Comment on egusphere-2022-1285

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> We explore how rock properties and channel morphology vary with rock type in Last Chance canyon, Guadalupe mountains, New Mexico, USA. The rocks here are composed of horizontally to near horizontally interbedded carbonate and sandstone. This study focuses on first and second order channel sections where the streams have a lower channel steepness index (ksn) upstream and transition to a higher ksn downstream. We hypothesize that differences in bed thickness and rock strength influence ksn values, both directly by influencing bulk bedrock strength but also indirectly through the production of coarse sediment. We collected discontinuity intensity data (the length of bedding planes and fractures per unit area), Schmidt hammer rebound measurements, and measured the largest boulder at every 40-foot elevation contour to test this hypothesis. Bedrock and boulder minerology was determined using a lab-based carbonate dissolution method. High resolution orthomosaics and DEMs were generated from drone photos. The orthomosaics were used to map channel sections with exposed bedrock. The high-resolution DEMs were used to measure channel slope and hillslope relief. We find that discontinuity intensity is negatively correlated with Schmidt hammer rebound values. Channel steepness is higher where reaches are primarily incising through more thickly bedded carbonate bedrock. Where there is more thinly bedded sandstone rock exposed, channel steepness tends to be lower. Furthermore, the effect that rock properties have on channel morphology is confounded by sediment input from hillslopes. Thickly bedded rock units on surrounding hillslopes contribute larger sized colluvial sediment to the channels, and these reaches have higher ksn. Larger and more competent carbonate sediment armors both the carbonate and the more erodible sandstone and dampens the negative effect sandstone bedrock has on channel steepness. We believe that in the relatively steep, high ksn downstream channel sections slope is primarily controlled by the coarse alluvial cover. We further posit that the upstream low ksn reaches have a baselevel that is essentially fixed by the steep downstream reaches, resulting in a stable configuration where channel slopes have adjusted to lithologic differences and/or sediment armor.