Controls on the distribution of channel reach morphology in selectively glaciated catchments

• Channel reach morphology reflects varied lithology and glaciation.
• A slope-area framework identified most bedrock and alluvial reaches correctly.
• The critical slope threshold was higher than other regions.
• Drier climate and coarse bed materials are hypothesised to increase the threshold.
• Most mixed bedrock-alluvial reaches occur in the bedrock reach domain.

To assess the controls on the distribution of channel reach morphology in a selectively glaciated landscape, we used field mapping and a geographical information system (GIS) in the River Dee catchment, northeast Scotland. Controls on channel morphology were investigated using (1) continuous longitudinal assessment of channel morphology distribution in relation to geology, glacial history, topography, and total stream power (Ω) in two subcatchments, and (2) slope (S), Ω, and a slope–drainage area (S–A) framework to understand the occurrence of 173 widely distributed bedrock, mixed bedrock–alluvial, and alluvial (three different types) reaches. The S–A framework used indicators of transport capacity (Qc) and sediment supply (Qs) to differentiate channel types. The study highlights the disjointed nature of channel reach distribution at the river scale that reflects variable lithology and glacial modification. Because of the subdued topography in contrast to other regions, colluvial forcing of channel morphology in the headwaters was lacking. However, in common with other glaciated landscapes, repeated sequences of channel reach type progression determined by valley steps were evident. The S–A analysis successfully discriminated 87.2% of alluvial and 91.4% of bedrock reaches despite the variable land use and glacial modification. Discrimination of the full range of channel types using S, Ω, or the S–A framework was poor however. Notably, a third of the transport alluvial reaches were located in the bedrock S–A domain, and the majority of mixed reaches were widely distributed mostly within the bedrock domain and not close to the critical slope (Sc). In comparison to other regions, the Sc above which Qc > Qs and bedrock reaches dominate, was notably higher. We hypothesise that a drier climate and the higher entrainment threshold of coarse, granite-dominated bed materials create a higher Sc.