The sensitivity of Scottish rivers and upland valley floors to recent environmental change

Rivers in humid zones are delicately adjusted, within narrow limits, to evacuate the water and sediment supplied from upstream without significant changes in channel type and the morphology of the valley floor. Prevailing climate and land use are the major controls determining the nature of that adjustment. Significant environmental change (either climate and/or land use) has the potential to change radically the channel type and destabilise the adjacent valley floor. Channel metamorphosis (in which a river shifts from an aggrading to a degrading regime, or from a single thread to multiple thread channel) often provides the clearest evidence that environmental change has occurred. This paper examines the stability of river channels and their associated valley floors in the uplands of Scotland since 1750. Changes in global-scale atmospheric circulation patterns during this period have resulted in regionally distinct “flood-rich” and “flood-poor” periods with potential geomorphic impacts. Changes in the planform of nine active rivers in the piedmont zone of upland Scotland are examined from series of rectified maps and aerial photographs at scales close to 1:10 000. At none of the sites (other than one affected by an artificial increase in base level) is there evidence of abrupt changes in river behaviour or channel metamorphosis. However, rates of lateral channel shift and extents of bare gravels do vary and these are clearly driven by changes in the incidence of floods since the mid 19th century. The scale of valley floor reworking is less dramatic than that recently reported for comparable sites in northern England. Differences in geomorphic history, land use and degree of slope-channel coupling are identified as the likely causes of marked differences in the behaviour of active rivers in Scotland and northern England over the last 250 years.