Morphological classification of fluvial environments: An investigation of the continuum of channel types

Bedrock-controlled channel systems exhibit considerable morphological varia tion. Both bedrock-influenced and alluvial morphological units coexist to f orma system of changing channel patterns in response to changes in the rela tive influence of the controlling process variables. This article investiga tes the morphological composition of the bedrock-influenced Sabie River, Mp umalanga Province, South Africa, mapping 25 km of river channel at the scal e of individual morphological units. Cluster and discriminant analyses defi ne 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 ana stomosed, pool-rapid, braided, and alluvial single-thread. The cluster anal ysis revealed that these channel types fit on a continuum from bedrock-domi nated channels to fully alluvial systems. Each channel type could also be c haracterized by a certain set of "dominant" morphological units, which chan ged from bedrock-influenced at one end of the continuum (bedrock anastomose d, pool-rapid) to alluvial deposits at the other (braided, alluvial single- thread). An investigation of the role of controlling process variables in d efining these channel types revealed a broad link between the degree of bed rock influence and the amount of available energy within the system as defi ned by indices such as the flow regime and water-surface slope variation. I t is clear that the bedrock-dominated channel types are characterized by en ergy levels in excess of those accepted for alluvial systems, and an extend ed river classification is presented for the Sabie River that includes thes e 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 o f an anabranched system, probably formed as a result of sediment accumulati on on top of a high-energy bedrock anastomosed channel template.