Large Wood Pieces Research Articles

Hydrogeomorphic response of steep streams following severe wildfire in the Western cascades, Oregon

AbstractSevere wildfire may alter steep mountain streams by increasing peak discharges, elevating sediment and wood inputs into channels, and increasing susceptibility to landslides and debris flows. In the Pacific Northwest, where mean annual precipitation is high and mean fire‐return intervals range from decades to centuries, understanding of steep stream response to fire is limited. We evaluate the hydrologic and geomorphic response of ~100‐m‐long steep stream reaches to the large‐scale and severe 2020 fires in the Western Cascade Range, Oregon. In the two runoff seasons after the fires, peak flows in burned reaches were below the 2‐year recurrence interval flood, a level sufficient to mobilize the median grain size of bed material, but not large enough to mobilize coarser material and reorganize channel morphology. Sediment inputs to study streams consisted of two road‐fill failure landslides, slumps, sheetwash, and minor bank erosion; precipitation thresholds to trigger debris flows were not exceeded in our sites. There was a 50% increase in the number of large wood pieces in burned reaches after the fires. Changes in fluxes of water, sediment, and wood induced shifts in the balance of sediment supply to transport capacity, initiating a sequence of sediment aggradation and bed‐material fining followed by erosion and bed‐material coarsening. Gross channel form showed resilience to change, and an unburned reference reach exhibited little morphologic change. Post‐fire recruitment of large wood will likely have long‐term implications for channel morphology and habitat heterogeneity. Below‐average precipitation during the study period, combined with an absence of extreme precipitation events, was an important control on channel responses. Climate change may have a complex effect on stream response to wildfire by increasing the propensity for both drought and extreme rain events and by altering vegetation recovery patterns.

Impact of large wood on sediment (dis)connectivity in a meandering river

Large wood (LW) can exert strong influence on sediment storage patterns in fluvial systems. However its specific impact on sediment routing is largely dependent on the stability of individual LW pieces or LW jams. We used sediment (dis)connectivity principles to quantify the impact of LW on sediment erosion and storage along a meandering river reach. Decadal spatiotemporal changes of LW and fluvial landforms are appraised along a 3.65 km long reach of the Odra River (Czechia). Detailed orthophotos at two-year intervals from 2012 to 2022 are analysed alongside field inventories. During this study period, there was a single flood event with a 5–10-year recurrence interval, and three smaller flow events with a recurrence interval of up to 2 years. We assessed the sediment deposition, bank protection, and erosion effects of LW, finding that the impact of LW on decelerating sediment fluxes was more pronounced. Specifically, LW-related deposition varied between 13.6 m2 and 36.7 m2 per 100 m of channel length during the individual two-year periods. In contrast, we detected only 0.5 m2 to 7.8 m2 per 100 m of channel length of newly eroded material during the same periods. Additionally, 6.3 % to 7.8 % of the lengths of the outer meander bends were protected by persistent LW elements against erosion during these two-year periods. Finally, we developed a three-stage conceptual model depicting the impact of LW on sediment (dis)connectivity in a meandering river at different relative timescales. Our model accounts for the dependence of interactions between LW and fluvial sediments based on the LW stability and frequency-magnitude of the hydrogeomorphic processes. In general, our observations suggest that the presence of stable LW in meandering rivers primarily leads to the deceleration of the lateral and longitudinal dimensions of sediment (dis)connectivity.

UAV Imagery for Measuring Large Wood Volume and Distribution: Ground Truthing Results and Sources of Error

abstract: UAV imagery offers great potential in determining precise environmental measurements. This study identifies and quantifies errors and their causes/sources between UAV imagery- and field-derived measurements of large wood (LW) and cross-sectional valley topography in several forested headwater stream valleys in the Ozark Highlands, Missouri. UAV imagery was collected at six sites following best practices for producing high resolution orthophotos and elevation models through Structure-from-Motion photogrammetry. Field-based measurements of LW were completed along 5 m wide valley cross-sections in which all LW > 1 m in length and 0.1 m in diameter were recorded. Topographic surveys were performed along the centerlines of the LW transects. The same assessments were performed using UAV imagery and ArcGIS. Our sites showed high (>= 0.85) R2 values between field and UAV-GIS topographic surveys. Differences between measurements of LW dimensions tended to be larger with R2 ranging from 0.42 to 0.88. Overall 78 percent of standing trees and 60 percent of LW pieces were identified in the UAV imagery. The main source of detection and measurement error was due to vegetation/canopy cover. This study furthers our understanding of the applications and limitations of UAV derived data for environmental assessments.

Large wood recruitment, retention and mobilization in low-order streams of the Brazilian Savanna

The delivery of wood to streams varies in space and time and may occur continuously or episodically. Once in a stream, wood may be retained or transported due to the balance between driving (water flow) and resisting forces (channel features and wood characteristics). Despite there being few studies directly measuring wood mobility, notable variations in annual transport rates and mean travelled distances have already been detected worldwide. In the tropics, due to wetter and warmer conditions and consequently higher decay rates and peak discharge, wood is believed that more readily degraded and transported downstream. However, there exists a knowledge gap regarding wood dynamics in tropical streams and even more considering tropical non-forested environments. In the present study, we aimed to document large wood (LW) recruitment, retention, and mobilization over a period of one year in eight low-order streams of the Cerrado (the South American Savanna) by tagging and tracing LW pieces. We detected high LW transport and recruitment rates in Cerrado streams, such that the total wood amounts remained constant over time. However, there were important differences between streams due to their distinct channel morphologies and the occurrence of episodic events. The LW length and diameter, original location and orientation were the most important factors in the prediction of the LW travelled distance, which ranged from zero to almost 100 m in one year. Our findings provide unprecedented information regarding wood mobility in tropical savanna streams in South America. To further advance knowledge about the dynamics of wood in Cerrado streams, future studies should characterize the great diversity of wood, especially with regard to densities that may strongly affect wood mobility. Furthermore, it is essential to perform long-term studies to document the dynamics of wood over years in these environments.

A high‐resolution inter‐annual framework for exploring hydrological drivers of large wood dynamics

AbstractRivers with alluvial bars store more wood than those without, supplied through channel shifting. However, wood dynamics (arrival or new deposits, departure or entrainment, and stable or immobile pieces) can vary substantially over time in response to critical hydrological drivers that are largely unknown. To evaluate them, we studied the dynamics of large wood pieces and logjams along a 12‐km reach of the lower Allier River using six series of aerial images of variable resolution acquired between 2009 and 2020, during which maximum river discharge fluctuated around the dominant flood discharge (Q1.5) that is potentially the bankfull discharge along this well‐preserved not incised reach. Individual wood departure was best correlated with water levels exceeding dominant flood discharge. The duration of the highest magnitude flood was best correlated with wood depositions, with shorter floods resulting in a higher number of deposits. Finally, most of the wood remained stable when river discharge did not exceed 60% of Q1.5 over a long period of time. Changes in inter‐annual wood budget (reach‐scale) depend on the duration over which discharge exceeded 60% of Q1.5. Hydrological conditions driving jam build‐up and removal were similar to those controlling individual wood piece dynamics. The results suggest that specific hydrological conditions influence the dynamics of large wood and log jams in the Allier River. Understanding the dynamics of large wood and its impact on river morphology is fundamental for successful river management and habitat restoration initiatives.

Large in-stream wood yield during an extreme flood (Storm Alex, October 2020, Roya Valley, France): Estimating the supply, transport, and deposition using GIS

During major floods, rivers erode their banks and thus recruit large wood pieces from the riparian zones. There is still a lack of knowledge about the transport of large wood, the volumes involved and the flux distribution, i.e. the large wood connectivity at catchment scale. During storm Alex (October 2020), the French Roya catchment (394 km2) experienced a paroxysmal morphogenic flood involving massive bank erosion. The riparian vegetation was largely recruited, with large wood contributing to logjams and bridge destruction. This paper presents a methodology for volumetric assessment of the large wood fluxes involved. Simple approaches are used to (i) quantify the inputs from stand density data from the national forest inventory and from source areas based on diachronic analysis of active channels highlighting the erosion of 87 ha of wooded areas; and (ii) quantify the volumes deposited via an exhaustive manual digitisation of 16,846 pieces of large wood deposited on 59 km of channels on the Roya and its tributaries. This catchment-scale, large wood connectivity analysis shows that the flood recruited and transported downstream a volume of around 14,000 m3 of large wood (uncertainty range: 7000–29,500 m3). Drone observations of the Roya River mouth in Italy and satellite images showing a raft of driftwood, several km long, drifting off the Roya River mouth in the aftermath of the flood corroborate our findings.

Large wood loads in channels and on floodplains after a 500-year flood using UAV imagery in Mark Twain National Forest, Ozark Highlands, Missouri

Anthropogenic climate change has increased the frequency of large floods in rivers draining the Ozark Highlands. This study assesses the effects of a > 500-yr flood in spring 2017 on riparian forests and large wood loads in the North Fork of the White River watershed, Missouri, for six stream reaches with drainage areas from 5 to 124 km2. Standing trees and large wood (LW) were assessed using unmanned aerial vehicle (UAV) imagery and calibrated by field surveys. Scaled flood magnitude (flood stage/bankfull depth) correlated with percent urban and agricultural land above each reach suggesting that land use may have contributed to forest damage. Canopy loss on the valley floor ranged from 7 to 63 % by reach and correlated with mean and cross-sectional stream power (p < 0.01). Standing tree density after the flood ranged from 50 to 243 trees/ha. The density of LW pieces ranged from 25 to 147 trees/ha. Most LW was aligned with stream flow, not in jams, and located on floodplains below riffles or bar heads, along channel bends, or in chutes. Wood loads on the valley floor increased downstream from 12 to 45 m3/ha. Channel loads were < 30 m3/ha while floodplain, terrace, and chute loads were > 30 m3/ha at drainage areas >50 km2. Channel LW loads increased with flood magnitude and in narrow valleys (p < 0.02), but not drainage area. Increased wood storage occurred on floodplains and terraces, but it is not clear if the stored wood will be available for downstream transport by future floods.

Small-Scale Modeling of Flexible Barriers. II: Interactions with Large Wood

During strong floods, rivers often carry significant amounts of sediment and pieces of large wood (LW). When bridges and hydraulic structures are unable to allow LW to pass through, it becomes necessary to trap LW through specific wood retention structures (e.g., flexible barriers). This paper presents a comprehensive analysis of the interactions between LW and flexible barriers using small scale models. A dimensionless criterion is first proposed to compute blockage probability of single logs. It is based on experiments varying log size and shape, channel slope (2%, 4%, and 6%), water discharge, and barrier bottom clearance. Based on runs using six mixtures of hundreds of logs, an equation is secondly provided to compute flow depth at a barrier accounting for the head losses related to large numbers of logs. Conditions leading to the release of LW when the barrier is severely overwhelmed are also studied. The deformation measured on the barrier proves to be lower with LW-laden flows than under full hydrostatic loading of a barrier obstructed by a plastic sheet. Overall, we demonstrate that flexible barriers are very relevant structures to trap LW. A companion paper shows how to design and manufacture a small scale flexible barrier in mechanical similitude with the prototype scale.

Transport Dynamics of Large Wood in a Stream Channel Affected by Spur Dikes

AbstractThe damage to river engineering structures induced by large‐wood (LW) jams is one of the significant problems in river management. For example, typical bridge damage in alluvial areas in Japan, such as wash away, is often caused by the accumulation of large wood materials. The densely transported large wood pieces are an important factor in the accumulation process. We investigated the influence of local flows generated by hydraulic structures on the initial stage of the concentration of large wood pieces. First, we experimentally investigated the flow structures generated by spur dikes in a 0.6 m wide channel and the trajectory of large wood pieces. Then, we numerically investigated the influence of external force on the transport mechanisms of large wood pieces. We employed a simplified Basset‐Boussinesq‐Oseen model to evaluate each term of the momentum equation of the particles and calculated the trajectories of the wood pieces previously observed experimentally. The results showed that the transported large wood pieces were concentrated in low‐vorticity regions of the water surface. This tendency was more prevalent in short pieces (10 mm) than in long pieces (40 mm) because the influence of the relatively small‐scale patterns of the pressure gradient of the surrounding water surface might be spatially averaged for long pieces. This tendency has not been explicitly elucidated in previous works; thus, this knowledge is expected to be key in controlling LW transport, such as detecting the possible regions where LW jams occur or designing efficient inlet sections of LW retention facilities.

Large wood retention in a large meandering river: Insights from a 5-year monitoring in the Odra River (Czechia)

Large wood (LW) is an integral component of watercourses. However, we still need to gain more knowledge of the geomorphic processes related to LW in large lower-energy, meandering rivers due to the limited number of field studies dealing with LW transport and retention in these widespread fluvial systems. This study aims to detect the predictors of the interannual variability of LW during a period of five years in a meandering river wider than the height of riparian trees. We employed four LW inventories between 2016 and 2021 in a 3.65 km long active meandering reach of the Odra (Oder) River, Czechia. We found a negligible interannual variation in LW volumes (8.3–9.2 m3/ha), but much higher in frequencies (4.4–7.0 LW/100 m). However, the variations of LW volumes in individual meander bends did not allow us to develop any robust model predicting LW volumes. We found a higher number of significant models with two independent variables that included variables from the at-site meander and from the first upstream meander in comparison to the second upstream meander. This suggests a higher importance of local “retention” and “recruitment” variables at the expense of variables of “recruitment” related to the more distant meanders. Variables derived from riparian stands were recognised as important predictors entering into 7 of 13 models with two independent variables. This points to the complex recruitment-retention function of riparian stands for LW. We found evidence that the presence of stable LW pieces can drive local channel morphodynamics, and the observed spatiotemporal variation in jams was not only a product of the suggested frequent transport of LW and instability of jams but also of the possible burial of jams in sediments. Our results documented the complex links between the morphodynamics of meandering rivers and the biogeomorphic impact of living or dead vegetation.

Observer‐bias and sampling uncertainties in riverine wood flux and volume estimation from video monitoring technique

AbstractWood is an integral part of rivers that can have both positive and negative impacts on natural systems and infrastructures. Different techniques have been developed to quantify wood flux or discharge in rivers. Among them, the stream‐side video monitoring technique has proven effective for at‐a‐station wood monitoring with a high temporal and spatial resolution over an indefinite time period. However, the visual annotation of wood pieces in the videos is subject to uncertainties due to observer bias or ‘vision limitations’, and video sampling or ‘time limitations. Vision limitations mean that there are patches in the recorded image that may or may not be considered as wood pieces depending on the judgement of the observer. Time limitations mean that the video record may be sampled to estimate the wood flux rather than completing a census of the full record due to the time‐consuming nature of continuous visual annotation. To assess these uncertainties, six flood events and 13 video segments corresponding to more than 37 days and 64,000 pieces of wood were analysed on two different rivers (Ain and Allier Rivers in France). The results show that while there is a significant difference between observers for the detection of small wood pieces (&lt; 1 m in length), no significant difference exists for the detection of large wood pieces (&gt; 1 m in length). The application of a truncation length (i.e., considering only wood pieces with a size higher than a certain threshold) reduces the piece number uncertainty significantly without resulting in a meaningful change in the total volume of wood. For the time limitation, it is shown that sampling uncertainty depends on wood flux related to water discharge and flood stages (rising versus falling), so a dynamic sampling strategy that depends on flood stage is recommended.

Large wood dynamics in a mountain river disturbed by a volcanic eruption

Volcanic eruptions can dramatically alter river basins and channel networks by delivering huge amounts of sediments into the channels and affecting the surrounding vegetated area. These disturbances modify the process chain of large wood (LW) recruitment, storage, and transport. We investigated the effects of the 2015 Calbuco volcanic eruption in southern Chile on the dynamics of LW along two reaches of the Blanco Este River, which drains the north-eastern flanks of the volcano. The two reaches, R1 and R2, were surveyed for almost four years, combining field and remote analyses to better comprehend LW dynamics. We framed our study around three research questions: i) what are the processes explaining the spatial and temporal variations of LW load into the active channel of the Blanco Este River? ii) can the longitudinal (dis)connectivity be considered a controlling factor rather than flood magnitude? iii) is it possible to infer a relationship between recruitment sources, and flood events, with LW load fluctuations? The abundance of individual LW pieces and wood jams (WJs) were always higher in the downstream reach (R2). LW source areas to R1 were limited to forested bank erosions, while in R2 LW was recruited from bank erosions with the addition of wood pieces entering in the main channel from a lateral affluent and from eroded old lahar deposits that re-connected this source to the active channel. WJs were highly unstable and there were not statistically significant relationships between LW fluctuations and water discharge. In the Blanco Este River, LW delivery and export can be classified as event-driven processes, but alternating periods with longitudinal (dis)connectivity led to events delivering wood into the channel which seemed to occur asynchronously with those exporting LW. Finally, LW load in R1 appeared to be in equilibrium, but this condition might change in the future since the area of upper forested streambanks is rapidly declining.

The geomorphic role of large wood in the coastal zone: Mobilization threshold and beach morphology impacts in the North American Great Lakes

While large wood has been extensively studied in fluvial settings, where it functions as a key component of both physical and biological systems, the role of large wood in the shoreline-nearshore zone has received little attention to date. To address this gap, we examined large wood as a geomorphic agent along an eroding beach shoreline of southwestern Lake Michigan using high-resolution orthoimagery and DEM datasets. Erosion of the site and the liberation of woody materials from the strand were enhanced by rising lake levels. We digitized positions and evaluated the physical characteristics of 2627 pieces of large wood from 5 aerial images (2009–2017) and 11 seasonal drone surveys (2018–2020). This high temporal data resolution allowed us to track 374 unique pieces of large wood over time, revealing common transport pathways and sedimentary impacts of large wood along a wave-dominated beach shoreline. Results indicate that, while large wood has local, transient impacts on beach geomorphology, its presence has little bearing on longer-term beach evolutionary trajectories. Large wood offers little natural shoreline protection along high energy, dynamic shorelines, but may promote backshore accretion and foredune growth under other hydrodynamic and sediment-supply conditions. Our analysis of decadal shoreline evolution reinforces the importance of antecedent topography and coastal habitat elevation as primary controls on rates of shoreline recession.

Untangling the controls on bedload transport in a wood‐loaded river with RFID tracers and linear mixed modelling

AbstractBedload transport is a fundamental process by which coarse sediment is transferred through landscapes by river networks and may be well described stochastically by distributions of grain step length and rest time obtained through tracer studies. To date, none of these published tracer studies have specifically investigated the influence of large wood in the river channel on sediment transport dynamics, limiting the applicability of stochastic sediment transport models in these settings. Large wood is a major component of many forested rivers and is increasing due to anthropogenic ‘Natural Flood Management’ (NFM) practices. This study aims to investigate and model the influence of large wood on grain‐scale bedload transport.We tagged 957 cobble to pebble sized particles (D50 = 73 mm) and 28 pieces of large wood (&gt; 1 m in length) with radio frequency identification (RFID) tracers in an alpine mountain stream. We monitored the transport distance of tracers annually over 3 years, building distributions of tracer transport distances with which to compare with published distributions from wood free settings. We also applied linear mixed modelling (LMM), to tease out the influence of wood from other controls on likelihood of entrainment, deposition, and the transport distances of sediments.Tracer sediments accumulated both upstream and downstream of large wood pieces, with LMM analysis confirming a reduction in the probability of entrainment of tracers closer to wood in all 3 years. Upon remobilization, tracers entrained from positions closer to large wood had shorter subsequent transport distances in each year. In 2019, large wood also had a trapping effect, significantly reducing the transport distances of tracer particles entrained from upstream, that is forcing premature deposition of tracers. This study demonstrates the role of large wood in influencing bedload transport in alpine stream environments, with implications for both natural and anthropogenic addition of wood debris in fluvial environments.

Downed wood dynamics in the riparian and littoral zone of small lakes in tolerant hardwood forests

Large wood (LW) is an important structural feature in forested lake ecosystems, but little is known about the production of LW in riparian forests and its movement across the forest–lake ecotone, both of which we describe here for eight small (&lt;30 ha) lakes in Ontario, Canada. The creation of snags through live-tree death varied by species, ranging from 0.24% year−1 for eastern white cedar to 5.60% year−1 for balsam fir. Snag breakage was best described as a function of snag height and snag fall described as a function of tree species, decay stage, and diameter. Long LW pieces and pieces generated close to the lake were more likely to be deposited at the shoreline. The density of LW at the shoreline was 125–550 pieces·km−1, and density and volume of LW were positively related to riparian slope. LW was a stable resource at the shoreline, with a recruitment rate about equal to the loss rate and average movement along the shoreline of 2.89 cm·year−1. LW volumes at the shoreline and in the littoral zone of the lake were positively related to lakebed slope. LW could reside in the littoral zone for centuries, providing physical structure for multiple generations of aquatic organisms in these small lakes. Our results indicate that critical habitat for fish that need aquatic LW extends at least 5 m into riparian forest and that removal of trees within 5 m of the forest edge would reduce LW input to the littoral zone.

Impacts of an extreme flood on large wood recruitment and transport processes

ABSTRACT This research investigates impacts of an extreme flood on recruitment and transport of large wood (LW) in sub-basins of the North Fork River, Missouri. Data collection took place two months after a >500 year flood to characterize LW conditions before natural recovery processes could obscure impacts. We used sites from previous LW studies in the region as reference to help identify flood impacts. Results showed 1) LW load volumes were no different than reference sites, but individual LW pieces comprised a greater percentage of the total load, 2) a high proportion of pieces at flood-impacted sites contained root wads, 3) transport capacity of the flood-impacted sites was high compared to reference sites, and 4) LW recruitment increased exponentially with flood magnitude. These results suggest that extreme floods have a significant impact on the composition of the LW load, and that geomorphic impacts of such floods may result in enhanced transport capacities. Based on these findings, we present two possible post-flood LW response/recovery scenarios; one in which elevated transport capacity serves to speed system recovery to the pre-flood LW regime, and one in which the enhanced LW piece composition results in a new post-flood LW regime with an enhanced load.

Age matters: substrate-specific colonization patterns of benthic invertebrates on installed large wood

Large wood (LW) is an indispensable element in riverine ecosystems, especially in lower river parts. The presence of LW significantly shapes local hydraulics, morphology, the nutrient budget; promotes overall river dynamics; and additionally presents a unique habitat for numerous benthic invertebrate species. Therefore, LW is recognized as valuable asset for river restoration measures. Experiences from previous projects show that ecological responses on LW implementation measures vary greatly. That complicates comparisons and estimations on the success of planned measures. Methodological inconsistencies and thus reduced transferability of the results is one major issue. Additionally, wood quality aspects are suspected to be important factors affecting benthic invertebrate colonization patterns. The focus of this study is therefore to consistently assess the ecological significance of installed LW and concrete samples of similar size and shape in terms of benthic invertebrate colonization and to further test, if the condition of wood affects the benthic invertebrate colonization. Our results show that (1) installed LW serves as an abundantly and heterogeneously colonized habitat, (2) the state of decay of LW pieces significantly affects benthic invertebrate colonization in terms of density and diversity and (3) even rare or threatened taxa closely associated to LW were abundantly present on the installed logs, emphasizing the suitability of the chosen approach.

Drivers of variability in large wood loads along the fluvial continuum of a Mediterranean intermittent river

AbstractAlthough in‐channel and floodplain large wood (LW) has been recognized as an important component of lotic ecosystems, there is still limited knowledge on the recruitment, mobility and retention of LW in rivers with an intermittent hydrological regime. In this study, we analysed the LW characteristics and related reach‐scale variables of 22 reaches in a Mediterranean intermittent river (Evrotas, Greece) in order to identify predictors of in‐channel and floodplain LW distribution. Our results indicated high downstream variation in LW volumes in the fluvial corridor (0.05–25.51 m3/ha for in‐channel LW and 0–30.88 m3/ha for floodplain LW). In‐channel and floodplain LW retention was primarily driven by the hydrological regime of the studied reaches (i.e. perennial or non‐perennial) with higher volumes of LW observed in perennial sections. The width of the riparian corridor was an important predictor of LW storage at the reach scale. Non‐perennial reaches had a disproportionally larger number of relatively small‐diameter living trees at the expense of mature trees with larger diameters typical for riparian stands functioning as LW recruitment areas in perennial reaches. The smaller dimensions of in‐channel LW in non‐perennial reaches, coupled with the dominance of loose LW pieces, implies frequent LW transport during ordinary flood events. Nevertheless, overall low LW retention in the fluvial corridor under non‐perennial flow regime predicts low volumes of mobilized LW. In contrast, the recruitment of relatively long and large‐diameter LW from mature riparian stands in perennial reaches, together with additional LW stabilization by banks, bed sediments, living trees or other LW pieces decreases the potential for further LW transport. © 2020 John Wiley &amp; Sons, Ltd.

Long‐term large wood load fluctuations in two low‐order streams in Southern Chile

AbstractThe eco‐hydrogeomorphic significance of large wood (LW) and its potential for increasing downstream hazards during extreme floods have been widely recognized. We used LW data collected for a 10‐year period from the two low‐order streams of Pichún (Pi) and Vuelta de Zorra (VZ) in Southern Chile to (a) determine if the abundance and dimensions of individual LW pieces change with time, (b) quantify wood load fluctuations during the 10‐year period, and (c) assess the role of LW recruitment from the riparian forests to explain wood load fluctuations during the study period. Nine years after the first survey, the number of LW pieces in Pi and VZ diminished by 60 and 40%, respectively. Despite the reduction in these numbers, in Pi, the LW dimensions did not change significantly during the study. In VZ, the dimensions exhibited statistically significant differences, despite being within the same class. In both catchments, the LW load fluctuated during a 10‐year period, but the drivers of change differed. Although tree toppling was the recruitment mechanism responsible for LW in both stream cases, the high wood load measured in Pi at the beginning of the study suggested massive tree recruitments before the first survey, followed by wood exports which were higher than inputs in the subsequent 10‐year period. In VZ, LW load decreased during the first 9 years (mean annual rate of ~9.2 m3 year−1) and then increased by ~12.1 m3 year−1 in year 10. At VZ, the inputs consisted of single trees that were recruited from the riparian area and by upstream flotation, while exports occurred by downstream fluvial transport. Wood inputs and exports occurred asynchronously and led to LW load fluctuations at decadal and annual intervals. Land management and tree species thus exert a major influence on wood inventory and budget in streams. © 2020 John Wiley &amp; Sons, Ltd.

Numerical simulation of large wood deposition patterns and responses of bed morphology in a braided river using large wood dynamics model

AbstractThis study discusses the reproducibility of a numerical model for simulating the morphodynamics involved in the transport of large pieces of wood in a braided river, considering the root wad effect and jam formation. The developed numerical model can simulate the behaviour of large pieces of wood using a two‐dimensional depth‐averaged Eulerian flow model that calculates the water flow and bed morphology. A Lagrange‐type wood transport model is used herein, and the applicability of the combined model is discussed through a comparison with obtained experimental results. From the simulation results, we calculate the total braiding index and estimate the deposition patterns of wood pieces for comparison with the experimental results. We then analyse the bed morphology responses and wood deposition patterns in terms of the root wad effect and input supply. Moreover, we discuss the advantages and limitations of the proposed model to predict the large wood dynamics considering the bed morphology. © 2019 John Wiley &amp; Sons, Ltd.