OBSERVATIONS ON THE ROLE OF LITHOLOGY IN STRATH TERRACE FORMATION AND BEDROCK CHANNEL WIDTH

Field observations from western Washington and eastern Tibet indicate a strong lithologic influence on strath terrace formation and highlight the response of bedrock channel width to spatial gradients in bedrock erodibility and/or rock uplift rates. Measurements of local bedrock bed and bank erosion rates together with observations of the role of weathering processes on erosion of siltstones and sandstones illustrate a mechanism that underlies a conceptual model that predicts a strong lithological control on strath terrace formation. Direct measurements of bedrock erosion rates in lithologies susceptible to accelerated erosion upon sub-aerial exposure show that lateral rates of bedrock bank erosion can substantially exceed vertical incision below the perennial flow level due to an asymmetry in erodibility between perennially submerged rock and rock exposed to cyclic wetting and drying on bedrock channel walls. Under such conditions, the rapid, weathering-mediated lowering of bedrock exposed above baseflow promotes development of a beveled bedrock platform that could become a strath terrace upon abandonment. In contrast to previous studies, channel widths measured across a substantial gradient in lithology, slope, and long-term erosion rates in eastern Tibet do not follow conventional relations where channel width scales as a power law function of drainage area. Instead, as they flow across a zone of rapid uplift these channels systematically narrow relative to the predicted width extrapolated from traditional power-law relations. Field studies from a wide range of geological settings and lithologies support the interpretation that, in general, strath terraces tend to be more extensive in rivers flowing over weak sedimentary rock and tend to be poorly developed and/or preserved where rivers flow over hard, erosion-resistant rock.