Corrigendum Status and trends of dam removal research in the United States I n Bellmore et al., 1 some of the numbers in Figure 1, which illustrate the number of dams per state in the U. S., were published with error. These revisions do not affect any of the conclusions of the paper. The corrected figure appears below: We apologize for this error.

Dam removal is widely used as an approach for river restoration in the United States. The increase in dam removals—particularly large dams—and associated dam‐removal studies over the last few decades motivated a working group at the USGS John Wesley Powell Center for Analysis and Synthesis to review and synthesize available studies of dam removals and their findings. Based on dam removals thus far, some general conclusions have emerged: (1) physical responses are typically fast, with the rate of sediment erosion largely dependent on sediment characteristics and dam‐removal strategy; (2) ecological responses to dam removal differ among the affected upstream, downstream, and reservoir reaches; (3) dam removal tends to quickly reestablish connectivity, restoring the movement of material and organisms between upstream and downstream river reaches; (4) geographic context, river history, and land use significantly influence river restoration trajectories and recovery potential because they control broader physical and ecological processes and conditions; and (5) quantitative modeling capability is improving, particularly for physical and broad‐scale ecological effects, and gives managers information needed to understand and predict long‐term effects of dam removal on riverine ecosystems. Although these studies collectively enhance our understanding of how riverine ecosystems respond to dam removal, knowledge gaps remain because most studies have been short (< 5 years) and do not adequately represent the diversity of dam types, watershed conditions, and dam‐removal methods in the U.S.

[1] Dam removal projects are playing an increasingly important role in stream restoration, and offer unparalleled opportunities to study sediment dynamics following disturbance. We used the removal of the $4-m high Merrimack Village Dam (MVD) on the Souhegan River in southern New Hampshire to measure processes and rates of channel evolution in a sandfilled impoundment. From 2007 to 2010, we repeatedly surveyed 11 cross sections and the longitudinal profile, and collected sediment samples to measure changes in channel morphology and bed texture. The dam removal in August 2008 resulted in a nearly instantaneous base level drop of 3.9 m and caused a two-phased channel response. The initial, process-driven phase (2 months) was characterized by rapid incision and removal of the impounded sand (up to 1013 t d ), followed by channel widening. Once incised to base level, the rate of sediment removal slowed (30.7 t d À1) and adjustments became eventdriven, and the former impoundment segmented into a nonalluvial section and an alluvial section with erosion and deposition influenced by vegetation on the channel banks. Two years after the dam removal and two high-magnitude floods, the river has excavated 79% of the original sediment. Continued response will be substantially influenced by the establishment of bank vegetation within the former impoundment and the magnitude and frequency of high discharge events. Initial channel development and sediment erosion occurs rapidly (weeks to months) in sand-filled impoundments, but excavation of the remaining sediment occurs more slowly depending on vegetation feedbacks and flood events.

Long‐term flow records for watersheds with minimal human influence have shown trends in recent decades toward increasing streamflow at regional and national scales, especially for low flow quantiles like the annual minimum and annual median flows. Trends for high flow quantiles are less clear, despite recent research showing increased precipitation in the conterminous United States over the last century that has been brought about primarily by an increased frequency and intensity of events in the upper 10th percentile of the daily precipitation distribution – particularly in the Northeast. This study investigates trends in 28 long‐term annual flood series for New England watersheds with dominantly natural streamflow. The flood series are an average of 75 years in length and are continuous through 2006. Twenty‐five series show upward trends via the nonparametric Mann‐Kendall test, 40% (10) of which are statistically significant (p < 0.1). Moreover, an average standardized departures series for 23 of the study gages indicates that increasing flood magnitudes in New England occurred as a step change around 1970. The timing of this is broadly synchronous with a phase change in the low frequency variability of the North Atlantic Oscillation, a prominent upper atmospheric circulation pattern that is known to effect climate variability along the United States east coast. Identifiable hydroclimatic shifts should be considered when the affected flow records are used for flood frequency analyses. Special treatment of the flood series can improve the analyses and provide better estimates of flood magnitudes and frequencies under the prevailing hydroclimatic condition.

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Dam removals with unmanaged sediment releases are good opportunities to learn about channel response to abruptly increased bed material supply. Understanding these events is important because they affect aquatic habitats and human uses of floodplains. A longstanding paradigm in geomorphology holds that response rates to landscape disturbance exponentially decay through time. However, a previous study of the Merrimack Village Dam (MVD) removal on the Souhegan River in New Hampshire, USA, showed that an exponential function poorly described the early geomorphic response. Erosion of impounded sediments there was two‐phased. We had an opportunity to quantitatively test the two‐phase response model proposed for MVD by extending the record there and comparing it with data from the Simkins Dam removal on the Patapsco River in Maryland, USA. The watershed sizes are the same order of magnitude (102 km2), and at both sites low‐head dams were removed (~3–4 m) and ~65 000 m3 of sand‐sized sediments were discharged to low‐gradient reaches. Analyzing four years of repeat morphometry and sediment surveys at the Simkins site, as well as continuous discharge and turbidity data, we observed the two‐phase erosion response described for MVD. In the early phase, approximately 50% of the impounded sediment at Simkins was eroded rapidly during modest flows. After incision to base level and widening, a second phase began when further erosion depended on floods large enough to go over bank and access impounded sediments more distant from the newly‐formed channel. Fitting functional forms to the data for both sites, we found that two‐phase exponential models with changing decay constants fit the erosion data better than single‐phase models. Valley width influences the two‐phase erosion responses upstream, but downstream responses appear more closely related to local gradient, sediment re‐supply from the upstream impoundments, and base flows. Copyright © 2017 John Wiley & Sons, Ltd.

Managers make decisions regarding if and how to remove dams in spite of uncertainty surrounding physical and ecological responses, and stakeholders often raise concerns about certain negative effects, regardless of whether these concerns are warranted at a particular site. We used a dam‐removal science database supplemented with other information sources to explore seven frequently raised concerns, herein Common Management Concerns (CMCs). We investigate the occurrence of these concerns and the contributing biophysical controls. The CMCs addressed are the following: degree and rate of reservoir sediment erosion, excessive channel incision upstream of reservoirs, downstream sediment aggradation, elevated downstream turbidity, drawdown impacts on local water infrastructure, colonization of reservoir sediments by nonnative plants, and expansion of invasive fish. Biophysical controls emerged for some of the concerns, providing managers with information to assess whether a given concern is likely to occur at a site. To fully assess CMC risk, managers should concurrently evaluate site conditions and identify the ecosystem or human uses that will be negatively affected if the biophysical phenomenon producing the CMC occurs. We show how many CMCs have one or more controls in common, facilitating the identification of multiple risks at a site, and demonstrate why CMC risks should be considered in the context of other factors such as natural watershed variability and disturbance history.

Recent studies document increasing precipitation and streamflow in the northeastern United States throughout the 20th and early 21st Centuries. Annual peak discharges have increased over this period on many New England rivers with dominantly natural streamflow -especially for smaller, more frequent floods. To better investigate high-frequency floods (<5-year recurrence interval), we analyze the partial duration flood series for 23 New England rivers selected for minimal human impact. The study rivers have continuous records through 2006 and an average period of record of 71 years. Twenty-two of the 23 rivers show increasing trends in peaks over threshold per water year (POT ⁄ WY) -a direct measure of flood frequency -using the Mann-Kendall trend test. Ten of these trends had p < 0.1. Seventeen rivers show positive trends in flood magnitude, six of which had p < 0.1. We also investigate a potential hydroclimatic shift in the region around 1970. Twenty-two of the 23 rivers show increased POT ⁄ WY in the post-1970 period when comparing pre-and post-1970 records using the Wilcoxon rank-sum test. More than half of these increases have p < 0.1, indicating a shift in flow regime toward more frequent flooding. Region wide, we found a median increase of one flood per year for the post-1970 period. Because frequent floods are important channel-forming flows, these results have implications for channel and floodplain morphology, aquatic habitat, and restoration.