Abstract:
This study addressed the effects of hilislope and fiuvial processes on spatial
patterns of stream bed particle size at the watershed, reach and within-reach scales.
The study was conducted in Lookout Creek watershed, a fifth-order, 64 km2 basin in
the Western Cascades 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 hilislope controls. At the watershed scale,
hydraulic controls explain around 50% of the variation 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 variation. 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.
