Processes and rates of bedrock erosion in Welsh rivers and the implications for long-term landsacpe development

Many studies of Welsh rivers have examined contemporary processes and late Quaternary histories in alluvial reaches but the development of shorter, steeper bedrock reaches remains poorly understood. It is unclear whether the characteristic gorges, rapids and waterfalls have developed since the last glacial or are older, inherited features that have been exhumed from beneath glaciofluvial outwash and little modified since. On the River Rheidol, west Wales, we investigated four bedrock reaches up to 524 m long (8% of total river length). The reaches have developed in interbedded, moderately resistant, shales and sandstones with varying degrees of jointing, folding and small-scale faulting. Bedrock erosion during floods is by plucking and abrasion. Seeding experiments in well-developed potholes indicate active gravel exchange during floods, although gravel volumes vary with pothole size, location relative to bedrock outcrop, and inundation frequency. Pothole morphometries suggest that they deepen faster than they widen but deviations from idealised growth trajectories result from preferential widening along bedding planes or from block plucking around pothole rims. Contemporary erosion rates are poorly constrained but some rock engravings near water level have survived for decades and minimal
bedrock erosion occurred during extensive flooding (Q >100 m3/s) in June 2012. Coupled with estimations of reach antiquity derived using a published equation for waterfall retreat rate, the Rheidol bedrock reaches have probably developed intermittently during part of the Quaternary, having been buried by sediment and/or ice during glacial advances and exhumed by river activity during glacial retreat. By influencing upvalley transmission of baselevel changes, the relative stability of bedrock reaches in the Rheidol and possibly other Welsh rivers has implications for wider landscape dynamics, including patterns and rates of alluvial terrace, river profile and hillslope development.