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Watershed controls on the export of large wood from stream corridors
Abstract
Large wood maintains in-channel and floodplain habitats by influencing the biophysical character of the river corridor. Large wood dynamics in a river corridor are a product of watershed wide processes and also of local recruitment, transport, and storage. This complexity of scales added to the logistical constraints in taking measurements limits our understanding of large wood dynamics through the watershed. To begin to unravel this issue, we compiled a data set of the volume of large wood deposited annually into 131 reservoirs across Japan and compared large wood export to flow discharge and watershed characteristics (watershed size, latitude, channel slope, percent forest, and forest type). We found that large wood was predominately transported during peak flow events. Large wood export increased logarithmically with watershed area. The decreasing export rate of large wood per watershed area is interpreted as a combination of annual export variability in upper watersheds, a non-significant increase in large wood recruitment along the longitudinal gradient (potentially human influenced), the increase in long-term storage on adjacent large floodplains, and significant decay/fragmentation downstream. Watersheds < 10–20 km2 had a highly variable large wood export pattern, conforming generally to previously published work that suggest transport limitation in smaller watersheds. The data suggest the existence of an export threshold (∼ 75 km2) where large wood export is no longer related to watershed size. Export across all watershed sizes was controlled by watershed characteristics (slope, percent forested, etc.) and peak discharge events. The connection with upstream watersheds and laterally with the floodplain increases the net flux of large wood through downstream transport and retransport of buried logs. Identifying rates of large wood transport from watershed connectivity as a potential key input process will improve our basic understanding of geomorphic and ecological patterns within the watershed. These results highlight the importance of understanding the local- and watershed-scale dynamics of large wood in creating and maintaining more heterogeneous riparian and aquatic habitat along the river corridor.
... This connectivity can be categorized into two types: structural and functional. Structural connectivity refers to the connection between adjacent geomorphological units, such as slopes, alluvial terraces, piedmonts, thalwegs, etc. Functional connectivity occurs when there is a linkage between units through hydro-geomorphological processes within and beyond the alluvial floodplain, including features like watershed floods, rill wash, and channel bars [2][3][4]. These two forms of connectivity facilitated the dispersal of human groups during the Middle Paleolithic, serving as pathways for accessing and stockpiling natural resources. ...
... This paper specifically focuses on the territorial dispersion at the regional watershed scale, with a particular emphasis on the Guadaíra River watershed [3]. The Guadaíra River is a left bank tributary of the Guadalquivir River, located in the province of Seville, Spain. ...
... Geosciences 2023, 13, x FOR PEER REVIEW 12 of Figure 4. Rozalejo (ROZ-1) lithic industry: handaxe (1); scrapers (2, 3, and 4); borer (5); notch denticulate (7); Levallois flakes (8, 9, and 10); Mousterian point (11). Raw materials: quartzites (1 and 6); flint (3,4,5,7,8,9,10, and 11). ...
This research addresses the geomorphological connectivity existing amid the piedmont’s karstic fillings (Sierra de Esparteros) and the Guadaíra and Guadalquivir Rivers’ alluvial terraces (SW of Spain), spotted with vestiges of human activities (Middle Palaeolithic). This study includes the analysis of 20 geoarchaeological sites and 28 lithic assemblages, with a total of 13,233 lithic pieces. Techno-typological and use–wear (SEM) analyses were conducted on these artifacts. Depending on the raw materials and the provenance of these lithic industries, two groups of assemblages were identified: one made of quartzite from the north, and another made of flint from the south. Two main geochronological periods were established (OSL and U/Th): (1) a short duration (MIS6/MIS5) and (2) a long duration (MIS5/MIS3). Techno-typological analysis showed three sorts of activities: (a) the provision and distribution of raw materials, (b) knapping, and (c) other activities that imply the use of a lithic workshop (LW), along with the settlement characteristics of habitual or recurrent (HS), temporary (TS), and indeterminate (IN). This geoarchaeological connectivity is called the “hominid alluvial corridor” (HAC). The underlined features are the geomorphological units, the continuum alluvial series, the raw material of the lithic industries as an indicator of provenance and transportation throughout the alluvial system, and the use–wear analysis of the tool-kit to interpret the functionality of the pieces.
... The Great Raft of the Red River (a tributary of the Mississippi), for example, was a 300-km wood accumulation that had long persisted during the nineteenth century (McCall, 1988). In modern times, having such large rafts is very unusual because of river management, but a few cases have been documented in reservoirs where a dam blocks the transfer of wood downstream (Moulin and Piégay, 2004;Le Lay and Moulin, 2007;Fremier et al., 2010). On the Rhône River, for example, great quantities of drifting wood produced large rafts on the upstream side of the Génissiat dam that disturbed normal operations (Moulin and Piégay, 2004;Le Lay and Moulin, 2007). ...
... Maximum spring discharge was used because it was the only parameter available and because it represents the highest discharge of the year in rivers of the Gaspé Peninsula. Although peak river discharge has been shown to explain most wood discharge (Piégay, 2003;Fremier et al., 2010), other parameters could also be important, e.g., flood duration, maximum fall discharge, and the number of geomorphological effective floods in a year. Winter conditions may also play a determinant role in the large wood dynamics in rivers (Keller and Swanson, 1979). ...
... The temporal variation of LW transport/kinetics is caused by the variation of LW volume recruitment, the intensity of episodic flood events, and the lag effects from previous flood events (Martin and Benda, 2001;Nakamura and Swanson, 2003;Moulin and Piégay, 2004). The lag effect and the influence of previous flood events are complex but poorly described aspects of LW kinetics (Fremier et al., 2010). Considering the high annual variability, interannual monitoring of LW is essential when examining the controlling factors of LW kinetics and also when preparing a detailed wood budget analysis. ...
This thesis is an analysis of the dynamics of large wood in river by a multi-scale large wood budget approach in a semi-alluvial river of cold regions: the Saint-Jean River, Gaspé. The study of the spatial and temporal variability of the dynamics of large wood was carried out through a methodological approach combining four years of field and by analyzing historical documents. The rivers of the Gaspé Peninsula produce annually and carry large amounts of large wood. This production comes from the high specific power of rivers and by banks composed of noncohesive sediment and having a generally dense riparian forest tree. Until 2015, the Delta of the Saint-Jean River had several very large jams. These jams are put in place since the 1960s, they represent a unique opportunity to quantify and apply a wood budget and to identify key variables related to the dynamics of large wood at multiple spatial and temporal scales.
In the first chapter, we described the function and the temporal evolution of large wood jams in the delta of the Saint-Jean River and presented the dynamics of large wood in the watershed. This chapter shows that the Saint-Jean River is characterized by high dynamics of large wood that promote large jams in the delta and along the fluvial corridor. Our results show that almost all large wood in river is produced by lateral migration and by the influence of the morphology. The density of trees in riparian areas is very high, which favors a major recruitment of large wood to the system. For the accumulations in the river corridor, two areas accumulate the majority of wood and these first results show a significant mobility, which can fluctuate substantially from year to year. With this chapter, we confirm that the case of the Saint-Jean River is a unique study site to apply and validate a large wood budget and to quantify the various components and key factors in the dynamics of large at the watershed scale.
In the second chapter, we analyzed the interannual variability of the dynamics of large wood and quantified the effect of low-recurrence hydro-meteorological events. To document the interannual variability of large wood in the Saint-Jean River, 4 annual surveys were made between 2010 to 2013 to locate and describe more than 1000 large wood jams and over than 2000 individual large wood along a river corridor of 60 km long. The intense hydrometeorological event in December 2010 resulted in a major flood in the area, causing significant lateral migration in the Saint-Jean River. The results indicate that the recruitment of large wood between 2010 and 2013 represents 57% of the total production for the period 2004 and 2013 and large wood volumes accumulated along the river corridor are 4 times higher in 2013 than in 2010. In terms of mobility, video analysis of three different events showed that the intensity of the transport (number of large wood per minute) can be higher to ten times during an event with mechanical ice-breakup, compared to an open water.hydroclimatic event.
In the third chapter, we conducted an analysis by large wood budget and analysis of eco-hydromorphological trajectory over more than 50 years. We quantified each component (input, output and accumulated) of a large wood budget at multiple spatial and temporal scales. We used the volumes of data accumulated in the delta of the Saint-Jean River since 1963 to close an analysis of large wood budget. These analyzes demonstrated and quantified the large variability of a large wood budget at multiple spatial and temporal scales. At the interannual scale or decadal, scale, the dynamics of large wood have periods when the input, storage and mobility of large wood differ according to eco-hydromorphological contexts. The eco-hydromorphological trajectory suggests an increase in river dynamics due to a significant change in the hydrology, resulting in higher production and mobility of large wood and increased in volumes accumulated in the corridor of the Saint-Jean River since the last decade.
... decomposition, breakage). Substantial effort has been put forward to characterize wood load characteristics (reviewed in Gurnell et al., 2002), including abundance or volume (Fox and Bolton, 2007;Cadol et al., 2009;Jones et al., 2011) and spatial distribution (Comiti et al., 2006;Magilligan et al., 2008;Wohl and Jaeger, 2009;Morris et al., 2010;Kraft et al., 2011), and identify the environmental parameters that influence wood load (Hedman et al., 1996;Morris et al., 2007;Fremier et al., 2010;King et al., 2013). ...
... In mountain systems, there appears to be some consensus of longitudinal trends in LW characteristics at river network scales (10 3-4 m), although substantial variation exists. In particular, wood loads tend to decrease in the downstream direction (Keller and Swanson, 1979;Magilligan et al., 2008;Wohl and Jaeger, 2009;Fremier et al., 2010) and individual pieces self-organize in non-random distributions that determine jam formation (accumulations ≥ 3 pieces) and spacing (Cadol et al., 2009;Kraft et al., 2011;Wohl and Cadol, 2011), which are strongly influenced by valley scale conditions (Morris et al., 2010;Wohl and Beckman, 2011). However, in the headwater regions of these streams, reach scale (10 1-2 m) controls on wood load appear less consistent across different hydroclimatic settings (Comiti et al., 2006;Mao et al., 2008;Cadol et al., 2009;Wohl et al., 2012a). ...
... Forest characteristics that influence wood recruitment include stand age, forest basal area, and natural or anthropogenic disturbance-related processes that includes tree mortality, treefall, blowdown, and management practices (Bragg, 2000;Gregory et al., 2003;May and Gresswell, 2003b;Morris et al., 2007;Wohl and Jaeger, 2009;Wohl and Cadol, 2011). Valley characteristics that influence wood recruitment includes drainage area, channel position within the network, hydrologic regime, valley confinement and gradient, presence of landslides and debris flows, and channel migration (Benda et al., 2003;Swanson, 2003;Hassan et al., 2005;Seo and Nakamura, 2009;Fremier et al., 2010;Wohl and Cadol, 2011;Bertoldi et al., 2013). ...
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 hemlock forests are expected to change because of infestation of hemlock woolly adelgid (HWA), an exotic, invasive insect. We examine 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. 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. This article is protected by copyright. All rights reserved.
... • Use remote sensing techniques to assess change on larger spatial and longer temporal scales (e.g., Bertoldi et al., 2013;Atha, 2014;Ulloa et al., 2015;Kramer et al., in press). • Conduct stratigraphic and/or other analysis of wood deposited in basins and floodplains to obtain long-term (decade to millennial scale) records of wood flux from watersheds (Guyette et al., 2008;Seo et al., 2008;Seo and Nakamura, 2009;Fremier et al., 2010;Boivin et al., 2015;Kramer and Wohl, 2015). • Use already existing data from unconventional sources. ...
... Heidorn (2008) calls these dark data. For example, Moulin and Piégay (2004); Seo et al. (2008);and Fremier et al. (2010) successfully used reservoir debris extraction records to indirectly estimate basin wood flux. In addition to finding and using dark data, a vast amount of unconventionally collected data is freely available on the internet. ...
To effectively manage wood in rivers, we need a better understanding of wood mobility within river networks. Here, we review primarily field-based (and some numerical) studies of wood transport. We distinguish small, medium, large, and great rivers based on wood piece dimensions relative to channel and flow dimensions and dominant controls on wood transport. We suggest further identification and designation of wood transport regimes as a useful way to characterize spatial-temporal network heterogeneity and to conceptualize the primary controls on wood mobility in diverse river segments. We draw analogies between wood and bedload transport, including distinguishing Eulerian and Lagrangian approaches, exploring transport capacity, and quantifying thresholds of wood mobility. We identify mobility envelopes for remobilization of wood with relation to increasing peak discharges, stream size, and dimensionless log lengths. Wood transport in natural channels exhibits high spatial and temporal variability, with discontinuities along the channel network at bankfull flow and when log lengths equal channel widths. Although median mobilization rates increase with increasing channel size, maximum mobilization rates are greatest in medium-sized channels. Most wood is transported during relatively infrequent high flows, but flows under bankfull can transport up to 30% of stored wood. We use conceptual models of dynamic equilibrium of wood in storage and of spiralling wood transport paths through drainage networks, as well as a metaphor of traffic on a road, to explore discontinuous wood movement through a river network. The primary limitations to describing wood transport are inappropriate time scales of observation and lack of sufficient data on mobility from diverse rivers. Improving models of wood flux requires better characterization of average step lengths within the lifetime travel path of a piece of wood. We suggest that future studies focus on: (i) continuous or high-frequency monitoring of wood mobility; (ii) monitoring changes in wood storage; (iii) using wood characteristics to fingerprint wood sources; (iv) quantifying volumes of wood buried within river corridors; (v) obtaining existing or new data from unconventional sources, such as citizen science initiatives, and (vi) creating online interactive data platforms to facilitate data synthesis.
... Direct observations of wood movement in small (Dixon and Sear, 2014;Jochner et al., 2015), medium-sized (Ruiz-Villanueva et al., 2016), large rivers (MacVicar and Piégay, 2012;Schenk et al., 2014;Ravazzolo et al., 2015), and great rivers (Kramer and Wohl, 2014) and measurements of LW transport into reservoirs (Moulin and Piégay, 2004;Seo et al., 2008;Seo and Nakamura, 2009;Fremier et al., 2010) indicate that LW movement occurs predominantly above a threshold discharge that has been expressed as a proportion of bankfull stage, an absolute value, or a recurrence interval. The transport threshold is partly a function of LW piece size (MacVicar and Piégay, 2012) but is also likely to be strongly influenced by the minimum stage necessary to float LW pieces deposited along the channel margins on bars, islands, and channel banks. ...
... Seo et al. (2008) found that LW export per unit area to reservoirs in Japan declined as watershed size increased, a pattern that they attributed to more abundant floodplain storage areas in large, lowland rivers (Seo and Nakamura, 2009) and lack of continuous riparian forest cover in larger watersheds, although this latter factor is likely to vary strongly with land-use history and channel type. Fremier et al. (2010) interpreted the same data as reflecting an export threshold of 75 km 2 drainage area, above which the rate of LW export per drainage area declines. Analogous drainage area thresholds have not been explored for other regions, but the specific threshold value is likely to vary greatly based on observations of massive LW export from much larger subbasins within the Mackenzie River drainage basin, for example (Kindle, 1921;Eggertsson, 1994;Kramer and Wohl, 2015). ...
Nearly 50 years of research focused on large wood (LW) in rivers provide a basis for understanding how wood enters rivers; how wood decays, breaks, and is transported downstream; and how at least temporarily stable wood influences channel geometry, fluxes of water, sediment, and organic matter, and the abundance and diversity of aquatic and riparian organisms. Field-based studies have led to qualitative conceptual models and to numerical stimulations of river processes involving wood. Numerous important gaps remain, however, in our understanding of wood dynamics. The majority of research on wood in rivers focuses on small- to medium-sized rivers, defined using the ratio of wood piece size to channel width as channels narrower than the locally typical wood-piece length (small) and slightly narrower than the longer wood pieces present (medium). Although diverse geographic regions and biomes are represented by one or a few studies in each region, the majority of research comes from perennial rivers draining temperate conifer forests. Regional syntheses most commonly focus on the Pacific Northwest region of North America where most of these studies originate. Consequently, significant gaps in our understanding include lack of knowledge of wood-related processes in large rivers, dryland rivers, and rivers of the high and low latitudes. Using a wood budget as an organizing framework, this paper identifies other gaps related to wood recruitment, transport, storage, and how beavers influence LW dynamics. With respect to wood recruitment, we lack information on the relative importance of mass tree mortality and transport of buried or surficial downed wood from the floodplain into the channel in diverse settings. Knowledge gaps related to wood transport include transport distances of LW and thresholds for LW mobility in small to medium rivers. With respect to wood storage, we have limited data on longitudinal trends in LW loads within unaltered large and great rivers and on fluctuations in LW load over time intervals greater than a few years. Other knowledge gaps relate to physical and ecological effects of wood, including the magnitude of flow resistance caused by LW; patterns of wood-related sediment storage for diverse river sizes and channel geometry; quantification of channel-floodplain-LW interactions; and potential threshold effects of LW in relation to physical processes and biotic communities. Finally, knowledge gaps are related to management of large wood and river corridors, including understanding the consequences of enormous historical reductions in LW load in rivers through the forested portions of the temperate zone; and how to effectively reintroduce and manage existing LW in river corridors, which includes enhancing public understanding of the importance of LW. Addressing these knowledge gaps requires more case studies from diverse rivers, as well as more syntheses and metadata analyses.
... The Great Raft of the Red River (a tributary of the Mississippi), for example, was a 300-km wood accumulation that had long persisted during the nineteenth century (McCall, 1988). In modern times, having such large rafts is very unusual because of river management, but a few cases have been documented in reservoirs where a dam blocks the transfer of wood downstream (Moulin and Piégay, 2004;Le Lay and Moulin, 2007;Fremier et al., 2010). On the Rhône River, for example, great quantities of drifting wood produced large rafts on the upstream side of the Génissiat dam that disturbed normal operations (Moulin and Piégay, 2004;Le Lay and Moulin, 2007). ...
... Maximum spring discharge was used because it was the only parameter available and because it represents the highest discharge of the year in rivers of the Gaspé Peninsula. Although peak river discharge has been shown to explain most wood discharge (Piégay, 2003;Fremier et al., 2010), other parameters could also be important, e.g., flood duration, maximum fall discharge, and the number of geomorphological effective floods in a year. Winter conditions may also play a determinant role in the large wood dynamics in rivers (Keller and Swanson, 1979). ...
... The methods of measuring LW volume and the assumptions used in calculating the LW biomass were given by Seo et al. [2008]. To remove the area-proportional effect of the watershed on LW export, all of the LW biomass data were transformed to express the LW export per unit watershed area (unit LW export) [Seo et al., 2008; Fremier et al., 2010]. [9] To examine the influence of precipitation variability on LW export, we compiled data on daily precipitation (DP), which was monitored directly by the local reservoir W03510 SEO ET AL.: LARGE WOOD EXPORT REGULATED BY PRECIPITATION AND LATITUDE W03510 management offices or was collected from the Japan Meteorological Agency station closest to each study watershed. ...
... However, landslides that occurred on hillslopes or at the heads of steep tributaries during intensive precipitation can deliver considerable amounts of LW into the channel, and these pieces are exported to the confluences with main stem channels by subsequent debris flows, unrestricted by the physical characteristics of small streams [Keller and Swanson, 1979; Benda and Cundy, 1990; Nakamura et al., 2000]. These LW pieces deposited at the confluence can be transported by peak flows of main stem channels to larger channels [Piégay, 2003; Fremier et al., 2010]. In general, mass movements (e.g., landslides and debris flows) begin to occur when the EP (T h ¼ 3 days) exceeds approximately 100 mm, and the risk becomes very high when the EP exceeds approximately 200 mm [National Research Institute of Earth Science and Disaster Prevention, 2002]. ...
1] We examined the relationships between large wood (LW) export and precipitation patterns and intensity by analyzing the data on the annual volume of LW removed from 42 reservoirs and the daily precipitation at or near the reservoir sites. We also calculated the effective precipitation by considering the antecedent precipitation. Both daily and effective precipitation data were used as explanatory variables to explain LW export. The model selection revealed that the precipitation pattern and intensity controlling LW export varied with latitude in the Japanese archipelago. In small watersheds with narrow channel widths and low discharges, mass movements, such as landslides and debris flows, are major factors in the production and transport of LW. In this case, the effective precipitation required to initiate mass movements regulated the LW export and did not vary with the latitude. In intermediate and large watersheds with wide channel widths and high stream discharges, heavy rainfall and subsequent floods regulated buoyant depth, influencing the initiation of LW movement. In southern and central Japan, intense rainfall accompanied by typhoons or localized torrential downpours causes geomorphic disturbances, which introduce abundant pieces of LW into the channels. However, these pieces continue to be removed by repeated rainfall events. Therefore, LW export is supply-limited and potentially produces less LW accumulation. Conversely, in northern Japan, where typhoons and torrential downpours are rare, LW export is transport-limited because LW pieces recruited by bank erosion, tree mortality, and windthrow accumulate and persist on valley floors. These pieces may be easily exported by infrequent flooding., Large wood export regulated by the pattern and intensity of precipitation along a latitudinal gradient in the Japanese archipelago, Water Resour. Res., 48, W03510, doi:10.1029/ 2011WR010880.
... Conversely, in many geomorphic studies on LW dynamics, wood volume or biomass are recognized to be more important than surface area. Except for a few studies that estimated the pile volume of LW pieces trapped behind dams (e.g., Moulin and Piégay 2004; Seo et al. 2008; Fremier et al. 2010) (Fig. 1), numerous field studies have established distinct definitions of piece size, while each study has handled the definitions slightly differently (Table 1). Specifically, Martin and Benda (2001) applied different definitions of LW size according to the channel size. ...
... Chen et al. (2006) estimated the total density, volume and mass of in-channel LW within different-sized streams of forested watersheds; they also examined the relationship between LW characteristics (size, amount, volume, mass, orientation and position) and channel features through the channel networks at the watershed scale. Furthermore, Seo et al. (2008) and Fremier et al. (2010) elucidated the hydrogeomorphic factors that control LW dynamics using a database of LW fluvial export volumes derived from 131 reservoirs with watershed areas ranging from 6 to 2,370 km 2 ; they then quantitatively examined variations in the fluvial export of LW as a function of watershed size. To verify these findings and explain the variations in LW fluvial export, Seo and Nakamura (2009) investigated the size, distribution and fragmentation/decay class of LW pieces in 78 channel segments distributed across a large basin. ...
Recent research has elucidated the positive ecological roles of large wood (LW) in fish-bearing channels. However, where logjams
increase local flooding and bank erosion, LW has negative impacts on public safety and property protection. Although our understanding
of reach-scale processes and patterns has increased dramatically in recent years, only a few studies have integrated this
knowledge at the watershed scale. Here we review variations in LW dynamics along a gradient of watershed sizes. In small watersheds,
a massive amount of LW, resulting from forest dynamics and hillslope processes, remains on the valley floor. These pieces
may persist for several decades and are eventually transported during debris flows. In intermediate watersheds, LW is dominantly
recruited by bank erosion from adjacent riparian areas. These pieces are continuously transported downstream with LW pieces
that are supplied from the upstream watershed by floods because these channels have a greater width and depth than the length
and diameter of the pieces, as well as a high stream power. This leads to fragmentation of the LW pieces, which increases
their transportability. In large watersheds, LW pieces are frequently recruited at locations where the channel is adjacent
to riparian forests. Floated LW pieces can accumulate along channels with wide floodplains. Storage in floodplains can lead
to more rapid decay than in an anaerobic environment, resulting in the subsequent removal of LW pieces from the system. Our
review presents a generalized view of LW processing at the watershed scale, and is relevant to ecosystem management, disaster
prevention and the identification of knowledge gaps.
KeywordsEcosystem management-Disaster prevention-Large wood dynamic-River systems-Watershed scale
... A final innovative approach to estimating wood mobility that is also applicable to "large" rivers, is repeat observations of large wood rafts, whether they occur naturally or accumulate behind dams (e.g. Fremier et al., 2010;Boivin et al., 2017b;Benacchio et al., 2017;Senter et al., 2017). Repeat monitoring of their spatial extent (approximate volume) yields estimates of net mobility between monitoring occasions. ...
This article explores the character and role of wood in fluvial systems. Following descriptions of the key characteristics of wood that affect its role and how it is measured, the quantities and styles in which wood is retained in fluvial systems are considered. This leads to discussions of wood budgets and wood mobility in fluvial systems and, finally, to relationships between wood and landforms. Throughout, discussions and descriptions focus on “small,” “medium” and “large” rivers where size relates to the ratio of wood piece length to channel width. The article concludes by drawing together some common aspects of wood and geomorphology across rivers with different physical and wood characteristics and then highlighting some areas that require more research attention.
... Several studies have monitored LW mobility using techniques such as radio tags and telemetry (Schenk et al. 2014;Ravazzolo et al. 2015), video or time-lapse photography (MacVicar and Piégay 2012; Kramer and Wohl 2014), repeat surveys using ground-based and/ or remote sensing data (Wohl and Goode 2008;Curran 2010;Kasprak et al. 2012;Dixon and Sear 2014), LW entering reservoirs (Fremier et al. 2010), or some combination of these techniques (MacVicar et al. 2009;. The results of these studies indicate that there is typically not a simple or linear relationship between discharge and LW transport because of the influence of at least four factors. ...
Large wood and beaver dams are fundamental components of forested stream ecosystems but can also create hazards. We present guidelines for identifying stream segments that maximize environmental benefits while minimizing hazards. We focus on lesser gradient stream segments, although wood can be ecologically beneficial anywhere in a river network. Stream segments can be targeted for field-based evaluation using checklists for scenarios of either retention or reintroduction for logjams or beaver dams. We also present the Wood Jam Dynamics Database and Assessment Model, which incorporates a machine-learning-based statistical analysis to predict wood jam dynamics and provides a standardized survey protocol for wood jams.
... Logjam studies have been conducted in steep, single-threaded mountainous settings [Wohl and Beckman, 2014] and low-gradient alluvial settings where wood-floodplain interactions are important [Wohl, 2013;Collins et al., 2012]. Logjams which grow to a size such that they span the entire channel width and are of significant length are referred to as 'log rafts' -these features can occur in low-gradient alluvial settings [Boivin et al., 2015], behind reservoir dams [Moulin and Piégay, 2004;Le Lay and Moulin, 2007;Fremier et al., 2010] and along coastlines [Kramer and Wohl, 2015]. To date, no studies have explored large, channel-spanning 'log rafts' in steep, mountainous, single-threaded reaches. ...
Colloquially, a "logjam" indicates a kinematic arrest of movement. Taken literally, it refers to a type of dense accumulation of wood in rivers widely recognized as bestowing numerous biological and physical benefits to the system but also present serious hazards to infrastructure. Despite this, no in-situ field measurements have assessed the degree of arrest in a naturally-formed logjam. Using time-lapse photography, repeat total station surveys and water level loggers, we provide an unprecedented perspective on the evolution of a logjam in central Idaho. Despite the namesake, we find that the logjam is not jammed. The ensemble of logs progressively deforms in response to shear and buoyant lift of flowing water, modulated by the rising limb, peak and falling limb of the snowmelt hydrograph. As water rises and log drag against the bed and banks decreases, they collectively translate downstream, generating a heterogeneous pattern of deformation. As streamflow recedes and the logs reconnect with the bed and banks, the coherent deformation pattern degrades as logs settle opportunistically amongst their neighbors. Field observations of continuous movement at a low rate are qualitatively similar to creep and clogging, behaviors that are common to a wide class of disordered materials. These similarities open the possibility to inform future studies of environmental clogging, wood-laden flows, logjams, hazard mitigation and the design of engineered logjams by bridging these practices with frontier research efforts in soft matter physics and granular rheology.
... Variations in hydrologic conditions associated with annual peak NBB í µí± explained 712 79% of the variation found among seven observed í µí± %&# values (Fig. 7, Eq. (5)). The 713 explanatory correlation suggests complex interrelationships between variability in 714 decadal and interannual hydrologic responses, the cyclic availability of wood, and a suite 715 of other watershed processes (Moulin and Piégay, 2004; Fremier et al., 2010; Marcus et 716 al., 2011; Benda and Bigelow, 2014). This discussion focuses on how í µí± %&# varies in 717 response to hydrologic and climatic variability. ...
The dynamics that move wood through and out of watersheds are complex and not yet fully understood. In this study, climatic conditions, hydrologic responses, and watershed processes were explored to better understand variations in wood export using aerial imagery, event-based video monitoring, and field measurements from the 1097 km² mountainous Mediterranean climate North Yuba River, California, watershed and its reservoir near the downstream outlet. Over a 30-year study period, 1985–2014, volumetric estimates of annual wood export into the reservoir, available for a subset of years, were used to investigate watershed-scale, wood export dynamics. Variations in annual peak discharge explained 79% of the variance in interannual wood export, with 84% of total observed wood export (ca. > 10,000 m³ of wood per event) delivered by years with discharge events of 19-year, 21.5-year, and 60-year flood recurrence intervals. Continuous video monitoring conducted during snowmelt season periods in 2010 and 2011 yielded wood discharge observations at minima 15% of statistical bankfull flow, while maximum daily discharge explained 55% of observed daily wood piece variation. No statistically significant wood discharge differences were found in snowmelt season observations, likely caused by domination of the hydrograph by diurnal pulses within the seasonal cycle. Wood piece sizes in upstream watershed locations were found to be significantly longer but not significantly larger in diameter than those observed in transport or measured in the reservoir, an indication that more downstream fining of length than diameter may occur during transport. A conceptual model and functional framework are introduced in support of a watershed-scale explanation of wood export, transport, and storage processes applicable to large, Mediterranean-climate, mountain watershed settings.
... The temporal variation of LW transport/kinetics is caused by the variation of LW volume recruitment, the intensity of episodic flood events, and the lag effects from previous flood events (Martin and Benda, 2001;Nakamura and Swanson, 2003;Moulin and Piegay, 2004). The lag effect and the influence of previous flood events are complex but poorly described aspects of LW kinetics (Fremier et al., 2010). Considering the high annual variability, interannual monitoring of LW is essential when examining the controlling factors of LW kinetics and also when preparing a detailed wood budget analysis. ...
Abstract Semi-alluvial rivers of the Gaspé Peninsula, Québec, are prone to produce and transport vast quantities of large wood (LW). The high rate of lateral erosion owing to high energy flows and noncohesive banks is the main process leading to the recruitment of large wood, which in turn initiates complex patterns of wood accumulation and reentrainment within the active channel. The delta of the Saint-Jean River (SJR) has accumulated large annual wood fluxes since 1960 that culminated in a wood raft of > 3-km in length in 2014. To document the kinetics of large wood on the main channel of SJR, four annual surveys were carried out from 2010 to 2013 to locate and describe > 1000 large wood jams (LWJ) and 2000 large wood individuals (LWI) along a 60-km river section. Airborne and ground photo/video images were used to estimate the wood volume introduced by lateral erosion and to identify local geomorphic conditions that control wood mobility and deposits. Video camera analysis allowed the examination of transport rates from three hydrometeorological events for specific river sections. Results indicate that the volume of LW recruited between 2010 and 2013 represents 57% of the total LW production over the 2004–2013 period. Volumes of wood deposited along the 60-km section were four times higher in 2013 than in 2010. Increases in wood amount occurred mainly in upper alluvial sections of the river, whereas decreases were observed in the semi-alluvial middle sections. Observations suggest that the 50-year flood event of 2010 produced large amounts of LW that were only partly exported out of the basin so that a significant amount was still available for subsequent floods. Large wood storage continued after this flood until a similar flood or an ice-breakup event could remobilise these LW accumulations into the river corridor. Ice-jam floods transport large amounts of wood during events with fairly low flow but do not contribute significantly to recruitment rates (ca. 10 to 30% early). It is fairly probable that the wood export peak observed in 2012 at the river mouth, where no flood occurred and which is similar to the 1-in 10-year flood of 2010, is mainly linked to such ice-break events that occurred in March 2012.
... In Japan, agencies responsible for local reservoir management remove LW pieces trapped by reservoirs, and typically estimate total annual volumes delivered to the reservoirs (see Seo et al., 2008Seo et al., , 2012Fremier et al., 2010). From these databases, Seo et al. (2012) examined variations in LW export as a function of precipitation pattern in watersheds >20 km 2 (see Figures 3c and 3d in Seo et al. (2012)). ...
Large wood (LW) plays an important role in river ecosystems, but LW-laden floods may cause serious damage to human lives and property. The relationship between precipitation patterns and variations in LW distribution and export at the watershed scale is poorly understood. To explore these linkages, we examined differences in LW distribution as a function of channel morphologies in six watersheds located in southern and northern Japan, and analyzed the impacts of different precipitation patterns on the fluvial export of LW from river catchments. In southern Japan, intense rainfalls caused by typhoons or localized torrential downpours initiate landslides and debris flows that introduce massive amounts of LW into channels. Gravel bars formed by frequent flood events are widely prevalent, and the LW temporarily stored on these bars is frequently moved and/or broken into smaller pieces by floods. In these systems fluvial export of LW is supply-limited, with smaller accumulations and shorter residence times than in northern Japan. Conversely, in northern Japan, where typhoons and torrential downpours rarely occur, LW is mostly recruited by bank erosion, tree mortality and windthrow into channels, rather than by landslides and debris flows. Recruited pieces accumulate in log jams on valley floors, particularly on floodplains supporting mature forests, resulting in larger accumulations and longer residence times. In these watersheds fluvial export of LW is transport-limited, and the pieces gradually decompose during long-term storage as log jams. This article is protected by copyright. All rights reserved.
... Deposited instream wood also interacts with river flows to produce other channel features, which are important habitats for aquatic biota, such as scour pools and sand banks, and thus has a role in maintaining the heterogeneity of habitats within the river (Abbe and Montgomery, 2003; Brooks et al., 2003; Webb and Erskine, 2003; Naiman et al., 2008). The recruitment and the downstream movement of in-stream wood are regulated by factors operating at multiple scales and are dependent on the structure and composition of the riparian vegetation, the individual architecture of in-stream wood (Gippel et al., 1996; Fremier et al., 2009), the nature of the stream channel (Lassettre et al., 2008) and hydrology. The recruitment of wood to the river from riparian forests is likely to occur sporadically, with the greatest input generally occurring not only during floods or periods of high flow via bank erosion and/or bank collapse (Lassettre et al., 2008; Pettit et al., 2006; Latterell and Naiman, 2007) but also during the falling limb of flood flows (Cadol et al., 2009). ...
The recruitment of wood from the riparian zone to rivers and streams provides a complex habitat for aquatic organisms and can influence both aquatic biodiversity and ecosystem function. The Daly River in the wet–dry tropics of northern Australia is a highly seasonal, perennially flowing sand-bed river where surveys of river wood aggregations at the reach scale (~2 km) in 2008 and 2009 recorded densities of 37–78 km À1 and identified distinct types of river wood aggregations: key pieces, standing trees, fallen trees, wrack and single pieces. After larger than average flows in the 2008/2009 wet season, between 46% and 51% of the surveyed river wood had moved. The distribution of wood age classes indicated continual recruitment and slow turnover of wood within the river. Surveys of fish and habitat characteristics at the mesohabitat scale (~100 m) showed fish species richness; diversity and fish abundance were not correlated to the proportion of wood present. Fish assemblage structure was associated with wood cover as well as other environmental variables such as stream width and depth. The importance of in-stream wood also varied for different species and age classes of fish. This study documents the dynamic nature of river wood aggregations and their complex and variable distribution and suggests their importance as fish habitat in this tropical river.
... [58] Figure 15 illustrates the conceptual model for LW storage, supply, and mobility described in the previous paragraphs. The model extends the one put forward by Comiti et al. [2006], Fremier et al. [2009] inasmuch it includes two possible types of supply-limited sections, both upstream and downstream from reaches with limited transport capacities, and builds on the results presented above as well as on unpublished field observations in several similar basins of the Eastern Alps. However, there is a need to more fully investigate LW storage and dynamics especially in the smaller basins (<1 km 2 ), i.e., those for which there are fewer data and so the hypothesized trend of LW storage versus drainage area is more speculative, as highlighted in Figure 15. ...
An understanding of the dynamics of large wood (LW) in mountain channels
provides the basis for evaluating natural morphological patterns as well
as managing potentially hazardous wood transport during flood events.
Few studies have investigated the distribution of LW in managed streams
of the Alps across a wide spatial scale. This paper presents extensive
field measurements of LW storage and channel morphology carried out in
13 channels of the Eastern Italian Alps with drainage areas ranging from
1.2 to 70 km2, mean bed slope between 0.03 and 0.38, and
channel width between 2 and 20 m. More than 9000 LW elements were
measured in the 33 reaches surveyed. A geostatistical, geographic
information system (GIS)-based model for wood recruitment from hillslope
instabilities was also developed and applied to the study basin. LW
storage in the study channels results as being much lower than in
seminatural basins of comparable size and climate, and only basins
characterized by extensive mass wasting processes contain high wood
loads with relevant morphological consequences. The statistical analysis
of LW storage at the reach scale indicates that unit stream power is
apparently the most significant hydromorphological factor influencing LW
storage, in agreement with studies in other world regions. However, we
argue that the effect of unit stream power on LW storage is not only
linked to flow transport capacity but also derives from its association
with LW supply and valley morphology. Both the GIS model and statistical
tests on field data indicate that hillslope instabilities connected to
the channel network dominate the LW recruitment volume and the
distribution of in-channel wood storage.
... [3] Riverine large wood has received much attention in recent years, with detailed analyses of LW storage and transport at reach [Gippel et al., 1996], watershed [Merten et al., 2010;Downs and Simon, 2001;Seo et al., 2010], and regional [Fremier et al., 2010;Cordova et al., 2006] scales. Although large-scale analyses have relied on empirical associations (such as an inverse relationship between LW retention and the LW length:channel width ratio), process-based analyses require estimation of fluid forces on wood, namely buoyancy, lift and drag [Braudrick and Grant, 2000;Wallerstein et al., 2001;Buxton, 2010]. ...
Large wood (LW) exerts an important influence on the geomorphology and
ecology of streams and rivers. The magnitudes of flow forces on LW are
needed to support stream management activities and are typically
computed using time mean lift and drag coefficients determined in
laboratory flumes using small, smooth cylinders. Herein we report
measurements of forces on LW of varying complexity (simple cylinder,
branching, and complex root wad) and surface (bark) roughness made in an
outdoor grassed channel under steady and unsteady flows. LW orientation
relative to the primary flow direction and LW relative submergence were
varied. Drag and lift coefficients for cylindrical (unbranched) LW
followed patterns reported by others for metal cylinders in wind
tunnels. Drag coefficients for cylindrical (unbranched) LW, corrected
for blockage effects, ranged from -0.05 to 1.29, and lift coefficients
ranged from -0.88 to 0.52, varying systematically with LW position
relative to the channel bed and incident flow direction. Measured drag
coefficients for the noncylindrical LW, corrected for blockage effects,
ranged from 0.22 to 6.27, while lift coefficients varied from -3.65 to
30.84. Systematic relationships between the relative submergence and
orientation of branching LW and the drag and lift coefficients were not
observed, but coefficients were greatest for LW with few branches and
converged on smaller values typical of blunt bodies as LW complexity
increased. For both simple and complex LW, maximum lift and drag forces
during the rising limb of unsteady flows were about 2-3 times greater
than steady flow temporal mean values.
... Deposited instream wood also interacts with river flows to produce other channel features, which are important habitats for aquatic biota, such as scour pools and sand banks, and thus has a role in maintaining the heterogeneity of habitats within the river (Abbe and Montgomery, 2003; Brooks et al., 2003; Webb and Erskine, 2003; Naiman et al., 2008). The recruitment and the downstream movement of in-stream wood are regulated by factors operating at multiple scales and are dependent on the structure and composition of the riparian vegetation, the individual architecture of in-stream wood (Gippel et al., 1996; Fremier et al., 2009), the nature of the stream channel (Lassettre et al., 2008) and hydrology. The recruitment of wood to the river from riparian forests is likely to occur sporadically, with the greatest input generally occurring not only during floods or periods of high flow via bank erosion and/or bank collapse (Lassettre et al., 2008; Pettit et al., 2006; Latterell and Naiman, 2007) but also during the falling limb of flood flows (Cadol et al., 2009). ...
The recruitment of wood from the riparian zone to rivers and streams provides a complex habitat for aquatic organisms and can influence both aquatic biodiversity and ecosystem function. The Daly River in the wet–dry tropics of northern Australia is a highly seasonal, perennially flowing sand-bed river where surveys of river wood aggregations at the reach scale (~2 km) in 2008 and 2009 recorded densities of 37–78 km À1 and identified distinct types of river wood aggregations: key pieces, standing trees, fallen trees, wrack and single pieces. After larger than average flows in the 2008/2009 wet season, between 46% and 51% of the surveyed river wood had moved. The distribution of wood age classes indicated con-tinual recruitment and slow turnover of wood within the river. Surveys of fish and habitat characteristics at the mesohabitat scale (~100 m) showed fish species richness; diversity and fish abundance were not correlated to the proportion of wood present. Fish assemblage structure was associated with wood cover as well as other environmental variables such as stream width and depth. The importance of in-stream wood also varied for dif-ferent species and age classes of fish. This study documents the dynamic nature of river wood aggregations and their complex and variable dis-tribution and suggests their importance as fish habitat in this tropical river.
... A potentially more useful, although spatially limited, data set is one maintained by local reservoir management agencies in Japan, termed here the Japanese Reservoir Database (JRD). Detailed information on channel characteristics associated with 131 dams, including stream gradient, along with annual surveys of sediment and woody debris has permitted spatial analysis of controls on woody debris export (Seo et al., 2008;Fremier et al., 2009;Seo and Nakamura, 2009). ...
Despite many decades of research on downstream effects of dams on rivers, we have few general models predicting how any particular river is likely to adjust following impoundment. In the absence of such a framework, most research on downstream effects is based on individual case studies. To integrate these studies, we develop a conceptual and analytical framework for predicting geomorphic response of rivers to dams. This conceptual model emphasizes the role of geologic setting and history as first-order controls on the trajectory of channel change. Basin geology influences watershed and channel processes through a hierarchical set of linkages, extending from the drainage basin to the valley and channel, which collectively determine the sediment transport and discharge regimes. Geology also directly shapes the suite of hillslope processes, landforms, and geomorphic disturbances that sculpt and define the channel and valley floor morphologies. These factors, in turn, affect the capacity for adjustment of the downstream channel. In particular, they determine the type, direction, and extent of channel adjustments, including incision, widening, and textural changes. To incorporate hydrologic and sedimentalogic factors, we extend this framework to include two dimensionless variables that, taken together, predict geomorphic responses to dams depending on the ratio of sediment supply below to that above the dam (S*) and the fractional change in frequency of sediment-transporting flows (T*). Drawing on examples from dammed rivers in the US and China, we explore how trajectories of geomorphic change, as defined by these two variables, are influenced by the geological setting and history of the river. This approach holds promise for predicting the magnitude and trend of downstream response to other dammed rivers, and can identify river systems where geological controls are likely to dominate.
... A potentially more useful, although spatially limited, data set is one maintained by local reservoir management agencies in Japan, termed here the Japanese Reservoir Database (JRD). Detailed information on channel characteristics associated with 131 dams, including stream gradient, along with annual surveys of sediment and woody debris has permitted spatial analysis of controls on woody debris export (Seo et al., 2008;Fremier et al., 2009;Seo and Nakamura, 2009). ...
Predicting changes downstream of a dam to a river's planform, hydraulic geometry, grain size distribution, and slope represents both a real world management problem and a fundamental test of geomorphic science. We consider the current state-of-science for predicting downstream geomorphic response, and whether the distinctive aspects of gravel-bed rivers might constrain the wider range of potential responses to impoundment, improving quantitative prediction. Analytical approaches based on how dams change flow regimes, transport competence, and capacity clearly improve estimates of the direction, trend, and magnitude of downstream channel adjustments, including predictions of direction and magnitude of bed incision, channel armoring, channel width, and changes at tributary junctions. Bed incision on most gravel-bed rivers below dams is modest, typically on the order of several metres or less. Mixed grain-sizes typical of gravel-bed streams create the opportunity for textural adjustments that would not be present if the grain-size distribution were more homogeneous. Predicting the timescale over which armoring develops, resulting grain size of the bed, and longitudinal extent of armoring below dams are less certain. New approaches to evaluating channel changes at tributary junctions below dams based on the calibre of the introduced sediment and ratio of mainstem to tributary competence also look promising. Although predicting downstream response of rivers to impoundment remains a challenge, these new tools provide a strong foundation for moving forecasts of fluvial response out of the realm of simple conceptual approaches, and toward more technically defensible predictive models.
Mobilization of large wood in river channels during floods represents a hazardous factor, augmenting flood risk and endangering infrastructures such as bridges, weirs, and reservoir dams. A hybrid modeling approach combining numerical models with field‐based surveys has been recently used to elucidate the processes of LW entrainment and deposition in rivers. We used two‐dimensional hydraulic modeling performed in HEC‐RAS to simulate LW entrainment in two valley bottom segments of the Dyje River, Czechia, where LW deposition is a significant hazard to the dam of the downstream Znojmo reservoir. We surveyed all LW pieces in the inundation area of the 2002 extreme (>Q100) flood and simulated their entrainment for eight flood scenarios (1–100‐year recurrence interval). We used the equations of Braudrick and Grant (2000, https://doi.org/10.1029/1999WR900290) to calculate the LW entrainment threshold; we introduced coefficient k accounting for the incomplete submersion of LW pieces resting on an inclined surface into the original equations. Four entrainment categories — stable, wetted stable, wetted buoyant, and wetted entrained — were defined, and the proportion of LW pieces in each category was calculated for the flood scenarios. We found marked differences in entrainment categories for respective flood scenarios between the two valley segments. These were attributed to the differences in pieces' dimensions, their spatial distribution within the inundation area, and valley‐bottom topography, which affects the hydraulic conditions for a given discharge. The presented approach enables the calculation of the LW quantity potentially mobilized by the flood of a certain magnitude indicating the degree of potential risk for the infrastructures located downstream.
Floating matter (FM) is a pivotal, albeit neglected, element along river corridors contributing to their ecological integrity. FM consists of particulate matter of natural (e.g. wood, branches, leaves, seeds) and anthropogenic (e.g. plastic, human waste) origin as well as of organisms that, due to its properties, is able to float on the water surface. In this paper, we provide a comprehensive overview of the FM cycle and the fundamental environmental functions FM provides along rivers. Indeed, FM serves as an important geomorphological agent, a dispersal vector for animals and plant propagules, a habitat, a resource, and a biogeochemical component. Furthermore, we collected data on the amount of FM accumulating at dams and in reservoirs, and related it to key characteristics of the respective catchments. River fragmentation truncates the natural dynamics of FM through its extraction at damming structures, alteration in the flow regime, and low morphological complexity, which may decrease FM retention. Finally, we identify key knowledge gaps in relation to the role FM plays in supporting river integrity, and briefly discuss FM management strategies.
In order to identify the factors related to runoff of woody debris, we analyzed the relationship between woody debris volume and some parameters which are basin area and inflow discharge by using the data of woody debris volume deposited annually into 1103 dam reservoir across Japan. After quality check of these data, we applied single regression analysis. The results showed that woody debris volume increased logarithmically with inflow discharge. To find further relationship between woody debris volume and other parameters, we applied multiple regression analysis. It showed that woody debris volume is related to forest area, inflow discharge and 5 year daily rainfall.
Large wood is an important physical component of woodland rivers and significantly influences river morphology. It is also a key component of stream ecosystems. However, large wood is also a source of risk for human activities as it may damage infrastructure, block river channels, and induce flooding. Therefore, the analysis and quantification of large wood and its mobility is crucial for understanding and managing wood in rivers. As the amount of large-wood-related studies by researchers, river managers, and stakeholders increases, documentation of commonly used and newly available techniques and their effectiveness has also become increasingly relevant as well. Important data and knowledge has been obtained from the application of very different approaches and has generated a significant body of valuable information representative of different environments. This review brings a comprehensive qualitative and quantitative summary of recent advances regarding the different processes involved in large wood dynamics in fluvial systems including wood budgeting and wood mechanics. First, some key definitions and concepts are introduced. Second, advances in quantifying large wood dynamics are reviewed, in particular how measurements and modeling can be combined to integrate our understanding of how large wood moves through and is retained within river systems. Throughout, we present a quantitative and integrated meta-analysis compiled from different studies and geographical regions. Finally, we conclude by highlighting areas of particular research importance and their likely future trajectories, and we consider a particularly under-researched area so as to stress the future challenges for large wood research.
In‐channel large woody debris (LWD) promotes quality aquatic habitat through sediment sorting, pool scouring and in‐stream nutrient retention and transport. LWD recruitment occurs by numerous ecological and geomorphic mechanisms including channel migration, mass wasting and natural tree fall, yet LWD sourcing on the watershed scale remains poorly constrained. We developed a rapid and spatially extensive method for using light detection and ranging data to do the following: (i) estimate tree height and recruitable tree abundance throughout a watershed; (ii) determine the likelihood for the stream to recruit channel‐spanning trees at reach scales and assess whether mass wasting or channel migration is a dominant recruitment mechanism; and (iii) understand the contemporary and future distribution of LWD at a watershed scale. We utilized this method on the 78‐km‐long Narraguagus River in coastal Maine and found that potential channel‐spanning LWD composes approximately 6% of the valley area over the course of the river and is concentrated in spatially discrete reaches along the stream, with 5 km of the river valley accounting for 50% of the total potential LWD found in the system. We also determined that 83% of all potential LWD is located on valley sides, as opposed to 17% on floodplain and terrace surfaces. Approximately 3% of channel‐spanning vegetation along the river is located within one channel width of the stream. By examining topographic and morphologic variables (valley width, channel sinuosity, valley‐ side slope) over the length of the stream, we evaluated the dominant recruitment processes along the river and often found a spatial disconnect between the location of potential channel‐spanning LWD and recruitment mechanisms, which likely explains the low levels of LWD currently found in the system. This rapid method for identification of LWD sources is extendable to other basins and may prove valuable in locating future restoration projects aimed at increasing habitat quality through wood additions.
Wood is recruited to rivers by a diversity of processes, including chronic mortality, windstorms, wildfires, bank erosion, landslides, and ice storms. Recruitment, storage, and transport of large wood in streams can be understood in terms of a mass balance, or quantitative wood budget, similar to the study of other material fluxes in watersheds. A wood budgeting framework is presented that includes numerical expressions for punctuated forest mortality by fire, chronic mortality and tree fall, bank erosion, mass wasting, decay, and stream transport. When used with appropriate parameter values derived for specific conditions or regions, the wood budget equations can be used to make predictions on the importance of various landscape processes on wood abundance in streams in any locale. For example, wood budgets can be used to predict how variations in climate (wet - dry), topography (steep - gentle), basin size (small - large), and land management could affect abundance and distribution of large wood in streams. Wood budgets also can be integrated into numerical simulation models for estimating the natural range of variability, specifically temporal fluctuations of wood supply driven by large storms, floods, fires, and mass wasting, and spatial variability driven by topographic heterogeneity and variations in wood transport. Field studies of wood in streams may be enhanced by the use of a wood budget framework. This includes specifying what measurements are required over what length of stream for estimating recruitment rates of all relevant inputs processes, wood loss by decay, and stream transport of wood. Finally, wood budgets can be used to estimate rates of bank erosion, forest mortality, and landsliding, given appropriate field measurements of wood in streams and riparian conditions.
Coarse woody debris (CWD) is an important component of temperate stream and forest ecosystems. This chapter reviews the rates at which CWD is added and removed from ecosystems, the biomass found in streams and forests, and many functions that CWD serves. CWD is added to ecosystems by numerous mechanisms, including wind, fire, insect attack, pathogens, competition, and geomorphic processes. Despite the many long-term studies on tree mortality, there are few published rates of CWD input on mass-area-1 time-1 basis. CWD is significantly transformed physically and chemically. Movement of CWD, especially in streams, is also an important but poorly documented mechanism whereby CWD is lost from ecosystems. Many factors control the rate at which CWD decomposes, including temperature, moisture, internal gas composition of CWD, substrate quality, size of CWD, and types of organisms involved. However, the importance of many of these factors has yet to be established in field experiments. CWD performs many functions in ecosystems, serving as autotrophic and heterotrophic habitat and strongly influencing geomorphic processes, especially in streams. It is also a major component of nutrient cycles in many ecosystems and is an important functional component of stream and forest ecosystems.
MOUNTAIN river networks often consist of both bedrock and alluvial
channels1-5, the spatial distribution of which controls
several fundamental geomorphological and ecological
processes6,7. The nature of river channels can influence the
rates of river incision and landscape evolution1,2, as well
as the stream habitat characteristics affecting species abundance and
aquatic ecosystem structure8-11. Studies of the factors
controlling the distribution of bedrock and alluvial channels have
hitherto been limited to anthropogenic badlands12. Here we
investigate the distribution of channel types in forested mountain
drainage basins, and show that the occurrence of bedrock and alluvial
channels can be described by a threshold model relating local sediment
transport capacity to sediment supply. In addition, we find that
valley-spanning log jams create alluvial channels- hospitable to aquatic
life-in what would otherwise be bedrock reaches. The formation of such
jams depends critically on the stabilizing presence of logs derived from
the largest trees in the riverside forests, suggesting that management
strategies that allow harvesting of such trees can have a devastating
influence on alluvial habitats in mountain drainage basins.
This study examined the temporal dynamics and longitudinal distribution of wood over a multi-decadal timescale at the river reach scale (36 km) and a meander bend scale (300–600 m) in the Ain River, a large gravel-bed river flowing through a forested corridor, and adjusting to regulation and floodplain land-use change.
At the 36 km scale, more wood was recruited by bank erosion in 1991–2000 than since the 1950s. The longitudinal distribution of accumulations was similar between 1989 and 1999, but in both years individual pieces occurred homogeneously throughout the reach, while jam distribution was localized, associated with large concave banks. A relationship between the mean number of pieces and the volume recruited by bank erosion (r2 = 0·97) indicated a spatial relationship between areas of wood production and storage.
Wood mass stored and produced and channel sinuosity increased from 1993 to 2004 at three meander bends. Sinuosity was related to wood mass recruited by bank erosion during the previous decade (r2 = 0·73) and both of these parameters were correlated to the mean mass of wood/plot (r2 = 0·98 and 0·69 respectively), appearing to control wood storage and delivery at the bend scale. This suggests a local origin of wood stored in channel, not input from upstream trapped by preferential sites.
The increase in wood since 1950 is a response to floodplain afforestation, to a change from braided to meandering channel pattern in response to regulation, and to recent large floods. We observed temporal stability of supply and depositional sectors over a decade (on a reach scale). Meander bends were major storage sites, trapping wood with concave banks, also delivering wood. These results, and the link between sinuosity and wood frequency, establish geomorphology as a dominant wood storage and recruitment control in large gravel-bed rivers. Copyright
A wood budget was constructed for the Game Creek basin (132 km) in southeast Alaska to identify spatial and temporal controls on the abundance and distribution of large woody debris (LWD). Field measurements of wood storage, size, and age were used to estimate volumetric rates of LWD recruitment and transport. Mortality recruitment did not follow a spatial pattern and ranged from 0.1 to 8.1 m·km·year (recruitment corresponded to forest mortality rates of 0.1–2.6% per year). Wood recruitment by bank erosion increased with increasing drainage area and ranged from 1 m·km·year at the smallest drainage areas to about 16 m·km·year at 60 km. Bank erosion recruitment exceeded the maximum mortality recruitment at a drainage area of approximately 20 km (about 10-m-wide channel). Recruitment from land-sliding was only locally significant. The contribution of fluvial transport (flux) to total LWD storage increased with drainage area to an asymptotic maximum of 50% at about 50 km (about 20-m-wide channel). Mean predicted transport distances for mobile LWD over the lifetime of individual pieces ranged from about 200 m in small, jam-rich streams to about 2,500 m in larger channels with fewer jams. Fluvial transport of LWD increased interjam spacing and jam size and decreased jam age with increasing distance downstream. Constructing LWD budgets at the watershed scale has numerous geomorphic and ecological implications, including identifying spatial controls on the abundance and diversity of aquatic habitats. In addition, information on LWD budgets may be useful for determining how and where to protect LWD sources to streams.
Transfer of large woody debris (>10 cm diameter) from old-growth Douglas-fir (Pseudotsugamenziesii (Mirbel) Franco) forests into five first-to fifth-order stream reaches (drainage areas of 0.1 to 60.5 km2) has ranged from 2.0 to 8.8 Mg•ha−1•year−1 in 7- to 9-year study periods. Amounts of large debris in these streams range from 230 to 750 Mg•ha−1 with generally lower values in larger channels. The addition of woody debris is widely scattered in time and space and comes mainly from single trees rooted away from the streambank. We infer that wind is a major agent for entry of wood into these streams. Downstream movement of debris is strongly related to length of individual pieces; most pieces that moved were shorter than bankfull width.
throughout the world, yet we know little about hydraulic thresholds for movement and transport of logs. We developed theoretical models of entrainment and performed flume experiments to examine thresholds for wood movement in streams. Both the model and the experiments indicate that log entrainment is primarily a function of the piece angle relative to flow direction, whether or not the log had a rootwad, the density of the log, and the piece diameter. Stability increased if the pieces had rootwads or were rotated parallel to flow. Although previously reported as the most important factor in piece stability, piece length did not significantly affect the threshold of movement in our experiments or our physically based model, for logs shorter than channel width. These physically based models offer a first-order approach to evaluating the stability of either naturally derived woody debris or material deliberately introduced to streams for various management objectives. Large woody debris is an integral component of forested, fluvial systems
1] The fluvial export of large woody debris (LWD) was monitored in 131 reservoirs throughout Japan. Published data on the fluvial export of dissolved and particulate organic carbon were used to estimate the contributions of LWD in carbon budgets. Of all variables tested, watershed area was most important in explaining LWD carbon (LWDC) export, followed by annual precipitation. LWDC export per unit area was relatively high in small watersheds, highest in intermediate-sized watersheds, and decreased in large watersheds. In small watersheds, a large proportion of LWD retained on narrow valley floors may fragment or decay and eventually be exported in forms other than LWD. In intermediate-sized watersheds, LWD supplied from upstream and recruited by bank erosion is consistently transported downstream. In large watersheds, LWD recruitment is limited and LWD transported from upstream is stored on large floodplains. These differences in LWD recruitment, retention and transport in watersheds of different sizes lead to the proportion of LWDC in organic carbon exports to be maximum in intermediate-sized watersheds and decline rapidly in large watersheds.
Hierarchical and branching river networks interact with dynamic watershed disturbances, such as fires, storms, and floods, to impose a spatial and temporal organization on the nonuniform distribution of riverine habitats, with consequences for biological diversity and productivity. Abrupt changes in water and sediment flux occur at channel confluences in river networks and trigger changes in channel and floodplain morphology. This observation, when taken in the context of a river network as a population of channels and their confluences, allows the development of testable predictions about how basin size, basin shape, drainage density, and network geometry interact to regulate the spatial distribution of physical diversity in channel and riparian attributes throughout a river basin. The spatial structure of river networks also regulates how stochastic watershed disturbances influence the morphology and ages of fluvial features found at confluences.
Wood load, channel parameters and valley parameters were surveyed in 50 contiguous stream segments each 25 m in length along 12 streams in the Colorado Front Range. Length and diameter of each piece of wood were measured, and the orientation of each piece was tallied as a ramp, buried, bridge or unattached. These data were then used to evaluate longitudinal patterns of wood distribution in forested headwater streams of the Colorado Front Range, and potential channel-, valley- and watershed-scale controls on these patterns. We hypothesized that (i) wood load decreases downstream, (ii) wood is non-randomly distributed at channel lengths of tens to hundreds of meters as a result of the presence of wood jams and (iii) the proportion of wood clustered into jams increases with drainage area as a result of downstream increases in relative capacity of a stream to transport wood introduced from the adjacent riparian zone and valley bottom. Results indicate a progressive downstream decrease in wood load within channels, and correlations between wood load and drainage area, elevation, channel width, bed gradient and total stream power. Results support the first and second hypotheses, but are inconclusive with respect to the third hypothesis. Wood is non-randomly distributed at lengths of tens to hundreds of meters, but the proportion of pieces in jams reaches a maximum at intermediate downstream distances within the study area. We use these results to propose a conceptual model illustrating downstream trends in wood within streams of the Colorado Front Range. Copyright © 2009 John Wiley & Sons, Ltd.
Amounts and structural characteristics of coarse woody debris (CWD) were examined in relation to stand age and site moisture condition in 196 Pseudotsuga menziesii stands, which ranged from 40-900 yr old and most, if not all, originating after fire. In a chronosequence from the Cascade Range, the amount of CWD followed a U-shaped pattern for stands <500 yr old, with moderate levels (92 Mg/ha) in stands <80 yr old, lowest levels (<50 Mg/ha) in stands 80-120 yr old, and highest levels (173 Mg/ha) in stands 400-500 yr old. After 500 yr the amounts of CWD declined to intermediate levels. In the S Coast Range, lowest levels (32 Mg/ha) of CWD were in the youngest stands. In the Cascade Range, levels of CWD inherited from preceding stands were highest in young stands and declined to near zero by 250 yr. The overall decay rate constant (k) for snags and logs in the Cascade Range, calculated directly from the chronosequence, was 0.029 yr-1. Volume and biomass of CWD differed significantly in old-growth stands (>200 yr old) among site moisture classes. Dry sites averaged 72 Mg/ha, moderate sites 137 Mg/ha, and moist sites 174 Mg/ha. The dynamics of CWD were modeled for 3 fire histories, each beginning with an initial fire in an old-growth stand but differing in number and severity of subsequent fires. All models exhibited low values of CWD between 80-200 yr. The lowest and most prolonged minimum in CWD during succession occurred when additional fires burned early in succession, which probably happened preceding many stands in the southern Coast Range. A steady-state condition in CWD may not be reached for >1000 yr. The nature and timing of disturbance play a key role in the dynamics of CWD in the region. -from Authors
Wood degradation in freshwater aquatic ecosystems differs from degradation in the terrestrial environment. Oxygen concentration in the interior of wet pieces of wood is not high enough to support fungi, the primary decomposing agents of wood in terrestrial environments. The primary decomposing agents of wood in streams and rivers are bacteria and actinomycetes limited to a thin layer on the surface of the piece. As a result, microbial decomposition of wood proceeds much more slowly in water, and changes in wood properties occur only on the surface. Fragmentation is an important degradation process in streams. Flowing water or abrasion from sediment removes wood softened by microbial action, and several types of invertebrates ingest or construct burrows in the softened, outer layer. Removing the outer layer exposes new surfaces to bacterial colonization. Pieces of wood more than 1,000 years old have been identified in rivers by using dendrochronological and radioisotope techniques, although most wood degrades within 100 years after it enters a stream. Nutrient content of wood is low relative to other types of organic matter. Wood abundance in streams is often an order of magnitude greater than that of other types of organic matter, however. Thus, wood does contain a significant proportion of the nutrients associated with organic matter. Wood also affects the nutrient dynamics of streams by controlling the rate of transport of materials downstream. Wood primarily affects nutrients being transported in a particulate form. Removing wood from a 200-m section of a headwater stream in New Hampshire resulted in a sevenfold increase in the transport of sediment and particulate organic matter the next year. The increase was the combined result of mobilizing stored particulate material and increasing the efficiency with which material in the channel was routed downstream. The effect of wood on material transport influences nutrient availability in many Pacific Northwest streams by retaining salmon carcasses, which contribute significant amounts of N and P to the systems where these fish spawn and die.
Disturbance relevant to dynamics of wood in rivers can take many forms. We consider effects of ecosystem disturbance related to wood in river systems in geographic settings, which include high-gradient, boulder-dominated streams, braided, gravel-bed streams, and low-gradient, sand-bed streams. Disturbances of forests affect delivery of wood to streams and rivers directly by causing wood input or wood removal and indirectly by limiting source material. Disturbance of the fluvial system, either the channel form or flow regime, alters the transport and standing crop of wood. Change in wood distribution by processes of deposition, transport, and removal can disturb riparian, benthic (streambed), hyporheic, and water-column habitat. Human actions, such as harvesting trees, building roads, and regulating water flow, can substantially alter the types, frequencies, spatial patterns, and severity of the natural disturbance regime. We summarize the current status of knowledge on these points and identify knowledge gaps in studies of wood in rivers within the context of ecological disturbance. Finally, we offer a framework for future work and management that integrates processes that shape the spatial and temporal dynamics of wood at a series of scales.
A large river, in relation to wood dynamics, has a width several times greater than the height of the trees in its riparian area. Large rivers undergo particular physical and biological processes related to wood that vary according to their condition (pristine or managed) and their locations in the landscape (upland area or downstream, tropical or temperate climates). In this chapter, several topics are developed to illustrate how large wood plays a significant role in the functioning of large rivers: production and transfer of wood, interactions between wood and channel geometry (accumulation sites, trapping efficiency, effects of wood on channel forms), and interactions between wood and human activities and management (effects of wood on fish resources, damages associated with the wood, restoration and maintenance strategies). Bank erosion, wind, beavers, floodplain forest clearing by overbank flow, meteorological events (snow, wind), and humans (dump areas, residuals of timber harvest) are the main vectors of wood introduction to large rivers. The amount of wood stored in the channel is usually low compared to what is introduced annually. Wood is mainly located at the edge of the floodplain (islands, concave banks, side channels), but differences exist in accumulation sites, according to the channel pattern and geometry. The effects of wood on island formation as well as its effects on meander cut-off have been observed but are variable from one river to another according to the size of the wood and the character of the floodplain. Fish abundance and diversity are influenced by wood structures, even in rivers with low amounts of wood. Wood obstructions and their associated geomorphic facies provide rearing sites and habitats for aquatic invertebrates. Because the amount of wood in large rivers is usually low, it rarely increases flooding risks locally. Wood affects navigation where unobstructed channels are favored; wood transport has then a cost because it must be removed. When the wood in rivers is enhancing fish resources, there is a need to define a balance between the ecological benefit of the wood and the need for removing it for safety purposes. In the 1980s, research was focused on habitat questions at a local spatial scale, linking hydraulic and geomorphic conditions, and characters of fish species. A research priority is now necessary to understand wood mobility and its downstream impacts.
Field surveys document the accumulation of large woody debris (LWD) into structurally distinctive jam types in the alluvial channel of the Queets River on the Olympic Peninsula of north west Washington. Calculations, field observations and historical evidence show that these jams can form stable structures controlling local channel hydraulics and providing refugia for riparian forest development over decades and possibly centuries. Distinctive spatial patterns of LWD, pools, bars and forested islands form in association with particular jam types. The deposition of 'key member' logs initiates the formation of stable bar apex and meander jams that alter the local flow hydraulics and thereby the spatial characteristics of scour and deposition leading to pool and bar formation. Historical evidence and the age structure of forest patches documents the temporal development of alluvial topography associated with these jam types. Bar apex jams, for example, are associated with a crescentic pool, an upstream arcuate bar and a downstream central bar that is the focus of forest patch development. Experimental and empirical studies in hydraulic engineering accurately predict channel scour associated with jams. Individual jams can be remarkably stable, providing long-term bank protection that creates local refugia for mature forest patches within a valley floor environment characterized by rapid channel migration and frequent disturbance. Processes controlling the formation, structure and stability of naturally occurring LWD jams are fundamental to the dynamics of forested river ecosystems and provide insights into the design of both habitat restoration structures and ecosystem-based watershed management.
Riparian systems epitomize heterogeneity. As transitional semiterrestrial areas influenced by water, they usually extend from the edges of water bodies to the edges of upland terraces. Riparian systems often exhibit strong biophysical gradients, which control energy and elemental fluxes, and are highly variable in time and space.These attributes contribute to substantial biodiversity, elevated biomass and productivity, and an array of habitats and refugia. Focusing on riparian systems of medium-sized floodplain rivers, we describe heterogeneity at multiple space and time scales, illustrate interactions among scales, and propose a conceptual model integrating major system components. We show how climatic and geologic processes shape an array of physical templates, describe how disturbances redistribute materials, and illustrate how soils and subsurface processes form and are sustained. Collectively, these processes strongly influence plant productivity and fluxes of channel-shaping large woody debris (LWD). Ultimately, riparian ecosystem function integrates climate (past and present), geologic materials and processes, soil development and attendant microbial transformations, subsurface characteristics, plant productivity, animal activities, and LWD—and the active, continuous and variable feedbacks between the individual components.
Geomorphological processes that commonly transport soil down hillslopes and sediment and woody debris through stream systems in steep, mountainous, forest landscapes can operate in sequence down gravitational flowpaths, forming a cascade of disturbance processes that alters stream and riparian ecosystems. The affected stream and riparian landscape can be viewed through time as a network containing a shifting mosaic of disturbance patches - linear zones of disturbance created by the cascading geomorphological processes. Ecological disturbances range in severity from effects of debris flows, which completely remove alluvium, riparian soil and vegetation along steep, narrow, low-order channels, to localized patches of trees toppled by floating logs along the margins of larger channels. Land-use practices can affect the cascade of geomorphological processes that function as disturbance agents by changing the frequency and spatial pattern of events and the quantity and size distribution of material moved. A characterization of the disturbance regime in a stream network has important implications for ecological analysis. The network structure of stream and riparian systems, for example, may lend resilience in response to major disturbances by providing widely distributed refuges. An understanding of disturbance regime is a foundation for designing management systems.
Floodplains of large alluvial rivers are often expansive and characterized by high volume hyporheic flow through lattice-like substrata, probably formed by glacial outwash or lateral migration of the river channel over long time periods. River water downwells into the floodplain at the upstream end; and, depending on bedrock geomorphology and other factors, groundwater from the unconfined aquifer upwells directly into the channel or into floodplain springbrooks at rates determined by head pressure of the water mass moving through the floodplain hydrologic system. These large scale (km3) hyporheic zones contain speciose food webs, including specialized insects with hypogean and epigean life history stages (amphibionts) and obligate groundwater species (stygobionts). Biogeochemical processes in the hyporheic zone may naturally load groundwaters with bioavailable solutes that appear to exert proximal controls on production and biodiversity of surface benthos and riparian vegetation. The effect is especially evident in floodplain springbrooks. Dynamic convergence of aquifer-riverine components adds physical heterogeneity and functional complexity to floodplain landscapes. Because reaches of aggraded alluvium and attendant ecotonal processes occur serially, like beads on a string, along the river continuum, we propose the concept of a hyporheic corridor in alluvial rivers. We expect predictable zonation of groundwater communities and other aquifer-riverine convergence properties within the corridor from headwaters to river mouth. The landscape-level significance and connectivity of processes along the hyporheic corridor must be better understood if river ecosystems, especially those involving large floodplain components, are to be protected and/or rehabilitated.
Everything VariesSignificanceGood and Bad HypothesesNull Hypothesesp ValuesInterpretationStatistical ModellingMaximum LikelihoodExperimental DesignThe Principle of Parsimony (Occam's Razor)Observation, Theory and ExperimentControlsReplication: It's the n's that Justify the MeansHow Many Replicates?PowerRandomizationStrong InferenceWeak InferenceHow Long to Go On?PseudoreplicationInitial ConditionsOrthogonal Designs and Non-orthogonal Observational Data
The flood pulse concept of Junk, Bayley and Sparks is a major contribution to our understanding of river–floodplain interactions and has become an important paradigm in lotic ecology. The concept is based mainly on large tropical lowland rivers. Floodplains may, however, develop in all geographical areas and at different locations along a river corridor. We extend this concept to temperate areas by including information derived from near-natural proglacial, headwater and lowland floodplains. Specific attention is directed to the role of temperature as a major determinant of floodplain ecology. Further attention is directed to the importance of expansion–contraction cycles occurring well below bankfull (‘flow pulse’ versus ‘flood pulse’). Selected examples are presented that highlight the complexity of expansion–contraction events and their consequences on habitat heterogeneity and functional processes. Habitat heterogeneity is mainly a product of shifting water sources, different flow paths and the relative importance of autogenic processes. In different floodplain systems, expansion may enhance habitat heterogeneity (e.g. glacial floodplain) or create homogeneity (e.g. Danubian floodplain). Further, the ecological consequences of episodic flow and flood pulses are discussed. Finally, a landscape approach is suggested in order to document expansion and contraction processes and to elucidate how these processes influence landscape heterogeneity and biodiversity patterns. Such a landscape-based ecosystem model can be applied to rigorously assess the ecological integrity of river–floodplain systems. Copyright © 2000 John Wiley & Sons, Ltd.
We propose that the Concept provides a framework for integrating predictable and observable biological features of lotic systems. Implications of the concept in the areas of structure, function, and stability of riverine ecosystems are discussed. -from Authors
Natural rates of input and depletion of large woody debris (LWD) in southeast Alaska streams were studied to provide a basis for managing streamside zones to maintain LWD for fish habitat after timber harvest. Debris was inventoried in a variety of stream types in undisturbed old-growth forest; 252 pieces of LWD were dated from the age of trees growing on them. Longevity of LWD was directly related to bole diameter: small LWD (10–30 cm in diameter) was less than 110 years old, whereas large LWD (>60 cm in diameter) was up to 226 years old. Assuming equilibrium between input and depletion of LWD in streams in old-growth forests and exponential decay of LWD, we calculated input and depletion rates from mean age of LWD. Input and depletion rates were inversely proportional to LWD diameter and ranged from 1%/year for large LWD in all stream types to 3%/year for small LWD in large, high-energy, bedrock-controlled streams. A model of changes in LWD after timber harvest (which accounted for depletion of LWD and input from second-growth forest) indicated that 90 years after clear-cut logging without a stream-side buffer strip large LWD would be reduced by 70% and recovery to prelogging levels would take more than 250 years. Because nearly all LWD is derived from within 30 m of the stream, the use of a 30-m wide, unlogged buffer strip along both sides of the stream during timber harvest should maintain LWD.
Publisher Summary
This chapter reviews the rates at which Coarse Woody Debris (CWD) is added and removed from ecosystems, the biomass found in streams and forests, and many functions that CWD serves. CWD is an important component of temperate stream and forest ecosystems and is added to the ecosystem by numerous mechanisms, including wind, fire, insect attack, pathogens, competition, and geomorphic processes. Many factors control the rate at which CWD decomposes, including temperature, moisture, the internal gas composition of CWD, substrate quality, the size of the CWD, and the types of organisms involved. The mass of CWD in an ecosystem ideally represents the balance between addition and loss. In reality, slow decomposition rates and erratic variations in input of CWD cause the CWD mass to deviate markedly from steady-state projections. Many differences correspond to forest type, with deciduous-dominated systems having generally lower biomass than conifer-dominated systems. Stream size also influences CWD mass in lotic ecosystems, while successional stage dramatically influences CWD mass in boat aquatic and terrestrial settings. This chapter reviews many of these functions and concludes that CWD is an important functional component of stream and forest ecosystems. Better scientific understanding of these functions and the natural factors influencing CWD dynamics should lead to more enlightened management practices.
Instream large woody debris (LWD) provides several critical functions in riverine ecosystems, including sediment and nutrient retention, salmonid habitat enhancement, and stable colonization sites for incipient floodplain vegetation. In this study, the size and species composition of LWD in the Queets River, Washington, USA, were examined and compared with the size and species composition of forest trees from which they originated, in order to determine a depletion rate for LWD in the active channel. Increment cores from instream LWD were crossdated against cores from riparian conifers to estimate the year each LWD piece was recruited to the river channel. Debris pieces that were decayed or otherwise incompetent to provide cores were dated using standard 14C techniques. Hardwood species (Alnus rubra, Populus trichocarpa,and Acer macrophyllum) were better represented among riparian forests than among instream LWD, and conifers (Picea sitchensis, Tsuga heterophylla, Pseudotsuga menziesii, and Thuja plicata) were better represented among LWD than in the adjacent riparian forest, suggesting that hardwoods were depleted from the channel faster than conifers. The depletion rate of coniferous LWD from the channel followed an exponential decay curve in which 80% of LWD pieces were ,50 yr old, although some pieces have remained for up to 1400 yr. Although most wood is depleted from the channel within 50 yr, some wood is apparently buried in the floodplain and exhumed centuries later by lateral channel migration. The calculated depletion constant of 0.030 is equivalent to a half-life of ;20 yr, meaning that virtually all of the wood will have disappeared within 50 yr. This rapid depletion suggests that harvesting large conifers from the riparian zones of large streams could have adverse impacts within three to five decades.
The distribution of coarse woody debris in a fifth-order Cascade Range (Oregon) stream system was examined from a geomorphic point of view. The number, volume, location, orientation, decay class, and pool formation roles of coarse woody debris were investigated. The processes of coarse woody debris production, transport, and storage, which vary with channel and valley floor geomorphology, are responsible for the pattern of coarse woody debris distribution on valley floors. Channel width and sinuosity are the main factors that control production, storage sites, and hydrologic effects of coarse woody debris. The amount of coarse woody debris and the number of pool-forming pieces are relatively high in wide, sinuous reaches, where a complex structure of floodplains and riparian forests develops in association with a braided channel pattern. These relations are transferable to other systems with similar relations of coarse woody debris piece length to channel width.
We compared the contribution of large wood from different sources and wood distributions among channel zones of influence in a relatively pristine fourth-order watershed in the central Coast Range of Oregon. Wood in the main stem of Cummins Creek was identified as coming from either (i) streamside sources immediately adjacent to the channel or (ii) upslope sources delivered by landslides or debris flows more than 90 m from the channel. About 65% of the number of pieces and 46% of the estimated volume of wood were from upslope sources. Streamside sources contributed about 35% of the number of pieces and 54% of the estimated volume of wood. The estimated mean volume of upslope-derived pieces was about one-third that of streamside-derived pieces. Upslope-derived pieces were located primarily in the middle stream reaches and in the zones of influence that had the most contact with the low-flow channel. Streamside-derived pieces were more evenly distributed among the examined reaches and were predominately in the influence zones that had the least contact with the low-flow channel. Our findings suggest that previous studies that examined only streamside sources of wood have limited applications when designing and evaluating riparian management approaches in landslide-prone areas. The failure to recognize the potential sources of wood from upslope areas is a possible reason for the decline of large wood in streams in the Pacific Northwest.
From headwaters to mouth, the physical variables within a river system present a continuous gradient of physical conditions. This gradient should elicit a series of responses within the constituent populations resulting in a continuum of biotic adjustments and consistent patterns of loading, transport, utilization, and storage of organic matter along the length of a river. Based on the energy equilibrium theory of fluvial geomorphologists, we hypothesize that the structural and functional characteristics of stream communities are adapted to conform to the most probable position or mean state of the physical system. We reason that producer and consumer communities characteristic of a given river reach become established in harmony with the dynamic physical conditions of the channel. In natural stream systems, biological communities can be characterized as forming a temporal continuum of synchronized species replacements. This continuous replacement functions to distribute the utilization of energy inputs over time. Thus, the biological system moves towards a balance between a tendency for efficient use of energy inputs through resource partitioning (food, substrate, etc.) and an opposing tendency for a uniform rate of energy processing throughout the year. We theorize that biological communities developed in natural streams assume processing strategies involving minimum energy loss. Downstream communities are fashioned to capitalize on upstream processing inefficiencies. Both the upstream inefficiency (leakage) and the downstream adjustments seem predictable. We propose that this River Continuum Concept provides a framework for integrating predictable and observable biological features of lotic systems. Implications of the concept in the areas of structure, function, and stability of riverine ecosystems are discussed.Key words: river continuum; stream ecosystems; ecosystem structure, function; resource partitioning; ecosystem stability; community succession; river zonation; stream geomorphology
We measured the carbon discharge from a small, forested watershed that is covered with deep snow in winter. Snowmelt and rainfall both cause high flows, and we compared their effect on carbon discharge characteristics. The particulate organic carbon (POC) concentration was stable (around 5 mgC/l) at times of low flow and increased rapidly during high flow periods. The maximum POC concentration exceeded 80 mgC/l during rainfall, but it did not exceed 20 mgC/l during snowmelt, even though the rate of water discharge was high for both events. The dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) concentrations fluctuated seasonally, and also temporarily during high flow events. The DIC concentration decreased from 4·0 to 1·4 mgC/l as the spring thaw progressed. It increased progressively to a high of 7 mg/l in August and September, and then gradually decreased again. The DOC increased rapidly from 2 to 4 mg/l at the beginning of the thaw, then plunged to 2 mg/l when the snowpack disappeared. It increased from July until October (>5 mgC/l) and then decreased once more. The DOC and DIC concentrations were not clearly synchronized during the snowmelt. During rainfall, the DIC concentration decreased in rising limbs of the hydrograph and increased in falling limbs, whereas the reverse was the case for the DOC concentration. In late November, however, both the DOC and DIC concentrations decreased with increasing water discharge. Values for the annual transport of water, POC, DIC, and DOC were 925 mm, 21 kgC/ha, 19 kgC/ha, and 33 kgC/ha, respectively. The fractions transported during the snowmelt, which spanned 62 days, were 69% for water, 54% for POC, 58% for DIC, and 65% for DOC. Excluding the snowmelt period, the fractions transported in the peak 15 days were 38% for water, 63% for POC, 30% for DIC and 46% for DOC. We discuss the effects of snowpack and of respiration by roots and heterotrophs on the carbon discharge patterns of the watershed.
A simple minimization problem yielding the ordinary sample quantiles in the location model is shown to generalize naturally to the linear model generating a new class of statistics we term "regression quantiles." The estimator which minimizes the sum of absolute residuals is an important special case. Some equivariance properties and the joint aymptotic distribution of regression quantiles are established. These results permit a natural generalization to the linear model of certain well-known robust estimators of location. Estimators are suggested, which have comparable efficiency to least squares for Gaussian linear models while substantially out-performing the least-squares estimator over a wide class of non-Gaussian error distributions.
Logs of standard dimensions from five species of trees were submerged in a stream to evaluate changes in strength and decomposition over a period of 5 years. Changes in structural properties occurred only for wood near the outer surface of the logs. Nearly all bark was removed from the logs within 12 months. Diameter loss for the five species ranged from 10.6 mm (western hemlock Tsuga heterophylla) to 21.8 mm (bigleaf maple Acer macrophyllum) after 5 years. Decreases in the density of surface wood for the five species ranged from 23% (red alder Alnus rubra) to 31% (western hemlock). Modulus of rupture, modulus of elasticity, and wood density did not change for wood more than 12 mm from the log surface for any of the species. Bigleaf maple exhibited the highest resistance to rupture, and western redcedar Thuja plicata exhibited the lowest. Western redcedar was also the most easily flexed. Microbial activity on the surface of the logs was highest at the start of the experiment and decreased rapidly with time of immersion. The two hardwood species (bigleaf maple and red alder) generally had higher levels of microbial activity than the conifer species (Douglas fir Pseudotsuga menzesii, western hemlock, western redcedar) from 12 months through 60 months of immersion. Differences in the rate of decomposition between conifer and hardwood logs were much less than in terrestrial environments. Our results suggest that hardwood logs can be used in stream enhancement projects where the wood will be submerged.
In second- to fifth-order streams that drain old-growth timber in western Washington, characteristics and function of woody debris changed in relation to stream size. Average diameter, length, and volume of pieces of wood increased as stream size increased, whereas the frequency of occurrence of woody debris decreased. In streams with channel widths less than 7 m, 40% of the pieces of debris were oriented perpendicularly to the axis of flow; in streams with channel widths over 7 m, more than 40% of the pieces were oriented downstream. The types of pools most commonly associated with pieces of wood changed from plunge pools in small streams (42%) to debris scour pools in larger systems (62%). Pool area was correlated with the volume of the piece of wood forming the pool in streams of all sizes. However, this relationship was most evident in larger channels. Nearly 40% of the pieces of wood in channels less than 7 m wide were associated with sediment accumulations. Less than 30% of the pieces retained sediment in channels from 7 to 10 m wide, and less than 20% retained sediment in channels greater than 10 m wide. Surface area of sediment accumulations and the volume of the piece of wood forming the accumulation were related in all streams, but the relationship was clearest in the larger channels. Accumulations of particulate organic matter associated with woody debris were more frequent in small streams but were larger in large streams. No relationship was observed between the volume of fine particulate organic matter accumulated by a piece of wood and the piece of wood's volume.
The annual fluvial export of large wood (LW) was monitored by local reservoir management offices in Japan. LW export per unit watershed area was relatively high in small watersheds, peaked in intermediate watersheds, and decreased in large watersheds. To explain these variations, we surveyed the amount of LW with respect to channel morphology in 78 segments (26 segments in each size class) in the Nukabira River, northern Japan. We examined the differences in LW dynamics, including its recruitment, transport, storage, and fragmentation and decay along the spectrum of watershed sizes. We found that a large proportion of LW produced by forest dynamics and hillslope processes was retained because of the narrower valley floors and lower stream power in small watersheds. The retained LW pieces may eventually be exported during debris flows. In intermediate watersheds, the volume of LW derived from hillslopes decreased substantially with reductions in the proportion of channel length bordered by hillslope margins, which potentially deliver large quantities of LW. Because these channels have lower wood piece length to channel width ratios and higher stream power, LW pieces can be transported downstream. During transport, LW pieces are further fragmented and can be more easily transported. Therefore, the fluvial export of LW is maximized in intermediate watersheds. Rivers in large watersheds, where the recruitment of LW is limited by the decreasing hillslope margins, cannot transport LW pieces because of their low stream power, and thus LW pieces accumulate at various storage sites. Although these stored LW pieces can be refloated and transported by subsequent flood events, they may also become trapped by obstacles such as logjams and standing trees on floodplains and in secondary channels, remaining there for decades and eventually decaying into fine organic particles. Thus, the fluvial export of LW pieces is low in large watersheds. Copyright © 2009 John Wiley & Sons, Ltd.
Effects of coarse woody debris (CWD) on channel morphology and sediment storage were investigated at five sites, representative of first-order to fifth-order streams. In the steep and bedrock-confined stream (first-second order), interaction between the channel and CWD was limited, except where breakage upon falling produced CWD pieces shorter than channel width. Channel widening, steepening and sediment storage associated with CWD were observed predominantly in third- to fifth-order streams. Variation in channel width and gradient was regulated by CWD. In the fifth-order stream, most of the CWD pieces derived from the riparian forest interacted directly with the channel without being suspended by sideslopes. In this system CWD promoted lateral channel migration, but sediment storage was temporary, with annual release and capture.
Woody debris is a structural element of river systems, which provides habitats for aquatic communities but may enhance flooding frequency and damage infrastructure. An abundant scientific literature highlights the role of the wood, particularly in the fields of ecology and geomorphology. In order to find a balance between woody debris preservation and reintroduction for ecological purposes, and the need for channel clearing for risk management, more research is now needed to understand woody debris delivery residence time and transport dynamics in regulated rivers.
This work uses reservoirs, which trap woody debris, to determine its geographical origin and temporal variability in relation to the flow regime. This approach is illustrated by data collected on the Genissiat dam on the upper Rhône river (France), which traps all the woody debris even during high floods.
The results show that the wood input increases with flood frequency but also depends on the position of the flood event in the hydrological series. The wood load is significantly high and removed from the reservoir when the flood recurrence attains one in 1.5 years. Qualitative description of the wood extracted from the reservoir indicates that 83% of the pieces, whatever the size, have a natural origin (broken, cut by beaver) and 17% has been cut by humans. The large pieces of wood (diameter >12.5 cm) more frequently have a human origin (40%).
Large pieces with roots are infrequent (10%) underlining the low contribution of bank erosion. Most of the pieces are strongly smoothed, without any branches, roots and bark confirming the effect of physical breakage of high energy rivers. Amongst 503 samples, 45% have a riparian origin (Populus, Fraxinus, Alnus). Copyright
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.
This research quantifies amounts and characteristics of woody debris in mountain channels of the Alps. All pieces of woody debris greater than 5 cm diameter and 0.3 m length have been surveyed along five channels of the Dolomites (Italian Alps) with channel slopes ranging from 0.04 to 0.26 (basin area 2.2–51 km2). During the survey, channels were divided into uniform reaches and the mean gradient, bankfull width and depth of flow were measured. Overall, 4900 woody debris pieces, single and jam-forming, were measured and classified with respect to several qualitative attributes, such as presence/absence of rootwads, orientation to flow, and position in the channel. Results show that very large variations in the volume of woody debris per unit area of the streambed occur even within single streams, and that inputs mainly result from slope instabilities adjacent to the channels. Amounts of woody debris are low compared to most previous data published from different world regions, but match with other mountain rivers of the Alps. Debris transport seems to be of minor importance compared to local inputs from the slopes, because reach characteristics are poorly correlated with the volume of woody debris in a reach. A strong negative correlation is, however, observed between basin area and average density of woody debris in the five channels, but debris removal in the larger channels likely affects such a trend. Finally, the frequency of log steps is inversely correlated to drainage area, and positively related to the density of debris.
The objectives of this study were: (1) to document spatial and temporal distributions of large woody debris (LWD) at watershed scales and investigate some of the controlling processes; and (2) to judge the potential for mapping LWD accumulations with airborne multispectral imagery. Field surveys were conducted on the Snake River, Soda Butte Creek, and Cache Creek in the Greater Yellowstone Ecosystem, USA. The amount of woody debris per kilometer is highest in 2nd order streams, widely variable in 3rd and 4th order streams, and relatively low in the 6th order system. Floods led to increases in woody debris in 2nd order streams. Floods redistributed the wood in 3rd and 4th order streams, removing it from the channel and stranding it on bars, but appeared to generate little change in the total amount of wood throughout the channel system. The movement of woody debris suggests a system that is the reverse of most sediment transport systems in mountains. In 1st and 2nd order tributaries, the wood is too large to be moved and the system is transport-limited, with floods introducing new material through undercutting, but not removing wood through downstream transport. In the intermediate 3rd and 4th order channels, the system displays characteristics of dynamic equilibrium, where the channel is able remove the debris at approximately the same rate that it is introduced. The spatial distribution and quantity of wood in 3rd and 4th order reaches varies widely, however, as wood is alternatively stranded on gravel bars or moved downstream during periods of bar mobilization. In the 6th order and larger channels, the system becomes supply-limited, where almost all material in the main stream can be transported out of the central channel by normal stream flows and deposition occurs primarily on banks or in eddy pool environments. Attempts to map woody debris with 1-m resolution digital four-band imagery were generally unsuccessful, primarily because the imagery could not distinguish the narrow logs within a pixel from the surrounding sand and gravel background and due to problems in precisely coregistering imagery and field maps.
We combine hydraulic modeling and field investigations of logjams to evaluate linkages between wood-mediated fluctuations in channel-bed-and water-surface elevations and the potential for lateral channel migration in forest rivers of Washington state. In the eleven unconfined rivers we investigated, logjams were associated with reduced channel gradient and bank height. Detailed river gauging and hydraulic modeling document significant increases in the water-surface elevation upstream of channel-spanning wood accumulations. Logjams initiated lateral channel migration by increasing bed-or water-surface elevations above adjacent banks. Because the potential for a channel to avulse and migrate across its floodplain increases with the size and volume of instream wood, the area of the valley bottom potentially occupied by a channel over a specified timeframe — the channel migration zone (CMZ) — is dependent on the state of riparian forests. The return of riparian forests afforded by current land management practices will increase the volume and caliber of wood entering Washington rivers to a degree unprecedented since widespread clearing of wood from forests and rivers nearly 150 years ago. A greater supply of wood from maturing riparian forests will increase the frequency and spatial extent of channel migration relative to observations from wood-poor channels in the period of post-European settlement. We propose conceptual guidelines for the delineation of the CMZs that include allowances for vertical fluctuations in channel elevation caused by accumulations of large woody debris.
Field surveys in the 724-km2 Queets river basin on the west slope of the Olympic Mountains in NW Washington reveal basin-wide patterns of distinctive wood debris (WD) accumulations that arise from different mechanisms of WD recruitment, hydraulic geometry, and physical characteristics of WD. Individual pieces of WD in an accumulation or jam can be separated into key, racked, and loose members. Ten types of WD accumulations are identified based on the mode of recruitment and the orientation of key, racked, and loose debris relative to the channel axis. Although some types of WD accumulation have few geomorphic effects, others form stable in-stream structures that influence alluvial morphology at both subreach- and reach-length scales ranging from less than 1 to greater than 10 channel widths. In the Queets river, stable accumulations of WD directly influence channel anabranching, planform geometry, flood plain topography, and establishment of long-term riparian refugia for old-growth forest development. The classification of wood debris accumulations in the Queets river basin is based on physical observations that offer a template potentially applicable to other forested mountain regions.
Floodplains are formed by a complex interaction of fluvial processes but their character and evolution is essentially the product of stream power and sediment character. The relation between a stream's ability to entrain and transport sediment and the erosional resistance of floodplain alluvium that forms the channel boundary provides the basis for a genetic classification of floodplains. Three classes are recognised: (1) high-energy non-cohesive; (2) medium-energy non-cohesive; and (3) low-energy cohesive floodplains. Thirteen derivative orders and suborders, ranging from confined, coarse-grained, non-cohesive floodplains in high-energy environments to unconfined fine-grained cohesive floodplains in low-energy environments, are defined on the basis of nine factors (mostly floodplain forming processes). These factors result in distinctive geomorphological features (such as scroll bars or extensive backswamps) that distinguish each floodplain type in terms of genesis and resulting morphology. Finally, it is proposed that, because floodplains are derivatives of the parent stream system, substantial environmental change will result in the predictable transformation of one floodplain type to another over time.
The importance of coarse wood to aquatic biota and stream channel structure is widely recognized, yet characterizations of large-scale patterns in coarse wood dimensions and loads are rare. To address these issues, we censused instream coarse wood (≥2 m long and ≥10 cm minimum diameter) and sampled riparian coarse wood and channel characteristics in and along 13 streams in western Montana. Instream coarse wood tended to be shorter but of larger diameter than riparian pieces, presumably because of fluvial processing. Instream coarse wood also displayed highly variable spatial patterns. Most segments lacked significant spatial correlation in coarse wood abundance in adjacent 50 m reaches and when present, coarse wood patch sizes (100–1200 m) were specific to particular streams. Estimation of instream and riparian piece dimensions within 25% of the mean required samples of 13–314 pieces, whereas estimation of wood loads instream segments required samples of 8–210 reaches (400–10 500 m). If these results are representative of other systems, few previous studies have used sample sizes adequate to characterize instream coarse wood loads.
Large woody debris (LWD) input, storage and distribution were studied along the Drôme River, a French Alpine river with an active shifting channel and a well-developed riparian forest. LWD input from the floodplain is low: 669.6 mg year−1 between 1948 and 1971 and 569.3 mg year−1 between 1971 and 1991. Based on estimates of average LWD mass per study plot, a range of 766–2122 mg year−1 of LWD were stored within the active channel (e.g., unvegetated bars and low-flow channel; 60 km course covering 492 ha). LWD accumulations are mainly observed on gravel bars at a limited number of preferential sites. They are relatively rare and are randomly distributed in the low-flow channel. Consequently, LWD location is mainly associated with the decrease of flow level in shallow sectors. In the low-flow channel, LWD stop-en-route is primarily caused by in-channel structures such as boulders or vegetated islets. General geomorphological factors (e.g., pattern, slope, etc.) provide less explanation of LWD distribution. Moreover, the residence time of LWD accumulations on the Drôme River is short (LWD storage=1.3–3.7 times the annual LWD input from the floodplain) and their morphogenic role is negligible: few of the accumulations are buried or characterized by vegetation shoots and associated pools.