(This is also the CEOAS Geology & Geophysics seminar for the week.)
Jim O’Connor, USGS
Eroding Cascadia—Sediment and solute transport and landscape denudation in western Oregon and northwestern California

Sediment and mass-transfer budgets provide an overarching framework for comprehending landscapes. Their applicability ranges from understanding regional landscape evolution over millions of years to managing effects of historical land use on individual rivers. Despite their core importance, comprehensive mass erosion and transport budgets are challenging to attain, especially at broad scales, because of the variety of processes and feedbacks involved and the difficulty in making critical measurements. Here we present a comprehensive assessment based on riverine measurements of sediment and solute transport. These measurements underly empirical basin-scale estimates of bedload, suspended sediment, and silicate solute fluxes for 100,000 km² of northwestern California and western Oregon. This spatially explicit sediment budget shows the multi-faceted control of geology and physiography on the rates and processes of fluvial denudation. Bedload transport is greatest for steep basins, particularly areas underlain by the accreted Klamath Terrane. Bedload flux commonly decreases downstream as clasts convert to suspended load by breakage and attrition, especially for softer rock types. Suspended load correlates strongly with lithology, basin slope, precipitation, and wildfire disturbance. It is highest in steep regions of soft rocks, and our estimates suggest that much of the suspended load is derived from bedload comminution. Dissolution, measured by basin-scale silicate solute yield, constitutes a third of regional landscape denudation. Solute yield correlates with precipitation and is proportionally greatest in low-gradient and wet basins and for high parts of the Cascade Range, where undissected Quaternary volcanic rocks soak in 2–3 meters of annual precipitation. Combined, these estimates provide basin-scale erosion rates, ranging from ~50 tons per square kilometer per year (0.02 millimeters per year) for low gradient basins such as the Willamette River, to 500 tons per square kilometer per year (0.2 millimeters per year) for steep coastal drainages. The denudation rates determined here from modern measurements are less than those estimated by longer-term geologic assessments, suggesting episodic disturbances such as fire, flood, seismic shaking, and climate significantly add to long-term landscape denudation.

See the 2021 Thomas Condon Lecture page for more information.