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1.
Habitat degradation in river ecosystems has considerably increased over the past decades, resulting in detrimental effects on aquatic and riparian communities. During the last two decades, the value of large wood as a resource for river restoration and recovery has been increasingly documented. However, post-project appraisal of the associations between restored large wood, morphological complexity and river ecology as a result of river restoration is extremely rare and thus scientific knowledge is essential. To investigate restored wood-induced morphological response and sediment complexity in an overwidened reach along a low gradient lowland river (River Bure, UK), two sub-reaches containing 12 jams initiated by wood emplacement in 2008 and 2010 and a sub-reach free of wood were studied. Wood surveys recording the dimensions and number of wood pieces in jams, geomorphological mapping of the reach illustrating the spatial distribution of features in and around the jams and in a section free of wood, and sediment sampling (analysed for particle size, organic content and plant propagule abundance) of five recurring patch types surrounding each jam (two wood-related patches and three representing the broader river environment) were performed. Wood jams partially spanned the river channel and contained large pieces of wood that created more open structures than naturally-formed wood jams. Where no wood was introduced, the channel remains wide and the gravel bed is buried by sand and finer sediment. In the restored reaches, fine sediment has accumulated in and around the wood jams and has been stabilised by vegetation colonisation, enhancing flow velocities in the narrowed channel sufficiently to mobilise fine sediment and expose the gravel bed. Sediment analysis reveals sediment fining with time since wood emplacement, largely achieved within the two wood-related patch types. Fine sediment retained around the wood shows a relatively higher plant propagule content than other patch types, suitable for sustaining plant succession as the vegetated side bars aggrade. Although channel narrowing and morphological adjustment has occurred surprisingly rapidly in this low energy, over-widened reach following wood introduction (2–4 years), sustaining the recovery in the longer term to suitably support flora and fauna communities depends on the continued delivery of wood by ensuring a natural supply of sufficiently large wood pieces from riparian trees both upstream and within the reach.  相似文献   

2.
This paper explores changes in suspended sediment transport and fine sediment storage at the reach and patch scale associated with the reintroduction of partial large wood (LW) jams in an artificially over‐widened lowland river. The field site incorporates two adjacent reaches: a downstream section where LW jams were reintroduced in 2010 and a reach immediately upstream where no LW was introduced. LW pieces were organized into ‘partial’ jams incorporating several ‘key pieces’ which were later colonized by substantial stands of aquatic and wetland plants. Reach‐scale suspended sediment transport was investigated using arrays of time‐integrated suspended sediment samplers. Patch‐scale suspended sediment transport was explored experimentally using turbidity sensors to track the magnitude and velocity of artificially generated sediment plumes. Fine sediment storage was quantified at both reach and patch scales by repeat surveys of fine sediment depth. The results show that partial LW jams influence fine sediment dynamics at both the patch and reach scale. At the patch‐scale, introduction of LW led to a reduction in the concentration and increase in the time lag of released sediment plumes within the LW, indicating increased diffusion of plumes. This contrasted with higher concentrations and lower time lags in areas adjacent to the LW; indicating more effective advection processes. This led to increased fine sediment storage within the LW compared with areas adjacent to the LW. At the reach‐scale there was a greater increase in fine sediment storage through time within the restored reach relative to the unrestored reach, although the changes in sediment transport responsible for this were not evident from time‐integrated suspended sediment data. The results of the study have been used to develop a conceptual model which may inform restoration design. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

3.
A 177 river km georeferenced aerial survey of in‐channel large wood (LW) on the lower Roanoke River, NC was conducted to determine LW dynamics and distributions on an eastern USA low‐gradient large river. Results indicate a system with approximately 75% of the LW available for transport either as detached individual LW or as LW in log jams. There were approximately 55 individual LW per river km and another 59 pieces in log jams per river km. Individual LW is a product of bank erosion (73% is produced through erosion) and is isolated on the mid and upper banks at low flow. This LW does not appear to be important for either aquatic habitat or as a human risk. Log jams rest near or at water level making them a factor in bank complexity in an otherwise homogenous fine‐grained channel. A segmentation test was performed using LW frequency by river km to detect breaks in longitudinal distribution and to define homogeneous reaches of LW frequency. Homogeneous reaches were then analyzed to determine their relationship to bank height, channel width/depth, sinuosity, and gradient. Results show that log jams are a product of LW transport and occur more frequently in areas with high snag concentrations, low to intermediate bank heights, high sinuosity, high local LW recruitment rates, and narrow channel widths. The largest concentration of log jams (21.5 log jams/km) occurs in an actively eroding reach. Log jam concentrations downstream of this reach are lower due to a loss of river competency as the channel reaches sea level and the concurrent development of unvegetated mudflats separating the active channel from the floodplain forest. Substantial LW transport occurs on this low‐gradient, dam‐regulated large river; this study, paired with future research on transport mechanisms should provide resource managers and policymakers with options to better manage aquatic habitat while mitigating possible negative impacts to human interests. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

4.
Downed large wood (LW) in floodplains provides habitat and nutrients for diverse organisms, influences hydraulics and sedimentation during overbank flows, and affects channel form and lateral migration. Very few studies, however, have quantified LW volumes in floodplains that are unaltered by human disturbance. We compare LW volumes in relatively unaltered floodplains of semiarid boreal lowland, subtropical lowland, and semiarid temperate mountain rivers in the United States. Average volumes of downed LW are 42.3 m3 ha?1, 50.4 m3 ha?1, and 116.3 m3 ha?1 in the semiarid boreal, subtropical, and semiarid temperate sites, respectively. Observed patterns support the hypothesis that the largest downed LW volumes occur in the semiarid temperate mountain sites, which is likely linked to a combination of moderate‐to‐high net primary productivity, temperature‐limited decomposition rates, and resulting slow wood turnover time. Floodplain LW volumes differ among vegetation types within the semiarid boreal and semiarid temperate mountain regions, reflecting differences in species composition. Lateral channel migration and flooding influence vegetation communities in the semiarid boreal sites, which in turn influences floodplain LW loads. Other forms of disturbance such as fires, insect infestations, and blowdowns can increase LW volumes in the semiarid boreal and semiarid temperate mountain sites, where rates of wood decay are relatively slow compared with the subtropical lowland sites. Although sediment is the largest floodplain carbon reservoir, floodplain LW stores substantial amounts of organic carbon and can influence floodplain sediment storage. In our study sites, floodplain LW volumes are lower than those in adjacent channels, but are higher than those in upland (i.e. non‐floodplain) forests. Given the important ecological and physical effects of floodplain LW, efforts to add LW to river corridors as part of restoration activities, and the need to quantify carbon stocks within river corridors, we urge others to quantify floodplain and instream LW volumes in diverse environments. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

5.
Large wood (LW) is a ubiquitous feature in rivers of forested watersheds worldwide, and its importance for river diversity has been recognized for several decades. Although the role of LW in fluvial dynamics has been extensively documented, there is a need to better quantify the most significant components of LW budgets at the river scale. The purpose of our study was to quantify each component (input, accumulation, and output) of a LW budget at the reach and watershed scales for different time periods (i.e. a 50‐year period, decadal cycle, and interannual cycle). The LW budget was quantified by measuring the volumes of LW inputs, accumulations, and outputs within river sections that were finally evacuated from the watershed. The study site included three unusually large but natural wood rafts in the delta of the Saint‐Jean River (SJR; Québec, Canada) that have accumulated all LW exported from the watershed for the last 50 years. We observed an increase in fluvial dynamics since 2004, which led to larger LW recruitment and a greater LW volume trapped in the river corridor, suggesting that the system is not in equilibrium in terms of the wood budget but is rather recovering from previous human pressures as well as adjusting to hydroclimatic changes. The results reveal the large variability in the LW budget dynamics during the 50‐year period and allow us to examine the eco‐hydromorphological trajectory that highlights key variables (discharge, erosion rates, bar surface area, sinuosity, wood mobility, and wood retention). Knowledge on the dynamics of these variables improves our understanding of the historical and future trajectories of LW dynamics and fluvial dynamics in gravel‐bed rivers. Extreme events (flood and ice‐melt) significantly contribute to LW dynamics in the SJR river system. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

6.
This study analyses large wood (LW) storage and the associated effects on channel morphology and flow hydraulics in three third‐order mountain basins (drainage area 9–12 km2) covered in old‐growth Nothofagus forests, ranging from the temperate warm Chilean Andean Cordillera to the sub‐Antarctic Tierra del Fuego (Argentina). Amount, characteristics and dimensions of large wood (>10 cm diameter, >1 m long) were recorded, as well as their effects on stream morphology, hydraulics and sediment storage. Results show that major differences in LW abundance exist even between adjacent basins, as a result of different disturbance histories and basin dissection. Massive LW volumes (i.e. >1000 m3 ha?1) can be reached in basins disturbed by fires followed by mass movements and debris flows. Potential energy dissipation resulting from wood dams is about a quarter of the total elevation drop in two streams, with a gross sediment volume stored behind wood dams of around 1000 m3 km?1, which appears to be of the same order as the annual sediment yield. Finally, the presence of wood dams may increase flow resistance by up to one order of magnitude. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

7.
Large wood (LW) affects several ecological and hydrogeomorphic processes in streams. The main source of LW is riparian trees falling inside channels. However, in confined valley floors, falling trees are more likely to be suspended above the channel. Eventually, these suspended trees will decompose and break to finally fall into the channel to better provide functions for streams. We evaluated changes in wood decay, length, diameter, and suspended status (suspended or non-suspended) 17 years post-harvest and nine years after the first sampling occurred in 2006 in 12 headwater streams of coastal British Columbia, Canada. We also evaluated whether changes differed among riparian management treatments (no-harvest buffers of 10 and 30 m in width, thinning, and unharvested reference sites), and identified the factors affecting wood changes and suspended status. Wood pieces advanced in decay, became shorter, and 34% of them (n = 108) changed status from suspended to non-suspended. Non-suspended wood pieces were more decayed and shorter than suspended wood. Suspended wood was longer, thicker, less decayed, and represented 46.5% (n = 147) of the wood sampled in 2006. Our findings revealed limited influences of riparian management on many aspects of wood changes considered in this study. Changes in wood characteristics were more likely for pieces that were smaller in diameter, longer, and suspended closer to the water. The transition from suspended to non-suspended LW can be a long-term process that can increase wood residence time and reduce LW in-stream functions particularly in confined stream valleys. The suspended stage is also an important mechanism underlying time lags in stream ecosystem responses to riparian tree fall. © 2020 John Wiley & Sons, Ltd.  相似文献   

8.
Dynamics and functions of large wood have become integral considerations in the science and management of river systems. Study of large wood in rivers took place as monitoring of fish response to wooden structures placed in rivers in the central United States in the early 20th century, but did not begin in earnest until the 1970s. Research has increased in intensity and thematic scope ever since. A wide range of factors has prompted these research efforts, including basic understanding of stream systems, protection and restoration of aquatic ecosystems, and environmental hazards in mountain environments. Research and management have adopted perspectives from ecology, geomorphology, and engineering, using observational, experimental, and modelling approaches. Important advances have been made where practical information needs converge with institutional and science leadership capacities to undertake multi-pronged research programmes. Case studies include ecosystem research to inform regulations for forest management; storage and transport of large wood as a component in global carbon dynamics; and the role of wood transport in environmental hazards in mountain regions, including areas affected by severe landscape disturbances, such as volcanic eruptions. As the field of research has advanced, influences of large wood on river structures and processes have been merged with understanding of streamflow and sediment regimes, so river form and function are now viewed as involving the tripartite system of water, sediment, and wood. A growing community of researchers and river managers is extending understanding of large wood in rivers to climatic, forest, landform, and social contexts not previously investigated. © 2020 John Wiley & Sons, Ltd.  相似文献   

9.
In the twentieth century Polish Carpathian rivers were considerably modified by channelization and gravel mining, with significant detrimental effects to their ecological integrity, vertical stability of the streambeds and flood hazard to downstream river reaches. Restoration of the rivers is thus necessary to improve their ecological status and re‐establish geomorphic dynamic equilibrium conditions. Various approaches to defining hydromorphological reference conditions, proposed to date in river restoration literature, have serious deficiencies. In particular, environmental changes that took place in the catchments of Carpathian rivers during the twentieth century invalidate the historical state of the rivers as reference for their restoration. This is illustrated by a change from bar‐braided to island‐braided channel pattern that occurred in the past century in unmanaged sections of the Czarny Dunajec in response to a reduction in flow and sediment dynamics of the river. We indicate that reference conditions should be defined as those which exist or would exist under present environmental conditions in the catchment but without human influence on the channel, riparian zone and floodplain of the river which is to be restored. This assumption was tested through the evaluation of hydromorphological river quality of the Czarny Dunajec according to the European Standard EN‐14614. The evaluation confirmed a high‐status hydromorphological quality in an unmanaged channel section, which can thus be used as a reference for restoration of impacted river sections. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

10.
Debris flow is one of the dominant processes distributing large wood (LW) within mountainous catchments. However, little has been reviewed on wood-laden debris flow (WLDF), presumably owing to limited reviewable works. This article, therefore, navigates the international readers through 40 years of WLDF studies, most of which have been published only in Japanese. Firstly, we reviewed the historical development of Japanese WLDF particularly focusing on the 1980s and the 1990s. A series of post-disaster fieldworks from the July 1982 Nagasaki flood to the July 1990 Kumamoto flood provided 32 catchment-scale wood budgeting data; empirical relationships among drainage area, dominant tree species, sediment yield, and wood loads associated with single debris flow disasters were illustrated. Secondly, the characteristics of WLDF were summarized based on relevant previous studies on the recruitment, transport, and deposition processes of LW during debris flows. Thirdly, we discussed the connectivity between those Japanese WLDF studies and international LW studies by relating/contrasting their research approaches and spatiotemporal scales. In contrast to global LW research trends, Japanese WLDF studies have almost exclusively regarded LW as hazardous materials (i.e., “driftwood” or “woody debris”) that need to be retained upstream of the inhabited areas. Those practice-oriented WLDF studies were concentrated on drainage areas of 10−2 to 100 km2, representing 1–6 orders of magnitude smaller spatial scales than those generally covered by existing international LW studies. Strongly motivated by engineering requirements, “dynamic” interactions between debris flows and LW during floods have also been physically presented, mainly based on unique laboratory experiments involving steep flume (> 0.05) and mobile bed conditions. Finally, some future works for WLDF were briefly stated from practical and scientific perspectives. By “rediscovering” those WLDF studies domestically developed in Japanese debris flow channels since the 1980s, a more comprehensive understanding of LW dynamics in the river system may be achieved.  相似文献   

11.
The importance of monitoring and analysing wood fluxes in mountain environments is widely recognized. However, there is a lack of information related to the long-term fluctuations in wood load and associated to the changes in large wood (LW) characteristics. The main aims of the research were to analyse (i) changes in wood characteristics, (ii) fluctuations in wood load, and (iii) the relationship between wood load and sub-reach settings, proposing an ad hoc roughness index (RI). Repeated field surveys to measure wood load and LW characteristics during a 13-year period were conducted within a 2.2 km-long reach of the Rio Toro (Chile), which flows through the Malleco Forest National Reserve that was affected by wildfires in 2002 and 2015. Two spatial scales (i.e. study reach and sub-reach) were considered. Irrespective of the adopted scale wood load fluctuated considerably, in both number and volume, with significant changes in LW characteristics (i.e. mean diameter and length). Moreover, a clear tendency to the aggregation in wood jams (WJs) was observed within the entire study reach. Based on our results, we conclude that 17 years after the first wildfire wood recruitment has started in the upper parts of the basin. Moreover, Generalized Linear Mixed Models (GLMM) analyses were carried out to investigate the relationship between RI and both number and volume of LW. Both models are characterized by high adjusted R2 of 74.9% and 72.8% for volume and number, respectively. This demonstrated that the RI can characterize the reaches in terms of tendency to deposition and trapping of wood. These results are promising, particularly in improving the knowledge related to potential deposition areas that can control wood load fluctuations, also permitting its management to be improved. © 2020 John Wiley & Sons, Ltd.  相似文献   

12.
Multiple stressors like alterations of water quality, hydrology and hydromorphology impact riverine ecosystems. To counteract its consequences, restoration measures are required, e.g. demanded by the EU Water Framework Directive. However, small-scale hydromorphological restoration measures often show little success. Besides overriding stressors, recolonization potential and insufficient time for development are often discussed as reasons for this lack of success. Over a period of 17 years a hydromorphological restoration measure in a German lowland stream reach was evaluated, excluding most of these confounding factors. The restoration measure was left to its self-dynamic development, i.e. there was no further intervention by water management, apart from some large wood installations after 8 years. In the accompanying studies, changes in hydromorphology and the organism groups macroinvertebrates (including both structural and functional diversities), fish and macrophytes were investigated.We proved a rapid and stable enhancement of ecological functions as indicated by a diverse and resilient macroinvertebrate biocoenosis. Both taxonomic and functional richness of macroinvertebrates significantly increased, but varied over time. Since the restoration measure allowed self-dynamic development, a taxonomic and functional endpoint was not achieved even after 17 years, even though near-reference conditions were attained after only 4 years. Deficits in fish communities were most probably due to their low recolonization potential.Our results underscore that small scale hydromorphological restoration can be successful if overriding stressors are absent. Furthermore, we proved the importance of self-dynamics in restoration measures, allowing hydromorphological and biological development.  相似文献   

13.
The importance of large wood (LW) to riverine functions is well established scientifically and increasingly recognized by river managers in many countries. However, public perceptions largely associate LW with elevated danger and/or need for intervention. Such perspectives are amplified amongst recreational river users (defined here as any individuals that recreate by floating on the water surface of a river) who interact more directly with rivers than the general public and commonly view wood in life-or-death terms. Given that human life occupies a highest-order charge for river managers, they are left in a difficult position when safety appears to conflict with environmental services. LW deficits are perpetuated partly because wood removal, often in the name of safety, is far easier than placing wood in rivers. Further, river restoration practitioners are frequently burdened with expectations and liability unparalleled in built environments. A fundamentally different mindset is necessary to achieve desired ecologic outcomes when working with rivers. Based on two decades of experience as boaters, LW practitioners, and emergency responders, we (1) discuss LW hazard and risk from recreational and management viewpoints, (2) discretize objective and measurable physical properties of LW hazards, and (3) propose a decision framework that implicitly addresses risk by considering LW hazards relative to river use and ambient hazards. The approach is structured to increase objectivity in LW hazard mitigation and diminish asymmetric biases that favor LW removal. Our intent is to build understanding and rational flexibility among risk-averse management, regulatory, and funding entities to facilitate implementation of scientific understanding without undue risk to river users. © 2020 John Wiley & Sons, Ltd.  相似文献   

14.
Large wood (LW) is an important component of forested headwater streams. The character of LW loads reflects a balance between adjacent valley processes that deliver LW to the channel (herein recruitment processes) and stream channel processes that either retain or transport LW through the reach (herein retention processes). In the central Appalachian Mountains, USA, LW characteristics in headwater streams located in eastern hemlocks (Tsuga candensis) forests are expected to change because of infestation of hemlock woolly adelgid (Adelges tsugae, HWA), an exotic, invasive insect. We examined LW characteristics in 24 headwater streams ranging from un‐infested to severe infestation, as determined by hemlock canopy health. The objectives of this work were to: (i) quantify wood loads; (ii) assess the relative importance of valley recruitment and in‐stream retention mechanisms in controlling reach‐scale wood loads; and (iii) assess if there was a detectable influence of HWA on LW loads. We hypothesized that LW loads would be similar to other forested streams in eastern USA and dominated by recruitment processes. In addition, higher LW loads would correspond with advanced HWA infestation. Mean wood frequency was 38 pieces/100 m ± 17 (standard deviation); mean wood volume was 3.69 m3/100 m ± 2.76. In general, LW load characteristics were influenced by both recruitment and retention parameters; jam (accumulations ≥ 3 pieces) characteristics were dominated by retention parameters. Results suggest that adjacent stand basal area influences LW loads and once LW is recruited to the channel, streams lack sufficient hydraulic driving forces, despite having lower resistance structures, to transport LW out of the reach. Sites in moderate decline had higher proportions of short (1–2 m and 1–4 m) and very long (>10 m) LW with higher frequency of jams that were low in volume. We present a hypothesized conceptual model of expected changes to LW loads associated with HWA infestation and hemlock mortality. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

15.
We use field measurements and airborne LiDAR data to quantify the potential effects of valley geometry and large wood on channel erosional and depositional response to a large flood (estimated 150-year recurrence interval) in 2011 along a mountain stream. Topographic data along 3 km of Biscuit Brook in the Catskill Mountains, New York, USA reveal repeated downstream alternations between steep, narrow bedrock reaches and alluvial reaches that retain large wood, with wood loads as high as 1261 m3 ha−1. We hypothesized that, within alluvial reaches, geomorphic response to the flood, in the form of changes in bed elevation, net volume of sediment eroded or aggraded, and grain size, correlates with wood load. We hypothesized that greater wood load corresponds to lower modelled average velocity and less channel-bed erosion during the flood, and finer median bed grain size and a lower gradation coefficient of bed sediment. The results partly support this hypothesis. Wood results in lower reach-average modelled velocity for the 2011 flood, but the magnitude of change in channel-bed elevation after the 2011 flood among alluvial and bedrock reaches does not correlate with wood load. Wood load does correlate with changes in sediment volume and bed substrate, with finer grain size and smaller sediment gradation in reaches with more wood. The proportion of wood in jams is a stronger predictor of bed grain-size characteristics than is total wood load. We also see evidence of a threshold: greater wood load correlates with channel aggradation at wood loads exceeding approximately 200 m3 ha−1. In this mountain stream, abundant large wood in channel reaches with alluvial substrate creates lower velocity that results in finer bed material and, when wood load exceeds a threshold, reach scale increases in aggradation. This suggests that reintroducing small amounts of wood or one logjam for river restoration will have limited geomorphic effects. © 2020 John Wiley & Sons, Ltd.  相似文献   

16.
We measured longitudinal spacing and wood volume of channel‐spanning logjams along 30 1‐km reaches of forest streams in the Colorado Front Range, USA. Study streams flow through old‐growth (> 200 year stand age) or younger subalpine conifer forest. Evaluating correlations between the volume and longitudinal spacing of logjams in relation to channel and forest characteristics, we find that old‐growth forest streams have greater in‐stream wood loads and more jams per kilometer than streams in younger forest. Old‐growth forests have a larger basal area close to the stream and correlate with larger piece diameters of in‐stream wood. Jam volume correlates inversely with the downstream spacing for ramp and bridge pieces that can act as key pieces in jams. Most importantly, old‐growth streams have shorter downstream spacing for ramp and bridge pieces (< 20 m). Our results suggest that management of in‐stream wood and associated stream characteristics can be focused most effectively at the reach scale, with an emphasis on preserving old‐growth riparian stands along lower gradient stream reaches or mimicking the effects of old growth by manipulating the spacing of ramp and bridge pieces. Our finding that average downstream spacing between jams declines as wood load increases suggests that the most effective way to create and retain jams is to ensure abundant sources of wood recruitment, with a particular emphasis on larger pieces that are less mobile because they have at least one anchor point outside the active channel. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

17.
Hyporheic restoration is of increasing interest given the role of hyporheic zones in supporting ecosystem services and functions. Given the prevalence of sediment pollution to waterways, an emerging restoration technique involves the removal of sediment from the interstices of gravel‐bed streams. Here, we document streambed sediment removal following a large, accidental release of fine sediment into a gravel‐bed river. We use this as a natural experiment to assess the impact of fine sediment removal on reach‐scale measures of transient storage and to document the responses of reaches with contrasting morphology (restored vs. unrestored) to changing discharge one‐field season. We conducted a series of conservative solute tracer experiments in each reach, interpreting both summary statistics for the recovered in‐stream solute tracer time series. Additionally, we applied the transient storage model to interpret the results via model parameters, including a Monte Carlo analysis to measure parameter identifiability and sensitivity in each experiment. Despite the restoration effort resulting in an open matrix gravel bed in the restored reach, we did not find the significant differences in most time series metrics describing reach‐scale transport and transient storage. We hypothesize that this is due to enhanced vertical exchange with the gravel bed in the restored reach replacing lateral exchange with macrophyte beds in the unrestored reach, developing a conceptual model to explain our findings. Consequently, we found that the impact of reach‐scale removal of fine sediment is not measureable using reach‐scale solute tracer studies. We offer recommendations for future studies seeking to measure the impacts of stream restoration at the reach scale.  相似文献   

18.
High severity wildfires impact hillslope processes, including infiltration, runoff, erosion, and sediment delivery to streams. Wildfire effects on these processes can impair vegetation recovery, producing impacts on headwater and downstream water supplies. To promote forest regeneration and maintain forest and aquatic ecosystem functions, land managers often undertake active post-fire land management (e.g., salvage logging, sub-soiling, re-vegetation). The primary objective of our study was to quantify and compare sediment yields eroded from (a) burned, (b) burned and salvage logged, and (c) burned, salvage logged, and sub-soiled plots following the 2015 Valley Fire in the northern California Coast Range. We distributed 25 sediment fences (~75 m2 contributing area) across four hillslopes burned at high severity and representative of the three management types. We collected eroded sediment from the fences after precipitation events for 5 years. We also quantified precipitation, canopy cover, ground cover, and soil properties to characterize the processes driving erosion across the three management types. Interestingly, during the second year after the fire, sediment yields were greater in the burned-only plots compared with both the salvage logged and sub-soiled plots. By the third year, there were no differences in sediment yields among the three management types. Sediment yields decreased over the 5 years of the study, which may have occurred due to site recovery or exhaustion of mobile sediment. As expected, sediment yields were positively related to precipitation depth, bulk density, and exposed bare soil, and negatively related to the presence of wood cover on the soil surface. Unexpectedly, we observed greater sediment yields on the burned-only plots with greater canopy closure, which we attributed to increased throughfall drop size and kinetic energy related to the residual canopy. While these results will aid post-fire management decisions in areas with Mediterranean climates prone to low intensity, long duration rainstorms, additional research is needed on the comparative effects of post-fire land management approaches to improve our understanding of the mechanisms driving post-fire erosion and sediment delivery.  相似文献   

19.
Wood additions to streams can slow water velocities and provide depositional areas for bacteria and fine particles (e.g., particulate organic carbon and nutrients sorbed to fine sediment), therefore increasing solute and particle residence times. Thus, wood additions are thought to create biogeochemical hotspots in streams. Added wood is expected to enhance in-stream heterogeneity, result in more complex flow paths, increase natural retention of fine particles and alter the geomorphic characteristics of the stream reach. Our aim was to directly measure the impact of wood additions on fine particle transport and retention processes. We conducted conservative solute and fluorescent fine particle tracer injection studies in a small agricultural stream in the Whatawhata catchment, North Island of New Zealand in two reaches—a control reach and a reach restored 1-year earlier by means of wood additions. Fine particles were quantified in surface water to assess reach-scale (channel thalweg) and habitat-scale (near wood) transport and retention. Following the injection, habitat-scale measurements were taken in biofilms on cobbles and by stirring streambed sediment to measure fine particles available for resuspension. Tracer injection results showed that fine particle retention was greater in the restored compared to the control reach, with increased habitat-scale particle counts and reach-scale particle retention. Particle deposition was positively correlated with cobble biofilm biomass. We also found that the addition of wood enhanced hydraulic complexity and increased the retention of solute and fine particles near the wood, especially near a channel spanning log. Furthermore, particles were more easily remobilized from the control reach. The mean particle size remobilized after stirring the sediments was ~5 μm, a similar size to both fine particulate organic matter and many microorganisms. These results demonstrate that particles in this size range are dynamic and more likely to remobilize and transport further downstream during bed mobilization events.  相似文献   

20.
This work investigates wood dynamics in braided streams through physical modelling in a mobile bed laboratory flume, with the specific objective to characterize wood storage and turnover as a function of wood input rate and of wood element type. Three parallel channels (1.7 m wide, 10 m long) filled with uniform sand were used to reproduce braided networks with constant water discharge and sediment feeding. Wood dowels with and without simplified root wads were regularly added at the upstream end of each flume at different input rates, with a 1:2:3 ratio between the three flumes. Temporal evolution of wood deposition patterns and remobilization rates were monitored by a series of vertical images that permitted the recognition of individual logs. Results show that wood tends to disperse in generally small accumulations (< 5 logs), with higher spatial density on top of sediment bars, and is frequently remobilized due to the intense morphological changes. The amount of wood stored in the channel depends on log input rate through a non‐linear relationship, and input rates exceeding approximately 100 logs/hour determine a sharp change in wood dynamics, with higher storage volume and augmented formation of large jams (> 10 elements) that are less prone to remobilization. Presence of root wads seems to play a minor role in wood deposition, but it reduces the average travel distance of logs. Turnover rates of logs were similar in the three flumes, independently of wood input rate and largely resembling the turnover rate of exposed bars. For the simulated conditions, significant effects of wood on bed morphology were not observed, suggesting that interactions with fine sediments and living vegetation are crucial to form large, stable wood jams able to bring about relevant morphological changes. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

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