Woody Debris

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Debris Flow Control Structures for Forest Engineering

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This report summarizes the results of a study on the use of debris flowcontrol structures in forest engineering. The purposes of the study were:

• to review and assess the viability and limitations of such structures; and
• to provide a number of conceptual designs that are appropriate for forest engineering.

Report on exotic forest debris management related to storm events in the Bay of Plenty

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In late 2010 and early 2011, the eastern Bay of Plenty was affected by a series of extreme storm events. These resulted in severe debris flows of woody material from plantation forests adversely impacting on waterways and properties downstream. Two plantation forests, at Taneatua and Omataroa, have been used as case studies to consider options for forest management practices to reduce the risk of potential problems from wood debris in the future. A joint working group comprising Council staff and representatives of the Bay of Plenty forest industry and Scion has developed a series of recommendations covering forest management practices and identification of high risk land in the Bay of Plenty region. In addition, Council staff members have identified further work required on clarification of critical storm thresholds, and internal consents and compliance procedures to minimise the adverse effects of debris flows from plantation forests as a result of severe storm events in the future.

Removal of Woody Debris May Affect Stream Channel Stability

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Sevveral western states mandate the removal of logging debris from streams in order to prevent accumulations impassable to anadromous fish. Monitoring a small western Washington stream revealed large changes in channel structure during the first high flow after cleaning. Nearly 60 percent of the monitored pieces of debris moved during this storm channel cross sections were substantially altered by movement of stored sediment, and the number, area, and volume of pools decreased. The degree of channel rearrangement was greater than in a comparable undisturbed stream. Subsequent storms caused much less debris movement and channel change than the first high flow, even though some of the later flows were of greater magnitude. An interim guide to stream cleaning is prescribed.

Public perception as a barrier to introducing wood in rivers for restoration purposes

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Reintroduction of wood in rivers for restoration purposes is now recognized in a positive way by scientists. Nevertheless, the perception of wood in riverscapes is strongly affected by the socio-cultural environment. This cultural influence might explain why wood reintroduction is accepted and promoted in some regions of the world but not
in others, despite the demonstrated ecological benefits. From an extensive student perception survey, we show that most of the groups from nine countries in the world considered riverscapes with wood to be less aesthetic, more dangerous, and needing more improvement than riverscapes without wood. By contrast, this way of thinking was not observed in Germany, Sweden, and Oregon (USA), where the first instances of wood reintroduction occurred.

Measuring woody debris in the small streams of New Zealand's pine plantations

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To assess the impact of harvesting on woody debris volumes in streams, a method was required with sufficient precision to provide meaningful evaluation and comparison of pre- and post harvest levels of woody debris. Before harvest, woody debris volumes were measured in 24 first- to third-order stream sites in New Zealand's mature pine plantations (22-34 years of age). An adaptation of the Van Wagner line intersect method was used to measure the small woody debris 1-9 cm in diameter (SWD). All large woody debris 10 cm in diameter (LWD) was measured for diameter and length. Woody debris volumes in the stream channel ranged from 2 to 345 m3 ha-1, averaging 112 m3 ha-1 (±34, 95% confidence interval (CI)). Woody debris surface areas averaged 2883 m2 ha-1 (±688), range 220-6769 m2 ha-1. Most of the woody debris volume (87%) was composed of LWD. Sixty-seven percent of the woody debris volume was located above the stream, the remainder was lying in-stream or on the floodplain. Woody debris volumes in streams of mature pine plantations in New Zealand were similar to woody debris volumes in streams of temperate native forests in New Zealand and North America. These sites will be remeasured after harvest to identify any changes in woody debris characteristics.

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

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1. 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.

Effects of forest harvesting and woody debris removal on two Northland streams, New Zealand

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The short-term effects of Pinus radiata forest harvesting to the stream edge followed by stream-cleaning (removal of woody debris from the stream channel), on instream light levels, stream temperature, dissolved oxygen concentrations, and aquatic invertebrates were assessed in streams draining partly (25% clear-cut) and totally (100% clearcut) harvested catchments, compared with nearby indigenous forest and mature pine plantation reference sites. There were marked increases in in-stream light levels and water temperatures following forest harvest and stream-cleaning at both sites. In-stream light levels increased from 8-13% to 60-90% and maximum monthly water temperatures increased on average by 5.6°C in the partly harvested and 3.6°C in the fully harvested catchment. Dissolved oxygen levels decreased at both sites shortly after harvest (94%-71% saturation in the partly harvested catchment; 72%-37% saturation in the totally harvested catchment), increasing to 75% and 81%, respectively, 1 year later. Although aquatic invertebrate mean density and taxa richness increased at both sites following harvest, the relative abundance of sensitive mayfly, caddisfly, and stonefly species decreased and community composition changed to one dominated by Chironomidae (midges) or Mollusca. Impacts relative to pre-harvest conditions were
not as marked in the totally harvested catchment, possibly because of pre-existing elevated stream temperatures and high levels of sand and silt. Any downstream protection provided by the forested headwaters of the partly harvested catchment was soon lost after the stream entered the clear-cut area, although these forested headwaters may provide a potential source of aquatic invertebrates for re colonisation in the future as water quality and habitat recover. Our results suggest that: (1) pre-existing
constraints on habitat quality can influence the magnitude of harvesting impacts; and (2) length of stream edge harvested may be a better indicator of impact on some aspects of stream ecology, such as lighting, stream temperature, dissolved oxygen, and aquatic invertebrate community composition, than percentage of catchment harvested. This study also highlights the importance of considering the hydrological and landscape context for mitigating harvesting and wood management impacts on stream

Does adding wood to agricultural streams enhance biodiversity? An experimental approach

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Riparian clearing and the removal of wood from channels have affected many streams in agricultural landscapes. As a result, these streams often have depauperate in-stream wood loads, and therefore decreased habitat complexity and lower levels of in-stream biodiversity. The introduction of wood was investigated as a possible rehabilitation technique for agricultural streams.Wood was re-introduced to eight streams in two separate high-rainfall, intensively grazed regions of Victoria, Australia and the effect on aquatic macroinvertebrate communities was measured. The addition of wood increased overall family richness and the richness of most functional feeding groups occupying edge and benthic habitats within the stream.Wood addition led to less overlap between benthic and edge macroinvertebrate communities, suggesting increased habitat heterogeneity within the stream ecosystem. Of all sampled habitats, wood supported the greatest density of families and was colonised by all functional feeding groups.Wood habitats also had the highest overall richness and supported the most taxa that were sensitive to disturbance. These findings suggest that re-introducing wood to agricultural streams is an appropriate rehabilitation technique where those streams are affected by reduced habitat complexity. Additional work is needed to confirm these findings over larger spatial and temporal scales.

Decomposition of Pinus radiata coarse woody debris in New Zealand

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The decomposition of Pinus radiata (D. Don) stems, coarse woody roots and stumps was studied in Tarawera forest, Bay of Plenty region, North Island, New Zealand. The study examined the residues from two thinning events with 6 and 11 years of decay. Changes in the mass of stems, and density of roots and stumps were used to estimate the decay rate constants using a single exponential model. The decay rate of stems was not significantly related to DBH and averaged 0.1374 year1 (22 years for 95% mass loss). The decay rate of coarse woody roots was not significantly different to stem decay and averaged 0.1571 year1 (19 years for 95% mass loss). A large range in stump decay rates was measured and a significantly lower decay constant was observed for stumps (0.1101 year1, 27 years for 95% mass loss), possibly due to the stumps being kept alive after felling through root grafting and a resistance to decay due to the presence of resin. The concentration of C remaining in stems and stumps increased with mass loss from 52% to 55% C after 11 years of decay. The C concentration in coarse woody roots initially increased but then declined near to the original level of 50% after 11 years of decay. Nitrogen concentrations increased substantially in all components with decay.

Decomposition and nutrient release from radiata pine (Pinus radiata) coarse woody debris

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The dynamics of decomposition of thinning slash and nutrient release were studied in a radiata pine (Pinus radiata D Don) plantation forest in New Zealand. This study examined decomposition of coarse woody debris (CWD) components (log-wood, log-bark, and side branches) originating from stands thinned between 1 and 13 years previously. Changes in component density were used to estimate the decay rates. Both chemical analyses and 13C nuclear magnetic resonance (NMR) spectroscopy were conducted to investigate relationships between decomposition and chemical composition. The rate of decomposition was the fastest for log-wood followed by log-bark, which in turn decomposed faster than side-branch material. After 13 years, log-wood, log-bark and side branches lost 59, 55 and 24% of their initial mass, respectively. Single exponential model analysis indicated that the half-life of total thinning slash (sum of log-wood, log-bark and side branches) was 13.25 years. Proximate analyses showed that the faster rate of decomposition of log-wood was mainly due to greater carbohydrate concentration, while greater concentrations of polyphenol and lignin were responsible for the slower decomposition rate of log-bark. The slow rate of decomposition of side branches was due to unfavorable micro-climate (most of the side branches were not in contact with soil even after 9 years of decomposition) as well as greater lignin and polyphenol concentrations. Carbon-13 NMR analysis revealed that during decomposition the relative proportions of O-alkyl and acetal C, which represent carbohydrates, decreased while N-alkyl, aromatic, and phenolic C, which represent tannins and acid insoluble compounds including lignin, increased in all thinning slash components. Net release of nutrients (N, P, K, Ca and Mg) occurred during thinning slash decomposition, in contrast to earlier studies, although the concentrations of most nutrients increased with time. Nutrient release was attributed to the nature of the thinning slash materials and the high proportion of bark material in particular. Although there was a net release, the rate of release of C and the majority of nutrients from thinning slash was slow making it an important C sink and long-term source of nutrients.

Colonisation and use of pine wood versus native wood in New Zealand plantation forest streams: implications for riparian management

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1. Riparian management is becomingly increasingly ecognized as an important tool for reducing harvesting impacts on plantation forest streams. To provide information for a riparian management decision support system, this study investigated effects of riparian tree type (plantation Pinus radiata D. Don versus four native species) on the development of epixylic biofilms, and colonization and feeding by invertebrates on wood at two contrasting stream sites in the central North Island, New Zealand.
2. Electron micrographs revealed a diverse microflora colonizing all wood types, which generally had similar ergosterol concentrations, microbial activity and algal biomass after 1–2 years immersion.
3. Wood type (pine versus native) did not have a significant effect on densities of total invertebrates or dominant taxa. Percentage abundance of some dominant invertebrate groups differed between wood species, apparently in relation to surface complexity, but overall physical habitat differences appeared to override any effects of wood type on community composition.
4. The mass of fine particulate material produced by larvae of the conoesucid caddis fly, Pycnocentria funerea McLachlan, differed significantly among wood species, but this was not related to whether the wood was from native trees or plantation pine.
5. Overall, the results indicate that local variations in habitat heterogeneity and differences in wood surface texture were more important than wood type in influencing epixylic biofilm development, and utilization by invertebrates. Alien pine wood generally provided habitat conditions and trophic resources during initial decomposition that were within the range of variation of native wood species expected to colonize riparian planting set-backs in northern New Zealand pine forests.

The physical and biological function of wood in New Zealand’s forested stream ecosystems

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Since the arrival of humans approximately 1000 years before present (B. P.), New
Zealand has lost approximately 80% of its forest cover and along with it, the contribution of wood to our aquatic ecosystems. The aim of this thesis was to undertake a large catchment-scale assessment of LW loadings, spatial distribution and morphological influence in an old-growth indigenous forest to provide some understanding on the natural characteristics of wood that would have been present in many river systems of New Zealand prior to human settlement. The second component of the thesis involved the experimental removal of wood from three small streams in order to provide some insight into what that loss of wood may have meant for fish and aquatic invertebrate communities. In the first part of the study, a catchment scale survey of large wood (LW) was completed in a 5thorder, old-growth forest river system. LW volumes ranged from 59-503 m3 ha-1 and declined down the river system along with the number of LW pieces suspended across the channel and LW influence on channel morphology, whereas piece frequency, number of pieces in debris dams and length increased. Nearly half the pieces were influencing channel morphology, particularly wood accumulation, sediment storage, bank armouring, and pool formation. These key pieces were larger, longer and more stable than average. LW contribution to habitat complexity was highest in the middle to upper sections of the river system. Four key zones of wood distribution and influence were identified in the river system. Zonal boundaries were influenced by changes in transport capacity, fluvial processes and channel morphology. In the second part of the study, a field trial was established in three small forested streams to measure the influence of wood and its experimental removal on channel morphology, and indigenous fish and aquatic invertebrate communities. Prior to wood removal there were no significant differences in the total density of fish between wood pools (pools with wood cover), open pools and riffles. Total fish biomass was marginally significant with most of the fish biomass located in wood pools. At the species level, the density and biomass of banded kokopu (Galaxias
fasciatus) and the weights of longfin eels (Anguilla dieffenbachii) were significantly
higher in wood pools. Species richness, density and biomass of bluegill bullies
(Gobiomorphus hubbsi), torrentfish (Cheimarrichthys fosteri) and the density of
redfin bullies (Gobiomorphus huttoni) was highest in riffles. Differences in fish
community composition were greatest between riffles and pools, whereas there was
considerable overlap between the two pool types. Total invertebrate density was 70% higher in debris dams than riffles prior to wood removal, but this difference was not significant. Densities of Trichoptera (caddisfly) and Plecoptera (stonefly), and five aquatic invertebrate taxa were significantly higher in debris dams which also contained greater numbers of less common taxa (< 1% total catch) than riffles. Only Deleatidium sp. (Ephemeroptera) densities were significantly higher in riffles than in debris dams. Aquatic invertebrate communities in debris dams differed significantly from those in riffles and season had a significant influence on aquatic invertebrate community structure. Removal of wood and associated debris dams from the treatment sections in
each of the three streams resulted in a simplified channel morphology, significantly
increasing the length and area in riffles and reducing the area of pools. The impact on
the fish community was greatest for the two larger fish, banded kokopu and large
longfin eels, whose abundance declined in the treatment sections. At the reach scale,
only banded kokopu biomass showed a significant decline following wood removal.
Invertebrates were less affected by wood removal and associated loss of debris dams.
Invertebrate composition in the remaining riffles in the treatment sections had a
higher proportion of Ephemeroptera and lower proportions of Trichoptera, Plecoptera
and Diptera with fewer rare species than remaining debris dams in the control
sections, but there were no discernable effects on invertebrate densities and functional
feeding groups at the reach scale. Public perception of wood in waterways is mainly negative and wood is managed primarily to reduce flood damage in New Zealand‟s streams. With continued research and advocacy on the environmental benefits, careful planning and judicial use, there is the potential to make better use of wood to rehabilitate and enhance New Zealand‟s stream environments. This thesis provides some insight into the contribution of wood to forested stream ecosystems in New Zealand and the implicit losses associated with forest removal. It also contributes to our global understanding on the role of wood, its contribution to habitat heterogeneity and influence on biological communities.

An experimental approach to quantify the contribution of invertebrates to the decay of dead wood in New Zealand plantations

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As part of the new Scion led programme “Protecting and Enhancing the Environment Through Forestry” we are attempting to quantify the contribution that insects make to the decomposition of dead wood in plantation forests around New Zealand. Eight sites have been selected that span two environmental gradients, temperature and moisture. It was deemed appropriate to control for these two variables as they are key determinants of dead wood decomposition and the diversity of understorey native plant species present in plantations (Brockerhoff et al. 2003, Garrett et al. 2007). At this stage we do not know the distribution of native invertebrates in plantations as a function of temperature or moisture.

Harvesting effects on woody debris and bank disturbance in stream channels

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Woody debris volumes and channel bank disturbance were measured in a 100-m section of stream channel, prior to and after harvesting, in 17 streams in pine plantations in five regions of New Zealand. These sites were harvested using four different harvest methods. Volumes of pre-harvest woody debris and woody debris produced during harvest averaged 105 m3/ha, and 147 m3/ha, respectively. Apart from the stream-cleaned sites where virtually all the pre-harvest and harvest woody debris was removed, postharvest volumes (pre-harvest + harvest) averaged 289 m3/ha and increased three-fold on average over pre-harvest levels. Most of the woody debris in the stream channel was positioned above the stream—69% of pre-harvest woody debris, 64% of harvest woody debris, and 66% of total post-harvest woody debris. The remainder lay in-stream or on the floodplain. The most significant change in woody debris characteristics after harvest was size distribution. Small woody debris <10 cm in diameter (SWD) increased from 13% of woody debris volumes at pre-harvest to 38% at post-harvest. The number of pieces of large woody debris >10 cm in diameter (LWD) increased significantly, and the average length and piece size decreased significantly after harvest. This was due mainly to the removal of the larger merchantable pieces of LWD from the stream channel. Harvest method had the most impact on harvest woody debris volumes in the stream channel, overriding the influence of riparian buffers which ranged in width from 1 to 30 m at four of these sites. Stream-cleaned sites had the lowest harvest woody debris volumes, followed by sites harvested with ground-based systems (15 m3/ha and 48 m3/ha respectively). When yarder systems were used to extract timber back from the stream edge, woody debris volumes averaged 104 m3/ha, whereas hauling across the stream channel resulted in the highest average woody debris volumes of 287 m3/ha. For hauling across the stream channel only, there was a relationship between stand volume and harvest woody debris volumes. Bank collapses accounted for 68% of all pre-harvest channel bank disturbances. Bank scuffing from felling and log extraction during harvest operations was the most common channel bank disturbance after harvest (46%). Harvest method did not show a clear relationship with the degree of channel bank disturbance.

Spatial distribution and influence of large woody debris in an old-growth forest river system, New Zealand

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A field survey was undertaken to determine the quantity, spatial distribution and influence of large woody debris (LWD) in a fifth-order river system in old-growth forest in New Zealand. LWD attributes were assessed at 25 sites distributed in the headwaters and along the main stem of the Whirinaki River system (73 km2). LWD volume, number of pieces, piece length and piece size, were positively correlated with bankfull width, whereas the number of pieces/unit area, LWD/unit area, number of pieces suspended across the channel and LWD influence on channel morphology, were negatively correlated. Pieces influencing channel morphology were larger, longer and more stable than average. We identified four key zones in the river system based on LWD spatial distribution patterns and influence on habitat complexity. Zonal boundaries occurred where there were changes in the transport capacity, fluvial processes, channel width and geomorphic structure of the channel. The results of this study highlight the need to understand the characteristics, spatial distribution patterns and influence of LWD at the catchment level when undertaking protective, management or rehabilitation programmes in forested river ecosystems.

Management of logging slash in streams of New Zealand

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One issue when harvesting around waterways is the potential for large amounts of logging slash to end up in the stream channel. Management options can include using harvest systems and practices to minimise the amount of logging slash entering the stream, removing the logging slash once it is in the stream channel, or leaving the logging slash in place. In order to fill the information gap identified in the literature review, a survey was carried out to identify the current practices being used to manage logging slash in New Zealand's plantation forest streams.

Distribution and abundance of coarse woody debris in some southern New Zealand streams from contrasting forest catchments

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Greater amounts of coarse woody debris (CWD) occurred in streams from old native forests than in streams from young native and pine forests in southern New Zealand. The size of CWD in the streams generally reflected the age of the surrounding
vegetation. More wood was present in pool than in non-pool sections of old native forest streams and the frequency of pools per unit length formed by woody debris was greatest in these streams. The volumes of pools formed by wood and those formed by inorganic substrates were similar. Amounts of woody debris in these streams were relatively small compared to values recorded from North America.

Decomposition and nutrient dynamics of green and freshly fallen radiata pine (Pinus radiata) needles

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Thinning and pruning operations in radiata pine (Pinus radiata) plantation forests result in the addition of large amounts of green needles to the forest floor. The decomposition of green and freshly fallen radiata pine needles and the effects of adding green needles to freshly fallen needles were examined in a microcosm experiment. Green needles lost 72% of the original mass after 10 months, compared with 27% for freshly fallen needles. The corresponding mass losses for 1:1 ratios of green and freshly fallen needles were 55% when mixed and 53% when layered. Nutrient concentrations generally increased during decomposition while total amounts of nutrients decreased with time. Decomposition was primarily influenced by needle lignin and N content, and by the holocellulose to lignocellulose quotient (HLQ). The results of this study indicate that addition of green needles does not significantly affect the decomposition of freshly fallen needles. This outcome was attributed to substrate preference by decomposer microorganisms. It is, therefore, concluded that forest management practices (thinning, pruning and harvesting) which result in significant inputs of carbohydrates and nutrients in the form of green needles will have little impact on decomposition of existing forest floor materials.

Influence of large woody debris on channel morphology in native forest and pine plantation streams in the Nelson region, New Zealand

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The influence of large woody debris (LWD) on channel morphology was assessed in five pine plantation and five native forest streams in the Nelson region of New Zealand. LWD volumes averaged 127 m3 ha–1 in pine plantation streams and 94 m3 ha–1 in native forest streams. Most of the LWD in pine and native streams had no influence on channel morphology (78 and 54%, respectively). Those LWD pieces influencing channel morphology contributed mainly to sediment storage in both pine and native streams as well as flow deflection and debris collection in the native streams. Wood aligned either perpendicular or obliquely to stream flow and positioned on or partly buried in the stream bed had the greatest influence on channel morphology. There
were twice as many pools in native streams as in pine streams, and scour processes formed the majority of pools. LWD influenced c. 50% of pool formation in both pine and native streams and increased pool variety. LWD volumes in these streams are low compared with Pacific north-west streams of the United States, similar to streams in the subalpine forest of Colorado, and higher than volumes in the more modified streams of Europe. Similarities in pool forming processes, pool type, and spatial arrangement of wood influencing channel morphology are apparent between the streams in this study and streams of the Pacific north-west and Colorado.