The effects of hillslope and fluvial processes on particle size of the stream bed at the watershed, reach, and within-reach scales in a fifth-order mountain stream

This study addressed the effects of hillslope and fluvial processes on spatial patterns of stream bed particle size at the watershed, reach , and within-reach scales. This study was conducted in Lookout Creek watershed, a fifth-order, 64 km² basin in the western Cascade mountains of Oregon. Stream bed particle size was measured at 25 sites on first- through fifth-order streams. Boulder density was measured from the headwaters of the mainstem of Lookout Creek to its mouth, approximately 16 kilometers of stream length. In Lookout Creek watershed, spatial patterns of particle size result from a hierarchy of hydraulic and hillslope controls. At the watershed scale, hydraulic controls explain around 50% of the variance in d50 and d84. Particle size is related to watershed-scale trends in stream power and stream competence. At the reach scale, debris flows and landslides leave a patchy signature on stream bed particle size. Patches of both high and low boulder density are associated with landslides. A peak in density is associated with a February 1996 debris flow. Hydraulic controls are less evident at this scale, although the degree of reach constraint may affect particle size in fourth- and fifth-order Lookout Creek. At the within-reach scale, hydraulic controls are responsible for around 20% of the observed particle size variance. The effects of the largest flood on record at Lookout Creek were documented at 7 sites that had been sampled before the flood. At the watershed scale, a fining in particle size was observed; this is consistent with the hypothesis that Lookout Creek has an armor layer which the flood disturbed. At the within-reach scale, changes in particle size appeared stochastic, and provided little information about the role of hydraulic controls at that scale.