Mixed bedrock-alluvial rivers: characteristics, controls, consequences Seminar

Traditionally, bedrock and alluvial rivers have been regarded as fundamentally different in character, with bedrock rivers being shaped primarily by lithological and structural controls, and alluvial rivers being shaped primarily by flow and sediment transport processes. Research into the role of sediment transport in bedrock channel incision, however, has led to an increasing realisation that many rivers worldwide in fact are transitional between fully bedrock and fully alluvial. In these rivers – variably termed ‘mixed bedrock-alluvial’, ‘semi-alluvial’, ‘bedrock-controlled’, ‘bedrock-influenced’ and so on – certain reaches may be characterised by alluvium that forms a thin or discontinuous veener over bedrock, so that bedrock commonly crops out in the channel bed or banks. In these rivers, empirical datasets and conceptual models developed for fully bedrock or fully alluvial rivers are not adequate for describing many aspects of channel-floodplain behaviour and broader fluvial landscape development.

Focusing on the South African interior, this talk will outline the variable characteristics of mixed bedrock-alluvial rivers, and explain the controls on their development. The consequences of mixed bedrock-alluvial river processes for fluvial landscape development will be illustrated through case studies of: i) river pattern development (e.g. meandering versus non-meandering); ii) floodplain and terrace sedimentology (including the implications for geoarchaeological investigations); and iii) long profile development (e.g. anabranching). The talk will conclude by emphasising the critical need for additional geochronological data to help determine rates and timings of bedrock and alluvial processes.