The existence of a channel-funning, 'dominant' or 'bankfull' discharge has been applied, with some success, to a variety of alluvial river systems in temperate areas. This paper presents the results of an investigation into the relationship between flow magnitude and frequency, and the geomorphological units of the Sable River in the Mpumalanga Province, South Africa. The river is perennial, however, it exhibits an extreme seasonal flow regime. The river has been subject to incision (100,000-10,000 years ago), which exposed bedrock areas as topographic highs within a wide macro-channel. Zones of deposition have created alluvial sections resulting in a morphologically diverse river system. Stage-discharge relationships have been observed and constructed for 23 monitoring sites on the Sable River. These have been used in conjunction with a 62-year calibrated simulated daily discharge record, to generate inundation frequencies for the morphological units present at each site. This includes results using annual maximum series data and time spans for activation, using the continuous daily flow data. The results reveal that the overall form of the cross-section is not related to a single channel-forming discharge, instead, a complex relationship exists whereby the section is influenced by the entire flow regime. The results do however suggest a poorly defined division of morphological units within the incised macro-channel, namely, those influenced by perennial, seasonal and infrequent flows. This relationship is best demonstrated by the morphological units associated with the perennial, active channels in alluvial sections. which correlate with the low flow regime (1-2-year return period on the annual maximum series), and larger macro-channel deposits which are related to rarer higher flows. Climatic wet and dry cycles, human influences and the physical effect of bedrock in the macro-channel further complicate the inundation pattern, resulting in disequilibrium conditions between channel form and contemporary flow regime on the Sable River. Macro-channel sedimentary deposits, ill particular, show no consistent relationship with a particular segment of the flow regime, being inundated by flows of between a 10- and 50-year return period. (C) 2001 Elsevier Science B.V. All rights reserved.
