Transient Response and Adjustment Timescales of Channel Width and Angle of Valley-Side Slopes to Accelerated Incision

Abstract

Studying bedrock rivers during their transient states helps understand the response of a fluvial system to changed boundary conditions. Although studies show how river form adjusts to changes in incision or rock uplift rates, field constraints on the timescale of this adjustment are limited. We present a method that uses knickpoint travel time to estimate the adjustment times of channel width and angle of valley-side slopes to accelerated incision. The travel time of knickpoints between their current positions and the points where changes in width or hillslope angle have just finished represents the time required for morphological adjustment after knickpoint passage. We documented channel slopes, channel widths, and hillslope angles along six rivers that cross an active normal fault in Iwaki, Japan, and identified river sections in a transient state. Channel slopes and basin-averaged erosion rates determined from 10 Be concentrations are distinct between rivers near and distant from the fault, suggesting that past increases in fault throw rates triggered the knickpoint formation and the observed transient response.Adjustment time depends on the slope exponent in the detachment-limited model and is 2-5 times greater for channel width than hillslope angle, indicating that catchment adjustment times can be much longer than times predicted only by knickpoint travel time. The fact that channel slope, channel width, and hillslope angle have distinct adjustment times underlines the importance of correctly identifying river sections that are fully adjusted to the new boundary conditions when inferring erosion or relative uplift rates for bedrock rivers.