Long‐term dynamics of large wood in old‐growth and second‐growth stream reaches in the Cascade Range of Oregon

Understanding large wood (LW; ≥1 m long and ≥10 cm in diameter) dynamics in rivers is critical for many disciplines including those assessing flood hazard and risk. However, our understanding of wood entrainment and deposition is still limited, mainly because of the lack of long‐term monitoring of wood‐related processes. The dataset presented here was obtained from more than 8 years of monitoring of 1,264 tagged wood pieces placed in 4 low‐order streams of the Chilean mountain ranges and was used to further our understanding of key factors controlling LW dynamics. We show that LW displacement lengths were longer during periods when peak‐flow water depths (Hmax) exceeded the bankfull stage (HBk) than in periods with Hmax ≤ HBkand that these differences were significantly higher for smaller wood pieces. LW length and length relative to channel dimensions were the main factors governing LW entrainment; LW displacement lengths were inversely related to the ratio of piece length to H15%(i.e., the level above which the flow remains for 15% of the time) and to the ratio of H15%to bankfull width. Unrooted logs and LW pieces located at the bankfull stage travelled significantly longer distances than logs with attached rootwads and those located in other positions within the bankfull channel. A few large logjams were broken during the period of observation, and in all occasions, LW from these broken logjams did not travel over longer distances than other pieces of LW moved in the same periods and in the same stream segments. Most importantly, our work reveals that LW dynamics tend to be concentrated within a few reaches in each stream and that reaches exhibiting high wood dynamics (extensive entrainment, deposition, or repositioning of LW) are significantly wider and less steep than less dynamic reaches.

Dynamics of large wood in an eastern U.S. mountain stream

The knowledge of large wood (LW) dynamics regarding factors controlling wood transport, preferential depositional sites and the timing and duration of wood flux, is crucial for the maintenance of a good ecological status of rivers and for the management of wood-related potential hazards. Besides field surveys, tracking experiments or physical modelling, numerical models represent an alternative and complementary approach to explore LW dynamics, to test hypotheses and to run scenarios. We used a 2D numerical model which simulates the transport of large wood together with flow dynamics. The model is able to predict and simulate wood transport and deposition, and reproduces interactions between wood, channel bed, floodplain surface and infrastructures. We applied this model combined with direct field observations to explore main factors controlling large wood dynamics in the Czarny Dunajec River in Poland. We simulated different types of logs under different flood magnitude scenarios (steady and unsteady flow conditions) to analyse wood transport, deposition and remobilization in two contrasting river morphologies. We summarized in this chapter the main outcomes from this work. Results illustrate that a wide range of quantitative information about LW transport and deposition can be obtained from the use of numerical modelling together with the proper assessment of inlet and boundary conditions and validation based on field data.

Comparison of piece length and dynamics of large wood in streams covered with coniferous and broadleaf forests using UAV

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

Factors controlling large-wood transport in a mountain river

Watershed controls on the export of large wood from stream corridors

<p>Large wood (LW), both as individual pieces and in accumulations (WJ), plays an important role in the morphology, hydrology, and ecology of rivers. However, LW dynamics in rivers affected by volcanic eruptions has been little studied. This study aims to investigate the changes of LW volumes along a segment of the Blanco-Este River (southern Chile) affected by the 2015 Calbuco volcanic eruption. The following research questions were addressed: a) what are the drivers that explain the spatial and temporal variability of the amount of LW along the river active channel? b) what is the level of connection between the potential source areas of wood and the channel? c) is it possible to infer a relationship between recruitment sources and floods, with fluctuations in the amount of wood along the channel? The study was conducted in two reaches, the upstream one more proximal to the volcano (hereinafter R1) and the downstream more distal from the volcano (R2). LW and WJ volume were calculated using the structure from motion (SfM) technique for several sampling campaigns performed between 2017 and 2020 using a drone. Data from a fluviometric station near the Blanco-Este River and time lapse camera records were used to interpret the dynamics of wood during floods. Finally, the stability of WJs was used to indirectly evaluate the mobility of LW in the study reaches. Results show that the amount of LW (n°/ha), WJ (n°/ha) and total wood volume (m<sup>3</sup>/ha) are considerably higher in R2 than in R1. In both reaches, the main recruitment source of LW to the channel is associated with erosions of the forested margins, but for R2 a tributary and erosions of old laharic deposits are also recruitment sources. LW volume in R1 did not vary much between campaigns (1.9-5.1 m<sup>3</sup>/ha) which would indicate that this reach is in an equilibrium condition of LW loading. Since the wood volume in R2 showed important variations between sampling campaigns (9.1-73.9 m<sup>3</sup>/ha), this reach does not seem to have reached this equilibrium condition yet. Results showed that there is no clear relationship between the wood fluctuations and the flood intensities (volume increases and decreases indistinctly associated to low or high peak flows), a fact confirmed from the time lapse cameras. However, wood supply appears, as might be expected, somehow controlled by floods, as well as wood transport. But, apparently, the floods competent to move logs are of lower magnitude than those generating bank erosions and subsequent wood recruitment. From the analyses of the drone images, it was observed that the stability of the WJs was very low in the Blanco-Este, which indicates a high LW mobility. A connection between the areas that supply LW to the river channel appears to occur during major flood events with sufficient competence to erode forested streambanks. The latter calls for the need to incorporate the analysis of longitudinal wood connectivity in channel studies. This study is part of the FONDECYT 1200079 project.</p>

This paper synthesizes information on the spatial and temporal dynamics of wood in small streams in the Pacific Northwest region of North America. The literature on this topic is somewhat confused due to a lack of an accepted definition of what constitutes small streams and what is the relative size of woody debris contained within the channel. This paper presents a matrix that defines woody debris relative to channel size and then discusses the components of a wood budget. Headwater streams are in close proximity to wood sources and, in steeplands, are often tightly constrained by steep hillslopes. Special consideration is given to ecosystem characteristics and to management practices that affect the wood dynamics in this context. Knowledge gaps and uncertainties that can be used to guide future research are identified. Very little is currently known about the role of mass wasting in wood recruitment and storage relative to other processes, such as bank erosion and mortality, in larger streams. Further, very little work has addressed the relative importance of different wood depletion processes, especially those associated with wood transport. The effect of other ecosystem variables on wood dynamics locally across a watershed (from valley bottom to mountaintop) and regionally across the landscape (from maritime to continental climates) is not addressed. Finally, the scientific community has only begun to deal with the effects of management practices on wood quantity, structure, and movement in small streams.

Large wood has a positive effect on channel morphodynamics and freshwater biodiversity of mountain streams. However, its presence can enhance the risks associated with extreme flood events in inhabited floodplains. This study reports on wood abundance, spatial distribution, recruitment and depositional mechanisms in three third-order basins of south-eastern Andes (Tres Arroyos, Rio Toro and Buena Esperanza). Major differences in large wood loads and quantity exist among the analyzed basins, due to different disturbance history and forest cover features. Marked logs in Buena Esperanza and Tres Arroyos were surveyed before and after floods. Ordinary events (recurrence interval < 2 years) moved only small and isolated logs (diameter < 0.25 m; length < 3 m) and for relatively short distances. The reported study basins are taken as paradigmatic cases to illustrate a management strategy for hazards associated with wood transport during major floods. Log removal and riparian vegetation cuts are not effective strategies because high-magnitude, infrequent events are able to recruit trees from hillslopes due to mass wasting processes. Flood-prone area should be preserved free from sensitive settlements. However, important localized infrastructures can be protected with specifically designed structural solutions. Their potential use in the context of the three analyzed Andean basins is discussed.

Abstract. Understanding and modelling the dynamics of large wood (LW) in rivers during flood events has spurred a great deal of research in recent years. However, few studies have documented the effect of high-magnitude flash floods on LW recruitment, transport and deposition. On 25 October 2011, the Magra river basin (north-western Italy) was hit by an intense rainstorm, with hourly rainfall rates up to 130 mm h−1 and event rain accumulations up to 540 mm in 8 h. Such large rainfall intensities originated flash floods in the main river channels and in several tributaries, causing severe damages and loss of lives. Numerous bridges were partly or fully clogged by LW jams. A post-flood survey was carried out along the channels of two catchments that were severely and similarly affected by this event, the Gravegnola (34.3 km2) and Pogliaschina (25.1 km2). The analysis highlighted a very relevant channel widening in many channel reaches, which was more marked in the Gravegnola basin due to highly erodible material forming the slopes adjacent to the fluvial corridor. Large wood recruitment rates were very high, up to 1270 m3 km−1, and most of it (70–80 %) was eroded from the floodplains as a consequence of channel-widening processes, while the rest came from hillslopes processes. Overall, drainage area and channel slope are the most relevant controlling variables in explaining the reach-scale variability of LW recruitment, whereas LW deposition appears to be more complex, as correlation analysis did not evidence any statistically significant relationship with the tested controlling variables. Indeed, in-channel LW displacement during the flood has been mostly limited by the presence of bridges, given the relatively large width attained by channels after the event.

We develop and test a conceptual model of wood dynamics in stream networks that considers legacies of forest management practices, floods, and debris flows. We combine an observational study of wood in 25 km of 2nd- through 5th-order streams in a steep, forested watershed of the western Cascade Range of Oregon with whole-network studies of forest cutting, roads, and geomorphic processes over the preceding 50 years. Statistical and simple mass balance analyses show that natural process and forest management effects on wood input, transport processes, and decomposition account for observed patterns of wood in the stream network. Forest practices reduced wood amounts throughout the network; in headwater streams these effects are fixed in stream segments bordered by cuts and roads, but in larger channels they are diffused along the channel by fluvial transport of wood. Landforms and roads limited delivery of wood by debris flows to mainstem channels. Network dynamics studies and watershed management plans should include spatial patterns of debris flow initiation and runout, flood redistribution, and reduction of wood in the network by forest cutting and intentional wood removal from channels on time scales of forest succession and recurrence of major floods.

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

Large wood promotes fundamental changes in river hydraulics and morphology, playing a relevant role in river ecology but also in flood hazard. Accurate predictions of large wood dynamics in terms of deposition patterns and travel distance are still lacking and only recently have numerical models been developed to this end. In this work we enhance the capabilities of the numerical model Iber‐Wood in reproducing large wood dynamics in shallow braided rivers and validate it by comparing simulations with the results of previous laboratory experiments. The flume experiments provide high‐resolution observations of wood travel distances and depositional patterns of wood. The comparison proves useful to improve the numerical simulation of (i) the interactions between wood pieces and the riverbed (e.g., when wood pieces are transported by dragging), (ii) wood pieces with roots, and (iii) the formation of wood jams (i.e., accumulations of greater than three wood pieces). A sensitivity analysis reveals the crucial role of bed topography, with limited effect played by drag and restitution coefficients. Taking advantage of a controlled environment with similar simplifying hypotheses, we combine the strengths of both physical and numerical modeling to explore the parameters that are most effective in controlling wood dynamics. We use the numerical model to explore the effect of unsteady flow conditions, with different wood supply input. The resulting wood depositional patterns, jam formation, and travel distances during floods may improve our understanding of some of the controls on biogeomorphic evolutionary trajectories of braided rivers.

Instream wood is a driver of geomorphic change in low-order streams, frequently altering morphodynamic processes. Instream wood is a frequently measured component of streams, yet it is a complex metric, responding to ecological and geomorphic forcings at a variety of scales. Here we seek to disentangle the relative importance of physical and biological processes that drive wood growth and delivery to streams across broad spatial extents. In so doing, we ask two primary questions: (1) is riparian vegetation a composite variable that captures the indirect effects of climate and disturbance on instream wood dynamics? (2) What are the direct and indirect relationships between geomorphic setting, vegetation, climate, disturbance, and instream wood dynamics? We measured riparian vegetation composition and wood frequency and volume at 720 headwater reaches within the American interior Pacific Northwest. We used ordination to identify relationships between vegetation and environmental attributes, and subsequently built a structural equation model to identify how climate and disturbance directly affect vegetation composition and how vegetation and geomorphic setting directly affect instream wood volume and frequency. We found that large wood volume and frequency are directly driven by vegetation composition and positively correlated to wildfire, elevation, stream gradient, and channel bankfull width. Indicator species at reaches with high volumes of wood were generally long-lived, conifer trees that persist for extended durations once delivered to stream habitats. Wood dynamics were also indirectly mediated by factors that shape vegetation: wildfire, precipitation, elevation, and temperature. We conclude that wood volume and frequency are driven by multiple interrelated climatic, geomorphic, and ecological variables. Vegetation composition and geomorphic setting directly mediate indirect relationships between landscape environmental processes and instream large wood. Where climate or geomorphic setting preclude tree establishment, reaches may remain naturally depauperate of instream wood unless wood is transported from elsewhere in the stream network.