WASTING (AND DISSOLVING) AWAY IN WESTERN OREGON

Broad-scale empirical assessments of measured bedload, suspended load, and solute loads for western Oregon and northwestern California show the physiographic and lithologic controls and magnitudes of sediment and solute production. Bed-material yield is a function of generalized rock type and catchment slope. Downstream bedload flux depends on upstream yield and in-transit comminution, which depends on rock type (O’Connor and others, 2014; GSAB, v. 26, p. 377–397). An empirical regression updated from Wise and O’Connor (2016; USGS SIR 5079) shows that suspended sediment flux similarly depends on rock type and catchment slope as well as catchment precipitation and the presence of impoundments. Solute load (of silicate-derived major cations) for 18 measurement stations is significantly correlated to catchment precipitation. Together these data show that for the measurement stations, solute load contributes 10–70% (mean 36%) of the total rock-mass loss. Bedload and suspended sediment fluxes also average about a third of the total flux at a given location within western Oregon and northwestern California, but bedload flux generally exceeds suspended load flux higher in watersheds and vice-versa downstream (as bedload becomes suspended load by comminution). Total modern (post ~1950) rock erosion rates for large watersheds range from 0.024 mm/yr for the Willamette River at Portland, Oregon to 0.06 mm/yr for the Klamath River near Klamath, California. Of the 18 measurement sites, the highest erosion rates are 0.11 mm/yr for the Smith River within the Klamath Mountains in northern California and the 0.12 mm/yr for the West Fork Millicoma River in the central Oregon Coast Range. Measurement-based modern erosion rates are 50–90% less than cosmogenically determined geologic landscape erosion rates, perhaps indicating the importance of episodic disturbances such as major regional flooding, wildfire, and earthquakes in significantly influencing longer-term overall landscape erosion.