Unravelling the frequency and magnitude of large wood mobility and transport distance across 11 low‐order streams over multiple years

Large wood mobility processes in low-order Chilean river channels

Temporal variations of large wood abundance and mobility in the Blanco River affected by the Chaitén volcanic eruption, southern Chile

Anthropogenic controls on large wood in streams and rivers, and its wider geomorphological and ecological consequences, have come under increasing scrutiny in recent years. However, while the anthropogenic controls on riparian vegetation have been extensively studied, the direct effect of removal of wood from rivers and its mobility have not been widely assessed. This paper specifically considers anthropogenic input, removal and mobility of large wood in rivers. The analysis is based on data from ten semi‐natural rivers in the Czech Republic. An increase in number of large wood pieces because of anthropogenic activity was documented in all of the river reaches studied. Anthropogenic activity was responsible for on average 9 per cent of large wood pieces within rivers, rising to over 20 per cent in extreme cases. Large wood unintentionally recruited to rivers by human activity was of smaller dimensions than natural large wood and therefore does not significantly contribute to total large wood volume. Deliberate removal of large wood was also a significant process and in some river reaches natural large wood loads have been reduced by almost 50 per cent. Large wood removal tends to target the largest wood pieces. All pieces of wood bearing signs of anthropogenic impact were susceptible to transport. This may have a negative effect on the public perception of in‐channel large wood. Despite the fact that the river reaches examined were classified as natural or semi‐natural (based on channel morphology and riparian vegetation), the human impact on large wood loads and dynamics (mobility) was surprisingly high. The results suggest that determination of natural large wood loads and dynamics in rivers flowing through the Central European cultural landscape remain difficult to quantify.

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

Previous research on large wood (LW) entrainment and mobility has shown that in small rivers most mobile pieces are shorter and thinner than bankfull width and depth, and are deposited perpendicular or oblique to the flow as individual pieces. LW size, quantity and position were investigated over several years in channel segments of four mountain experimental catchments in southern Chile. Every wood piece found within the bankfull channel with more than 10 cm in diameter and 1 m in length was measured, its position was referenced and several of these elements were tagged. The channel segments were re-surveyed after consecutive winter seasons and every tagged wood piece that had moved downstream from its initial position was re-referenced to investigate log mobility and calculate travel distance. LW mobility (the percentage of tagged wood pieces that had moved) was higher in periods where maximum water level exceeded channel bankfull depth, although the difference was not statistically significant, but there was no difference in LW diameter, length or travel distance in periods with maximum during which water levels did or did not exceed bankfull stage. A significant (R = 0.603) linear regression relationship was found between LW mobility (in %) as dependent variable and the ratio Hmax/HBk (Hmax the maximum water level registered during each winter period and HBk the bankfull stage). The ratio between LW travel distance and LW piece diameter decreased with an increase in the ratio between LW piece length and the mean channel bankfull width, although this relationship was not statistically significant.

Wood density and moisture sorption and its influence on large wood mobility in rivers

Studies presenting long-term observations of the recruitment and mobility of large wood in mountain watercourses are scarce, but they can considerably contribute to the knowledge of river/riparian forest interactions and the assessment of flood hazard resulting from wood mobility during floods. Widespread dieback of riparian forest along the headwater course of Kamienica Stream in the Polish Carpathians, caused by bark beetle infestation of spruce trees, has raised concerns about potential increases of large wood recruitment to the stream and of the flood hazard to downstream valley reaches. In October 2009, 429 trees growing along three sections of the stream were tagged with numbered metal plates and monitored over 10 years to determine the timing and causes of their delivery to the channel and the lengths of their displacement during individual flood events. Moreover, in 2012 the mode of location of wood deposits and a degree of wood decay were determined in the second- to fourth-order stream reaches. The monitoring of tagged trees indicated that trees were recruited to the channel during highintensity meteorological and hydrological events, mostly as a result of bank erosion during floods or windthrow. With 22% of tagged trees recruited to the channel during 10 years, the rate of turnover of the riparian trees was estimated at 45 years. As the riparian area is overgrown with trees with ages up to ~160 years, the rate evidences substantial intensification of large wood recruitment to the channel in the recent period. Results of large wood inventory and the 10-year-long monitoring of tagged trees indicated variable mobility of large wood along the upper course of the stream. Wood mobility was negligible in the second-order stream reach, very small in the third-order reach, and greater, but still limited in the fourth-order reach. Wood is transported longer distances only during major floods. However, the advanced state of decay of most pieces leads to their disintegration during floods, precluding distant transport. Thus, large wood retained in the upper stream course does not constitute an important flood hazard to downstream, inhabited valley reaches.

We present a comparative study of large wood (LW) mobility in two steep channels (0.06<S<0.20) of contrast European landscapes during major hydro-geomorphic events of similar magnitude. We investigated a headwater perennial stream draining Central European medium-high mountain relief (the Klepáčský, Hrubý Jeseník Mts, Czech Republic; A≤2.5 km2) and an ephemeral Mediterranean steep stream (the Sfakiano Gorge, Crete, Greece; A≤52 km2). Both studied hydro-geomorphic events were partially accompanied by debris-flood or debris-flow character of sediment transport including significant content of LW. The minimal estimations of transported LW volume were 71.3 m3 along 1.7 km long reach in the Klepáčský and 49.4 m3 along 4 km long reach in the Sfakiano Gorge. Despite completely different characters of hydrologic regimes, riparian zones and valley confinement settings, living trees in the valley floor played crucial role in wood deposition and development of large jams in both environments. The trees living in the valley floor were the main source of LW in the studied Mediterranean channel, whereas long-term LW recruitment from very steep hillslopes or rock cliffs was quite negligible. On the other hand, previous windstorms in the Klepáčsky caused notable delivery of LW from adjacent hillslopes and LW recruitment by bank erosion occurred only at spatially-limited semi-confined parts of the valley.

1. Large wood forms an important component of woodland river ecosystems. The relationship between large wood and the physical characteristics of river systems varies greatly with changes in the tree species of the marginal woodland, the climatic and hydrological regime, the fluvial geomorphological setting and the river and woodland management context.2. Research on large wood and fluvial processes over the last 25 years has focussed on three main themes: the effects of wood on flow hydraulics; on the transfer of mineral and organic sediment; and on the geomorphology of river channels.3. Analogies between wood and mineral sediment transfer processes (supply, mobility and river characteristics that affect retention) are found useful as a framework for synthesising current knowledge on large wood in rivers.4. An important property of wood is its size when scaled to the size of the river channel. ′Small′ channels are defined as those whose width is less than the majority of wood pieces (e.g. width < median wood piece length). `Medium' channels have widths greater than the size of most wood pieces (e.g. width < upper quartile wood piece length), and `Large' channels are wider than the length of all of the wood pieces delivered to them.5. A conceptual framework defined here for evaluating the storage and dynamics of wood in rivers ranks the relative importance of hydrological characteristics (flow regime, sediment transport regime), wood characteristics (piece size, buoyancy, morphological complexity) and geomorphological characteristics (channel width, geomorphological style) in `Small', `Medium' and `Large' rivers.6. Wood pieces are large in comparison with river size in `small' rivers, therefore they tend to remain close to where they are delivered to the river and provide important structures in the stream, controlling rather than responding to the hydrological and sediment transfer characteristics of the river.7. For `Medium' rivers, the combination of wood length and form becomes critical to the stability of wood within the channel. Wood accumulations form as a result of smaller or more mobile wood pieces accumulating behind key pieces. Wood transport is governed mainly by the flow regime and the buoyancy of the wood. Even quite large wood pieces may require partial burial to give them stability, so enhancing the importance of the sediment transport regime.8. Wood dynamics in `Large' rivers vary with the geometry of the channel (slope and channel pattern), which controls the delivery, mobility and breakage of wood, and also the characteristics of the riparian zone, from where the greatest volume of wood is introduced. Wood retention depends on the channel pattern and the distribution of flow velocity. A large amount is stored at the channel margins. The greater the contact between the active channel and the forested floodplain and islands, the greater the quantity of wood that is stored.

Understanding large wood dynamics is critical for a range of disciplines including flood risk management, ecology and geomorphology. Despite the importance of wood in rivers, our understanding of the mobility of large wood remains limited. In this study individual pieces of large wood were tagged and surveyed over a 32 month period within a third and fourth order lowland forest river. Individual pieces of wood were found to be highly mobile, with 75% of pieces moving during the survey period, and a maximum transport distance of 5.6 km. Multivariate analyses of data from this study and two other published studies identified dimensionless wood length as the important factor in explaining likelihood of movement. A length threshold of 2.5 channel widths is identified for near functional immobility, with few pieces above this size moving. In addition, for this study, wood type, branching complexity, location and dimensionless wood diameter were found to be important in determining mobility only for sinuous reaches with readily inundated floodplains. Where logjams persist over multiple years they were shown to be reworked, with component pieces being transported away and replaced by newly trapped pieces. The findings of this study have implications for river management and restoration. The high mobility observed in this study demonstrates that only very large pieces of wood of length greater than 2.5 channel widths should be considered functionally immobile. For pieces of wood of length less than the channel width the possibility of high rates of mobility and long transport distances should be anticipated.

If a tree falls in an urban stream, does it stick around? Mobility, characteristics, and geomorphic influence of large wood in urban streams in northeastern Ohio, USA

Large wood is inherently mobile in naturally functioning river corridors, yet river management commonly introduces wood that is anchored to limit hazards. Wood that is periodically mobilized is important for: replacing stationary large wood that performs diverse physical and ecological functions; contributing to the disturbance regime of the river corridor; diversifying wood decay states; dispersing organisms and propagules; providing refugia during floodplain inundation and in mobile‐bed channels; dissipating flow energy; and supplying wood to downstream environments including lakes, coastlines, the open ocean, and the deep sea. We briefly review what is known about large wood mobility in river corridors and suggest priorities for ongoing research and river management, including: structural designs that can pass mobile wood; enhancing piece diversity of introduced wood that is anchored in place; quantifying wood mobilization and transport characteristics in natural and managed river corridors; and enhancing documentation of the benefits of wood mobility.

Applicability, advantages and limitations of a range of methods applied to determine large wood dynamics in Kamienica Stream and the Czarny Dunajec River, Polish Carpathians, are discussed. Results of a 6-year-long monitoring suggest an increased rate of wood recruitment to Kamienica Stream caused by recent bark beetle infestation of the spruce forests in the valley. However, both monitoring of wood transport and wood inventories indicate that the mobility of large wood in the stream is low and can increase only during major floods. Thus flood hazard to downstream valley reaches potentially resulting from the considerable amounts of large wood stored in the upper stream reach is limited. In the Czarny Dunajec, wood inventories, a tracking experiment with logs tagged with radio transmitters, and numerical modelling indicated high potential for wood transport in the narrow river reaches formed by channelization or channel incision, and high potential for wood deposition in the wide, multi-thread channel. Vegetative regeneration of living willow wood considerably reduces its remobilization by subsequent floods. Efficient transport of large wood along narrow river reaches implicates that during floods substantial amounts of wood may be delivered from distant sources to the channel sections located downstream of the narrow reaches. Wide, multi-thread reaches operate as natural wood traps, considerably limiting further transfer of wood to vulnerable sites/reaches.

Characteristics of large wood in a headwater channel after an extraordinary event: The roles of transport agents and check dams

Large wood (LW) plays a fundamental role in maintaining the health and functionality of river ecosystems. LW influences hydrodynamics by altering flow patterns, contributes to sediment transport processes by trapping and redistributing sediment, and shapes diverse river channel forms. Moreover, LW enhances habitat complexity and diversity, sustaining biodiversity. Understanding and accurately quantifying LW storage is vital for a range of river management activities, including designing effective habitat restoration projects and implementing flood mitigation strategies. However, traditional field surveys and manual analysis of aerial imagery are labour-intensive, time-consuming, and limited in spatial and temporal scope. Advancing tools and techniques for LW quantification is therefore critical to enabling more efficient and widespread integration of wood into river restoration efforts.This study introduces a fully automated method integrating high-resolution drone imagery and advanced machine learning algorithms to detect and quantify instream LW. Leveraging convolutional neural networks (CNNs), we trained a YOLOv10 model for wood detection and a YOLOv8 model for wood segmentation using datasets from eight rivers in the Swiss Alps and Argentinean Andes. An independent dataset from the Avançon de Nant River in Switzerland was used for method validation, ensuring the robustness and generalizability of the approach. Our detection model achieved a 90\% accuracy in wood volume estimation and identified 97\% of wood pieces in the largest size bracket at a 0.3 confidence threshold, demonstrating high detection reliability. The segmentation model reached a mean Average Precision (mAP) of 70\%, effectively distinguishing wood pixels from background pixels despite slight underestimations in wood diameters for short and wide pieces. By automating both detection and volume estimation, our method addresses the limitations of traditional field-based approaches and significantly reduces human effort and potential for error.The approach effectively detected wood across different environmental conditions, although challenges such as differentiating wood from similar-coloured substrates and accounting for partially submerged pieces remain. Expanding the training dataset to include more diverse environmental scenarios could enhance model accuracy and reliability.This scalable and efficient method has substantial implications for monitoring river wood dynamics over large spatial and temporal scales. It provides a powerful and easy tool for scientists, conservationists, and river managers to understand wood storage better, improve habitat restoration efforts, and implement more informed flood risk management practices. Integrating UAV technology and machine learning significantly advances fluvial geomorphology studies, enabling consistent data collection in complex natural environments and informing sustainable management strategies.