Physical and Biological Responses to an Alternative Removal Strategy of a Moderate‐sized Dam in Washington,USA

Dam removal is an increasingly practised river restoration technique, and ecological responses vary with watershed, dam and reservoir properties, and removal strategies. Moderate‐sized dams, like Hemlock Dam (7.9 m tall and 56 m wide), are large enough that removal effects could be significant, but small enough that mitigation may be possible through a modified dam removal strategy. The removal of Hemlock Dam in Washington State, USA, was designed to limit channel erosion and improve fish passage and habitat by excavating stored fine sediment and reconstructing a channel in the former 6‐ha reservoir. Prior to dam removal, summer daily water temperatures downstream from the dam increased and remained warm long into the night. Afterwards, a more natural diel temperature regime was restored, although daily maximum temperatures remained high. A short‐lived turbidity pulse occurred soon after re‐watering of the channel, but was otherwise similar to background levels. Substrate shifted from sand to gravel–cobble in the former reservoir and from boulder to gravel–cobble downstream of the dam. Initially, macroinvertebrate assemblage richness and abundance was low in the project area, but within 2 years, post‐removal reaches upstream and downstream of the dam had diverse and abundant communities. The excavation of stored sediment and channel restoration as part of the dam removal strategy restored river continuity and improved benthic habitat while minimizing downstream sedimentation. This study provides a comparison of ecological effects with other dam removal strategies and can inform expectations of response time and magnitude. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.     

Downstream benthic responses to small dam removal in a coldwater stream

Increased awareness of the negative effects of dams combined with an aging dam infrastructure has led to an increase in dam removals. However, ecological responses of downstream biota to such actions are poorly understood. We examined the influence of a pair of small dam removals on downstream periphyton and macroinvertebrates in Boulder Creek, WI (USA). The dams were 180 m apart and both were removed on 9 July 2003. We monitored algae and macroinvertebrates as well as habitat characteristics for approximately 2 months before and after the removals upstream and downstream from the two dams, and continued our observations over a similar period (mid‐May to mid‐July) the following summer. After the removals, an initial release of sediment significantly increased the proportion of fine sediments in the downstream reach and buried benthic substrate. This burial led to a 60% reduction in chlorophyll in the downstream reach the week following removal, while concentrations increased in the upstream reference reach. Similarly, macroinvertebrate densities 2 weeks post‐removal were lower relative to pre‐removal densities and were associated with declines of ephemeropterans, trichopterans and dipterans. Examination of Trichoptera genera demonstrated substantial changes in this assemblage associated with diminished densities of the formerly dominant genus Brachycentrus. Algal and invertebrate populations increased in the weeks after the dam removal, but did not reach densities similar to the upstream reference reach. In the following year, both periphyton and invertebrate densities were lower in the downstream reach, suggesting a long‐term effect of the removal. Thus, effects of the dam removal were alternatively positive or negative over time, and indicate that the time scale of consideration has a strong effect on the interpretation of the consequences of this management activity. Copyright © 2008 John Wiley & Sons, Ltd.     

LONG‐TERM TAXON‐SPECIFIC RESPONSES OF MACROINVERTEBRATES TO DAM REMOVAL IN A MID‐SIZED SWEDISH STREAM

Dam removal to restore ecologically impaired rivers is becoming increasingly common. Although the target often is to facilitate fish migration, dam removal has also been assumed to benefit other types of organisms. Because few studies thus far deal with effects of dam removal on stream macroinvertebrates and because results have been equivocal, we investigated both short‐ and longer‐term dam‐removal effects on downstream macroinvertebrate communities. We did this in a before‐and‐after study of the removal of a dam located in a south Swedish stream. We sampled the benthic fauna 6 months before dam removal and both 6 months and 3.5 years after the dam was removed. We compared species composition, taxonomic richness, total densities and densities of macroinvertebrate groups before and after dam removal and between downstream and reference sites. We found that dam removal reduced some macroinvertebrate taxa at the downstream site, but we found no effect on community composition. Although this corroborates results from previous short‐term studies, we also found a reduction of taxonomic richness and that some dam‐removal effects persisted or even increased over time. The most likely explanation for the suppression of benthic macroinvertebrate richness following dam removal is a significantly increased sediment transport from the former reservoir and a subsequent loss of preferred substrates. Our results indicate that adverse dam‐removal effects may be long lasting but taxon specific. We therefore call for longer‐term studies on a variety of organisms to better understand how dam removal may influence downstream macroinvertebrate communities. Copyright © 2012 John Wiley & Sons, Ltd.     

Vegetation community response to hydrologic and geomorphic changes following dam removal

Dam removal can restore fish passage, natural flow regimes, sediment transport in streams, dispersal of organic matter, and drift of aquatic insects. However, dam removal also impacts the riparian vegetation, with both immediate and delayed responses. In this study, we measure vegetation change at the Merrimack Village Dam site on the Souhegan River in Merrimack, NH, USA. The August 2008 removal caused a ~3‐m drop in water level and rapid erosion of impounded sediment, with ~50% removed in the first 3 months. Terrace, floodplain, and wetland communities were surveyed in summer 2007, 2009, 2014, and 2015. Temporal change was quantified using Analysis of Similarity on the Bray–Curtis dissimilarity matrix. Only herbaceous vegetation closest to the river channel and in the off‐channel wetland changed significantly. The herbaceous plots directly adjacent to the impoundment eroded to bare sand in 2009, but by 2014, the original riparian fringe community had re‐established in the newly developed floodplain. Between 2007 and 2014, the off‐channel wetland area changed from aquatic species to a stable terrestrial community that persisted without significant change in 2015. The vegetation response was greatest in areas with the largest geomorphic and hydrologic change. These included the channel margin where erosion and bank slumping created an unstable scarp. The mid‐channel island and off‐channel wetland were strongly affected by the lowered water table. However, large unvegetated areas never persisted nor did the areal coverage of invasive species expand, which are two frequent concerns of dam removal stakeholders.     

An assessment of sediment reuse for sediment management of Gallito Ciego Reservoir,Peru

Gallito Ciego Reservoir, with a surface area of 14.2 km² and mean depth of 40.3 m, is located in the Jequetepeque River basin in north‐western Peru. It is rapidly filling with sediments, endangering its main purpose of supplying agricultural irrigation water. A sediment volume corresponding to 70% of the dead water volume has accumulated in the reservoir up to 2007, with reservoir bottom outlet becoming blocked. Below the dam, 35 000 ha of irrigated cropland, supporting 115 000 habitants, are endangered. This study was conducted to evaluate the possibilities of sediment management, especially the use of the sediment for agricultural purposes in the Jequetepeque River basin. Sediment samples were collected from littoral and profound sites in the reservoir. Suspended sediments also were collected. Physical parameters were investigated, and the nutrient and heavy metal concentrations were determined. By comparing grain‐size distribution and nutrient content, as well as pollutant concentration of the sediments, to those of terrestrial soils near the reservoir, an assessment of the potential for applying the sediments on the cropland was undertaken. Texture investigations of profound sediments revealed a strong grain‐size classification within the reservoir. Because the heavy metal concentrations in the sediments were below toxic thresholds, and the concentrations of nutrients also were low, using the sediments for agricultural purposes would not constitute risks, although they cannot replace fertilizer. As a soil amendment and building material, the sediments do have an economical value. Its dredging and use, however, must be seen as only one component of more holistic sediment management of the Jequetepeque Watershed.     

Toledo Bend reservoir and geomorphic response in the lower Sabine River

Downstream geomorphic responses of stream channels to dams are complex, variable, and difficult to predict, apparently because the effects of local geological, hydrological, and operational details confound and complicate efforts to apply models and generalizations to individual streams. This sort of complex geomorphic response characterizes the Sabine River, along the Texas and Louisiana border, downstream of the Toledo Bend dam and reservoir. Toledo Bend controls the flow of water and essentially prevents the flux of sediment from three‐quarters of the drainage basin to the lower Sabine River. Although the channel is scoured immediately downstream of the dam, further downstream there is little evidence of major changes in sediment transport or deposition, sand supply, or channel morphology attributable to the impoundment. Channels are actively shifting, banks are eroding, and sandbars are migrating, but not in any discernibly different way than before the dam was constructed. The Sabine River continues to transport sand downstream, and alluvial floodplains continue to accrete. The relatively small geomorphic response can be attributed to several factors. While dam releases are unnaturally flashy and abrupt on a day‐to‐day basis, the long‐term pattern of releases combined with some downstream smoothing creates a flow regime in the lower basin which mimics the pre‐dam regime, at least at monthly and annual time scales. Sediment production within the lower Sabine basin is sufficient to satisfy the river's sediment transport capacity and maintain pre‐dam alluvial sedimentation regimes. Toledo Bend reservoir has a capacity: annual inflow ratio of 1.2 and impounds 74% of the Sabine drainage basin, yet there has been minimal geomorphic response in the lower river, which may seem counterintuitive. However, the complex linked geomorphic processes of discharge, sediment transport and loads, tributary inputs, and channel erosion include interactions which might increase as well as decrease sediment loads. Furthermore, if a stream is transport‐limited before impoundment, the reduced sediment supply after damming may have limited impact. Copyright © 2003 John Wiley & Sons, Ltd.     

Restoration of Hydrochory Following Dam Removal on the Elwha River,Washington

Hydrochory, seed dispersal by water, affects riparian vegetation by contributing to downstream community composition and diversity. However, dams can block hydrochory, reducing downstream species diversity and fragmenting riparian corridors. Dam removal is becoming more prevalent for economic and ecological reasons and is expected to restore hydrochory; however, this has never been documented in rivers. The largest dam removal project to date was the 2011 to 2014 removal of the Glines Canyon and Elwha dams on the Elwha River in Washington. Prior to dam removal, hydrochory was lower below Glines Canyon Dam compared with an upstream reach; our objective was to test the hypothesis that dam removal would restore downstream hydrochory to levels observed in the upstream reach. To test this, we collected seeds in nets above and below the dam during three sample periods (early July, late July and early August), growing out seeds in a greenhouse and comparing seed abundance and species richness above and below dams, before and after dam removal. We found that after dam removal, the average number of hydrochorous seeds and species increased below Glines Canyon Dam to levels similar to or higher than that of the upstream reach; hydrochory levels in the upstream reach did not change. This study is the first to document the restoration of hydrochory in rivers following removal of a large dam. Restoration of hydrochory may ultimately increase downstream vegetation diversity and play a role in the recolonization of reservoir sediments deposited in the riparian zone in the years following dam removal. Copyright © 2016 John Wiley & Sons, Ltd.     

Geomorphic Responses to a Large Check‐Dam Removal on a Mountain River in Taiwan

The ability to understand and predict the impacts of dam removal in river systems is important, especially as dam decommissioning is becoming increasingly popular. In this study, we document the morphological and sediment impact of the removal of Chijiawan Check Dam in May 2011; a 13‐m‐high dam located on a coarse‐grained, steep mountain river channel in Taiwan. An estimated 0.2 million m³ of sediment had accumulated within the impoundment before its removal. Longitudinal and bankfull cross‐sectional surveys and a detailed sediment textural survey were undertaken along a 3.2‐km study reach of the Chijiawan Creek between 2010 and 2012. A rotating knickpoint with migration rates of up to 22 m/day was observed along the study reach, following dam removal. The rate and character of channel change, associated with the dam removal, appear to be driven as much by channel morphology and distance from the dam as by the hydrology variability. Our results suggested that relatively small amounts of sediment were eroded during the first 3 weeks following dam removal because of low discharge conditions. However, after 1 and 15 months, 10 and 75% of the sediment that had accumulated within former impounded was eroded, respectively. Sites near the former dam had a sediment texture that reflected the transport of released sediment, and this suggested that basin‐wide sediment processes exerted a strong influence. The removal of Chijiawan Dam offers unique insight on how sediment processes can drive river channel responses to sediment pulses may vary with discharge and sediment load, in areas subject to remarkably high flows and sediment loads. Copyright © 2015 John Wiley & Sons, Ltd.     

Rapid CHannel Widening Following Weir Removal due to Bed‐Material Wave Dispersion on the River Monnow,Wales

Kentchurch Weir, a low‐head weir on the river Monnow, Wales, was demolished in August 2011, releasing a sediment wave that had formed behind the structure for at least a century. We surveyed channel topography and bed‐material composition through a 1.5‐km long reach prior to weir removal and then periodically over a 2‐year period. The fill material was finer than the ambient bed material with all particles mobilized by bankfull flows. Rapid degradation of the 1460‐m³ sediment fill in the previously impounded reach occurred as bed material appeared to disperse downstream, consistent with other studies of sediment waves in gravel‐bed rivers. The riverbed profile was gradually smoothed through the study reach by degrading the elevated fill as a migrating knickpoint and aggrading the channel bed and bars immediately downstream of the former weir location. Extensive bank erosion was evident in the previously impounded reach with up to 10 m of widening following a single flow event, increasing channel width by more than 20%. Mitigation measures to enforce the riverbanks have been required as the gradual dispersion of the sediment wave continues to force flow diversion towards the riverbanks. The evolution of sediment stores behind flow obstructions follows that of sediment waves and theory available to describe wave evolution should do much to improve management efforts that seek to minimize channel widening following weir removal. Copyright © 2014 John Wiley & Sons, Ltd.     

FINE SEDIMENT INFILTRATION DYNAMICS IN A GRAVEL‐BED RIVER FOLLOWING A SEDIMENT PULSE

Pulses of fine sediment in gravel‐bedded rivers can cause extensive fine sediment infiltration (FSI) into void spaces in coarse bed material, potentially altering river morphodynamics and aquatic ecosystems. Previous work suggests a conceptual model of FSI whereby FSI occurs to a limited depth as a function of the relative grain size of bed sediment compared with infiltrating sediment and is influenced by fine sediment supply and local flow dynamics. Our study applies this conceptual model to a complex reach of a wandering, medium‐sized, gravel‐bed river to investigate the spatial and temporal controls on FSI. To constrain the timing of FSI, we use the release of contaminated sediment from an upstream dam removal and complementary field methods (bulk sampling, freeze cores and infiltration bags) to capture sediment across varied depositional settings. Our results indicate that, even in a morphologically complex reach, fine‐sediment content in the bed does not vary significantly among deposition settings or vertically below the bed surface. We also found that the most contaminated fine sediments released into our study river by a dam removal are not present within the bed material and that substrate has likely been reworked over the period between the release of contaminated sediment and sampling. Our observations also suggest that seals of fine sediment causing void pore space at depth, which have previously been associated with FSI, are not evident in our field area. This suggests that in natural systems, high sediment supply and mobile beds may limit seal formation and persistence. Copyright © 2013 John Wiley & Sons, Ltd.     

Short-term effects on Unionid mussel density and distribution before and after low-head dam removal in northern New York

Unionid mussel populations have declined in many environments, particularly those that were altered by human activities such as dam construction and removal. The Fort Covington Dam blocked the Salmon River in northern New York upriver of its confluence with a smaller tributary, the Little Salmon River. This dam was removed in 2009. My study compared the mean ranked density of mussels in both rivers for the pre-removal period (2005–2008) to the post-removal period (2009–2012) with special attention to Lampsilis cariosa and Margaritifera margaritifera. Systematic sampling was used at six riffles and double sampling at four glides divided between the Salmon and Little Salmon rivers from 2005 through 2012. The Little Salmon River served as a control. Mean ranked adult mussel density was not significantly different between the two rivers in the pre-removal period but was significantly greater in the Little Salmon River after dam removal. Living mussels of 13 species were collected dominated by Elliptio complanata. Dam removal did not affect sediment sorting, porosity, water chemistry, or mussel species distribution. Rapid dewatering of the reservoir led to stranding and death of mussels in the reservoir and in the adjacent ponds. Increased water velocity, exacerbated by rain, mobilized sediment that buried a potentially large mussel bed downstream of the dam where Lampsilis cardium and L. cariosa were abundant, which showed little recovery by 2013.     

水库报废生态环境影响及其修复

为研究水库报废对生态环境的影响,将水库报废拆坝影响划分为三个层次:以水沙过程为初始影响,视社会和经济影响为最终效应,两者之间为次生影响,并揭示了拆坝影响的综合性、矛盾性、时空延续性及不确定性等特点。从物理、化学、生态三方面明晰了拆坝生态环境影响:物理影响体现对水文、泥沙和地形地貌的影响,拆坝可完全逆转建坝带来的水文影响,淤积的泥沙将重新运移,通过溯源侵蚀使大坝上下游河段逐渐达到相对稳定的坡降;化学影响主要表现为对河流水质的影响;生态影响即可能使水生生物的生境质量得以改善,也可能由于水质退化、物种入侵对水生生境产生负面影响。针对性地提出了拆坝生态修复对策和措施:应通过重新引入乡土植物群落、控制外来物种及顺应植被演替规律修复库区及下游河道濒水植被;清除或降低污染物对生态环境的不利影响;重建河流近岸水文、地貌环境,保护和恢复水生生物;通过生态环境监测,应适时对修复措施进行调整。研究所得成果为报废水库的生态修复提供参考。     

Internal nutrient flux in an inland water supply reservoir, New South Wales, Australia

Nutrient dynamics at the water–sediment interface in the Suma Park Reservoir, Australia, was assessed under simulated laboratory conditions using intact sediment cores. This laboratory experiment demonstrated that the nutrient influx between the sediment and the water column, in both oxic and anoxic environments, contributed substantially to the total nutrient budget and overall recycling of the biologically available nutrients in the reservoir. This study also confirmed that the bottom sediments act as a source of ammonium‐nitrogen (NH₄‐N) and filterable reactive phosphorus (FRP), but function as a sink for nitrate‐nitrogen (NO₃‐N). Extrapolation of the experiment data revealed that the highest nutrient flux was obtained under a summer‐anoxic incubation, with the internal loads of FRP and NH₄‐N accounting for ≈ 365% and 338% of their external annual loads, respectively. The internal loss of NO₃‐N from the summer anoxic incubation was ≈ 7% of its external annual load. The temperature and dissolved oxygen concentration were the most important factors influencing the nutrient flux and internal loading. Denitrification was believed to be an eminent route of nitrate loss from the reservoir.     

Sediment‐bound nutrient export from five small reservoir catchments and its implications for the Sudan savanna zone of Ghana

A study was carried out in the Sudan savanna zone in the Upper East Region of Ghana to assess the rate of sediment‐bound nutrient export (NE) into five small reservoirs (Dua, Doba, Zebilla, Kumpalgogo and Bugri) and to analyse the implications of this export. The catchment soils and reservoir sediments from the various study sites were sampled and analysed for their bulk density, particle size distribution and nutrient content. Assessment of the nutrient concentrations indicated that the reservoir sediments were richer not only in nutrients and organic carbon, but also in clay and silt, than the catchment soils, having enrichment ratios >1. Nutrient export rates (NE; kg ha^?1 year^?1) from the reservoir catchments ranged from 0.755 (±0.264) for OC, 0.104 (±0.0245) for N, 0.0020 (±0.0003) for P, 0.016 (±0.0038) for K, 0.009 (±0.0024) for Na, 0.113 (±0.017) for Ca and 0.027 (±0.0093) for Mg. These rates were lower than those of other studies, likely due to the low nutrient content in the catchment soils. The relationships established between NE and specific sediment yield (SSY) indicated the NE was positively correlated with SSY (R² = 0.66–0.98). The derived empirical equations can be satisfactorily used to predict the quantity of sediment‐bound plant nutrients lost from similar catchments and subsequently stored in the reservoir sediments. The study results also suggest the need for sustainable land management practices to forestall erosion in the catchment areas and to reduce reservoir sedimentation, for enhancement of the livelihoods of the communities in the study area.     

Large dam removal and early spontaneous riparian vegetation recruitment on alluvium in a former reservoir: Lessons learned from the pre‐removal phase of the Sélune River project (France)

The restoration of ecological continuity along the Sélune River (Normandy, France) involves the removal of two tall hydroelectric dams (36 m removed in 2019 and 16 m in 2021), a project without precedent in Europe. During the pre‐removal phase (2014–2018), we performed scientific monitoring of the vegetation that was colonizing alluvium in the former dam reservoir (length: 19 km; surface area: 151 ha). Our study aimed to analyse if spontaneous vegetation could ecologically restore the riparian zone and help maintain fine sediment after dam removal. We used colonization indicators related to vegetation structure, taxonomic richness and diversity, and composition. These indicators were calculated at two spatial scales (local, at a single site, and broad, along the reservoir). The aim was to (a) characterize the spontaneously established species pool; (b) analyse longitudinal patterns in vegetation colonization; and (c) assess temporal changes in the species community. Our results show that diverse plant communities have developed. Slight differences in longitudinal and lateral patterns existed; they were linked with habitat heterogeneity and the reservoir's slow pace of draining. We observed fast spontaneous terrestrialization, which has resulted in cover stabilization, decreased diversity, and the development of herbaceous riverbank communities, with very few invasive species. This finding suggests stabilization potential is high and passive ecological restoration could occur, at least locally. Further analyses focusing on functional traits could help inform future management decisions regarding revegetation on reservoir alluvium.     

DISTRIBUTION AND ABUNDANCE OF STREAM FISHES IN RELATION TO BARRIERS: IMPLICATIONS FOR MONITORING STREAM RECOVERY AFTER BARRIER REMOVAL

Dams are ubiquitous in coastal regions and have altered stream habitats and the distribution and abundance of stream fishes in those habitats by disrupting hydrology, temperature regime and habitat connectivity. Dam removal is a common restoration tool, but often the response of the fish assemblage is not monitored rigorously. Sedgeunkedunk Stream, a small tributary to the Penobscot River (Maine, USA), has been the focus of a restoration effort that includes the removal of two low‐head dams. In this study, we quantified fish assemblage metrics along a longitudinal gradient in Sedgeunkedunk Stream and also in a nearby reference stream. By establishing pre‐removal baseline conditions and associated variability and the conditions and variability immediately following removal, we can characterize future changes in the system associated with dam removal. Over 2 years prior to dam removal, species richness and abundance in Sedgeunkedunk Stream were highest downstream of the lowest dam, lowest immediately upstream of that dam and intermediate farther upstream; patterns were similar in the reference stream. Although seasonal and annual variation in metrics within each site was substantial, the overall upstream‐to‐downstream pattern along the stream gradient was remarkably consistent prior to dam removal. Immediately after dam removal, we saw significant decreases in richness and abundance downstream of the former dam site and a corresponding increase in fish abundance upstream of the former dam site. No such changes occurred in reference sites. Our results show that by quantifying baseline conditions in a small stream before restoration, the effects of stream restoration efforts on fish assemblages can be monitored successfully. These data set the stage for the long‐term assessment of Sedgeunkedunk Stream and provide a simple methodology for assessment in other restoration projects. Copyright © 2011 John Wiley & Sons, Ltd.     

Sources and leaching of manganese and iron in the Saigon River Basin, Vietnam

High concentrations of manganese and iron in the Saigon River are major problems for the water supply in Ho Chi Minh City (HCMC), Viet Nam. To identify their sources and leaching processes, we surveyed water quality along the Saigon River and ran batch leaching tests using soil and sediment samples. Two important leaching processes were identified: acidic leaching from acid sulfate soil (ASS) in the middle reaches of the river, and Mn dissolution and Fe reduction from sediments in the downstream reaches. Low pH caused the concurrent release of Fe and Mn from the ASS. In contrast, anoxia caused the release of Fe but not Mn from the sediments, whereas low pH facilitated Mn dissolution. Sediments are a more important source of Mn because of their higher Mn contents (10 times) and release rates (14 times) than those from ASS.     

Beaver assisted river valley formation

We examined how beaver dams affect key ecosystem processes, including pattern and process of sediment deposition, the composition and spatial pattern of vegetation, and nutrient loading and processing. We provide new evidence for the formation of heterogeneous beaver meadows on riverine system floodplains and terraces where dynamic flows are capable of breaching in‐channel beaver dams. Our data show a 1.7‐m high beaver dam triggered overbank flooding that drowned vegetation in areas deeply flooded, deposited nutrient‐rich sediment in a spatially heterogeneous pattern on the floodplain and terrace, and scoured soils in other areas. The site quickly de‐watered following the dam breach by high stream flows, protecting the deposited sediment from future re‐mobilization by overbank floods. Bare sediment either exposed by scouring or deposited by the beaver flood was quickly colonized by a spatially heterogeneous plant community, forming a beaver meadow. Many willow and some aspen seedlings established in the more heavily disturbed areas, suggesting the site may succeed to a willow carr plant community suitable for future beaver re‐occupation. We expand existing theory beyond the beaver pond to include terraces within valleys. This more fully explains how beavers can help drive the formation of alluvial valleys and their complex vegetation patterns as was first postulated by Ruedemann and Schoonmaker in 1938. Copyright © 2010 John Wiley & Sons, Ltd.     

Sedimentology of New Fluvial Deposits on the Elwha River,Washington, USA,Formed During Large‐Scale Dam Removal

Removal of two dams 32 m and 64 m high on the Elwha River, Washington, USA, provided the first opportunity to examine river response to a dam removal and controlled sediment influx on such a large scale. Although many recent river‐restoration efforts have included dam removal, large dam removals have been rare enough that their physical and ecological effects remain poorly understood. New sedimentary deposits that formed during this multi‐stage dam removal result from a unique, artificially created imbalance between fluvial sediment supply and transport capacity. River flows during dam removal were essentially natural and included no large floods in the first two years, while draining of the two reservoirs greatly increased the sediment supply available for fluvial transport. The resulting sedimentary deposits exhibited substantial spatial heterogeneity in thickness, stratal‐formation patterns, grain size and organic content. Initial mud deposition in the first year of dam removal filled pore spaces in the pre‐dam‐removal cobble bed, potentially causing ecological disturbance but not aggrading the bed substantially at first. During the second winter of dam removal, thicker and in some cases coarser deposits replaced the early mud deposits. By 18 months into dam removal, channel‐margin and floodplain deposits were commonly >0.5 m thick and, contrary to pre‐dam‐removal predictions that silt and clay would bypass the river system, included average mud content around 20%. Large wood and lenses of smaller organic particles were common in the new deposits, presumably contributing additional carbon and nutrients to the ecosystem downstream of the dam sites. Understanding initial sedimentary response to the Elwha River dam removals will inform subsequent analyses of longer‐term sedimentary, geomorphic and ecosystem changes in this fluvial and coastal system, and will provide important lessons for other river‐restoration efforts where large dam removal is planned or proposed. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.     

Tracing fine sediment sources in small mountain catchment

Fine sediment represents an important diffuse source pollutant in surface waters, due to its role in governing the transfer and fate of many substances, including nutrients, heavy metals, pesticides and other organic contaminants, and its influence on aquatic ecology. Therefore, catchment management strategies frequently need to include provision for the control of sediment mobilization and delivery. The sediment tracing concept provides a valuable framework for assisting the management and control of diffuse source sediment pollution by identifying the key sources and demonstrating the importance of intermediate storages and the likely impact of upstream mitigation strategies on downstream sediment and sediment associated contaminant fluxes. In this research, fine sediment sources were identified using tracing method. By field works, sediments were sampled from dam reservoir, different sources were also sampled. Fifteen tracers were first selected for tracing which are: The amounts of N, P, C, Cr, Co, Mg, K, Na, smectite, chlorite, illite, kaolinite, and two magnetic properties consisting of LOW Frequency Magnetic Susceptibility (X(LF)) and Frequency Dependent Magnetic Susceptibility (X(FD)). The samples were analyzed in the laboratory for these parameters and different statistical methods were applied to the data including Non-parametric Kruskal-Wallis Test and Stepwise Discriminant function analysis. The results provide important information on the relative importance of fine sediment sources to the reservoir sediments, which can be used to support model validation and the targeting of management and control strategies.