Bedrock landsliding, river incision, and transience of geomorphic hillslopechannel coupling: Evidence from inner gorges in the Swiss Alps

[1] The formation of inner gorges cut into bedrock has been explained as relief rejuvenation by fluvial incision in response to rapid base level drop, repeated glaciations, frequent pore pressure–driven landsliding focused at hillslope toes, or catastrophic outburst flows from natural dam failures. Prominent inner gorges occur in soft Mesozoic Bündner schist and lower Tertiary flysch units of the formerly glaciated Alpenrhein catchment, eastern Swiss Alps. Their channel and hillslope morphologies differ from basins characterized by strong glacial or landslide imprints, while formally resembling the theoretically predicted transient response of detachment-limited bedrock rivers to rapid base level fall by headward knickpoint migration. Assuming a postglacial onset of fluvial bedrock incision into a Last Glacial Maximum surface in response to base level drop induced by downwasting of trunk valley glaciers requires downcutting rates E > 20 mm yr 1 and requires hillslopes to adjust by frequent landsliding toward development of a threshold state. We test this scenario using data on surface uplift, geomorphometry, geomorphic hillslope coupling, and probabilistic slope stability models. We find that adjustment of inner gorge walls through landsliding is mainly strength limited and structurally controlled, and threshold conditions are restricted to the lower 25% of local hillslope relief. Mass movement processes on upper hillslopes remain largely decoupled from channel incision despite inferred postglacial specific sediment yields of 10 m km 2 yr 1 from the studied basins. Conversely, several constraints imposed by fluvial bedrock detachment, postglacial sediment yields, and bedrock landsliding argue for a pre-Holocene origin for at least some of the inner gorges in the area. This implies partial protection of fluvial gorge topography by subglacial sediment fill during the last (Würm) extensive glaciation and implies that glaciers were insufficient to fully eradicate fluvially sculpted bedrock topography. This leads us to conclude that lithology and major climate oscillations should be considered as further alternative controls on inner gorge formation.