Downstream Fining in a Mountain Stream Channel Affected by Debris Flow

Grain size of particles tend to become smaller in the downstream direction. Abrasion and selective transport are two sets of processes commonly accepted as explanations for observed trends in fining of sediment. Most recent studies have emphasized the effectiveness of selective transport in producing downstream fining in streams with abundant sediment supply. The contribution of abrasion to particle fining of the coarsest class of particles was investigated in Finney Creek, a high gradient mountain stream in northwest Washington that has a high incidence of sedimentation from debris slides and debris flows. Two dominant rock types comprise the coarsest bed material in the studied reach; foliated particles, which are derived from the local bedrock, and non-foliated particles, which are derived from glacial valley fill. Four distinct downstream trends of particle fining are spatially associated with sources of recent deposits of coarse clasts in the channel. While particle sizes of both rock types diminish rapidly from the debris source, overall fining trends are influenced most by the fining trend evident in the foliated class of particles. The primary fining mechanisms are different for the two rock types, and are related most strongly to the inherent durability of each rock type. Selective transport is probably most important for non-foliated particles, and active but overwhelmed by abrasion for foliated particles. Field observations and experimental abrasion studies indicate that abrasion is the dominant set of processes responsible for the reduction of sizes of foliated particles, which abrade at about 10 times the rate of non-foliated particles.