Strath terrace and knickpoint formation in a coastal basin Draining to the Cascadia subduction margin, Smith River, Northern California

Strath terrace and knickpoint formation along mainstem channels are indicators of landscape response to tectonic and climatic forcing in the Smith River watershed in northwestern California. Knickpoints in the Smith River basin are manifest as a distinct point where there is a break in slope of the longitudinal profile at the upstream end of an oversteepened reach, the knickzone. Two types of knickpoints occur. At larger drainage areas in the lower portion of the basin, knickpoints and associated downstream knickzones occur within rock types with no significant difference in rock strength as confirmed by Schmidt hammer measurements. There is a distinct upstream convergence of the modern channel with the lowest elevated strath surface along these knickzones, resulting in elevated strath surfaces downstream of the knickzones. We infer that the knickpoints are migratory and that the straths are more vertically separated from the modern channel only after the knickpoint has migrated upstream. Reconstructed paleo-longitudinal profiles project to oxygen isotope stage 5e marine terraces near the coast and suggest that base-level fall as a result of falling sea level following late Pleistocene eustatic sea level highstands may engender migratory knickpoints now preserved upstream. At smaller drainage areas in the upper portion of the basin, the only knickpoints present in channels are those associated with large landslides that mobilize entire hillslopes into the channel and thereby force a channel response. Extensive preservation of strath terraces throughout the basin indicates that strath terraces record base-level lowering and subsequent incision of the river. In lower portions of the basin incision is achieved by knickpoint migration, producing strath terraces with long profiles that are nonparallel to the modern river profile. In upper portions of the basin periods of vertical incision and lateral planation are related to climate-driven fluctuations in sediment production on channel hillslopes resulting in strath terrace long profiles that do parallel the modern river profile.