Coarse sediment movement in the vicinity of a logjam in a neotropical gravel-bed stream

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.