Strath terrace formation: the influence of rock type, climate, and humans

Abstract

Strath terraces record cycles of bedrock river incision and planation, and thus reflect how mountainous landscapes evolved in response to changing climate, tectonics, rock type, and other forcings. Previous work has used strath terrace age and geometry to back-calculate uplift rates and sediment production, but tends to focus on long wavelength rock uplift rates or marine isotope stage glaciation and so neglects short term (<10 ky) terrace formation. Here, I investigate the role of rock type and structure, interglacial climate, and human action on terrace formation using field studies and literature reviews with a focus on Holocene terraces. Through field mapping, terrace dating, and geospatial analyses of the Willapa River, WA, and Nehalem River, OR, I find that rock type controls valley width and the potential for terrace formation and preservation. Slaking prone rocks erode rapidly in transport-limited conditions, and durable bedload from the headwaters can enhance erosion rates such that an extensive 10 ky terrace and an inset 100 yr terrace formed. Following up on the 100 yr terrace, I used field mapping and terrace dating in the central Cascades, WA, to find that strath terrace formation was caused by anthropogenic wood loss ca. 100 yr ago. Loss of wood decreased sediment retention and led to river incision by exposing previously covered bedrock. Anthropogenic terrace formation through wood loss is plausibly a global phenomenon; my literature review reveals terrace formation in the late Holocene is often coincident with deforestation. However, interglacial climate fluctuations coincide with and are also likely to contribute to late Holocene terrace formation, which expands on the prevalent theory that terrace formation in response to climate is dominated by glacial-interglacial cycles. My results show that basins are more sensitive to river incision than previously recognized, and that smaller amplitude climatic forcings, as well as anthropogenic forcings, can have a large impact on the topography of fluvial systems. That impact is sensitive to the internal basin characteristics, such as rock type and structure, that set the potential for erosion and landform preservation.