Eroding Cascadia—Sediment and Solute Transport and Landscape Denudation

Year: 2023
Presenter/s: Jim O'Connor
Symposium Session: 2023 - 01 Exploring Deep Geologic Time and Relevance to River Processes
Topics covered: california and sediment transport


Riverine measurements of sediment and solute transport give 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 tonnes per square kilometer per year (0.02 millimeters per year) for low gradient basins such as the Willamette River, to 500 tonnes 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.

This abstract derives from O’Connor and others (2021), Geological Society of America Bulletin,