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Abstract: Wood influences channel morphology by increasing channel roughness, altering flow hydraulics, and decreasing sediment transport in most fluvial systems where it is present. In many locations, the collection of wood into jams leads to even greater interruption of sediment transport by means of sediment impoundment and the creation of low-velocity backwater flow zones. We examined tracer clast movement near a wood jam in a neotropical gravel-bed stream in order to evaluate the effect of wood mobility and piece transience on the ability of the jam to impede sediment transport. We found that the jam had minimal influence on the passage of any individual clast, with tracer clast mobilization and travel distance being best predicted by flow magnitude and grain size rather than location relative to the jam. The rate of clast passage through the jam did not correlate with either the piece retention rate of the jam or with the density of the jam. The jam did, however, temporarily trap a sediment wedge during a period when fluvially imported wood pieces blocked flow under a key wood piece, diverting flow to a secondary flow path. These results suggest that there is an upper bound on the flows in which wood is able to disrupt sediment transport, and unlike for bedrock roughness elements, this bound is frequently crossed at our study site. [Copyright &y& Elsevier]

Abstract: Wood in tropical streams has the potential to be more mobile than wood in otherwise similar temperate streams because of the warm and humid conditions that promote decay and the more frequent and flashier floods of the tropics. To test this hypothesis, we monitored all large wood pieces for 2.3years in 10 50-m-long reaches of old-growth headwater streams in La Selva Biological Station, Costa Rica. Annual wood retention rates for pieces ranged from 0.55 to 0.91 among the sites, and retention rates by volume ranged from 0.67 to 0.99. Assuming steady state wood load, which is reasonable for La Selva, these rates are equivalent to mean residence times of 2.2–10.6years for pieces, and 3.0–83.2years for a volume of wood. Calculating mean residence time from the weighted average of retention rates gives an average residence time of 4.9years for a piece of wood and 6.9years for a volume of wood. These values are less than those reported for old-growth temperate forests, supporting our hypothesis. Mobility of individual pieces was best predicted by piece length relative to stream width (l r, higher l r led to lower mobility), channel gradient (s, higher s led to higher mobility), and piece integration into the channel (unattached pieces were 2.6 times more mobile than attached, ramp, or bridge pieces). Temporal variation in retention rates was well explained by variation in peak flow. All four of these factors have also been observed to influence mobility in the temperate zone. The higher mobility of wood in our study site relative to the temperate rainforest of the Pacific Northwest may be explained by the flashy and frequent floods, the high decay rate, or the branching morphology of the native trees; but differentiating the role of these factors, particularly flow and decay, will be complicated by their covariation across climates. [ABSTRACT FROM AUTHOR]

The article presents a study which assesses the distribution of in-stream wood in forested headwater streams in La Selva, Costa Rica. As the study methodology, data on the wood load in the headwater steams as well as the stream's drainage area and gradient were collected and statistically analyzed. Results reveal that as wood load at La Selva ranges from 3 to 34·7m3/100m. It was revealed that wood load increases as the ratio of wood diameter/channel depth increases.