In soil-mantled landscapes, the length of hillslopes is set by the relative efficiency of hillslope transport processes and incision by channelized flows. In cases where rapid channel incision exposes less-weathered, more-resistant rocks, incision should slow, creating a damping feedback that prevents runaway channel expansion. We present observations from the northern Gabilan Mesa (central Coast Ranges, California), which is underlain by mud-rich rocks that provide less resistance to erosion than the soils produced from them. This strength inversion arises because unconfined, mud-rich units of the Pancho Rico Formation undergo a process called slaking, in which pervasive fractures, spaced centimeters apart, develop when subjected to wet-dry cycles. In contrast, consecutive wetting and drying cycles have produced a cohesive regolith with higher tensile strength than the slaked bedrock. Exposure of the relatively weak bedrock may create a positive feedback, where the acceleration of erosion within the bedrock undermines the cohesive regolith and allows channels to expand headward at the expense of hillslopes. Geomorphic mapping reveals numerous channelized features that have incised deeply into bedrock, and crosscutting relationships and topographic analyses indicate that these features have consumed regions of the landscape previously occupied by hillslopes. However, these features are only present in south-flowing drainages, where soil is thinnest and therefore bedrock is most likely to be exposed by increases in erosion rate. Preferential headward expansion of these south-flowing channels has established a profound topographic asymmetry, where south-flowing tributaries occupy ∼75% of drainage basin width. These results suggest that processes operating at the scale of the critical zone, which may be unique to certain rock types, conspire to produce an instability that shapes the morphology of drainage basins.

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