Engineered Log Jams Research Articles

Understanding how restoration reduces competition for habitat by combining theory, observation, and experiment.

Habitat selection theory enables inferences about species habitat choice across a range of observed population densities. However, it is relatively uncommon to use habitat selection theory in studies of habitat restoration efficacy to understand the effect of restoration on habitat competition. We combined observational density data and resource selection functions to analyze habitat correlations with both habitat selection theory and a mark-recapture experiment to show how habitat restoration can mitigate competition between species with similar habitat preferences. To restore degraded and channelized riverine habitat for juvenile Chinook salmon (Oncorhynchus tshawytscha) and steelhead trout (Oncorhynchus mykiss) engineered log jams (ELJs) have been installed to create pools to enhance growth and rearing. Application of habitat selection theory first showed that both species share a preference for ELJ-treated habitat over unrestored habitat. Linear models showed that steelhead are generalists with respect to depth in unrestored habitat, whereas both species' abundance varies along a depth gradient in ELJ-treated habitat. Selective versus opportunistic use of deep and shallow ELJ pools was density-dependent. We found a range of densities at which a "ghost of competition" exists, where Chinook are selective on deep ELJ-treated pools and steelhead are selective on shallow pools. A mark-recapture experiment confirmed that steelhead limit Chinook movement into unrestored habitat, but this competitive effect vanished in ELJ-treated habitat where selection occurred with respect to pool depth. The experiment, combined with theory, enabled (1) the identification of a mechanism allowing for shared preference of restored habitat and (2) the description of how restoration can mitigate competition.

Influence of Channel‐Spanning Engineered Logjam Structures on Channel Hydrodynamics

AbstractNature‐based solutions to flood risk management, such as engineered logjams (ELJs), contribute to the reintroduction of wood in rivers. As part of stream restoration, and utilized in tributaries, ELJs increase upstream water levels, causing the flow to spill onto surrounding floodplains, resulting in the desynchronization of peak flows in a river network. To understand the effect of ELJs on local river hydrodynamics, we experimentally investigate the flow field upstream and downstream of six ELJs, using acoustic Doppler velocimetry and flow visualization. We consider channel‐spanning structures designed with a gap (b0) underneath, allowing unhindered baseflow. Our results revealed that upstream of the logjams, flow diverted toward the lower gap, creating a primary jet exiting underneath the structures, whose strength depends on the physical logjam design. Maximum jet velocities remained constant until a downstream distance of 4b0 for all logjams. The upper wake was structure‐dependent, with logjam structures allowing distinct internal flow paths generating secondary jets, which influenced near wake decay (x < 4b0) and turbulent mixing. The highest turbulence in the near wake was found for the non‐porous and short, porous logjam designs, while the upper wake of all long, porous logjams was characterized by low turbulent kinetic energy levels. Far wake decay (x > 4b0) was self‐similar for all logjams and resulted in near flow recovery at downstream streamwise distances greater than 35b0. ELJs are likely to enhance bed shear stress, increasing the risk of local scour and sediment mobilization. Our study expands the current knowledge of ELJ hydrodynamics and highlights potential implications for the riverine ecosystem.

“If You Build It…”: Methodological Approaches to Detect Postrestoration Responses in Stream Fishes

Habitat restoration to recover fish populations takes place worldwide, yet studies of its efficacy are fraught with challenges. One difficulty comes from studies of fish responses that are too focused on whether abundance has increased in the restored habitat, limiting the methodology to observational data of fish density. However, many other tools are available for evaluation of restoration. I review a set of conceptually based methods that colleagues and I used to assess restoration efficacy in a model river system to show the strength of considering alternative approaches. Although it was relatively easy to determine that fish are attracted to instream restoration structures (engineered log jams), spatial and temporal variability in the results requires approaches that more closely examine the distinction between increases in density and increases in habitat capacity. Furthermore, I show that efficacy research should be augmented with consideration of behavior, the spatial scale of the post‐restoration studies, the response of life history traits (e.g., growth), and the applicability of habitat selection theory. These detailed studies often, but not always, identify benefits of restoration that go beyond simple observations of fish abundance. They are therefore methods that are worth considering by researchers involved in postrestoration fish monitoring.

Effectiveness monitoring of juvenile Chinook salmon restoration projects in south Lake Washington, Washington State

Tabor RA, Liermann MC, Gearns HA, Moore ZJ, Lynch KD, Kurko K, Crittenden J, Shoemaker ME. 2022. Effectiveness monitoring of juvenile Chinook salmon restoration projects in south Lake Washington, Washington State. Lake Reserv Manage. 38:180–196. We evaluated 4 large restoration projects in south Lake Washington that were designed to improve nearshore habitat for lake-rearing juvenile Chinook salmon (Oncorhynchus tshawytscha). Restoration projects included shoreline modification, substrate enhancement, addition of engineered log jams (ELJs), and nonnatal tributary improvements. For most projects, we used a BACI (before-after-control-impact) study design. Projects were evaluated by determining the nighttime abundance of juvenile Chinook salmon through visual observations (primarily snorkeling with some surface observations in small nonnatal tributaries). Juvenile Chinook salmon are typically concentrated in the south end of the lake near their natal stream, and the project that was the farthest away from the natal stream did not appear to be beneficial, likely because few Chinook salmon were present. The highest observed abundance of juvenile Chinook salmon was associated with ELJs, where the abundance in February to April was typically 2 to 5 times higher than along adjacent, open shorelines. From January to April, juvenile Chinook salmon were usually more abundant in a daylighted nonnatal tributary (i.e., reconfigured from an unusable underground culvert to a more natural stream channel at the surface) than in 2 reference sites. Overall, all 4 project types improved juvenile use of the restored area, with substrate enhancement generally having the weakest response and ELJs the strongest response. Although most restoration projects for salmonids in the Pacific Northwest have focused on lotic systems, our results suggest projects in lentic systems may also be beneficial and should be considered by land-use managers. Supplemental data for this article is available online at https://doi.org/10.1080/10402381.2022.2027054 .

Development of a CFD model and procedure for flows through in-stream structures

Large wood (LW) has been widely used in river restoration projects due to its ecological benefit. 3D modeling has rarely been conducted. In this study, a 3D modeling procedure is proposed and a new 3D model is developed. The model incorporates advanced CFD techniques and is easy to apply. An engineered log jam (ELJ) is constructed and an experiment is conducted. The data are used to demonstrate the procedure and validate the CFD model. A comparison of the predicted and measured velocity shows that agreement is good with the Nash-Sutcliffe coefficient about 0.4 and index of agreement about 0.85. The model is further applied to a curved channel with ELJ on the outer bank, demonstrating the use of the 3D model to guide the proper representation of ELJs in a 2D model. The 3D model may be used to reveal complex flow patterns generated by LW for ecological benefit analysis.

Quantifying the hydrological implications of pre- and post-installation willowed engineered log jams in the Pennine Uplands, NW England

Nature Based Solutions (NBS), including Natural Flood Management (NFM) schemes are becoming an important component of many governmental and organisation responses to increases in flood and aridity risk. NFM structures may take multiple forms to slow, store, disconnect and filter distributed overland flow pathways within a catchment that coalesce to generate a flood-wave downstream and runoff rather than infiltrate groundwaters. To date few studies have conducted observations pre- and post-installation monitoring at river reach-scales, despite widespread and frequent installation, to investigate the efficacy of willowed engineered log jams (WELJs) interventions used in abating flood-flows, through backing-up flood-pulses with consequent reductions in downstream discharges. This paper examines the efficiency, before and after installation of five 1 m high WELJs incorporating 1,000 Bay willow (Salix pentandra) saplings supporting the dead horizontal timber, across a total of 130 linear metres spanning the floodplain of a decommissioned reservoir. One rain gauge, two fixed point time-lapse wildlife cameras and three water level stations were installed: upstream-of, within, and downstream-of all WELJs. The findings demonstrate a substantial reduction is achieved for most events, with an average of 27.3% reduction in peak discharge being achieved post-installation. The time to peak is little impacted, however there is demonstrable evidence of a longer and higher recessional limb to the events. These findings quantify for the first time the role that WELJs can play in a move towards re-naturalisation of water level regimes, with lower peak water flows achieved, and waters released from the river-reach more slowly. Furthermore, baseflow during dry periods is also elevated by 27.1%, offering greater resilience to dry periods and droughts. Consequently, over the river-reach scale (0–130 m), WELJs play an important role in alleviating flood and drought risk through suppressing flood peaks and increasing baseflow during low flows; steps towards improved hydro-morphological quality overall.

Riverine large woody debris introduced for natural flood management leads to rapid improvement in aquatic macroinvertebrate diversity

Natural flood management interventions, such as Large Wood Debris (LWD) or engineered log jams, are being increasingly deployed throughout the UK and elsewhere. In addition to alleviating flood risk, it is anticipated that they may influence the ecology of freshwater river systems, including macroinvertebrate populations. This study explores macroinvertebrate assemblages, water quality parameters, and sediment size distribution in a headwater stream following the addition of LWD as part of a natural flood management scheme. The study area consists of 6 sites within the intervention zone where LWD log jams were implemented, with comparative control sites upstream and downstream (3 sites each). Macroinvertebrate communities, sediment size distribution, and water chemistry were sampled 3 and 10 months following the addition of LWD. Our findings revealed increased macroinvertebrate abundance and taxa richness in the LWD intervention zone versus control, with an increased BMWP score reflecting the increased taxa richness. Average Score Per Taxon, and water chemistry showed no change, revealing invertebrate changes to be independent of water quality. NMDS and hierarchical clustering analysis on invertebrate data showed a clear separation of communities where LWD was present from those with no LWD, while SIMPER analysis showed that LWD addition led to the rapid establishment of taxa (Hydraenidae, Rhyacophilidae, Scirtidae, and Elmidae) that were otherwise absent. Ten months after LWD addition, improved biodiversity was also found in areas below the intervention zone, suggesting the positive impacts of LWD extend downstream. LWD also altered sediments, with sites immediately upstream of LWD log jams having a greater percentage of fine sediment than those immediately downstream. These results suggest that biological complexity and niche availability increased within the in-channel zone as a result of introduced LWD, thus revealing wider aquatic habitat improvement potential of LWD for natural flood management. The use of LWD as an intervention for flood management is recommended for its additional benefits for ecosystem health.

Flow and Scour around Idealized Porous Engineered Log Jam Structures

AbstractEngineered log jams (ELJs) are used widely in practice as a river restoration measure. One desired feature of an ELJ is its porous nature, which plays an important role in hydrodynamics and...

Effects of an engineered log jam on spatial variability of the flow field across submergence depths

AbstractEngineered log jams (ELJs) are commonly implemented in rivers to diversify fish habitat, but few studies have verified the hydraulic benefits of ELJs by assessing their effects on the spatial variability and turbulence structure of the flow field. This field and modelling study assessed the effects of an ELJ on various characteristics of the flow field with increasing submergence. A 2‐D hydrodynamic model was applied to simulate flow conditions of the Calapooia River, Oregon, with an ELJ installed at the upstream end of a meander. Output was analyzed using coherent flow structure identification, maps of turbulence measures, and wavelet analysis along linear transects. At the highest discharge (62% ELJ submergence), more and smaller coherent flow structures were identified than in the no ELJ model run, but the magnitudes of turbulence metrics were unaffected. At low flow (1% jam submergence), the presence of the ELJ did not affect the number of coherent flow structures identified and produced only small and localized effects on turbulence measures. Taken together, results indicate that the ELJ primarily affected the flow field by increasing the number of flow structures at higher discharges and submergence depths. However, these changes were small and local to the ELJ. The limited effects of this wood jam over limited spatial scales raise questions about the conditions under which such structures increase hydraulic diversity and highlight the need for more detailed understanding on spatial variability around habitat structures with complex shapes.

Effects of Different In-Stream Structure Representations in Computational Fluid Dynamics Models—Taking Engineered Log Jams (ELJ) as an Example

In-streamstructurescontributegreatlytothebiodiversityinstreamsandplayanimportant role in restoring and protecting rivers. They usually have complex geometries. To evaluate their impact and effectiveness, computational models are increasingly used. However, how to faithfully represent them in computer models remains a challenge. Often, simplifications have to be made. This work evaluated the effects of geometric simplification of an example in-stream structure, an engineered log jam (ELJ), in computational models. Three different representations were considered, namely full resolution, the porous media model and the solid barrier model. The turbulent flow was resolved with large eddy simulation (LES). First, the simulations were validated with a physical experiment in a flume. Then, the results from the three models were comparedandanalyzedonvariousaspectsrelatedtothestabilityandfunctionalitiesofthestructures. Unsurprisingly, it is found that the porous media model and the solid barrier model, which are computationally economic, can describe the flow dynamics only to some extent. From the calibration ofdragforceandwakelength,wefoundthattheequivalentgrainsized50 intheporositymodelshould scale as the key element diameter for the simulated ELJ. A wake length scale analysis was performed for the semi-bounded flow around this in-stream structure near the bank. The length estimator in the literature for unbounded vegetation patches can be used with modifications. The results also show that the flow passing through the porous in-stream structure has a significant impact on mean velocity, turbulence kinetic energy, sediment transport capacity and integral wake length. Since geometrically-fully-resolved simulations are not currently feasible for engineering practices, the following suggestions are made based on this study. If the near-field and wake are important for the purpose of the structure, the well-calibrated porosity model seems to perform better than the solid barrier model. However, care needs to be taken when interpreting the results because this work also identified substantial loss of physical information with the porosity model. When the emphasis is the far field away from the structure, both the porosity model and the solid barrier model give comparable results

An assessment of engineered log jam structures in response to a flood event in an upland gravel‐bed river

AbstractEngineered log jams (ELJs) are employed to address river restoration goals and a range of river management problems including coarse sediment movement. In the Bowmont Water, a dynamic wandering gravel‐bed river in the Scottish Borders, 33 previously untested ELJs primarily designed to capture and store coarse sediment, were installed on a trial basis. Using repeated topographical surveys and field observations, the performance of the ELJs in response to a 5–10 year recurrence interval flood that occurred on the 25 September 2012 was evaluated at two reaches with catchment areas of 28 km2 and 57 km2. Three of the structures were damaged due to scour of surrounding material that exposed the pile anchors and all the timbers of one structure were completely displaced downstream. Sixteen structures induced geomorphic responses and only four induced significant deposition (>0.3 m) above that which would occur naturally within the adjacent active gravel bar deposition zones. The placement in gravel bars, minor channel blockage ratio created by the structures and their porous nature limited the hydraulic interference and in turn geomorphic responses. Therefore the ELJ placement goal of increasing sediment storage was not fully met. This study contributes to the empirical evidence base for ELJ performance evaluation of different designs in a range of physiographic settings needed to validate performance and refine design. Using these initial findings and knowledge gained from other studies, recommendations for improving the design and placement strategy are proposed. Copyright © 2016 John Wiley & Sons, Ltd.

Flow, turbulence, and drag associated with engineered log jams in a fixed-bed experimental channel

Engineered log jams (ELJs) have become attractive alternatives for river restoration and bank stabilization programs. Yet the effects of ELJs on turbulent flow and the fluid forces acting on the ELJs are not well known, and such information could inform design criteria. In this study, a fixed-bed physical model was constructed to assess the introduction of ELJs along the Big Sioux River, SD. Two ELJ types were examined, referred to as ELJ-1 and ELJ-2. Both types were deflector jams, where ELJ-1 was rectangular and ELJ-2 was triangular, and oriented with one side attached to the channel bank. They were deployed either as single structures or in groups of two or three on the same side of the channel and at different separation distances. Results show that (1) time–mean and turbulent velocities and bed shear stresses were measurably altered near the ELJ, but spatially averaged flow just upstream and downstream of the structure was unaffected; (2) streamwise drag forces measured for the ELJs were significantly larger than the transverse forces, and the derived drag coefficients for the single structures were 2.72±0.19 for ELJ-1 and 1.60±0.37 for ELJ-2; and (3) the presence of an upstream structure created a near-bank wake region that extended a distance of more than 30 flow depths downstream, which greatly reduced drag forces and drag coefficients observed for the downstream structure by as much as 80%. These observations are further evidence of the efficacy of ELJs in providing near-structure scour pool development and bank protection downstream, and they can be used to inform and assess the design of ELJs for use in river restoration and bank stabilization projects.

Physical-scale model designs for engineered log jams in rivers

Stream restoration and river engineering projects are employing engineered log jams (ELJs) increasingly for stabilization and in-stream improvements. To advance the design of these structures and to evaluate their morphodynamic effects on corridors, the basis for physical-scale models of rivers with ELJs is presented and discussed. The prototype selected is the Big Sioux River, SD, chosen because ELJs will be used to mitigate excessive bank erosion. The underlying theory of physical-scale modeling and all primary and secondary scaling ratios are presented for two boundary conditions, a fixed- and movable-bed, given the experimental constraints of the intended facility. The scaling ratios for the movable-bed model sediment are relaxed, allowing for the use of typical experimental flows, facilities, and materials. Proposed ELJ designs are based on proven field installations, and these structures also are scaled to natural timber dimensions to be used in the prototype. Preliminary results from these physical models show that (1) ELJs greatly decelerate flow near the structure and accelerate flow in the main portion of the channel, yet spatially averaged flow velocity and depth remain unchanged, (2) derived drag coefficients for the ELJs based on force measurements vary from 0.3 to 0.7 depending on the scaling velocity employed, and (3) while significant localized erosion and deposition occurred in the vicinity of the ELJ, these effects extended well downstream of the structure and across the entire channel. Although physical experimentation using wood is not uncommon, the use of physical scaling theory appears to be employed infrequently, which potentially could limit the applicability of the results obtained. It is envisioned that the procedures outlined here would become more widely used in experimental research of rivers and in river restoration design.

Effects of configuration of instream wood on autumn and winter habitat use by fish in a large remeandering reach

This study examined the effects of the configuration of wood structures on the use of local autumn and winter habitats by fish in a remeandering reach of a large lowland river. Fish diversity was higher at the simple wood-structure (SWS) and the log-jam (LJ) sites than at the no-wood (NW) sites during both seasons. In particular, the diversity at the LJ sites was higher than that at the SWS sites during the winter. The abundance of the four dominant fish species was generally higher at the LJ sites than at the NW sites during both seasons. The SWS and LJ sites were characterized by greater depths, finer bed materials, and more diverse flow conditions during the autumn. During the winter, the LJ sites were characterized by slower currents and finer bed materials. These habitat features satisfied the various habitat requirements of the fish observed in our study reach; during the autumn, areas with fast and slow currents were favorable for juvenile salmon, silt and sand bed materials for lamprey, and deep areas for one species of goby, while slow currents were critical in the winter for abovementioned fish species. Engineered log jams were also effective in supporting the colonization of fish during both seasons. These findings suggest that a larger and more complex wood structure, particularly log jams, plays important roles in creating local habitats suitable for various fish species during the autumn and winter and in preserving fish abundance and diversity in large lowland rivers.

JUVENILE SALMON RESPONSE TO THE PLACEMENT OF ENGINEERED LOG JAMS (ELJS) IN THE ELWHA RIVER, WASHINGTON STATE, USA

ABSTRACTEngineered log jams (ELJs) are increasingly being used in large rivers to create fish habitat and as an alternative to riprap for bank stabilization. However, there have been few studies that have systematically examined how juvenile salmonids utilized these structures relative to other available habitat. We examined Chinook salmon (Oncorhynchus tshawytscha), coho salmon (O. kisutch) and trout (O. mykiss and O. clarki) response to the placement of engineered log jams (ELJs) in the Elwha River, Washington State, USA. We used summer snorkel surveys and a paired control‐treatment design to determine how engineered log jams in a large river system affect the density of juvenile salmon. We hypothesized that densities of juvenile salmonids would be greater in habitats with ELJs than in habitats without ELJs in the Elwha River and that this ELJ effect would vary by species and size class. Juvenile salmonid density was higher in ELJ units for all control‐treatment pairs except for one pair in 2002 and one pair in 2003. Positive mean differences in juvenile salmon densities between ELJ and non‐ELJ units were observed in two of four years for all juvenile salmon, trout greater than 100 mm and juvenile Chinook salmon. Positive mean differences occurred in one of 4 years for juvenile coho salmon and trout less than 100 mm. The results suggest that ELJs are potentially useful for restoring juvenile salmon habitat in the Elwha River, Washington State, USA. Copyright © 2011 John Wiley & Sons, Ltd.

Fish habitat rehabilitation using wood in the world

To provide river managers and researchers with practical knowledge about fish rehabilitation, various studies of fish habitat rehabilitation that used wood were reviewed. The review focuses on fish responses, wood installation methods, and geomorphic features of the rehabilitation sites. Most studies were conducted in moderately sized (small and medium) streams with relatively high bed gradients and aimed to improve the habitats of salmonid species. In this stream type, structures spanning the full (log dam) and partial (log deflector) width of the river were most common, and wood structures that created pools and covers were successful in improving fish habitat. Some projects were conducted in moderately sized low-gradient streams, in which wooden devices used to create instream cover were effective for fish assemblages. There were few studies in other aquatic ecosystems. However, well-designed large wood structures, known as engineered log jams, were used in rehabilitation projects for large rivers. In slack-water or lentic systems such as side-channels, estuaries, and reservoirs, small and large wood structures that created cover were used to improve habitat for many fish species. For successful fish habitat rehabilitation projects, the hydrogeomorphic conditions of rehabilitation sites should be carefully examined to avoid physical failure of wood structures. Although artificial wood structures can be used to improve fish habitat in various aquatic ecosystems, they should be considered to be a complementary or interim habitat enhancement technique. The recovery of natural dynamic processes at the watershed scale is the ultimate target of restoration programs.

Periphyton and invertebrate response to wood placement in large pacific coastal rivers

AbstractWood abundance in aquatic systems has been dramatically reduced compared to historical levels due to anthropogenic activities that led to wood removal and stream simplification. As a result, reintroduction of wood to aquatic systems is now a widely used and relatively well‐studied restoration technique for increasing habitat complexity. Although stream periphyton (biofilm) and invertebrates serve as food sources for a variety of predators including fish, birds and bats, data on how lower trophic levels respond to wood placement are relatively scarce. The purpose of this study was to test the hypothesis that periphyton biomass and aquatic invertebrate density were higher on Engineered Log Jams (ELJs) than on inorganic substrates in two large Pacific Northwest river systems. Among years and rivers, periphyton biomass and invertebrate densities were significantly higher on ELJs than on cobbles within the same reach. Invertebrate communities on ELJs were dominated by meiofauna (<500 µm), whereas cobbles were dominated by larger chironomids. We attribute these trophic level differences to substrate type, as we did not detect taxonomic differences between cobbles in reaches with and without ELJs. We show that adding wood to reaches with little or no naturally occurring wood increased overall habitat surface area and thereby the potential for increased productivity relative to reaches with low levels of wood. Finally, wood supports a unique community of invertebrates that are often overlooked in lotic system studies but may be contributing substantially to overall biological diversity. Published in 2009 by John Wiley & Sons, Ltd.

Confronting hysteresis: Wood based river rehabilitation in highly altered riverine landscapes of south-eastern Australia

This study evaluates an experiment in river rehabilitation which uses large wood to stimulate and emulate natural system processes in an effort to reverse channel degradation, excess sediment transport and habitat simplification that has resulted from two centuries of human induced disturbances, particularly desnagging. The experiment involved the reintroduction of 436 logs (350 t) within 20 engineered log jams (ELJs) over an 1100 m reach. Commencing in 1999, the experiment was set up as a standard BACI design, with a control reach 3 km upstream. In the 5 years since implementing the rehabilitation strategy, the study reach has experienced five floods greater than the mean annual, and a further five events capable of mobilising the gravel bed. Five surveys of channel terrain have been completed since treatment implementation, and the changes to net sediment storage and morphologic diversity assessed in comparison to the control reach. Seven surveys of the fish population in the reach have also been undertaken during the project to measure the ecological response to the introduced wood. The experiment has demonstrated the effectiveness of ELJ technology in achieving engineering and geomorphic goals. To date, the treatment has halted further degradation of the river and increased sediment storage, with the test reach now storing, on average, 40 m 3/1000 m 2 more sediment than in the control. These values, it would appear, represent a new reach-scale dynamic equilibrium storage level over decadal timescales. Additional sediment storage amounts to 3.5 m 3/m 3 of wood added. At the reach scale this additional storage represents a reduction of just 2% or less of the post-European expansion in channel capacity, suggesting far greater efforts are required than those employed here to reverse the cumulative effects of 180 years of channel erosion and simplification. Pool and bar area in the test reach increased by around 5% and 3.5%, respectively, while the corresponding values in the control were around 1.5% and 1%, respectively. Two indices of morphologic diversity were measured for each bed survey: the standard deviation of 3D residuals of change compared with the baseline survey (SD iΔ3D); and the standard deviation of thalweg residuals from the line of best fit (SD iTR). The SD iΔ3D index shows both reaches increased in complexity through the study with the treatment increasing more than the control (0.37 and 0.29, respectively). The SD iTR index does not detect clear changes because of the low signal to noise ratio, however, it does suggest the test reach was more complex than the control at the outset. The observed increase in fish abundance after the first 12 months of monitoring, reported previously, is now far less distinct 4 years on — a pattern seemingly reflecting the relatively minor increases in critical pool habitat and habitat diversity over the same period. Although no significant differences were detectable in fish species richness or total abundance from the reach aggregate data after 4 years, analysis of individual structures show them to be high quality habitat for native fish compared to the rest of the reach and the upstream control. These results highlight the challenges river managers face in achieving measurable improvements in the health of aquatic ecosystems in highly altered rivers. Managers must confront hysteresis in a biophysical and institutional sense when attempting to reverse the degradation of rivers. The scale of treatment implemented in this experiment was at the upper end of the spectrum of rehabilitation efforts currently being undertaken in Australia, suggesting that far greater resources and longer timescales are required to achieve the levels of improvement in the diversity of stream habitat expected by the community. The study also highlights problems with the strategy of attempting to meet multiple objectives within a reach scale rehabilitation project. While this treatment successfully met some geomorphic study objectives, maximising the benefits for fish habitat would require a strategy focused primarily on the creation of complex woody habitat within deeper pools, particularly pools immediately below riffles.

Experimental reintroduction of woody debris on the Williams River, NSW: geomorphic and ecological responses

AbstractA total of 436 logs were used to create 20 engineered log jams (ELJs) in a 1.1 km reach of the Williams River, NSW, Australia, a gravel‐bed river that has been desnagged and had most of its riparian vegetation removed over the last 200 years. The experiment was designed to test the effectiveness of reintroducing woody debris (WD) as a means of improving channel stability and recreating habitat diversity. The study assessed geomorphic and ecological responses to introducing woody habitat by comparing paired test and control reaches. Channel characteristics (e.g. bedforms, bars, texture) within test and control reaches were assessed before and after wood placement to quantify the morphological variability induced by the ELJs in the test reach. Since construction in September 2000, the ELJs have been subjected to five overtopping flows, three of which were larger than the mean annual flood. A high‐resolution three‐dimensional survey of both reaches was completed after major bed‐mobilizing flows. Cumulative changes induced by consecutive floods were also assessed. After 12 months, the major geomorphologic changes in the test reach included an increase in pool and riffle area and pool depth; the addition of a pool–riffle sequence; an increase by 0.5–1 m in pool–riffle amplitude; a net gain of 40 m3 of sediment storage per 1000 m2 of channel area (while the control reach experienced a net loss of 15 m3/1000 m2 over the same period); and a substantial increase in the spatial complexity of bed‐material distribution. Fish assemblages in the test reach showed an increase in species richness and abundance, and reduced temporal variability compared to the reference reach, suggesting that the changes in physical habitat were beneficial to fish at the reach scale. Copyright © 2004 John Wiley & Sons, Ltd.