Characterisation of channel reach morphology and associated controls in deglaciated montane catchments in the Cairngorms, Scotland

The morphological characteristics of 50 channel reaches were surveyed in montane streams in the River Dee catchment to test an existing process-based typology. Included in the analysis were common reach types documented elsewhere (e.g., cascade, step-pool, and pool-riffle). In addition, mixed bedrock–alluvial, alluvial transitional, and wandering reach types (that have received less attention) were also investigated given their common occurrence in the study area. Differences in geometry, substrate character, and flow resistance are evident particularly between principal types, although differentiation of mixed bedrock–alluvial and transitional types was less marked. Wandering reaches occupy a slope range that overlaps with other types but are distinguished by a grain size distribution that is intermediate between plane-bed and pool-riffle reaches and a high channel width for a given discharge. The quantitative differences observed, support visually based differentiation of channel types based on their bedform assemblages. Morphological characteristics were also related to a suite of variables to determine first-order controls on reach type. Channel cross-sectional geometry is strongly correlated with discharge, whilst grain size and indices of flow resistance are strongly correlated with slope and boundary shear stress in particular. In addition, the paraglacially conditioned geomorphic setting constitutes an important local control. The morphological properties of common channel types (bedrock, cascade, step-pool, plane-bed, and pool-riffle) broadly correspond with those observed elsewhere, although channel slope ranges were markedly lower. We hypothesise that channel types exist in a lower domain of slope because of a lower sediment supply rate that reflects the distinctive landscape evolution of the Cairngorms area.

Research Highlights
► We investigated the morphology of 50 reaches in the glaciated Cairngorm Mountains, Scotland.
► The study included mixed bedrock-alluvial, alluvial transitional and wandering reaches.
► The relevance of an existing channel typology and the importance of regional controls are discussed.