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Dam removal is often proposed as way to restore ecological integrity to rivers and streams, but ecological responses to dam removals are poorly understood, especially for downstream benthic communities. We examined the responses of benthic macroinvertebrate and algal assemblages in downstream reaches to the removal of a small, run-of-river dam on Manatawny Creek, Pennsylvania. Benthic macroinvertebrates, algae, and habitat characteristics were monitored upstream and downstream of the dam for 4 mo before removal, 3 mo after partial removal (i.e., when the impoundment was largely eliminated but sediment remained trapped behind the remaining structure), and 12 mo after complete dam removal. Macroinvertebrate density,...
A substantial increase in fluvial sediment supply relative to transport capacity causes complex, large-magnitude changes in river and floodplain morphology downstream. Although sedimentary and geomorphic responses to sediment pulses are a fundamental part of landscape evolution, few opportunities exist to quantify those processes over field scales. We investigated the downstream effects of sediment released during the largest dam removal in history, on the Elwha River, Washington, USA, by measuring changes in riverbed elevation and topography, bed sediment grain size, and channel planform as two dams were removed in stages over two years. As 10.5 million t (7.1 million m3) of sediment was released from two former...
The rates and styles of channel adjustments following an abrupt and voluminous sediment pulse are investigated in the context of site and valley characteristics and time-varying sediment transport regimes. Approximately 10.5 x 106m3 of stored gravel and sand was exposed when Barlin Dam failed during Typhoon WeiPa in 2007. The dam was located on the Dahan River, Taiwan, a system characterized by steep river gradients, typhoon- and monsoon-driven hydrology, high, episodic sediment supply, and highly variable hydraulic conditions. Topography, bulk sediment samples, aerial photos, and simulated hydraulic conditions are analyzed to investigate temporal and spatial patterns in morphology and likely sediment transport...
Total suspended solids (TSS), dissolved organic carbon (DOC), and total dissolved nitrogen (TDN) loads were calculated for all stages of the dam removal process (dewatering, breaching, and removal) at various points upstream, within, and downstream of Lowell Mill Impoundment on the Little River, North Carolina. The impoundment dewatering exported loads of TSS, DOC, and TDN which were all 1–2 orders of magnitude less than loads associated with historic floods. Conversely, floods exiting the former impoundment following dam removal produced TSS, DOC, and TDN loads comparable to, but slightly greater (1.2–1.75 times) than historic floods. Exported loads were greatest following the complete removal of the dam, most...
Although dam removal has been increasingly used as an option in dam management, and as a river restoration tool, few studies provide detailed quantitative assessment of the geomorphological response of rivers to dam removal. In this study, we document the response of the Pine River, Michigan, to the gradual removal of Stronach Dam. In 1996, prior to the initiation of removal, 31 permanent cross-sectional transects were established in the 10-km study area. These transects were surveyed annually during the course of the removal (1996–2003) and for the three years following removal (2004–2006). Dam removal resulted in progressive headcutting of sediments in the former impoundment, extending upstream 3.89 km of the...
The removal of the numerous ageing dams in the United States has become an important stream restoration technique. The extent to which the ecological damage done to streams by dams is reversed upon removal is unknown, especially on decadal time scales. The objectives of this study were to determine if macroinvertebrate assemblages within rivers recover following the removal of a dam and to estimate the time needed for recovery. A space‐for‐time substitution approach was used on eight rivers in various stages of recovery following a dam removal, ranging from <1 to 40 years post‐removal. Within each river, macroinvertebrates were sampled in a zone unaffected by the dam removal (reference zone) and two zones impacted...
Seismic signals near rivers are partially composed of the elastic waves generated by bedload particles impacting the river bed. In this study, we explore the relationship between this seismic signal and river bedload transport by analyzing high-frequency broadband seismic data from multiple stations along the Chijiawan River in northern Taiwan following the removal of a 13 m check dam. This dam removal provides a natural experiment in which rapid and predictable changes in the river's profile occur, which in turn enables independent constraints on spatial and temporal variation in bedload sediment transport. We compare floods of similar magnitudes with and without bedload transport, and find that the amplitude of...
The ability to predict the effects of dam removal in highly sediment-filled systems is increasingly important as the number of such dam removal cases continues to grow. The cost and potential impacts of dam removal are site-specific and can vary substantially depending on local conditions. Of specific concern in sediment-impacted removals is the volume and rate of reservoir deposit erosion. The complexity and potential accuracy of modeling methods used to forecast the effects of such dam removals vary substantially. Current methods range from predictions based on simple analysis of pre-dam channel geometry to sophisticated data-intensive, three-dimensional numerical models. In the work presented here, we utilize...
On 26 October 2011, engineers blew a hole in the base of Washington state's Condit Dam, loosing nearly a million cubic meters of water in an instant. The surge of water, sand, and mud marked the end of the 38-meter-tall concrete dam that had blocked the White Salmon River for nearly 100 years and the beginning of a rapid transformation of the downstream environment. Using river gauge measurements, time-lapse photography, and other techniques, Wilcox et al. tracked how the White Salmon River evolved in the wake of the breach.
The benefits of gradually removing a dam (through multiple notches) are to reduce the total project cost and reduce possible environmental effects by allowing the impounded sediment to slowly move downstream, and a stable stream and revegetated floodplain to form upstream. Notching, in this study of a dam on Brewster Creek, near St. Charles, Illinois, involves cutting a given height (in five 12–18 inch notches over approximately a 9 month period) across the length (or some portion of the length) of the dam. Brewster Creek is a tributary of the Fox River in northeastern, Illinois. Sediment, dissolved oxygen, and geomorphic response are being monitored before, during, and after a gradual (notching) removal of the...
The Pawtuxet River in Rhode Island, USA, has a long history of industrial activity and pollutant discharges. Metal contamination of the river sediments is well documented and historically exceeded toxicity thresholds for a variety of organisms. The Pawtuxet River dam, a low-head dam at the mouth of the river, was removed in August 2011. The removal of the dam was part of an effort to restore the riverine ecosystem after centuries of anthropogenic impact. Sediment traps were deployed below the dam to assess changes in metal concentrations and fluxes (Ag, Cd, Cr, Cu, Ni, Pb, and Zn) from the river system into Pawtuxet Cove. Sediment traps were deployed for an average duration of 24 days each, and deployments continued...
Although ?70 dams have been decommissioned in Wisconsin over the past 30 y, little is known about the physical and ecological effects of dam removal on riverine ecosystems. The purpose of our study was to document changes in channel form and macroinvertebrate assemblages following the removal of a low-head, run-of-river dam from the Baraboo River,Wisconsin, in January 2000. We surveyed cross sections and collected benthic macroinvertebrate samples in 6 reaches (an upstream reference reach, reaches immediately above and below the dam that was to be removed, and sequential unimpounded and impounded reaches further downstream) in a multiple-dam sys- tem. Surveys were conducted in December 1999, before dam removal,...
In the late 1970s, the construction of weirs in Norwegian regulated river systems for aesthetic reasons was common. However, today, the focus of river restoration has shifted towards improving biological functionality and biodiversity. In the present study, two weirs, originally built to create a stable water level, were removed on a residual flow reach in a Norwegian regulated river as a measure to restore river connectivity and to re-establish the local population of Atlantic salmon. The removal design was based on hydraulic modelling, and biological monitoring was implemented before and after the weir removal to evaluate the biological response to weir removal. The results demonstrated that salmon spawning sites...
Dams create barriers to fish migration and dispersal in drainage basins, and the removal of dams is often viewed as a means of increasing habitat availability and restoring migratory routes of several fish species. However, these barriers can also isolate and protect native taxa from aggressive downstream invaders.We examined fish community composition two years prior to and two years after the removal of a pair of low-head dams from Boulder Creek,Wisconsin, U.S.A. in 2003 to determine if removal of these potential barriers affected the resident population of native brook trout (Salvelinus fontinalis). Despite the presence of other taxa in the downstream reaches, and in other similar streams adjacent to the Boulder...
A slow draining reservoir on the U.S. East Coast was monitored to identify the processes governing channel evolution upstream of a dam removal. Channel evolution was documented through cross section surveys, sediment size analysis, discharge measurements, and visual assessments of vegetative growth. The reservoir drained slowly, allowing for an analysis of channel evolution and identification of the morphometric parameters defining the path and time required for a channel to reach dynamic equilibrium. Channel evolution was a multidirectional process, and evolving channel reaches actively migrated laterally while alternating between aggradation and degradation. Channel formation was dominated primarily by the hydrologic...
Dam removal is often implemented without adequate baseline monitoring to distinguish background variability from channel changes due to the removal. This study evaluated aerial photos as substitutes for multiple-year pre-removal field data to assess downstream channel changes associated with a small dam removal. The Brownsville Dam, a 2.1 m tall concrete dam on the Calapooia River, Oregon, was removed in 2007. We mapped bars and the low flow channel downstream from the dam and in an upstream control reach using aerial photos (1994-2008) and in the field prior to (2007) and following (2008) removal. The locations and magnitudes of changes in bar area and wetted width, relative to errors, indicate that downstream...
The Anaconda and Union City Dams oil the Naugatuck River in Connecticut were removed in February and October 1999. A detailed Study of the sites prior to removal was undertaken including sediment testing and predictions Of upstream channel formation post-darn removal. The 3.35-m-high timber crib/rock fill spillway of the Anaconda Dam partially breached during a storm prior to the dam's scheduled removal allowing a portion of the impounded sediment to move down through the river system. This event changed the removal plans and the remainder of the spillway was removed under an emergency order in the course of 4 days. The Union City Dam, a 2.44-m-high timber crib/rock fill dam capped with concrete and stone, was removed...
Previous studies of vegetation establishment in dam removal sites have shown that natural vegetation community establishment is highly variable and frequently includes species often considered undesirable in restorations. In this article, we examined two case studies where dam removal sites were planted with native species following dam removal in an effort to promote native species establishment and exclude invasive species. Some planted species established soon after the dam removals, but surveys four years later showed a decline in planted species and an increase in non-native species. In both cases, reed canarygrass (Phalaris arundinacea) became well established in the interval between surveys. A seedbank analysis...
In this paper, we synthesize a series of small dam removal studies to examine how changes in channel form can affect riparian vegetation, fish, macroinvertebrates, mussels, and nutrient dynamics. Each of the ecosystem attributes responded to the disturbance of dam removal in different ways and recovered at very different rates, ranging from months to decades. Riparian vegetation appeared to require the greatest time for recovery, while macroinvertebrates had the least. Mussel communities were the most adversely affected group of species and showed no signs of recovery during the time period of the study. Based on these and other studies, we suggest that ecosystems may follow two trajectories of recovery following...