KNICKPOINT RETREAT ON SMALL BEDROCK RIVER CATCHMENTS AS A CONSEQUENCE OF BASE-LEVEL FALL: THE CASE OF JURA (WEST SCOTLAND, UK)

Shifts in river base-level are indicative of major changes in prevailing tectonic and/or climatic conditions. Base-level fall of rivers can trigger knickpoints that propagate upstream transmitting disequilibrium to the landscape. Understanding knickpoint propagation and channel transience is fundamental to understanding how landscapes respond to changes imposed by tectonics and climate. We evaluated the effect of base-level drop in small bedrock rivers catchments (<100 km²) caused by a glacio-isostatic rebound after melting of the British Ice Sheet (~16000 yr B.P.) on the Isle of Jura (west Scotland). Using stream profile analysis and measuring sediment samples on river beds we evaluated: (1) knickpoint retreat distance from the former base-level prior to its drop ~13500 yr B.P. (2) stream profile morphology (concavity) in transient reaches; and (3) sediment size through a knickpoint reach. We found that the distance of knickpoint retreat from the former post-glacial shoreline scales closely with drainage area (a proxy for stream discharge) in a power law function D_r=cA^b. Extracting channel concavity by a normalized index measured from knickpoint lip to post-glacial shoreline we identified a drainage area of ~2-4 km², below which rivers seem unable to respond fully to base-level fall, Evaluating the coarse fraction (D₉₅) of sediment sampled on channel bed through the knickpoint reach to assess if rivers are adjusting to base-level fall, we found no fining of sediment from upstream of the knickpoint to the river’s outlet. Lack of downstream fining of sediment suggest a secondary effects of kickpoint propagation by destabilizing hillslopes which supply sediments to rivers. Our results highlight the sensitivity of fluvial processes in small bedrock river catchments to changes imposed by tectonic forces and/or climate.