River channels and floodplains of two coastal rivers, NSW

River channels and floodplains of two coastal rivers, NSW

P. Rustomji

During 2002, significant progress has been made on the collection and analysis of geomorphologic data from two river catchments along the NSW coast, namely the Macdonald and Tuross Rivers. Both catchments are of about 2000 km2 in size, have highly confined floodplains, similar landuses, and climatic regimes, yet appear to have undergone different geomorphic evolution throughout the twentieth century despite similar climatic histories (though with different flood histories).

Along the Macdonald River, topographic surveying of the channel and associated flood plains has allowed clarification of the relationship between discharge events of various return periods and the various flood plain units evident in the valley. The sediments of the various morphologic units have been studied and described, and samples collected for OSL dating, which is underway. In addition, hydraulic modelling exploring boundary shear stress and mean stream power rates under various channel morphology and flow scenarios has begun, leading to estimates of the magnitude of these variables associated with different geomorphic processes such as bench accretion, minor channel scour and major bank erosion.

On the Tuross River, similar topographic surveying has been conducted on the channel and associated flood plains. The present day thalweg of the Tuross River from the lowest bedrock bar to mean sea level displays a concave upwards profile, which differs from the generally straight profile found along the Macdonald River. In addition, the Tuross River has a cobble bed along the upper lengths of the study reach (a feature absent along the lower Macdonald River). The relationship of this cobble bed to the flood plain units is under investigation. In a similar fashion to the Macdonald River, a number of morphologic flood plain units are evident along the Tuross River, each with a characteristic sedimentary profile. Sediment samples have been collected for OSL dating and being processed.

Striking similarities in flood plain stratigraphy and morphology occur between these two river systems. They share a high level alluvial bar feature with flood basins on their distal sides, and a lower alluvial bench unit apparently inset within this high level bar. Along both rivers, the high alluvial bars have similar sedimentology, consisting of fine to medium sands, dark grey to brown in colour, whilst the inset benches are commonly yellow sands with clearly defined dark horizons of finer sediments separating sand layers. The OSL dating of these features may indicate whether or not the formation of these morphologically and sedimentologically similar alluvial deposits was synchronous in time.