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Status and trends of dam removal research in the United States

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Aging infrastructure coupled with growing interest in river restoration has driven a dramatic increase in the practice of dam removal. With this increase, there has been a proliferation of studies that assess the physical and ecological responses of rivers to these removals. As more dams are considered for removal, scientific information from these dam‐removal studies will increasingly be called upon to inform decisions about whether, and how best, to bring down dams. This raises a critical question: what is the current state of dam‐removal science in the United States? To explore the status, trends, and characteristics of dam‐removal research in the U.S., we searched the scientific literature and extracted basic information from studies on dam removal. Our literature review illustrates that although over 1200 dams have been removed in the U.S., fewer than 10% have been scientifically evaluated, and most of these studies were short in duration (<4 years) and had limited (1–2 years) or no pre‐removal monitoring. The majority of studies focused on hydrologic and geomorphic responses to removal rather than biological and water‐quality responses, and few studies were published on linkages between physical and ecological components. Our review illustrates the need for long‐term, multidisciplinary case studies, with robust study designs, in order to anticipate the effects of dam removal and inform future decision making. WIREs Water 2017, 4:e1164. doi: 10.1002/wat2.1164

Distribution of dams in the contiguous U.S. (a), the number of dams removed (b), and the number of published dam removal studies (c), by state. The number of dams from the National Inventory of Dams database does not accurately reflect all of the dams in the U.S. (see text).
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Pairwise co‐occurrence patterns of different response metrics monitored in dam removal studies, assigned to physical, biological, and water‐quality categories (outer ring). Ribbons inside the circle connect metrics that were measured in the same papers. The base of each ribbon has a width proportional to the number of studies in which that metric was monitored in conjunction with the metric at the other end. To read the figure, start with a given metric of interest (e.g., fish), and compare the width of the different ribbons at base of the metric. Wider‐based ribbons connect to metrics that were frequently measured in conjunction with the selected metric, whereas narrow ribbons connect to metrics that were less frequently measured in conjunction with the selected metric. Ribbon colors denote co‐occurrence patterns both within and among categories (see legend). Inset Venn diagram shows the number of studies per category, with the amount of overlap proportional to co‐occurrence.
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Proportion of dam removal studies (n = 139) classified by type of response metrics measured (a–c) and the total number of metrics measured per study (d).
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Summary statistics of dam removal research showing proportion of studies by (a) type of publication, (b) type of study design, and (c) the number of years of before/after dam removal data collection. Note that for study designs with no pre‐dam removal data (i.e. control/impact and impact categories) ‘years before’ data are included as a zero (light gray bar).
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Compilation of dams removed (orange) and dams with at least one published study (blue) by: (a) cumulative frequency distribution by year removed (exclusive of dams with no known date of removal), with a count of the number of dam removal studies published each year inserted below the x‐axis, and (b) relative frequency (percentage) in each dam height category. Data from American Rivers and Bellmore et al.
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