Removal Of Large Woody Debris Research Articles

Contrasting associations between habitat conditions and stream aquatic biodiversity in a forest reserve and its surrounding area in the Eastern Amazon

Streams of protected areas should be subjected to less environmental degradation than surrounding areas and consequently support greater aquatic biodiversity. To test this, 186 environmental and landscape variables were measured in 34 streams within the Caxiuana National Forest (CNF) and its surrounding zone in the eastern Amazon. We expected that streams inside the CNF protected area would have more riparian forest cover and large woody debris (LWD) that increase instream habitat complexity and aquatic biodiversity. Several environmental variables differed between streams in the CNF and surrounding zone; however, the major difference was greater LWD, leaf litter, and channel depth in CNF streams. Richness of fish, Chironomidae, EPT (Ephemeroptera + Plecoptera + Trichoptera), and all-groups combined were positively associated with LWD. Assemblage taxonomic composition was correlated with several variables, but most groups revealed no clear differentiation between the two areas. This lack of differentiation may be explained by relatively minor environmental impacts in areas surrounding the CNF given the region’s small human population. The most notable impact to streams outside of the CNF was removal of LWD to facilitate boat passage. To conserve aquatic biodiversity, we recommend expansion of protected areas and adoption of policies governing land use in surrounding zones.

Uptake of dissolved organic carbon by biofilms provides insights into the potential impact of loss of large woody debris on the functioning of lowland rivers

Summary Substantial amounts of large woody debris have been removed from lowland rivers worldwide. This large woody debris would have been colonised by biofilms and hence would have formed potentially important sites of chemical transformations (particularly carbon uptake) in these riverine environments. It is possible that one of the reasons that allochthonous production appears to be relatively unimportant to metazoan food webs in some river systems is due to the lack of sites suitable for biofilm growth. This paper explores a key aspect underpinning this idea, the kinetics of dissolved organic carbon (DOC) uptake by bacterial biofilms. Laboratory experiments were used to determine the kinetics of uptake of a natural source of DOC in the presence of a range of densities of biofilm‐coated surface at different temperatures. Changes in DOC speciation induced by biofilm carbon uptake were examined using 3‐dimensional excitation–emission fluorescence spectroscopy with post hoc PARAFAC deconvolution. The first‐order rate constant for carbon uptake increased linearly with amount of biofilm present. The rate of uptake is more sensitive to temperature (increasing approximately logarithmically) in the presence than in the absence of added biofilms. Furthermore, it appears that biofilms preferentially remove amino acid‐like components from the DOC. Based on these results, we derive a number of testable hypotheses regarding the impact of the reduction of sites of transformation on the functioning of lowland rivers. Specifically, our results suggest that there are both a critical biofilm‐coated surface area density and a critical temperature below which carbon uptake in a river reach will be substantially reduced. This implies that both removal of large woody debris and changes in flood seasonality may have altered the potential for incorporation of allochthonous carbon as a basal food resource for riverine food webs.

Hyporheic temperature dynamics and heat exchange near channel‐spanning logs

The flow of river water around large woody debris (LWD) creates pressure gradients along the riverbed that drive river‐groundwater mixing, or hyporheic exchange, and heat transport within the hyporheic zone. We quantify hyporheic fluid and heat exchange induced by current interaction with channel‐spanning logs using two approaches: laboratory flume experiments and numerical simulations that link turbulent open‐channel fluid flow, porous fluid flow, and heat transport. Flume and numerical experiments show that logs produce a characteristic diel temperature pattern within sediment that shifts with log blockage ratio (a fraction of the channel depth blocked by the log), channel Froude number, and sediment permeability. Upstream of a log, downwelling water transports the river's diel thermal signal deep into the sediment. Downstream, upwelling water from shallow flow paths has a thermal signal similar to the surface water, while upwelling water from deep flow paths forms a wedge of buffered (low‐amplitude) temperatures. Since most hyporheic water emerges from shallow flow paths, upwelling water has limited potential to buffer surface water temperature. Because of historic channel clearing practices, modern rivers have unnaturally low densities of LWD. A key implication is that LWD removal has contributed to thermal homogenization and potential degradation of hyporheic habitats. LWD reintroduction is a promising strategy to improve vertical connectivity in rivers and increase thermal patchiness within the hyporheic zone. However, hyporheic exchange near LWD may not impact diel surface water temperatures at the reach scale.

Hyporheic exchange due to channel‐spanning logs

The flow of river water around large woody debris (LWD) creates pressure gradients along the riverbed that drive a large zone of river‐groundwater mixing, or hyporheic exchange. Flume experiments and numerical simulations show that river water downwells into the riverbed upstream of a channel‐spanning log and upwells downstream. Exchange rates are greatest near the log and decay exponentially with distance upstream and downstream. We developed equations for bed pressure profiles and hyporheic exchange rates in the vicinity of a channel‐spanning log that can be used to evaluate the impact of LWD removal or reintroduction on hyporheic mixing. The magnitude of pressure disturbance along the bed (and thus hyporheic exchange) increases with the fraction of channel depth blocked by the log and channel Froude number. Exchange rates are relatively insensitive to relative depth of the log (gap ratio). At natural densities, LWD in lowland streams drives reach‐averaged hyporheic exchange rates similar to a ripple‐covered bed. However, the length scales and residence times of hyporheic exchange due to LWD are greater. By removing LWD from streams, humans have altered patterns and rates of hyporheic exchange, which influence habitat distribution and quality for invertebrates and fish.

Influence of experimental removal of large woody debris on spatial patterns of three‐dimensional flow in a meander bend

AbstractThis study reports the results of a large woody debris (LWD) removal experiment in a meander bend along a low‐energy stream in the Midwestern United States. The LWD obstacle was located in the center of the channel at the bend exit and consisted of a mature tree with an intact soil‐covered root wad and a large accumulation of logs, branches and pieces of lumber on top of and adjacent to the main tree. The results indicate that the LWD obstruction influenced 3D flow structure in this bend at all flow stages. The main effect of LWD is to dramatically decelerate flow throughout the majority of the bend, while locally accelerating flow where it passes through the narrow chute at the downstream end of the LWD obstruction. Results from the LWD removal experiment indicate that patterns of three‐dimensional flow structure in meander bends are sensitive to complete removal of LWD. After the removal of LWD from the bend, both downstream and secondary velocities increased and, though still weak, secondary flow intensified. Large, relatively stable, obstructions that span a significant portion of the channel may act as natural dams, effectively ponding water upstream of the LWD, thereby producing substantial convective deceleration of the flow. This research is the first to document three‐dimensional flow structure before and after a controlled removal of LWD from a meander bend. Studies of the type reported here represent a first step toward determining the ensemble of process interactions between LWD and bend dynamics. Copyright © 2006 John Wiley & Sons, Ltd.

Distribution and dynamics of large woody debris and organic matter in a low-energy meandering stream

This field-based study, using a combination of bed sediment sampling and large woody debris (LWD) tracking, explores the storage of organic matter and mobility of LWD in Poplar Creek, a low-energy meandering river that contains a diverse arrangement of large woody debris (LWD). This research addresses the following questions: 1) Does LWD influence the spatial pattern of the storage of benthic organic matter in meander bends? 2) How mobile is the LWD in low-gradient meandering systems? 3) Do characteristics of LWD such as length/bankfull channel width ratio, affect movement? Based on detailed sampling of sediment and LWD monitoring, this study provides a detailed representation of the influence of LWD on the storage of benthic organic matter in meander bends, as well as a detailed documentation of the mobility of LWD in the study reach. This study has demonstrated that: 1) LWD strongly influences patterns of storage of benthic organic matter in low-gradient meander bends; 2) removal of LWD from meander bends in low-gradient systems can substantially reduce the storage of benthic organic matter; and 3) the LWD within the study reach at Poplar Creek is highly mobile. The high mobility of LWD, observed in this study, contrasts with the rates observed in high-gradient systems and suggests that LWD-influenced patterns of storage of organic matter are temporally variable and that storage of benthic organic matter occurs in pulses, interspersed with transport, that are keyed to the mobility of LWD. Although LWD may exert an equally significant control over patterns of storage of organic matter in high- and low-gradient systems, the differences in the mobility rates of LWD suggest a fundamental difference in the dynamics of LWD-mediated organic matter between low- and high-gradient systems.

A practical scientific approach to riparian vegetation rehabilitation in Australia

The clearance of indigenous riparian vegetation and removal of large woody debris (LWD) from streams combined with the planting of exotic plant species has resulted in widespread detrimental impacts on the fluvial geomorphology and aquatic ecology of Australian rivers. Vegetation exerts a significant influence on fluvial geomorphology by affecting resistance to flow, bank strength, sediment storage, bed stability and stream morphology and is important for aquatic ecosystem function. As the values of indigenous riparian vegetation are becoming better recognised by Australian river managers, large amounts of money and resources are being invested in the planting of indigenous riparian vegetation as part of river rehabilitation programs. This paper summarises the results of an investigation into the survival, growth and regeneration rates of a series of trial native riparian vegetation plantings on in-channel benches in the Hunter Valley of southeastern Australia. The trials were poorly designed for statistical analysis and the paper highlights a number of shortcomings in the methods used. As a result, a new approach to riparian vegetation rehabilitation is outlined that promotes the use of scientific principles and understanding. Appropriate species should be selected using a combination of remnant vegetation surveys, historical records, palynology and field trials. A number of important factors should be considered in the rehabilitation of riparian vegetation to achieve worthwhile results. These include flood disturbance, vegetation zonation, vegetation succession, substrate composition, corridor planting width, planting techniques, native plant regeneration, LWD recruitment and adaptive ecosystem management. This approach, if adopted, revised and improved by river managers, should result in greater success than has been achieved by previous riparian vegetation rehabilitation efforts in Australia.

Desnagging to resnagging: new directions in river rehabilitation in southeastern Australia

AbstractExtensive desnagging (removal of large woody debris and living riparian vegetation) and associated river improvement works were conducted in rivers of southeastern Australia (Victoria and New South Wales) between at least 1886 and 1995. Swamp drainage, large woody debris removal and vegetation clearing were strongly supported by legislation, government funding and institutional arrangements in both states. As a result, large amounts of large woody debris were removed from rivers, regenerating indigenous vegetation was cleared from within designed alignment widths and, ironically, huge numbers of exotic trees, especially willows, were planted. The environmental impacts of desnagging have only been documented on a few impacted rivers but have included increased flow velocity, spatially extensive bed degradation, massive channel enlargement and loss of fish habitat. Recognition of the need for more integrated land and water management, and new research on the hydraulic, geomorphic, biogeographic and ecological significance of large woody debris and the values of indigenous riparian vegetation during the 1980s led to a major shift in river rehabilitation. We have drawn on our own and other published research to further develop a set of guidelines for the incorporation of large woody debris into river rehabilitation plans. Our guidelines extend those recently prepared for southeastern Australia and address site selection, where to place timber, the amount to be introduced, how to distribute it, techniques of introduction and woody debris sources. However, in the long term, riparian vegetation rehabilitation within the potential recruitment zone is essential to supply large woody debris. Given that our results demonstrate that very large woody debris makes a significant contribution to the total loading, it will be a very long time (>100 years) before natural recruitment can be recreated. Copyright © 2003 John Wiley & Sons, Ltd.

The distribution of large woody debris accumulations and pools in relation to woodland stream management in a small, low-gradient stream

This paper focuses upon the natural dynamics of large woody debris (LWD), the impact of management on LWD dynamics, and the impact of LWD removal and channelization on the distribution and size of pools in a British, second to third order, headwater catchment. The study stream is rather different from those subject to LWD accumulations which have been studied in North America. The most important contrast is that it is surrounded by predominantly deciduous rather than coniferous woodland. In terms of its width (1·8–4·5 m) and gradient 0·013 m m−1), it falls within the lower range of channels studied in North America. Nevertheless, there are similarities in LWD dam and pool spacing with some North American studies. The information on LWD dynamics during a period without management and on recovery of LWD dams after clearance covers a 16 year period (1982–1997). The paper illustrates that seven to eight years after clearance the total number of LWD dams has recovered but the most hydraulically active dam type has not recovered to pre-clearance levels. An analysis of geomorphological maps of the channel surveyed in 1982 and 1996/97 shows an overall decrease in the number and size of pools along the section that was cleared of LWD dams. The magnitude of the decrease and the associated adjustments in pools through changes in their size and location differ according to location with respect to a section of the study stream which was channelized in c. 1966 and which has subsequently incised its bed. Copyright © 1998 John Wiley & Sons, Ltd.

Effects of large woody debris removal on physical characteristics of a sand‐bed river

Abstract1. Removal of large woody debris (LWD) is one of the most widely practised stream alterations, particularly in sand‐bed rivers of the south‐eastern USA. Selective removal of LWD has been proposed as an alternative to orthodox non‐selective clearing in order to conserve ecological resources, but methods for comparing hydraulic and environmental effects of selective and non‐selective removal have not been developed. Conservation of stream habitats requires quantification of LWD removal impacts on physical habitat.2. Physical characteristics of straightened, sand‐bed reaches of the South Fork Obion River in western Tennessee, USA that were rich in LWD were compared with those in similar reaches where debris had recently been removed using selective removal guidelines.3. The mean volume of LWD per unit water volume was 0.0545 in the uncleared reaches, but nearly 60% lower (0.0225) in the cleared reach.4. A simple technique for predicting hydraulic roughness in channels with varying amounts of LWD was developed. Hydraulic roughness, as measured by the Darcy‐Weisbach friction factor, was about 400% greater in uncleared reaches at base flow but declined to a level about 35% greater than for the cleared reaches at higher flows. Predicted friction factors were within 35% of measured friction factors at higher flows.5. Physical habitat diversity in this channelized sand‐bed stream was strongly related to the density of LWD. Flow conditions in the uncleared reaches were more heterogeneous than in the cleared reach, especially at low flow. At low flow, uncleared reaches tended to be shallower, have lower velocities, slightly finer bed material, and more heterogeneous conditions overall. Shannon indices based on depth and velocity were an average of 48% higher in uncleared reaches.6. Bed sediments underneath and immediately adjacent to LWD formations were finer and contained more organic matter than sediments distant from LWD. However, when all bed samples were considered, organic content was positively correlated with median grain size.

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.