CONTROLS on SEDIMENT TRANSPORT INTERRUPTION BY A LOGJAM IN A TROPICAL GRAVEL-BED STREAM

In most fluvial systems where wood is present it increases channel roughness, alters flow hydraulics, and decreases sediment transport. In many locations the collection of wood into jams leads to even greater interruption of sediment transport by impounding sediment behind barriers and creating low-velocity backwater flow zones. We examined tracer clast movement near a wood jam in an unmanaged 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 poorly predicted by clast location relative to the jam, but better predicted by flow magnitude and grain size, factors which previous researchers have found to control tracer clast movement. The rate of clast passage through the jam did not correlate with either the piece retention rate of the jam, which showed consistently high piece turnover, or with the changing 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 to the flows in which wood is able to disrupt sediment transport. During moderate flows at our site, gravel is mobilized while the jam is stable, enabling the jam to interrupt gravel transport if its structure is favorable. During higher flows, those that mobilize cobble size clasts, wood is also mobilized and is thus less able to interrupt sediment transport. This bound appears to be frequently crossed at our study site. Thus we infer that compared to bedrock or boulder roughness elements, which are stable to much higher flows, wood is less able to interrupt sediment transport or increase flow resistance in high energy settings. In low energy settings, however, wood may be as effective as bedrock or boulders, and possibly more important because of its potentially greater abundance.