Instream Wood Research Articles

Wood jam characteristics influence but do not fully explain wood jam morphologic functions

In-stream wood jams alter their surrounding channel morphology, thus setting riverscape morphology and ecosystem function. While wood jams clearly scour pools, retain sediment, and influence bank erosion, there is a lack of observational evidence of how wood jam characteristics themselves control morphologic effects. Here, I analyzed field observations of hundreds of wood jams to test the hypothesis that wood jam characteristics can predict local morphologic effects such as sediment retention, bar forcing, and pool scour. I found mixed support for this hypothesis: While jam characteristics such as porosity, channel blockage ratio, thalweg occupation, and having rootwads and multiple trunks significantly predicted morphologic effect occurrence and magnitude, they only explained a small portion of the variance in those morphologic effects. While wood jam characteristics are relevant in controlling their overall morphologic functions, those functions are both inherently variable and likely affected by numerous other factors, such as reach-scale topography, the sediment transport capacity to supply ratio, and interactions with surrounding wood and vegetation. Wood function restoration may be more effective if it targets reach-scale objectives across dynamic wood jam assemblages, instead of individual jams.

Influence of Adding Small Instream Wood on Fishes and Hydraulic Conditions in Channelized Agricultural Headwater Streams

Influence of Adding Small Instream Wood on Fishes and Hydraulic Conditions in Channelized Agricultural Headwater Streams

The effect of planting grass filter strips on fishes in agricultural headwater streams

Abstract Grass filter strips (i.e., herbaceous riparian buffers) are a widely used conservation practice in the United States to mitigate the impacts of agriculture on streams and rivers. Yet, only a limited amount of information is available on the effects of grass filter strips on stream fishes and instream habitat conditions and long‐term assessments are rare. The objective of this research was to document the long‐term effects of planting grass filter strips adjacent to channelized agricultural headwater streams on fish community structure and instream habitat. Fishes, instream habitat and riparian characteristics were sampled from three channelized agricultural headwater streams without grass filter strips, three channelized agricultural headwater streams with grass filter strips and two unchannelized streams having forested riparian habitats in central Ohio from spring of 2006 to autumn of 2015. Fish species richness, abundance, darter species richness, per cent darters, sunfish species richness, minnow species richness and per cent headwater fishes did not differ among riparian habitat types. Darter species richness and per cent darters differed annually. Mean water depth, velocity, wetted width, instream habitat diversity, dominant grain size and per cent instream wood did not differ among riparian habitat types. Mean water depth, water velocity, wetted width and instream habitat diversity exhibited annual differences. Additionally, mean riparian width, woody vegetation density and per cent canopy cover were greater in forested streams than the unplanted and grass filter strip streams. Riparian characteristics did not differ annually. The lack of fish community responses 10–13 years after planting grass filter strips is probably due to the lack of corresponding changes in hydrological characteristics, substrate and instream wood amounts. These results suggest that grass filter strips should not be implemented alone but in conjunction with conservation practices that will improve physical habitat quality and water quality.

Current progress in quantifying and monitoring instream large wood supply and transfer in rivers

AbstractLarge wood drives both the form and function of gravel‐bed rivers draining forested basins. Previously overlooked benefits of wood in rivers are now widely recognized. Together with flow and sediment regimes, the wood regime controls rivers' physical and ecological integrity. Yet large quantities of wood transported during floods can pose additional hazards, potentially damaging infrastructures like bridges or dams and exacerbating flooding. However, unlike the water and sediment regimes intensively studied over the past decades, the instream wood regime or budgeting has been only recently defined and thus is still rarely quantified. The instream wood budget describes the cascading processes from supply or recruitment, entrainment, and transport to deposition, storage and decay (i.e., fragmentation or decomposition). These processes show high spatial and temporal variability but can be characterized by magnitude, frequency, timing, duration and mode. Instream wood budgeting is challenging, primarily because of the lack of observations, monitoring stations, and standardized protocols to acquire data. This contribution reviews the most recent advances to quantify the different instream wood budget components, notably the wood supply, and transfer. Case studies showing applications of biogeochemistry, videography, artificial intelligence, numerical modelling or tracking illustrate the current progress. Because critical challenges remain, we identify and describe some of them and discuss how the wood in riverine sciences may develop in the future.

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

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

Wood Increases Greenhouse Gas Emissions and Nitrate Removal Rates in River Sediments: Risks and Opportunities for Instream Wood Restoration

Wood Increases Greenhouse Gas Emissions and Nitrate Removal Rates in River Sediments: Risks and Opportunities for Instream Wood Restoration

Geospatial modelling of large-wood supply to rivers: a state-of-the-art model comparison in Swiss mountain river catchments

Abstract. Different models have been used in science and practice to identify instream large-wood (LW) sources and to estimate LW supply to rivers. This contribution reviews the existing models proposed in the last 35 years and compares two of the most recent geographic information system (GIS)-based models by applying them to 40 catchments in Switzerland. Both models, which we call here the empirical GIS approach (EGA) and fuzzy-logic GIS approach (FGA), consider landslides, debris flows, bank erosion, and mobilization of instream wood as recruitment processes and compute volumetric estimates of LW supply based on three different scenarios of process frequency and magnitude. Despite being developed following similar concepts and fed with similar input data, the results from the two models differ markedly. In general, estimated supply wood volumes were larger in each of the scenarios when computed with the FGA and lower with the EGA models. Landslides were the dominant process identified by the EGA, whereas bank erosion was the predominant process according to the FGA model. These differences are discussed, and results are compared to available observations coming from a unique database. Regardless of the limitations of these models, they are useful tools for hazard assessment, the design of infrastructure, and other management strategies.

A large-scale field experiment across six rivers illustrates how the effects of resource enrichment are context dependent

Resource supplementation can increase species richness and change the faunal composition of communities, but experiments have produced variable outcomes. An often overlooked element is that species richness can only increase if new taxa can disperse to resource-rich locations and invade established, local communities. We experimentally increased a basal resource (detritus) in six rivers in south-eastern Australia by driving wooden stakes into the riverbed to increase retention of detritus. Control sites were left untreated. Sites were located in agricultural sections with mostly cleared vegetation, but with intact (uncleared) reference sites upstream to provide sources of prospective colonists. We measured channel retentiveness and sampled benthic detritus and invertebrates before and after manipulation. We tested whether: greater retentiveness increased detritus densities, species richness and abundances and altered faunal composition; manipulation sites reached bio-equivalence with reference sites; new species arose from upstream reference areas; and whether outcomes were consistent across rivers. Only three rivers gained increases in detritus densities. All had low pre-existing amounts of in-stream wood compared with rivers that did not respond to treatment. Two rivers (Hughes Creek, Seven Creeks) gained higher species richness and invertebrate densities within 12 months and reached bio-equivalence with reference sites. In contrast, Turtons Creek showed species turnover through replacement of individuals. Only in Hughes Creek was there evidence of successful dispersal from the upstream reference area. The outcomes show that the effects of resource supplementation vary between rivers and suggest that pre-existing conditions (e.g. channel retentiveness) may cause these differences, providing clear evidence of context dependence.

An automated approach to detecting instream wood using airborne laser scanning in small coastal streams

Instream wood is a critical component of proper aquatic ecosystem function. The accurate detection and mapping of instream wood is of key importance for sustainable forest management due to the impact that instream wood features have on stream morphology, sediment distribution, and habitat availability for numerous aquatic species. The increasing availability of Airborne Laser Scanning (ALS) data allows for the development of methods to automatically detect and map instream wood. Herein, we develop and test a novel framework to map instream wood using ALS in coastal forested watersheds, and investigate the effects of wood properties and riparian vegetation characteristics on the accuracy of instream wood detection. Our focus sites are the Artlish and Nahmint watersheds on Vancouver Island, British Columbia, Canada. The location, length, width, submerged depth, and position relative to stream banks of instream wood were measured in nine streams within the watersheds. The method has three key steps: point cloud filtering, skeletonization, and validation. Our method uses advanced ALS processing to filter point clouds based on point height, intensity, classification, and the linear relationship to neighbouring points. Results indicated that our method was able to delineate instream wood with moderate overall accuracy ranging from 37 to 87%, (and a mean of 63%). Logjams were detected with high overall accuracy (83%) and individual wood pieces with an average accuracy of 49%. We found that the percentage of ALS returns classified as ground and the submerged depth of the wood had a significant effect of the detection accuracy of instream wood (p < 0.05). The detected instream wood features could be used as inputs for fish habitat modeling and to assess how different management practices impact the distribution of these features.

Pool Formation and The Role Of Instream Wood In Small Streams In Predominantly Second-growth Forests.

Extensive and intensive forest harvesting during the 20th century often had, and continues to have, negative impacts on stream structure and ultimately fish. Loss of instream wood and pools has been identified as a pathway by which riparian forest management can negatively impacts salmonids. Passive riparian restoration (allowing forests and streams to recover through natural processes) through the use of riparian buffers when conducting forest harvests has often been employed to promote stream habitat recovery. In this study, we evaluate the effects of 25 years of passive restoration on instream wood and pool recovery in 74 reaches in small streams (1st through 3rd order) across three channel types (pool-riffle, step-pool, and cascade) in a predominantly second-growth forest. Gradient and stream size were the most important variables for determining pool density, while stream size was the most important variable for determining pool depth. Instream wood formed 39% of pools in pool-riffle, 30% of pools in step-pool and 25% of pools in cascade channels types. Instream wood volume had a significant (R2 = 0.261, P ≤ 0.001) positive relationship with pool volume in wood-formed pools. Dimensions of individual wood pieces were associated with their likelihood of pool formation: for each additional 1 meter length within the bankfull channel the odds of forming a pool increased by 25.6%, and for each additional centimeter in diameter the odds of pool formation increased by 2.0%. When compared with studies conducted in both second- and old-growth forests, we concluded that streams associated with second-growth forests hadve both reduced densities and sizes of instream wood and pools. Young forests are likely continuing to negatively impact stream habitat and fish populations. Because riparian zones in many forests are now dominated by relatively young forest stands, and given the timeline for recovery using passive restoration alone (e.g., 100 s of years in Pacific Coastal forests), active restoration could be explored to accelerate instream wood and pool recovery.

Aquatic invertebrate responses to riparian restoration and flow extremes in three degraded intermittent streams: An eight‐year field experiment

Abstract Worldwide, riparian zones of many intermittent rivers and ephemeral streams (IRES) have been severely degraded. However, compared with perennially flowing waters, there have been few restoration efforts (e.g., revegetation, livestock removal) to halt or reverse this degradation. Even rarer are assessments of whether desired responses occurred and, if so, their underlying mechanisms, especially where flow extremes such as drying and flooding in IRES might alter or delay ecological responses to restoration. In order to assess how aquatic invertebrate communities responded to riparian replanting and livestock removal in three degraded lowland IRES in southeastern Australia, we conducted an eight‐year experiment (2004–2013), sampling paired control (unrestored) and treatment (restored, between 2005 and 2007 across creeks) sites twice yearly before and after restoration. To evaluate likely mechanisms underlying invertebrate responses to riparian restoration, we used a range of structural (e.g., richness) and functional (e.g., trait‐based) metrics. We also monitored environmental variables predicted to be likely drivers of aquatic invertebrate responses such as macrophyte and canopy cover, water quality, instream organic matter and instream wood. The study coincided with the last few years of a decade‐long drought that broke with severe floods (in 2010–2012), so we opportunistically examined how these hydrological extremes affected invertebrate responses to riparian restoration. At all three sites, macrophyte and canopy cover, water quality, organic matter and instream wood were comparable at treatment and control sites throughout the eight‐year experiment. There were also no significant differences in structural or functional metrics to indicate aquatic invertebrate responses to restoration after 6–8 years. This lack of predicted invertebrate responses apparently reflected how severely the drought impeded the riparian restoration, delaying even short‐term responses. Pervasive effects of catchment‐scale degradation may have further diminished the predicted benefits of the reach‐scale restoration efforts. By contrast, invertebrate metrics responded strongly to flow extremes, especially the brief floods that occurred as drought broke. These responses were evident for functional groups and individual families (especially rheophilic ones), but were not detected by a commonly used integrative measure of stream condition. This highlights the importance of selecting appropriate indicators in restoration monitoring programs and recognising their variability in responsiveness to restoration versus flow extremes. IRES are abundant and, in many parts of the world, increasingly common. Understanding how their inherent hydrological extremes may delay, modify and even overwhelm predicted responses to riparian restoration is crucial to setting realistic goals for IRES in the Anthropocene. Furthermore, assessing the success of riparian restoration in IRES requires a long‐term view given the extended time frames over which responses may occur, especially when expectations are often based on results of riparian restoration of perennial streams and rivers.

WOOD PREDICTORS IN NEOTROPICAL STREAMS: ASSESSING THE EFFECTS OF REGIONAL AND LOCAL CONTROLS IN AMAZON AND CERRADO CATCHMENTS.

Large wood plays a critical role providing complex habitat structure in rivers and streams. The instream wood regime consists of wood recruitment, transport, retention, and decay in river corridors. In tropical streams, transport and decay are thought to be the dominant influences on the amount instream wood stored, and these are driven by upstream forest cover, as well as catchment hydroclimatic and geomorphic characteristics. Lack of studies of the tropical wood regime leave many uncertainties. Notably, the wood regimes in the neotropical Amazon and Cerrado biomes are not known, and rapidly changing land-use threatens efforts to understand their natural wood regime. We investigated predictors of instream wood in catchments of the Amazon and Cerrado subject to a wide range of agricultural land use to identify the critical factors controlling wood recruitment and load. Using the structural equation modelling technique, we disentangled the complex net of regional and local controls. Contrary to our expectations, local drivers, such as the relation between the piece size and channel dimensions, discharge, stream power and riparian forest were the most important predictors of instream wood. The amounts of wood found in these streams were primarily the result of the wood delivered by the local riparian forest and how much of that wood remains trapped. Therefore, the preservation of the forested riparian zones in Amazon and Cerrado streams is crucial for maintaining the sources of wood as well as the channel morphology capable of trapping and retaining instream wood. Further research should compare reference and disturbed streams to quantify the influence of anthropogenic activities on instream wood and its primary influences. This information would facilitate assessing the extent of human alteration and developing mitigating measures to arrest or reverse changes that reduce instream wood and degrade aquatic and riparian habitat in neotropical rivers and streams.

Large Wood in Small Channels: A 20‐Year Study of Budgets and Piece Mobility in Two Redwood Streams

AbstractLarge wood (LW) influences geomorphic and ecological processes. However, most field data sets describing LW dynamics are conducted over the span of only a few years. We present a unique long‐term data set from northern California, USA of LW volumes, inputs, and transport in two small headwater streams, the North Fork (NF) and South Fork (SF) of Caspar Creek. We used data collected approximately every 2 years from 1998 to 2018 to assess how LW budgets change over time between catchments, what factors influence inputs from standing trees to the streams, and controls on LW piece mobility. We find that the SF, which experienced stream‐side logging and instream wood removal in the 1970s, continued to have lower LW volumes (range of 101–153 m3 km−1 over the 20‐year period) compared to the NF (range of 297–313 m3 km−1), which experienced logging in the 1980s but with buffer strips left near the channel. If current trends continue, it will likely take ∼100–200 years from the time of logging for the SF to reach NF instream wood volumes. We show that windstorms combined with precipitation events, very high winds, and hillslope steepness are strongly associated with LW piece inputs from standing trees. LW moves infrequently in the channel, with piece characteristics, the number of wood pieces per meter surrounding an LW piece, and discharge metrics influencing LW entrainment and displacement lengths. Our data set provides strong support to previously identified controls on LW dynamics in streams while also capturing variability over a 20‐year period.

Wood stock in neotropical streams: Quantifying and comparing instream wood among biomes and regions.

Instream wood plays important chemical, physical and ecological functions in aquatic systems, benefiting biota directly and indirectly. However, human activities along river corridors have disrupted wood recruitment and retention, usually leading to reductions in the amount of instream wood. In the tropics, where wood is believed to be more transient, the expansion of agriculture and infrastructure might be reducing instream wood stock even more than in the better studied temperate streams. However, research is needed to augment the small amount of information about wood in different biomes and ecosystems of neotropical streams. Here we present the first extensive assessment of instream wood loads and size distributions in streams of the wet-tropical Amazon and semi-humid-tropical Cerrado (the Brazilian savanna). We also compare neotropical wood stocks with those in temperate streams, first comparing against data from the literature, and then from a comparable dataset from temperate biomes in the USA. Contrary to our expectations, Amazon and Cerrado streams carried similar wood loads, which were lower than the world literature average, but similar to those found in comparable temperate forest and savanna streams in the USA. Our results indicate that the field survey methods and the wood metric adopted are highly important when comparing different datasets. But when properly compared, we found that most of the wood in temperate streams is made-up of a small number of large pieces, whereas wood in neotropical streams is made up of a larger number of small pieces that produce similar total volumes. The character of wood volumes among biomes is linked more to the delivery, transport and decomposition mechanisms than to the total number of pieces. Future studies should further investigate the potential instream wood drivers in neotropical catchments in order to better understand the differences and similarities here detected between biomes and climatic regions.

Trees and shrubs as components of the storage of coarse particulate organic matter and instream wood in Mediterranean intermittent streams

Coarse particulate organic matter (CPOM) and large instream wood (LW) belong to elements transported by stream flows, but their pattern of storage in nonperennial streams remains unclear, especially in the relationship with living trees and shrubs living in the active channels. This study evaluates the role of woody plants that occupy relatively wide active channels of nonperennial streams on the storage of CPOM and LW. All CPOM, LW and woody plants were inventoried along 18 transects in three mutually distinct reaches of intermittent streams in Peloponnese, Greece. We observed high variations of the CPOM (4.85 m3/ha - 15.65 m3/ha) and LW volumes (1.95 m3/ha - 13.35 m3/ha) deposited in the studied reaches with their highest abundance in the confined reach upstream of a landslide-related knickpoint. Woody plants were recognised as an important trapping element for CPOM in the case of a sufficient frequency of shrubs and trees in the active channel, which was not the case for the lowland reach with the far lowest measured basal area of the stems (1.84 m2/ha) in contrast to the other reaches (9.80 m2/ha and 6.32 m2/ha). A positive association was found between the basal area of the stems and the volume of CPOM deposits. However, the importance of woody plants in LW storage was confirmed only for the transects with abundant large trees with diameters greater than 0.1 m occupying the channel. The function of trees and shrubs not only as sources, but also as retention elements, should be carefully considered in the establishment of CPOM and LW budgets in nonperennial fluvial systems.

Associations between large wood and streambed complexity in headwater streams in the western Upper Peninsula, Michigan

Habitat complexity is an important feature of fluvial ecosystems, and in forested streams large wood can indirectly and directly contribute to complexity that supports biota. In this study of six forested streams of Michigan's western Upper Peninsula we quantified large instream wood and streambed morphology analyzed as variograms to describe the complexity of the streambed. Our goal was to determine the extent to which the streambed complexity could be explained by the volume and number of instream large wood objects. Based on previous studies in other systems, we expected a positive relationship between large wood volume and streambed complexity within the channel. Across the study sites, large wood volumes (standardized by area) ranged from approximately 0.0036 m3 m−2 to 0.029 m3 m−2 and the sites aligned into three groups based on overall complexity in channel morphology as derived from variograms. However, variation in streambed complexity was not clearly associated with the volume of large wood present in the channels. In contrast to our hypothesis, the two sites with the highest substrate complexity did not also have the greatest wood volume. Complexity was inversely related to large wood abundance and the relationship between standardized wood volume and complexity also appeared to follow this trend. Our findings may be a result of the low stream power that likely occurs in these streams, as well as because wood in the study streams was generally smaller and in lower abundance than typically reported for other streams and rivers. We suspect that the smaller diameter wood measured in these actively managed forests of northern Michigan would have shorter lifespans (i.e., degrade more quickly) and weaker hydrological influence, and thus a less significant influence on streambed complexity.

Seizing resilience windows to foster passive recovery in the forest-water interface in Amazonian lands

Forest regeneration has increased in many tropical abandoned lands and current restoration commitments in this region aim to restore over 1,400,000 km2 of degraded land by 2030. Although regenerating forests recover biomass, biodiversity, and processes with time, the recovery trajectories may be uncertain due to past disturbances. Currently, there is a lack of knowledge to sustain the effectiveness of passive regeneration for the recovery of riparian forests and the adjacent waterbodies in the tropics, which may compromise the outcomes of ongoing and future tropical riparian restoration programs. We evaluated the drivers of riparian forest structural recovery and how this relates to stream conditions in 12 abandoned pasturelands in eastern Brazilian Amazonia. These pasturelands range across regeneration age (pasture (PA) — 0 to 4 years; young regeneration (YR) — 8 to 12 years; old regeneration (OR) — 18 to 22 years) and years of past land-use (PA — 23.25 average years of past land-use, YR — 18.25, OR — 7). We compared the conditions of these sites to 4 reference sites with conserved forests (REF, >100 years), where there was no recorded pasture use in the past. Short-term responses of forests and streams to passive regeneration indicated high ecosystem resilience after low to intermediate past land-use intensity, reflected in the improvement of stream ecosystems. Such high resilience is possibly attributable to low- to intermediate-intensity pasture-related disturbances, remaining forest matrix, and residual structures (e.g. roots, sprouts, and in-stream wood) observed in the area. Our results suggest a recovery by 12 to 20 years for riparian forests of this region. However, areas degraded by intensive land-use apparently showed delayed recovery. We conclude that seizing resilience windows (defined here as the period when ecosystems retain high potential resilience) is essential to foster passive recovery of riparian forests and streams more cost-effectively in the tropics.

Characterizing stream morphological features important for fish habitat using airborne laser scanning data

Understanding changes in salmonid populations and their habitat is a critical issue given changing climate, their importance as a keystone species, and their cultural significance. Terrain features such as slope, gradient, and morphology, as well as forest structure attributes including canopy cover, height, and presence of on ground coarse wood, all influence the quality and quantity of salmonid habitat in forested ecosystems. The increasing availability of Airborne Laser Scanning (ALS) data for forest applications offers an opportunity to utilize these data for assessing the quality and quantity of habitat, which is often costly and difficult to characterize. ALS data provides detailed and accurate Digital Elevation Models (DEMs) under forest canopies, which in turn enable the characterization of detailed stream networks, as well as stream and terrain attributes important to salmonids. At the Nahmint watershed on Vancouver Island, British Columbia, Canada, we sampled six, 200 m long stream reaches, describing a range of terrain and stream features following standard data collection protocols. Our objective in this research was to use ALS data to estimate three attributes from the 3D point cloud and DEM that are known to be important for salmonids, including bankfull width,instream wood and discrete stream morphological units. Results indicate that ALS-based estimates had strong, significant, correlations with field-measured attributes (with Pearson's correlation of 0.80 and 0.81 for bankfull width and instream wood, respectively). Bankfull width was slightly underestimated using the ALS data (Bias = −1.01 m; MAD = 1.89 m; RMSD = 2.05 m) and 80% of instream wood pieces were detected. Using ALS-derived predictors in a Random Forest model, discrete stream morphological units (i.e. pools, riffles, glides, cascades) were classified with an overall accuracy of 85%, with pools having the highest user's class accuracy at 96%. Results presented herein indicate that ALS data can be used to provide a fine scale characterization of stream attributes that are required to identify salmonid habitat, providing critical information for sustainable forest management decision making, and providing a foundation for advanced salmonid habitat modeling.

Sampling-Based Approaches to Estimating Two-Dimensional Large Wood Area from UAS Imagery

Large in-stream wood (LW) is a critical component of riparian systems that increases heterogeneity of flow regimes and provides high habitat for salmonids and other fishes. We present four sampling-based methods to estimate two-dimensional LW for a 61-hectare river restoration project on the South Fork McKenzie River near Rainbow, OR (USA). We manually delineated LW area, from unoccupied aircraft systems (UAS) multispectral imagery for 40 randomly selected 51.46 m2 hexagonal plots. Seven auxiliary variables were extracted from the imagery and imagery derivatives to be incorporated in four estimators by summarizing spectral statistics for each plot including Random forest (RF) classification of segmented imagery (Cohen’s kappa = 0.75, balanced accuracy = 0.86). The four estimators were: difference estimator, simple linear regression estimator with one auxiliary variable, general regression estimator with seven auxiliary variables, and simple random sample without replacement. We assessed variance of the estimators and found that the simple random sample without replacement produced the largest estimate for LW area and widest confidence interval (17,283 m2, 95% CI 10,613 - 23,952 m2) while the generalized regression approach resulted in the smallest estimate and narrowest confidence interval (16,593 m2, 95% CI 13,054 - 20,133 m2). These methods facilitate efficient estimates of critical habitat components, that are especially suited to efforts that seek to quantify large amounts of these components through time. When combined with traditional sampling methods, classified imagery acquired via UAS promises to enhance the temporal resolution of the data products associated with restoration efforts while minimizing the necessity for potentially hazardous field work.

Local geomorphic effects in the presence of accumulations of different densities

Mobile components of dynamic river systems are varied, and more than fundamental water and sediment interactions must be considered to assess morphodynamic changes. Here, the effects of instream wood and plastic accumulations on the riverbed are modelled and assessed in the presence of a hydraulic structure. We observe a recent tendency towards considering instream wood and plastics as integral riverine system components. Such shift is driven by a multidisciplinary interest in instream wood and riverine plastic dynamics, including those addressing flood hazards and risk assessments. Like sediment, instream wood and plastics can be transported along the river channels during floods, increasing flooding risks and hazards by accumulating near hydraulic structures. Collection racks, a commonly encountered hydraulic structure, are used for trapping and removing various accumulated material, mitigating any potential risks. However, accumulations at collection racks can cause other risks because of potential backwater rise and local scour development. Accumulations of different densities, such as a combined presence of instream wood and plastics, add further complexity to the process understanding. In this study, a physical model is used to explore the geomorphic effects of accumulations of two different densities at a collection rack. Local geomorphic changes (i.e., scour and deposition), their evolution and the spatial extent around a vertical collection rack were analysed with Structure from Motion photogrammetry. The results show a strong relationship between the accumulation density and the hydraulic and geomorphic changes. Light floating material, forming accumulations closer to the water surface, are associated with the largest geomorphic changes, in contrast to dense material, forming accumulations close to the riverbed. In the presence of different densities, intermediate geomorphic changes were observed. Finally, to discuss the observations, we present a conceptual flow field for different scenarios of accumulation densities. Ultimately, outcomes from this work are essential to improve design practices for river managers.