Morphological Classification of Fluvial Environments: An Investigation of the Continuum of Channel Types

Bedrock-controlled channel systems exhibit considerable morphological variation. Both bedrock-influenced and al- luvial morphological units coexist to form a system of changing channel patterns in response to changes in the relative influence of the controlling process variables. This article investigates the morphological composition of the bedrock- influenced Sabie River, Mpumalanga Province, South Africa, mapping 25 km of river channel at the scale of individual morphological units. Cluster and discriminant analyses define objective reach–scale “channel type” assemblages based on morphological unit composition. A number of robust clusters emerged that could be broadly classified into five channel types, namely, bedrock anastomosed, mixed anastomosed, pool-rapid, braided, and alluvial single-thread. The cluster analysis revealed that these channel types fit on a continuum from bedrock-dominated channels to fully alluvial systems. Each channel type could also be characterized by a certain set of “dominant” morphological units, which changed from bedrock-influenced at one end of the continuum (bedrock anastomosed, pool-rapid) to alluvial deposits at the other (braided, alluvial single-thread). An investigation of the role of controlling process variables in defining these channel types revealed a broad link between the degree of bedrock influence and the amount of available energy within the system as defined by indices such as the flow regime and water-surface slope variation. It is clear that the bedrock-dominated channel types are characterized by energy levels in excess of those accepted for alluvial systems, and an extended river classification is presented for the Sabie River that includes these bedrock channels. The mixed anastomosing channel type on the Sabie River is characterized by higher available energy levels than braided or alluvial single-thread reaches. As such, it appears to be a higher-energy example of an anabranched system, probably formed as a result of sediment accumulation on top of a high-energy bedrock anastomosed channel template