Influence of mass wasting on bed-surface armoring, lag formation, and sediment storage in mountain drainage basins of western Washington State

Abstract

This dissertation uses field studies and analyses of digital topography in western Washington State to investigate (1) hydraulic controls on the spatial variation in bedsurface armoring, (2) grain-size controls on the formation of resistant "lag" deposits, and (3) grain-size and lithologic controls on the dispersion rate and residence time of sediment in mountain channels. Field and topographic analyses document systematic downstream coarsening of the median bed-surface grain size in channels with a drainage area < 1 km2 and a subsequent shift to the conventional pattern of downstream fining at a drainage area of about 10 km2. The grain-size maximum corresponds with the maximum unit stream power and the inflection in the drainage area-slope relation thought to represent the transition from debris flow-dominated channels to fluvially dominated channels. The results suggest that basin-wide trends in bed-surface armoring are hydraulically controlled by systematic variations in unit stream power and should therefore be common in headwater channels where debris-flow processes set the channel gradient. Results of a four-year case study of channel recovery from blockage by a landslide dam indicate that, in addition to hydraulic controls, trends in bed-surface armoring are forced by the temporary accumulation of lag deposits arising from the reworking of mass-wasting deposits. Results indicate that the caliber of supplied sediment relative to the flow competence of the receiving channel can influence the rate of sediment-pulse dispersion and the residence time of sediment storage. Measurements of sediment volumes stored in recent and ancient landslide dams in western Washington indicate that the coarsest deposits retain > 80% of initial debris after 103 years, exhibit long-term incision rates that are within one to two orders of magnitude of regional bedrock-lowering rates, and create backwater lakes that provide sediment "capacitance" to mountain drainage basins. Results of this research have practical implications for understanding the residence time of sediment storage in mountain river systems, the disruption of sediment routing, the enhancement of fluvial relief through the shielding of bedrock surfaces, and the timescale of channel recovery following natural and anthropogenic disturbances.