation at public meetings regarding dam removal, and textual analysis of hundreds of documents.
Results
Our compilation from states agencies and NGOs indicate that the number of dams documented in New England by the NID (∼4,000) significantly underestimates the actual number of dams equally more than xiv,000 inventoried dams are currently peppered throughout the New England mural (Table 1; Effigy 1A) generating a density of ∼viii dams per 100 km2. Most of these inventoried dams are in Connecticut, Maine, Massachusetts and New Hampshire, with the fewest in Rhode Island and Vermont (Table 1). These existing structures obstruct an array of watershed types that possess orphaned mill dams, to small headwater water supply dams, to larger hydropower facilities (Effigy 2). Based on our detailed compilation, 127 dams accept been removed in New England over the past several decades (upwards to 2013), presently averaging ∼12 yr-1. Nigh of the removed dams were small with ∼45% of them being less than iv yard in height (Figure 3); for comparison, across New England 35% all existing dams are less than iv grand (Effigy 3). The frequency of removed small dams (< four grand) is slightly larger than the binned values for existing dams; however, it is important to keep in listen the ii orders of magnitude difference between the number of removed dams (102) relative to the sheer number of existing dams (tenfour). For example, although the frequency of removed dams in the 6–viii chiliad height category seems loftier (eight.viii%), this occurrence simply corresponds to 8 dams, while at that place still remains more than 500 dams of this size regionally. Even with the prevalence of small dams among those eliminated, removals have non been exclusively restricted to small dams; over 40 dams > 4 m accept been removed, including five > 8 m. These removed dams occurred widely across and within drainages. Moreover, despite the predominance of small dams among those removed, these dams were not restricted to headwater locations; well-nigh (38%) occurred in medium-sized watersheds having upstream drainage areas between 100–1,000 kmii (Figure iv), with 8% formerly impounding watersheds between ane,000–10,000 km2.
Table ane.
Number of existing (in the National Inventory of Dams (NID) and in country records) and removed dams in each land in New England a
Figure 1.
Location map of existing and removed dams.
(A) All dams in New England, (B) Removed dams mapped past height and ecoregion.
Figure 1.
Location map of existing and removed dams.
(A) All dams in New England, (B) Removed dams mapped by height and ecoregion.
Close modal
Figure two.
Examples of removed dams.
(A) Kendrick Dam, VT; (B) Pelham Dam, MA; (C) Montsweag Dam, ME (photo courtesy of the Chewonki Foundation, Maine); and (D) Veazie Dam, ME.
Figure two.
Examples of removed dams.
(A) Kendrick Dam, VT; (B) Pelham Dam, MA; (C) Montsweag Dam, ME (photo courtesy of the Chewonki Foundation, Maine); and (D) Veazie Dam, ME.
Close modal
Effigy 3.
Dam pinnacle.
Dam height (m) for existing and removed dams in New England.
Effigy iii.
Dam summit.
Dam summit (grand) for existing and removed dams in New England.
Close modal
Figure 4.
Watershed area.
Watershed surface area (km2) upstream of all removed dams.
Figure four.
Watershed area.
Watershed area (km2) upstream of all removed dams.
Close modal
Within New England, near of the removed dams were located in Massachusetts (31) with Maine and New Hampshire bookkeeping for 28 and 26 removed dams, respectively. Connecticut, which has over 3,600 documented dams, has only removed 21 dams and Rhode Isle, an important littoral state, has merely removed iv dams. Vermont has removed 17 dams, many of them being very small run-of-river facilities. At the broader regional calibration, the two dominant ecoregions of New England are the Northeastern Coastal Zone (NCZ) and the Northern Highlands, which account for 40% and 38% respectively of all dam removals in New England (Figure 1B). Yet, despite the NCZ having the most removals, most of the removed dams were by and large far inland (Figure 1B), and this ecoregion likewise houses a large number of existing dams, with many of the removal sites possessing multiple dams downstream farther fragmenting the watersheds and disconnecting them from the sea. Near of the removed dams occurred at elevations beneath 100 m with a meridian between 0 – 25 k (Figure five) – suggesting a most coastal location – although 23% of the removals occurred in more upland (> 200 m) settings. Upstream bowl slope clusters between 0.5 – 1.0%, although 21% occurred in low gradient sections (Effigy vi).
Figure 5.
Dam elevation.
Peak (1000) of locations of former dams.
Figure v.
Dam summit.
Elevation (m) of locations of former dams.
Shut modal
Figure half dozen.
Watershed slope.
Relief ratio (i.e. watershed slope upstream of former dam) of sites of one-time dams.
Effigy 6.
Watershed slope.
Relief ratio (i.east. watershed gradient upstream of old dam) of sites of former dams.
Close modal
The specific location of the dam within a watershed relative to the location of the next upstream barrier, in combination with upstream watershed structure (drainage density, number of tributaries, etc.), best represents the caused do good of dam removal in opening access to upstream river reach. Our comparison of the removed and existing dams databases shows that ∼iii,770 river km accept been re-continued in New England waterways by dam removal (Effigy 7). With New England possessing ∼104,000 km of total river length, this liberated infinite of three,770 km by dam removal represents 3.61% of all river lengths. Areas upstream of the one-time dams are well forested with petty development or agriculture (Figure 8). About of the liberated gratis-flowing river space has occurred in Maine. Removal of the Sandy River, Fort Halifax, Pleasant River and Bangor hydroelectric dams stand for ∼1500 km of the reconnected river lengths in Figure 7. With Maine accounting for the few viable remaining runs of the federally-endangered Atlantic salmon, these dam removals have provided meaning access to upstream habitat (Hogg et al., 2013, 2015; Pess et al., 2014). The shape of the cumulative river length curve also reveals how most of the gain in reconnected river length is due to the removal of a small number of key barriers. Nevertheless, even in instances where considerable re-connectivity has occurred, equally for example the gain of 130 river km during the removal of the Homestead Woolen Mills Dam on the Ashuelot River in NH, several large dams occur both upstream and downstream (Gartner et al., 2015), ultimately blocking diadromous fish from accessing this newly available habitat.
Figure 7.
Liberated river kilometers.
Cumulative length (km) made available past dam removal in New England.
Figure seven.
Liberated river kilometers.
Cumulative length (km) made available by dam removal in New England.
Close modal
Figure 8.
Watershed land apply and land cover.
Box and whisker plots of land use and land encompass types for portions of the watershed at present made accessible by dam removal (from National Land Cover Database (Jin et al., 2013). The solid line in the box is the fiftyth percentile while the box edges define the 25th and 75th percentiles. The circumvolve is the mean and the whiskers place the minimum and maximum values.
Figure eight.
Watershed land employ and state cover.
Box and whisker plots of land employ and state encompass types for portions of the watershed now made accessible past dam removal (from National Land Comprehend Database (Jin et al., 2013). The solid line in the box is the fiftyth percentile while the box edges define the 25th and 75thursday percentiles. The circle is the mean and the whiskers identify the minimum and maximum values.
Close modal
Discussion
Besides documenting the general characteristics of removed dams at a regional assessment, our results provide, for the first time, the magnitude and extent of watershed calibration re-connectivity resulting from dam removal. This, in plow, provides a basis for assessing how dam removal may heighten watershed resilience in the confront of multi-scalar human impacts. As discussed beneath, nosotros perceive dam removal every bit a potentially disquisitional tool for not merely re-establishing connectivity at watershed scales, just for enhancing the capacity of of import upstream catchments to reply and adjust to regional and global anthropogenic changes (e.g., climate alter) in ecologically desirable means.
Regional benefits from dam removal
In terms of the general features of the removed dams, our data signal that dam removal in the northeastern U.S. is not restricted to dams isolated in small headwater locations. Although well below the height of virtually menstruum regulation structures, most (30%) of the removed dams were between 2–4 k with 22% of them between 4–6 yard – a height by and large sufficient to run into the requirement to exist registered in the NID. Additionally, they formerly dammed moderate size watersheds, generally between 100–1,000 km2, respective in size to a HUC-4 or HUC-5 USGS Hydrologic Unit Code. Moreover, the gain in longitudinal re-connectivity is farther augmented by the associated quality of the now liberated upstream watershed. In a sense, dam removal will allow the region to capitalize on over a century of water quality improvements. These new liberated sections of the watershed are well forested with minimal evolution (Figure 8), thus providing access to loftier quality habitat. Too providing important fish passage, these newly liberated watershed sections may thus serve to be important refugia every bit they are in some of the to the lowest degree disturbed watershed sections. The lack of development and broader wood comprehend may further serve to maintain the resilience of these ecosystems at a time of increasing concern of shifting freshwater thermal regimes regionally (Hayhoe et al., 2007). For case, distributions of native coldwater-dependent species (such as salmon and trout) are predicted to motility upstream in response to downstream warming. Removing barriers such as dams is critical to this response, and in New England our assay indicates that the high levels of forest cover and depression development pressure of these upstream sites will increase the likelihood that they will serve equally refugia (Kanno et al., 2014). Further, given that the scope for mitigating or ameliorating change via wood or h2o quality restoration may exist limited (due to existing high quality and extensive cover), dam and bulwark removal may be the nigh effective remaining adaptation strategy.
Although much of the public attention has been focused on large dam removal (Lovett, 2014; O'Connor et al., 2015; Service, 2011a, 2011b), our results elucidate the important singular and cumulative benefits of removing run-of-river dams. In their national assessment of river restoration projects Bernhardt et al. (2005) certificate the associated expense and limitations of the diverse approaches for restoration, and dam removal, on average, is cheaper than channel reconfiguration or other restoration measures. Not all of our removals provided firm costs for removal, but we judge from our regional compilations that, in general, dam removal costs ∼$80k per vertical meter of dam meridian, corresponding to inflation-adapted values reported past Born et al. (1998). This base removal cost, even so, becomes progressively more expensive for any sediment remediation requirements. Equally dam removal becomes more prevalent in New England and other regions, such cost estimates provide an important additional consideration for environmental managers and other concerned actors. In addition, not merely are restoration efforts (eastward.1000. bank stabilization, channel reconfiguration, etc.) more plush, they oftentimes are site-specific generating, at best, attain scale improvements; whereas dam removal offers a greater spatial scale of watershed rehabilitation.
Geomorphic and ecological implications
Nosotros were not able to appraise any geomorphic adjustments or specific ecological benefits with each dam removal as post removal monitoring in New England, as elsewhere, has more often than not been absent. In fact, in a recent national assessment of dam removal monitoring, Bellmore et al. (2015) document that of the > 1,100 dams removed nationally, only 139 had any monitoring whatsoever, and only 35 had both geomorphic and ecologic assessments. Detailed post-removal geomorphic assessments have been done on just a few rivers in New England (Pearson et al., 2011; Magilligan et al., 2016), and these studies testify that most of the geomorphic adjustments occur inside the first year driven primarily by the initial base level adjustment – a trend typical of dam removals elsewhere (Grant and Lewis, 2015; Sawaske and Freyberg, 2012). Because of the thin alluvial cover over bedrock in New England, upstream knickpoint migration is commonly express to dam proximal reaches (Pearson et al., 2011; Gartner et al., 2015) suggesting that post-removal prolonged sediment production may not occur or impair downstream geomorphic and ecological stability. Although no published sediment data exist for these removals, the modal bowl gradient of 0.v – 1% (Figure vi) corresponds mostly to aqueduct bed sedimentological environments typical of a sand to sandy gravelly matrix – typical of the coarse-grained Pleistocene deposits that drape almost of New England. The lack of fines regionally (Rainwater, 1962) and at the sites of removed dams further serves to assuage environmental concerns that pollutants sorbed to fine-grained sediment (silts and clays) may be flushed downstream following postal service-removal sediment evacuation. Although New England has experienced significant historical industrialization, downstream dispersal of contaminated sediments post-obit dam removal has not been generally documented, in part due to the fibroid (sand to gravel) nature of the released sediment where few bounden sites – relative to effectively grained material – exist for sorbing pollutants and because of the express monitoring of these effects. Additionally, the length scale for sediment dispersal is grain size dependent; in a detailed analysis of downstream travel distance, Grant and Lewis (2015) document the spatially express transport distances for the coarse fraction following dam removal. Contaminated sediment may potentially be a more significant outcome in southern New England where lower slopes and effectively-grained sediment exist.
From an ecological standpoint, the pattern revealed in Figure 1B depicts, perhaps, an operational disconnect between direction goals to restore diadromous fish populations and the dam removal process. Although virtually of the dams have been removed in Massachusetts and in the Northeastern Coastal Zone, very few of them were along the most important reaches for marine-freshwater substitution – i.e. the virtually downstream watershed positions. Our analysis does not include efforts to restore fish passage (through ladders, lifts, and bypasses) at existing structures, which have been implemented widely throughout the region. Withal, in contrast to dam removal, the efficacy of these passage structures is highly variable beyond sites, years, and species (Haro and Castro-Santos, 2012), and fifty-fifty when they practise facilitate adequate passage, they exercise not ameliorate the effects of dams on habitats and sediment regimes (Bunt et al., 2012). Further, the presence of downstream dams does not mean that dam removal has been an ineffective agent in the restoring aquatic ecosystems. Instead, with more than 3,770 river km now made accessible, resident fish and at-gamble species have greater access to previously unavailable habitat, and re-establishing the continuity of sediment transport has probable facilitated the development of of import fluvial habitats (confined, banks and floodplains) downstream.
Landscape of strategic opportunism
From an ecology management and policy perspective, the pattern axiomatic for dam removals in New England (Figure 1B) evokes a landscape of strategic opportunism more than a well-articulated management scenario planned out and implemented past local and regional stakeholders. This opportunistic strategy reflects the advertising hoc nature of dam removals whereby NGOs and state agencies essentially react and respond to willing dam owners who – either past environmental awareness, business/personal foresight, FERC requirements, or economic liability – decide that removal may the best economical or personal option. The lack of tiptop-down or bottom-up driven processes may appear to be an effective strategy as information technology has generated over 125 removals (∼10% of national removals), but the ecological/geomorphic effectiveness of removals as an intervention would almost certainly be enhanced past more programmatic approaches. In that location is an emerging literature engaging with various management scenarios, ranging from a "hit list" arroyo (Hoenke et al., 2014) to more market based strategies for prioritizing barrier removal (Kemp and O'Hanley, 2010; Neeson et al., 2015). With the advent of geospatial databases and associated spatial algorithms, these approaches offer salient management strategies to prioritize dam removal as it is at present possible to quantify which dam, once removed, may liberate the greatest number of gratuitous-flowing river km or offer the greatest opportunity for watershed or river restoration. For case, the U.s.a. Ground forces Corps of Engineers (USACE) is proposing to investigate dam removals as part of an overall "portfolio" approach to ecosystem restoration at the watershed calibration equally evidenced in their contempo comprehensive plans for fish passage forth the multi-dammed Blackstone River (USACE, 2015).
Both the "strategic opportunism" and the "priority list" approaches, withal, assume a social loonshit free of political confrontation, an institutional construction or organisation that has the political and economic will and resources to make environmentally informed decisions, and/or a general agreement that dam removal is the most effective restoration strategy (Fox et al., 2016). Indeed, current research on dam removals in New England highlights the vagaries of efforts to promote watershed resilience through dam removal and outlines the numerous pathways that local political processes can derail even the well-nigh thoughtful restoration efforts (Fox et al., 2016). Our results point to mayhap, by default, a "happy medium" wherein dam removal – and its associated environmental gains – has progressed incrementally through political, economic, and environmental expediency to achieve some of the stated management and river restoration goals. However, lacking any significant sustained monitoring or post-removal assessment, it is difficult to specifically determine the bodily gains ecologically from removal.
Assessments are farther complicated by the lack of common ecological metrics or stated management goals to evaluate the success of a given restoration strategy or effect (Bernhardt et al., 2005; Palmer et al., 2005; Bernhardt et al., 2007), a condition common to management efforts in the Anthropocene (Seidl et al., 2013). NOAA does provide baseline metrics for assessment (Wildman, 2013), but in nearly instances, the benchmark may be merely the re-introduction of migratory and resident fish species to previously unattainable watershed locales (Hogg et al., 2013, 2015; Pess et al., 2014) or the dam removal may generate more re-connected coupled geomorphic and ecological attributes (Magilligan et al., 2016). The lack of a singular metric of restoration success for dam removals likewise manifests as dams are removed for numerous socio-economic reasons (e.g. safe, liability, aesthetics, etc.) that often lack a clearly articulated ecological goal. Even when environmental reasons are stated, dam removals rarely identify clearly stated ecological standards. In some instances, the implicit assumption past many pro-removal NGOs is that a river with a removed dam is in a more "natural" state and therefore the restoration goal is cocky-axiomatic. More ofttimes, the stated main goal, like at the recent big dam removals on the Elwha (WA) and Penobscot (ME) Rivers, is re-connecting migratory fish runs, yet the metric of success is unclear: is information technology fish presence, fish abundance, or long-term viability of populations? Because these demographic metrics are difficult to measure or assess scientifically, and, in the case of diadromous fish, are as well determined by coastal and marine influences, NGOs (e.grand. American Rivers) and federal agencies (e.g. NOAA) that fund dam removals will commonly only present the number of river kilometers opened up by the removal – once again signaling that a watershed with more than kilometers of a "re-continued" free-flowing access is in a more natural state. Although still limited in regional or temporal scope, recent literature in New England suggests that "if you lot take information technology down, they [fish] will come" (Hogg et al., 2013, 2015; Pess et al., 2014). Our results highlight some of these ambiguities in river restoration, but also show that measurable "gains" can result from dam removal – at least from a watershed resilience perspective based on achieved gains in attainable river habitat. This is particularly salient when considering the Anthropocene context where baseline ecological knowledge is hard if non incommunicable to determine and man-altered systems are the new ecological reality.
A forever dam(n)ed landscape?
From a management and restoration perspective, New England remains a dammed landscape (Figure 1A). "Striking list" or bowl-oriented perspectives may initially announced attractive just the optimal coalescence of price, ecological gain, and political/institutional arrangements may not manifest in strategic and meaningful ways. For example, the much heralded removals of 2 dams on the Penobscot River required sustained negotiations and political maneuvering (Day, 2006; Opperman et al., 2011), yet our GIS-based results point that due to the watershed structure – where few major tributaries enter the mainstem and the remaining presence of the upstream dam – very few river kms were made available past these removals. Fish passage along the mainstem may have been achieved, but information technology remains to be seen whether successful access to a greater extent of upstream spawning habitat tin can or will occur.
Management strategies are slowly becoming more than coordinated in New England and there has been some motility away from the atypical removal to think in a more than "portfolio" approach to removals. For example, the near collapse of the ∼200 twelvemonth former Whittendon Dam on the Mill River well-nigh Taunton MA in 2005 generated an initial political frenzy to "save the dam", only the response slowly shifted to a more holistic strategy to remove each of the three abandoned manufacturing plant dams over a period of 3–v years. Rather than merely removing the decomposable Whittendon Dam – that would accept offered minimal gains equally the Hopewell Mills Dam however existed downstream – a partnership of nonprofit groups and state and federal agencies coalesced to initiate the removal of 3 closely spaced dams that will now liberate more than 50 km of free flowing river length. The same plan, spearheaded by the Corps of Engineers, to systematically appraise the viability of multiple dam removals and other rehabilitation measures for the entire Blackstone River watershed in Massachusetts and Rhode Island is another example of the more coordinated arroyo. Although still politically charged and institutionally fragmented, these broad-scale prioritizations can offer greater opportunities for river restoration regionally and tin can help guide the removal procedure.
Toward watershed resilience in the Anthropocene
The concept of resilience as practical within ecology and the direction of complex social-ecological systems (SES) has been the subject of decades of contend and refinement (see Curtin and Parker (2014) for an overview). Nosotros understand resilience equally "the capacity of a system to absorb disturbance and reorganize while undergoing change then as to still retain substantially the same function, structure, identity and feedbacks" (Walker et al., 2004). While there take been some efforts to apply resilience to watershed systems (see McCluney et al., 2014; Nemec et al., 2014), these efforts have failed to clarify what watershed resilience might expect like and how information technology might be measured. Nigh research quantifies the loss of resilience due to acute and chronic human interventions (Nemec et al., 2014; Waldman et al., 2016), only less attempt has addressed the proceeds in, or metric of, resilience following some restoration mitigation effort such as dam removal. Some metrics used in a SES approach to assess a watershed'south improved resilience mail service-removal are difficult if not incommunicable to decide with certainty. Other metrics (e.g., presence of formerly absent diadromous fish species), while superficially appealing and potentially important, may not reveal much about the overall integrity of a watershed but, instead, reveal more about fish community resilience following dam removal (Waldman et al., 2016). These limitations are specially significant in regions such equally New England with a relatively long history of intensive human being modification. Instead, watershed resilience is perhaps all-time characterized past the capacity of a watershed, equally a complex social-ecological system, to absorb an on-going or time to come stressor in a style that limits deposition of non only the habitats of aquatic organisms but also an array of biophysical functions, structures, and processes.
Our results underscore the chapters of dam removal to certainly raise watershed resilience in the sense of river stretches opened up and greater connectivity. These are the expected benefits of ecological interventions such as dam removal. Even so, ane of the less expected benefits of removal has been improvement to critical habitats in the upstream catchments of New England's watersheds. A brief example illustrates this signal. Every bit climatic change progresses and average annual temperatures proceed to climb over the coming decades, viable habitat for cold-water fish species becomes even more crucial. The removal of dams in upper catchments of New England'southward rivers capitalizes on decades-long improvements in woods comprehend (Effigy 8) and water quality, implying that the resilience of these systems in the face up of broader-calibration and long-term anthropogenic changes may be enhanced. In a very real sense, dam removal links the scale of catchment and reach level biophysical dynamics (e.g., connectivity, fish migrations) to broader scales of environmental alter (east.1000., climate change). While impossible to predict the longer-term impacts of dam removal given the apace changing social-ecological contexts of the Anthropocene, at that place is some room for cautious optimism that dam removal and other interventions may really enhance watershed resilience in important and unexpected means.
Conclusion
There has been considerable attempt over the past several decades to re-establish longitudinal and lateral watershed connectivity and to, more broadly, "restore" rivers in the United States and globally. River restoration is a broad (and often contested) term that may have multiple forms and is ofttimes scale dependent: these strategies are oft localized and occur primarily at the reach scale (Bernhardt et al., 2005, 2007). At somewhat larger scales, river restoration may refer to the re-institution of river connectivity by environmental flow releases (Arthington et al., 2006; Hughes and Mallory, 2008; Poff and Zimmerman, 2010), but these efforts can be fraught with design, implementation, and direction issues surrounding sediment flux and ascertaining the correct magnitude and timing of flow releases (Mahoney and Rood, 1998; Richter and Thomas, 2007; Schmidt et al., 2001). Moreover, environmental menstruation releases may serve to heighten downstream lateral connectivity, but because the dam remains, they practice non un-fragment watersheds or promote longitudinal connectivity. For these reasons, perhaps a plow towards thinking near dam removals every bit a kind of ecological intervention designed to enhance watershed resilience offers an opportunity to more than carefully situate dam removal within the broader goals and activities of ecological restoration.
Our region-wide analysis points to the greater calibration of restoration associated with dam removal, and its ability to regenerate a suite of riverine processes including enhanced sediment connectivity, unfragmenting watersheds to let fish passage, and the opening up meaning river length and of import habitat for resident and diadromous fish. Dam removal is progressively becoming office of the management toolkit nationally, and our results point to the greater potential for re-connectivity at the watershed scale and, perhaps more importantly, for enhanced watershed resilience. Appropriately, our results betoken to some unexpected biophysical benefits of undamming New England rivers. Dam removal is at best presented by restoration advocates as a means of enhancing fish passage and returning watersheds to some previous country that is virtually impossible to determine with precision. Some of these claims are accurate, only there is a value added to dam removal that is rarely voiced. This value is related to the chapters of dam removal to increment watershed resilience—every bit evidenced by the opening upward of critical upstream habitats for sure fish species—in the context of big-scale and enduring anthropogenic changes (e.grand., climate alter). To be certain, additional research is necessary to specify how the multiple-scale dynamics associated with dam removal function. The results presented here represent a step in that direction, and bolster the notion that dam removal is a potentially critical tool non only for restoration activities but also for thinking about and assessing watershed resilience.
Data accessibility argument
The dataset on existing dams was generated from the National Inventory of Dams (NID) and from country inventories (where bachelor). Data on dam removals for New England were compiled from the American Rivers database and from public documents searched using Google. The dam removal dataset tin exist made available upon request.
Copyright
© 2016 Magilligan et al. This is an open-access commodity distributed under the terms of the Artistic Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Acknowledgments
Alex Jospe and Erik Martin at The Nature Conservancy (TNC) provided invaluable assist with the TNC database. We thank Sam Creek from the Chewonki Foundation for the utilize of the photo of the lower Montsweag Brook dam in Effigy 2. This paper benefited tremendously from ongoing discussions with colleagues at the USGS Powell Center in Ft. Collins, CO.
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