Quantifying Fluvial Erosional Response to Volcanic Deposition in the Southern Cascades

Prior research on stream incision has focused on tectonic and climatic forcing factors, whereas ultra-fast aggradation of volcanic material has received far less attention. Over a ca. 3 Ma span on the western flank of the southernmost Cascade arc of western North America, several pulses of volcanic material with varying composition and volume inundated stream channels causing the gradient to increase and reset incision. Incision rates associated with volcanic deposition were estimated using stratigraphic and geomorphic relationships. Stream incision rates were high over timescales associated with multiple inset subunits from the Maidu and Lassen Volcanic Centers. Measuring stream interaction with these repeated flows show stream incision rates ranging from 48.286 mm/yr averaged over ~7 ka to 0.299 mm/yr averaged over ~400 ka. Incision rates show an asymptotic decrease with elapsed time after volcanic deposition. In much of the region, the lack of detailed volcanic stratigraphy required estimation of stream incision rates using the age of a volcanic center and the time elapsed since the eruptive episode, yielding relatively lower incision rates ranging from 0.2491 to 0.0315 mm/yr over relatively longer time intervals ranging from ~1080 to ~3400 ka. Stream profile analysis in the southernmost Cascades shows multiple knickpoints in the various streams, whereas streams in the northern Sierra Nevada, a region with well documented tectonic uplift, typically have one major knickpoint. The multiple knickpoints along southernmost Cascade streams are associated with drainages that lack hanging tributaries, in contrast to the northern Sierra streams where hanging tributaries are typical. These differences suggest knickpoints in the southernmost Cascades formed in response to volcanic aggradation instead of tectonic uplift, consistent with the lack of incision into basement beneath the volcanics. This contrasts the northern Sierra streams that have significant (up to 1.2 km) incision into basement beneath the base of volcanic deposits, as well as hanging tributaries.