WHEN WATER IS SCARCE AND ROCKS ARE FRAGILE, EQUATOR-FACING VALLEY NETWORKS OUT-STRETCH THEIR POLE-FACING COUNTERPARTS

Differences in the lengths of pole- and equator-facing slopes express links between landscape morphology and local climate. These topographic differences can result from variations in the efficiency of mass-transport processes sensitive to hillslope-scale, insolation-driven climate effects and the byproducts of these ‘micro-climates’ (e.g., vegetation differences). In the Gabilan Mesa (central Coast Ranges, CA), south-flowing valley networks are often > 2x longer than their north-flowing counterparts. Field relationships and topographic analysis suggest that south-flowing networks are lengthened by headward retreat of bare-bedrock knickpoints (hereafter ‘arroyos’). Arroyos are almost exclusively observed along south-facing slopes, which host thin soil mantles. When exposed, the local bedrock is pervasively fractured by slaking, causing it to lose its strength. Slaking likely results from the hydration of expansive minerals within the bedrock. We propose that weakening of exposed bedrock via slaking allows unstable expansion of arroyos and resultant lengthening of south-flowing channels. In contrast, bedrock along north-facing slopes is protected from slaking by a thick, relatively strong soil mantle. This hypothesis requires that valley network asymmetry is sensitive to rock type, which we test by comparing valley asymmetry in neighboring drainages developed in traditionally ‘weak’ and ‘strong’ rocks. Valley network asymmetry arises from the equator-ward alignment of low-order valley networks and is more severe in rocks expected to offer little resistance to erosion than in more resistant rocks when controlling for climate. This suggests that local pedogenic processes can sometimes shape the macroscopic structure of landscapes, and that aspect-driven differences in surface processes are sensitive to underlying rock type.