Quantification of the transient response to base-level fall in a small mountain catchment: Sierra Nevada, southern Spain

We integrated optically stimulated luminescence dating and 10Be cosmogenic nuclide measurements to quantify short‐to‐medium‐term (102–104 years) catchment dynamics and response to active tectonics. In the 27 km2 Río Torrente catchment, Sierra Nevada, southern Spain, rapid base‐level fall has triggered knickpoint migration up both trunk and tributary channels, resulting in two distinct geomorphic zones: (1) a steep lower catchment with concordant rates of hillslope erosion and channel incision over both short (100 years: 5–8 mm yr−1) and medium (12 ka: 5 ± 1 mm yr−1) timescales and (2) a low‐relief soil‐mantled upland topography with uniformly low bedrock and hillslope erosion rates (0.05 ± 0.02 mm yr−1). The uniformity of erosion in this upland surface indicates that catchment topography was previously in steady state. Rapid river incision below the channel knickpoints has resulted in the development of steep landslide‐dominated hillslopes that are essentially “tracking” the incising channel. The magnitude of base‐level fall required to generate these steep hillslopes is >100 m; at least 50 m of this base‐level fall occurred during the past ∼21 ka. These steep hillslopes are eroding back into the low‐angled upland surface at a much slower rate than the channel knickpoint. Consequently, the trunk channel knickpoint has already reached the catchment headwaters while hillslopes continue to adjust to the new base level, indicating that the channel profile will regain equilibrium form long before hillslopes. Thus hillslopes are the limiting factor for the duration of landscape transience in this small mountain catchment.