Effects Of Large Woody Debris Research Articles

Juvenile salmonid abundance in a diamictic semi‐fluvial stream in Norway—does stream bed shelter beat large woody debris?

AbstractThis study investigates the effect of large woody debris (LWD) on the abundance of juvenile Atlantic salmon (Salmo salar, L.) and anadromous brown trout (Salmo trutta, L.) in semi‐alluvial side channels of the river Aurlandselva (Norway) using point electrofishing and microhabitat mapping. Not the presence of LWD, but stream bed shelter availability and the distance to spawning grounds affected the fish abundance (fish/point), independent of other habitat components. LWD showed only an effect on fish abundance when in interaction with other habitat components. This discrepancy can be explained by the availability of cavities in the shelter‐rich coarse substrate which provide sufficient cover and territory for juvenile fish at the given carrying capacity of river Aurlandselva. Whilst LWD may be most effective to provide shelter in lowland streams (bed slope <0.005), maintaining or restoring shelter‐rich coarse substrates should be considered a key priority in steeper salmonid rivers and associated semi‐fluvial streams.

Large eddy simulation of turbulence and solute transport in a forested headwater stream

The large eddy simulation (LES) module of the Virtual StreamLab (VSL3D) model is applied to simulate the flow and transport of a conservative tracer in a headwater stream in Minnesota, located in the south Twin Cities metropolitan area. The detailed geometry of the stream reach, which is ∼135 m long, ∼2.5 m wide, and ∼0.15 m deep, was surveyed and used as input to the computational model. The detailed geometry and location of large woody debris and bed roughness elements up to ∼0.1 m in size were also surveyed and incorporated in the numerical simulation using the Curvilinear Immersed Boundary approach employed in VSL3D. The resolution of the simulation, which employs up to a total of 25 million grid nodes to discretize the flow domain, is sufficiently fine to directly account for the effect of large woody debris and small cobbles (on the streambed) on the flow patterns and transport processes of conservative solutes. Two tracer injection conditions, a pulse and a plateau release, and two cross sections of measured velocity were used to validate the LES results. The computed results are shown to be in good agreement with the field measurements and tracer concentration time series. To our knowledge, the present study is the first attempt to simulate via high‐resolution LES solute transport in a natural stream environment taking into account a range of roughness length scales spanning an order of magnitude: from small cobbles on the streambed (∼0.1 m in diameter) to large woody debris up to ∼3 m long.

Does large woody debris affect the hyporheic ecology of a small New Zealand pasture stream?

Rehabilitation of vertical hydrological exchange in small pasture streams is expected to improve hyporheic habitat, invertebrate life and ecosystem functioning, and may be achieved by adding large woody debris (LWD). This study investigated whether hyporheic improvement within 10 to 50 cm below streambed surface is evident around existing wood structures in a small New Zealand pasture stream in the Whatawhata area near Hamilton. Samples were taken around LWD and from areas where no wood was present in the pasture and from a control stream. At the pasture stream site, where the hyporheic zone was impacted by sedimentation and suffered from low dissolved oxygen (DO) levels (on average 8.4, 4.3 and 2.4 mg L−1 at 10, 30 and 50 cm depth, respectively), LWD was not associated with a significant improvement in vertical hydrological exchange, hyporheic habitat quality or invertebrate richness. However, higher densities in hyporheic invertebrates were found where LWD was present (100 vs. 56 individuals per L of sediment), in particular Leptophlebiidae and Oligochaeta. Potential pumping effects of LWD may have been hampered by large-scale deep groundwater upwelling in this reach. We conclude that in pasture streams where sedimentation is a legacy of land clearance on unstable slopes and where low hyporheic DO concentrations prevail, wood addition may not be an effective rehabilitation tool.

Effects of large woody debris on surface structure and aquatic habitat in forested streams, southern interior British Columbia, Canada

AbstractIt is well known that large woody debris (LWD) plays an important functional role in aquatic organisms' life. However, the influence of LWD on channel morphology and aquatic environments at watershed levels is still unclear. The relationships between wood and surface structure and aquatic habitat in 35 first through fifth order streams of southern interior British Columbia were investigated. Study streams in the channel networks of the study watersheds were classified into four size categories based on stream order and bankfull width: Stream size I: bankfull width was less than 3 m, Stream size II: 3–5 m, Stream size III: 5–7 m, Stream size IV: larger than 7 m. We found the number of functional pieces increased with stream size and wood surface area in stream sizes I, II and III (24, 28 and 25 m2/100 m2, respectively) was significantly higher than that in stream size IV (12 m2/100 m2). The contribution of wood pieces to pool formation was 75% and 85% in stream sizes II and III, respectively, which was significantly higher than those in stream size I (50%) and size IV (25%). Between 21% and 25% of wood pieces were associated with storing sediment, and between 20% and 29% of pieces were involved in channel bank stability in all study streams. Due to long‐term interactions, LWD in the intermediate sized streams (Size II and III) exhibited much effect on channel surface structure and aquatic habitats in the studied watersheds. Copyright © 2008 John Wiley & Sons, Ltd.

The Effect of Large Woody Debris on Macroinvertebrate Communities and Epilithon Detritus Composition in a Channelized Headwater Stream

ABSTRACT Large woody debris (LWD) was placed in a channelized second order reach of Cedar Creek, Michigan, to determine if its presence would alter substrate composition and macroinvertebrate community. Pre- and pos-treatment cores were analyzed for macroinvertebrate composition, substrate particle sue, epilithon, and detritus development. The addition of LWD in experimental sections increased the number of macroinvertebrates by 19% and total biomass by 10%. Species richness and the Shannon-Wiener diversity index increased. Inorganic substrate composition and epilithon development were not altered; however, course particulate organic matter retention was significantly increased in larger particle size classes (> 4 mm).

Grain-Size Sorting in Meander Bends Containing Large Woody Debris

Few studies have investigated the effects of large woody debris (LWD) on patterns of grain-size sorting in meander bends. In unobstructed meander bends, a characteristic pattern of sediment sorting has been observed in which the coarse and fine particles "cross over" through a bend. This field-based study examines the influence of LWD on spatial patterns of grain-size sorting in three meander bends of a low-energy stream in northern Illinois. The results of this study indicate that, in some cases, LWD substantially modifies patterns of grain-size sorting in low-gradient meander bends. Grain-size patterns in one study bend generally correspond with those observed in studies of other meander bends, but patterns in the two other bends are lacking the clear, characteristic crossover pattern of coarse and fine sediments. The detailed nature of the modifications appears to be highly dependent on the particular arrangement of LWD in a bend.

Effect of large woody debris configuration on three‐dimensional flow structure in two low‐energy meander bends at varying stages

Few process‐based studies have investigated the effects of large woody debris (LWD) on the fluvial dynamics of obstructed meander bends. This field‐based study examines the influence of LWD on spatial patterns of three‐dimensional (3‐D) flow in two meander bends of a low‐energy stream in northern Illinois. The results of this study indicate that LWD substantially modifies 3‐D flow structure through low‐gradient meander bends. However, the detailed nature of the flow modification is highly dependent on the particular arrangement, density, and mobility of LWD in a bend. In one of the study bends the LWD primarily functions as roughness elements that reduce the velocity of flow and constrain the high‐velocity core and helical motion to the center of the channel. In the other study bend, LWD obstructions act to strongly deflect the flow away from the outer bank rather than simply increasing the resistance to flow along this bank. While results indicate that LWD influences patterns of three‐dimensional flow at all stages, the degree of influence appears to change with stage; the overall pattern of three‐dimensional flow remains constant, but the spatial extent of the pattern increases with increasing stage.

Deforesting the riverscape: the effects of wood on fish diversity in a Venezuelan piedmont stream

While deforestation of tropical ecosystems has been shown to have significant impacts on terrestrial habitats, its effects on aquatic habitats are poorly studied. Deforestation dramatically reduces the input of woody debris to streams, and given the importance of large woody debris to fish communities in temperate streams, this might be one mechanism by which logging could affect aquatic ecosystems in the tropics. To examine the effects of large woody debris on the diverse fish assemblage of a tropical stream, we surveyed pools with and without wood at Rio Las Marias, Venezuela. Pools containing wood contained greater numbers of individuals and more species of fish than pools without wood, and the two types of pools differed in their composition. To test whether these results were due to the presence of woody debris, we conducted an experimental wood addition. Pools to which wood was added showed marked increases in both fish abundance and species richness relative to wood-free pools, and the composition of the fish assemblage in experimental pools approached that of pools with naturally occurring woody debris. These results demonstrate that large woody debris plays a major role in structuring fish communities in tropical streams. As a consequence, logging practices that reduce the input of woody debris to tropical streams or directly remove wood from streams could have serious impacts on aquatic habitats, affecting both the diverse fish communities and local economies dependent on stream fisheries.

Physical Aquatic Habitat II. Pools and Cover Affected by Large Woody Debris in Three Western Oregon Streams

Large woody debris (LWD) is important in affecting stream channel morphology and aquatic habitat. Although the greatest effects on streams of the Pacific Northwest have been by LWD from large conifers, many riparian forests in the region are dominated by red alder Alnus rubra. The effects of the small size and short life of LWD from red alders on channel morphology may be different from that of conifers and are poorly understood. We added LWD (primarily red alder) to three third-order streams in the Oregon Coast Range and used digital terrain models to evaluate physical habitat for salmonids over 3 years. Total residual pool volume increased in two streams, but in the one with the lowest gradient it did not change in the treated portion and even decreased in the untreated portion. In all streams, both the relative proportion and absolute amount of residual pool volume from deep pools increased from their pretreatment values. Cover from LWD in pools increased after treatment and remained high, but the absolute amount of cover was poorly predicted by the volume of LWD. Overall, the changes in stream channel morphology and habitat were consistent with the effects of LWD, and these case studies indicate that small, red alder LWD can effectively modify physical aquatic habitat.

Flume study of the hydraulic effects of large woody debris in lowland rivers

AbstractAs a part of a study investigating the hydraulic effects of large woody debris (LWD) in lowland rivers, a series of small‐scale experiments were conducted in a rectangular glass‐walled recirculating flume. These experiments were undertaken to determine the order of magnitude of the increase in flood levels caused by LWD at different positions within a channel cross‐section. Position variables that were considered in these experiments were height above bed, angle to flow direction, and separation distance in the direction of flow. This study was undertaken to quantify the hydraulic benefits (primarily reduced flood levels) gained by the removal of LWD from lowland rivers, which is a common practice in several countries. From an integrated river management perspective it is necessary to weigh any hydraulic benefits of LWD removal up against the environmental costs of loss of faunal habitat, and possible geomorphic instability.The results of these experiments indicate that the levels of LWD commonly occurring in the lowland rivers of southeastern Australia seldom cause any significant effect on flood levels. However, where LWD occur at channel constrictions, or where unusually high densities of LWD are present, the effect on flood levels will be significant.